Very Important Paper

Two-Dimensional Transition Metal Chalcogenides for AlkaliMetal Ions StorageYingxi Zhang+,[a, d] Liao Zhang+,[a, b] Tu’an Lv+,[c] Paul K. Chu,*[d] and Kaifu Huo*[a]

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1. Introduction

Owing to the ever-growing global energy demand coupled

with excessive consumption of fossil fuels and exacerbatingenvironmental impact, efficient and environmentally friendly

electrochemical energy storage (EES) technologies are highly

desirable.[1] Batteries and capacitors are two dominant types ofelectrochemical energy-storage devices. Since the commercial

introduction by Sony in 1991, rechargeable lithium-ion batter-ies (LIBs) have been widely implemented in mobile electronics

such as cell phones, portable computers, electric vehicles, andstorage of renewable clean energies.[2, 3] However, the uneven

distribution and limited resources of Li lead to the high cost of

LIBs, thereby restricting more extensive application of LIBs forlarge-scale energy storage of renewable energy generated by

wind power and solar energy, and smart grid.[4, 5] Na and K are

in the same alkali-metal group, following Li, that have the

same outermost ns valence electron configuration. Consideringthat their physicochemical properties are similar to Li, their low

cost and earth abundance, Na-ion batteries (SIBs) and K-ionbatteries (PIBs) have recently been proposed as alternatives to

LIBs, especially for large-scale grid storage.[6, 7]

EES devices contain two electrodes, the anode and the cath-ode, which largely determine the performance of rechargeable

batteries and hybrid supercapacitors. A typical commercial LIBcombines a Li-intercalation compound, for example, LiCoO2 or

LiFePO4, as cathode with a graphite anode. During the dis-charging and charging process, Li ions are intercalated/de-in-

tercalated into/from the interlayers of the graphite anode

through the formation of graphite intercalation compounds.[8]

Although graphite has dominated the market of anode materi-

als in LIBs so far, its low theoretical specific capacity of372 mAhg@1 results in a small energy density of 120–

150 Whkg@1 in LIB packs, which cannot satisfy the demand forhigh-energy LIBs.[9–12] Therefore, much effort has been made toexplore novel anode materials with higher specific capaci-

ties.[13] The Na- or K-ion-storage chemistry in SIBs and PIBs isquite similar to that of Li ions in LIBs[14–16] and knowledge

gained from LIBs can be applied to SIBs and PIBs.[17–19] Howev-er, the larger ionic radius of Na and K ions compared to Li

(1.38 a for K+ , 1.02 a for Na+ vs. 0.76 a for Li+) cause largevolume variations, sluggish ionic diffusion and electrochemical

reaction kinetics during charging/discharging, thus renderingthe direct transfer of LIB technology to NIBs and PIBs morecomplex and challenging.[20,21] For example, graphite materials

used in commercial LIBs can hardly accommodate Na+ inser-tion causing an insufficient capacity (lower than

35 mAhg@1) ;[22] therefore, it is crucial to develop high-per-formance versatile anode materials to overcome the bottle-

neck plaguing LIBs, SIBs, and PIBs.

Transition-metal dichalcogenides (TMDs) with a sandwich-like MX2 structure (M=Mo, V, W, Nb and X=S, Se, Te) are typi-

cal layered materials consisting of a hexagonally packed layerof metal atoms sandwiched between two layers of chalcogen

atoms by strong chemical bonds. The sandwiched layers arestacked together by weak van der Waals forces.[23] Bulk TMDs

On the heels of exacerbating environmental concerns andever-growing global energy demand, development of high-

performance renewable energy-storage and -conversion devi-ces has aroused great interest. The electrode materials, which

are the critical components in electrochemical energy storage(EES) devices, largely determine the energy-storage properties,and the development of suitable active electrode materials is

crucial to achieve efficient and environmentally friendly EEStechnologies albeit the challenges. Two-dimensional transition-

metal chalcogenides (2D TMDs) are promising electrode mate-rials in alkali metal ion batteries and supercapacitors becauseof ample interlayer space, large specific surface areas, fast ion-transfer kinetics, and large theoretical capacities achieved

through intercalation and conversion reactions. However, they

generally suffer from low electronic conductivities as well assubstantial volume change and irreversible side reactions

during the charge/discharge process, which result in poor cy-cling stability, poor rate performance, and low round-trip effi-

ciency. In this Review, recent advances of 2D TMDs-based elec-trode materials for alkali metal-ion energy-storage devices with

the focus on lithium-ion batteries (LIBs), sodium-ion batteries

(SIBs), potassium-ion batteries (PIBs), high-energy lithium–sulfur (Li–S), and lithium–air (Li–O2) batteries are described.

The challenges and future directions of 2D TMDs-based elec-trode materials for high-performance LIBs, SIBs, PIBs, Li–S, and

Li–O2 batteries as well as emerging alkali metal-ion capacitorsare also discussed.

[a] Y. Zhang,+ L. Zhang,+ Prof. K. HuoWuhan National Laboratory for OptoelectronicsHuazhong University of Science and TechnologyNo.1037 Luoyu Road, Wuhan 430074 (P.R. China)E-mail : kfhuo@hust.edu.cn

[b] L. Zhang+

China-EU Institute for Clean and Renewable EnergyHuazhong University of Science and TechnologyNo.1037 Luoyu Road, Wuhan 430074 (P.R. China)

[c] T.’a. Lv+

The Key Laboratory of Refractories and Metallurgy and Institute of Ad-vanced Materials and NanotechnologyWuhan University of Science and TechnologyNo. 947, Heping Avene, Wuhan 430081 (P.R. China)

[d] Y. Zhang,+ Prof. P. K. ChuDepartment of Physics, Department of Materials Science and Engineering,and Department of Biomedical EngineeringCity University of Hong KongTat Chee Avenue, Hong Kong (P.R. China)E-mail : paul.chu@cityu.edu.hk

[++] These authors contributed equally to this manuscript.

The ORCID identification number(s) for the author(s) of this article canbe found under:https://doi.org/10.1002/cssc.201903245.

This publication is part of a joint Special Issue with Batteries & Supercapsfocusing on “2D Energy Storage Materials”. Please visit the issue athttp://doi.org/10.1002/cssc.v13.6.

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exist in a wide variety of polymorphs and stacking prototypes,and the most common polymorphs are 1T, 2H, and 3R, where

the letters correspond to trigonal, hexagonal, and rhombohe-dral, respectively, and the digit indicates the number of X–M–X

units in the unit cell, that is, the number of layers in the stack-ing sequence (Scheme 1).[24] TMDs possess similar layered

structures as graphite but have larger inter-layer spacing (gen-erally larger than 6 a) and weaker van der Waals interaction inthe out-of-plane direction. The larger interspacing and electro-

static stabilization effects of the negatively charged X2@ ionsenable them to accommodate Li+ and larger Na+ and K+ ionsfor efficient electrochemical energy storage.[25] Moreover, theinterlayer distance in TMDs can be adjustable by changing the

synthetic conditions or carbon intercalation to enhance metal-ion storage.[26–28] Storage of Li+ , Na+ , or K+ involves multi-elec-

tron mechanisms including intercalation, conversion, and pseu-

docapacitance[29–31] making it possible for TMDs to have alarger theoretical capacity than graphite. Compared to the

metal-oxide counterparts, TMD insertion tends to be moreelectrochemically reversible due to the faster charge-transfer

kinetics.[25] These characteristics make TMDs promising versatileelectrode materials in LIBs, SIBs, PIBs, and hybrid alkali metal-

ion capacitors (AICs). In fact, around 1970 researchers discov-ered that a range of electron-donating molecules and ions

could be intercalated into the layered dichalcogenides, in par-ticular, TaS2.

[32] The first LIBs patented in 1970–1980 (Exxon)

employing TMDs such as TiS2, NbSe2, NbSe3, or TaS2 as cathodematerials and Li metal and LiAl alloys as anodes.[33] In 1977–1979, Exxon marketed button cells with LiAl anodes and TiS2cathodes for watches and other small devices. However, suchcells posed several practical challenges, including serioussafety concerns; consequently, Exxon decided to halt the proj-ect.[34]

As an analogue of graphene, few-layer two-dimensional (2D)TMDs possess some unique chemical, physical, and electronic

properties different from the bulk counterparts.[35] The small

thickness and large surface-to-volume ratio of 2D TMD nano-sheets provide abundant active sites, low intercalation barriers,

and short diffusion paths for metal-ion storage, consequentlyenabling efficient accommodation of Li+ as well as larger Na+

and K+ with a large capacity and sufficient electrochemical re-actions.[36] Compared to traditional insertion or conversion re-

actions of anode materials such as graphite and bulk TMDs,

the electrochemical reactions of 2D TMDs can be both surfacefaradaic and non-faradaic processes, resulting in larger pseudo-

capacitance and faster reaction kinetics.[37] Moreover, thestrong polarity and catalytic properties of TMDs render them

possible applications in high-energy Li–S and Li–O2 batteries.Furthermore, the thin and flexible characteristics of 2D TMD

nanosheets make it possible to construct flexible LIBs, SIBs,

and emerging EES devices.[38,39] Therefore, 2D TMDs havearoused great research interest and are considered promising

versatile electrodes in EES devices, including LIBs, SIBs, PIBs,and AICs[30,36,40] as well as high-energy Li–S and Li–O2 batteries

(Scheme 2).[41–43]

Various 2D TMDs-based electrode materials have been pre-

pared via mechanical exfoliation,[44] chemical vapor deposition

(CVD),[45,46] liquid exfoliation,[47–49] and chemical synthesis.[30,50]

However, the van der Waals attraction between TMD nano-

sheets makes them prone to aggregate during electrode fabri-cation, reducing the accessible surface areas and underminingthe electrochemical performance unique to the 2D structuresin LIB, SIBs, and PIBs.[51] Moreover, most of the TMDs such as

2H phase MoS2 and WS2 have poor electronic conductivity,large voltage polarization, and poor rate capability during cy-cling.[30,52,53] Under constant alkali-ion intercalation in LIBs, SIBs,

and PIBs, the conversion reaction takes place in TMDs to pro-duce soluble alkali-ion chalcogenides (such as Li2S, Na2S or

K2S).[54–56] Similar to Li–S batteries, the soluble polychalcoge-

nides intermediates produced in situ could be dissolved in

ether-based electrolytes, giving rise to the polychalcogenide

shuttle effect,[57,58] which results in large irreversibility, effusionof active anode materials, inferior cycling stability, and low

round-trip efficiencies.[59] Introduction of carbon materials,metal oxides, or MXene nanosheets to form 2D TMDs compo-

sites or 2D heterojunction has been demonstrated to be an ef-fective strategy to improve the energy storage characteristics

Paul K. Chu received his PhD in

chemistry from Cornell University. He

is Chair Professor of Materials Engi-

neering in the Department of Physics,

Department of Materials Science and

Engineering, and Department of Bio-

medical Engineering at City University

of Hong Kong. He is a Fellow of the

American Physical Society (APS), Amer-

ican Vacuum Society (AVS), Institute of

Electrical and Electronics Engineers

(IEEE), Materials Research Society

(MRS), Hong Kong Institution of Engineers (HKIE), and Hong Kong

Academy of Engineering Sciences (HKAES). His research interests

are quite diverse, encompassing plasma surface engineering, mate-

rials science and engineering, surface science, and functional mate-

rials. He is a highly cited researcher (2016–2019) in materials sci-

ence/joint field according to Clarivate Analytics.

Kaifu Huo received his BS in applied

chemistry from China University of Pe-

troleum in 1997 and his PhD in physi-

cal chemistry from Nanjing University

in 2004. He is currently a Professor in

the National Laboratory for Optoelec-

tronics at Huazhong University of Sci-

ence and Technology. He is a Fellow of

the Royal Society of Chemistry (FRSC).

He has authored/co-authored more

than 160 papers in international refer-

eed journals, which have been cited

more than 6900 times (current H-index: 45). His main research ac-

tivities encompass bioactive nanomaterials and nanostructured

electrode materials for electrochemical energy storage and biosen-

soring.

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of 2D TMDs in LIBs, SIBs, PIBs as well as high-energy Li–S andLi–O2 batteries.[43, 60–62] Recently, several reviews summarized

the preparation methods of 2D TMD nanomaterials as well astheir electrochemical applications in LIBs, supercapacitors, andelectrocatalysts.[51,62–64] However, few of them focus on 2D

TMDs-based hybrid materials for the alkali-metal-ion storageproperties, especially for SIBs, PIBs, AICs, and high-energy Li–S

batteries. In this Review, recent progress pertaining to thepreparation strategies of 2D TMDs and their hybrid materials

as well as the alkali metal ion storage properties for LIBs, SIBs,

PIBs, AICs, and Li–S and Li–O2 batteries is reviewed (schemati-cally shown in Scheme 3). The challenges and future directions

of 2D TMD-based electrode materials for emerging EES devicesare also discussed.

2. Synthesis of 2D TMDs and Compo-sites

The discovery of graphene in 2004 has aroused enor-

mous research interest in 2D nanomaterials.[65] TMDspossess lamellar structures similar to graphite but

have weaker van der Waals interactions between theX–M–M layers. Therefore, single- and few-layer 2DTMDs can be produced via top-down exfoliation of

layered bulk crystals including mechanical cleav-age,[66] chemical Li intercalation,[67] exfoliation with n-butyllithium (BuLi),[68] electrochemical Li intercala-tion,[69] and liquid-phase exfoliation by direct sonica-

tion in solvents.[20] Alternatively, 2D TMDs and theirhybrid nanomaterials can be prepared using bottom-

up approaches such as CVD,[46,70,71] hydro(solvo)ther-

mal synthesis[51,72,73] and electrospinning.[74] Sincethere are already several reviews on the preparation

methods of 2D TMDs nanosheets,[75,76] herein we onlyfocus on the preparation strategies for 2D TMDs and

3D hybrid architectures based on 2D TMD nanomate-rials.

2.1. Exfoliation

TMDs have a sandwich-like MX2 structure, in which

the intralayer M@X bonds are predominantly covalentand the sandwiched X–M–X layers are linked by weak van der

Waals forces so that single- and few-layer 2D TMDs can be pro-duced by mechanical and solution exfoliation from the bulk

flakes similar to graphene.[75] In 2004, Novoselov et al. preparedsingle-layer 2D MoS2 and NbSe2 from the layered bulk materi-

als by mechanical cleavage.[77] However, the low throughput ofmechanical exfoliation has restricted practical implementations.Liquid chemical exfoliation methods are likely to be better for

Scheme 1. Structures of A) layered graphite as well as those of B) 1T, C) 2H, and D) 3R-phase TMDs together with the corresponding top and side views.

Scheme 2. TMD electrodes in alkali-ion batteries.

Scheme 3. Synthesis strategies of TMDs and their applications in LIBs, SIBs,PIBs, and emerging AICs and Li–S and Li–O2 batteries.

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the production of few-layer 2D TMDs with a high yield. Bydirect sonication of layered TMDs in suitable solvents, 2D

MoS2, WS2, MoSe2, NbSe2, TaSe2, and MoTe2 nanosheets havebeen prepared by this simple method.[47,67, 78,79] The sonication-

assisted exfoliation technique is based on the solvent or sur-factant to overcome the van der Waals forces between adja-

cent layers, meaning that the solvents must have surface ener-gies that are comparable to those of the target materials.[80] Al-though sonication-assisted liquid exfoliation can produce 2D

TMDs in large amounts, the yield of single-layer nanosheets isrelatively low and the lateral size of the nanosheets is relativelysmall as well.[81] Recently, Truong et al. demonstrated a super-critical fluid (SCF) process to synthesize few-layer (1–10 layers)

MoS2 and MoSe2 (Scheme 4) in dimethylformamide (DMF).[82]

The SCF solvent offers advantages including low interfacial ten-

sion, excellent surface wetting, and large diffusion coefficients,rendering it a superior medium for diffusion between the

layers of TMDs and interlayer expansion. As a result, rapid and

high-yield exfoliation can produce a large amount of few-lay-ered MoS2 or MoSe2 nanosheets in a short reaction time of

1 h.[82] The chemical Li-intercalation method is another effectivemethod for single-layer 2D TMDs by using n-butyl lithium dis-

solved in hexane as the intercalation agent. The intercalated Limetal reacts violently with water, producing hydrogen gas that

causes the MX2 layers to separate, producing a suspension of

single-layer TMDs in water. During this process, Li intercalationresults in local rearrangement of the atomic structure of MoS2from the 2H to the 1T phase and it is difficult to control the lo-cation and amount of the 1T phase through controlled expo-

sure of single-layer TMD to Li. To address this issue, Chhowallaet al. developed a controllable electrochemical Li intercalation

method by controlling the cut-off voltage of the cell usingbulk TMDs as the electrodes and single-layer 2D nanosheets ofMoS2,WS2, TiS2, TaS2, and ZrS2 and few-layer NbSe2 nanosheets

were prepared by controllable Li intercalation and subsequentexfoliation in water or ethanol.[85] However, Li-intercalated

TMDs are very air sensitive, flammable, and reactive during ex-foliation and thus intercalatants that are less sensitive to ambi-

ent conditions have been explored. For example, Jeffery and

co-workers prepared ammoniated MoS2 and WS2 by reactingLixMS2 (M=Mo, W) with a saturated solution of NH4Cl, in which

NH3/NH4+ is intercalated into the layered LixMS2. The ammoni-

ated TMDs exfoliate readily in polar solvents, yielding colloidal

dispersions of MoS2 or WS2 nanosheets with large lateral di-mensions (Scheme 5).[83] When layered bulk TMDs are exfoliat-

ed into 2D thin sheets, the prismatic edges and basalplanes are exposed and edge termination by either

M or X atoms depends on the chemical potential ofthe growth environment and type of coordination

bonds at the edges. Recently, 2D MoS2 with averagethicknesses between 0.8–1.5 nm were prepared via

ligand conjugation of chemically exfoliated MoS2. The

zeta-potential and surface functionality of the MoS2sheets can be tuned to produce improved physical

and chemical properties boding well for broad appli-cation (Scheme 6).[84] To further optimize the properties specific

to a given application, ultrathin 2D alloyed TMD nanosheetssuch as MoS2xSe2(1@x), Ta2NiS5, and Ta2NiSe5 were prepared by

exfoliating the bulk layered crystals.[86] For example, Xie and

co-workers demonstrated that single-layer MoxW1@x S2 alloynanosheets can be prepared by the exfoliation method.[87]

2.2. Hydro(solvo)thermal method

The solvothermal/hydrothermal-based bottom-up approachesare simpler and safer for the synthesis of 2D TMDs.[36,88, 89] The

Scheme 4. Exfoliation mechanism of TMDs by SCFs. Reproduced with permission fromRef. [82] Copyright 2017 American Chemical Society.

Scheme 5. Steps in the synthesis, exfoliation, and restacking of ammoniatedMS2 (M=Mo, W). Reproduced with permission from Ref. [83] Copyright 2014American Chemical Society.

Scheme 6. Structural models illustrating ligand conjugation of the MoS2sheets. Reproduced with permission from Ref. [84] . Copyright 2013 Ameri-can Chemical Society.

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solvothermal/hydrothermal synthesis is carried out by dissolv-ing the precursors in water or an organic solvent before intro-

duction into an autoclave that is then sealed and heated to atemperature normally less than 250 8C. Under hydrothermal/

solvothermal conditions, M and X ions (or complexes) react toform MX2 nuclei that accumulate and grow into 2D nano-sheets.[90] The solvothermal/hydrothermal synthesis can pro-duce 2D TMDs with a high yield at a relatively low cost. How-ever, flower-like and few-layer nanosheets and other hierarchi-

cal architectures such as spheres, flowers, boxes, and tubes arenormally obtained rather than single-layer TMDs by this

method.[17,91–93] Li and co-workers synthesized hierarchical VS2nanosheet assemblies (NSA) comprising aligned ultrathin nano-

sheets using vanadyl acetylacetonate (VO(acac)2) and cysteineas the precursors in N-methyl-2-pyrrolidone (NMP)

(Scheme 7).[94] During the reaction, cysteine decomposes grad-

ually and releases H2S to react with the vanadium precursorand the as-generated individual nanosheets with a thickness of

a few atomic layers are hierarchically arranged with minimized

stacking to form the VS2 NSA. NMP as the reaction solventplays a critical role because it has a suitable surface energy

very close to that of VS2, thus promoting the formation andstabilization of the ultrathin TMD nanosheets during the solvo-thermal reaction. Electrochemical measurements show that the

VS2 nanosheet assembly can serve as a versatile host to enablerapid and durable storage of Li+ , Na+ , or K+ . Hydro(solvo)ther-

mal synthesis can also be used to prepare 3D TMDs/graphenehybrid materials with assistance of carbon-containing precur-sors,[95,96] graphene oxide (GO),[97,98] or 3D graphene foam.[99]

For example, Chang and Chen fabricated layered MoS2/G com-

posites by an l-cysteine-assisted solution-phase method andsubsequent annealing in H2/N2. The layered MoS2 is anchoredon the graphene surface (Scheme 8) whereas graphene inhibits

the growth of layered MoS2 crystals in the composites, espe-cially along the (002) plane of MoS2, during the hydrothermal

process and annealing.[96] The MoS2/G composites that exhibita 3D architecture consisting of curved nanosheets show a spe-

cific capacity of ~1100 mAhg@1 at a current density of

100 mAg@1 and no capacity fading after 100 cycles as LIBanodes. During the hydro(solvo)thermal process, the solvent

molecules or surfactant in the solution can be trapped be-tween the layers of the MX2, and if organic molecules are inter-

calated between the MX2 interlayers, a 2D TMD/C atomic het-erojunction is obtained after the thermal treatment. For in-

stance, Jia et al. prepared MoS2/N-doped-C porous bamboo-

like tubes (Scheme 9A) by annealing MoS2/oleylamine (OAm)tubes that were synthesized by a solvothermal method using

MoO3 as the molybdenum source, S powder as thesulfur source, OAm as the additive, and an ethanol–

water mixture as the solvent.[17] The porous MoS2/N-doped-C bamboo-like tubes are constructed of MoS2sheets sandwiched between N-doped-C layers with

an expanded interspacing of 10 a between the MoS2nanosheets. Insertion of N-doped C decreases the ad-

sorption energy of K+ from @1.29 to @2.73 eV(Scheme 9B). Benefiting from the synergetic effects

of N-doped-C layers and single-layer MoS2, the MoS2/N-doped-C tubes are high-capacity and stable anode

materials for PIBs showing a high specific capacity of

451 mAhg@1 at 50 mAg@1 in the 1st cycle as well asstable cycling performance maintained at 330 mAhg@1 after

50 cycles. Using the hydro(solvo)thermal synthesis combinedwith macrosized templates such as 3D graphene foams,

carbon cloths, carbon fibers, commercial paper towels, andcarbon nanotube papers, freestanding 3D TMD-based compo-

sites were obtained.[51,100–102] Xie et al.[103] prepared MoS2 nano-

sheets that are vertically aligned on the carbon paper derivedfrom paper towels by hydrothermal processing and subse-quent annealing (Figure 1A). The large surface-to-volume ratioof the nested structure with nanoreservoirs between adjacent

MoS2 nanosheets (Figure 1B) facilitate the interactions be-tween the MoS2 and electrolyte and provide short ion-diffusion

pathways while the carbon fibers is a good current collectorand provides electrical contact for the active materials and alow charge-transfer resistance (Figure 1C). As a result, the 3DMoS2@C architecture exhibits a high reversible capacity of446 mAhg@1 at 20 mAg@1, an initial coulombic efficiency (ICE)

of 79.5%, and rate capability of 205 mAhg@1 at a high currentdensity of 1000 mAg@1 for Na-ion storage.[103] In addition to 2D

TMD/carbon composites, heterojunction structures can also

improve the conductivity and reaction kinetics by adjustingthe semiconducting behavior.[105] Zhao et al.[104] synthesized a

carbon-free nanocomposite consisting of MoO2 nanoparticlesembedded between MoSe2 nanosheets (MoO2@MoSe2). The

MoO2 nanoparticles increase the electron conductivity of thelamellar composite whereas the self-built electrical field at the

Scheme 7. Synthesis and spectroscopic characterization of VS2 NSA. Reproduced withpermission from Ref. [94] Copyright 2017 John Wiley and Sons.

Scheme 8. Parallel growth of the MoS2 nanosheets on graphene by the hy-drothermal method. Reproduced with permission from Ref. [96] . Copyright2011 American Chemical Society.

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interface between MoO2 and MoSe2 further promotes diffusionof Li ions, favoring insertion and extraction due to the hetero-junctions; this can induce a new electric field from the MoSe2nanosheets to MoO2 nanoparticles to enhance migration of Liions (Scheme 10).[104] Moreover, Li et al.[106] prepared crystallineVOOH-coated VS2 microflowers (c-VS2@VOOH) consisting of

nanosheets by a one-step hydrothermal method. The VOOHcoating with a stable structure not only relieves the volume ex-pansion of VS2 during the sodiation/desodiation process but

also enhances transportation of both Na+ and e@ due to thepseudocapacitive nature. The c-VS2@VOOH composite as SIB

anode shows excellent cycling stability of 330 mAhg@1 after150 cycles at 0.2 Ag@1 and high reversible capacities of 356

and 224 mAhg@1 even at 0.5 and 1.0 Ag@1, respectively.[106]

2.3 Chemical vapor deposition

Compared to the hydro(solvo)thermal synthesis, CVD produces

2D TMDs with high quality and precisely controlled layer num-bers.[107] MoO3 and S powders are the typical precursors for

Figure 1. A) Schematic illustration of the preparation of MoS2 vertically aligned on carbon paper including MoS2 loading by hydrothermal deposition followedby annealing under Ar. B) SEM images of the vertically aligned MoS2@C composite. C) Schematic illustration showing the paths of Na-ion diffusion and elec-tron conduction in the MoS2@C electrode. Reproduced with permission from Ref. [103] Copyright 2016 Wiley.

Scheme 10. Enhanced ion transport mechanism of the MoO2@MoSe2 hetero-junction structure in LIBs. Reproduced with permission from Ref. [104] Copy-right 2016 Royal Society of Chemistry.

Scheme 9. A) Synthesis of MoS2/N-doped C tube. B) K adsorption in MoS2/MoS2 and MoS2/N-doped C/MoS2 interlayer with the formation energies. The green,yellow, black, blue, and purple balls represent the Mo, S, C, N, and K atoms, respectively. Reproduced with permission from Ref. [17] Copyright 2018 Wiley.

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CVD deposition of single-layer MoS2 on planar SiO2/Si.[108] Re-

cently, graphite paper,[109] carbon cloths,[110] and graphene

foams[111] were employed as templates to produce 2D TMDs toform 3D TMDs.[51] For example, Geng et al. prepared hierarchi-

cal 3D MoS2 nanosheets on graphene-mediated Ni foam byCVD[112] and Shi et al. synthesized MoS2/graphene hybrid heter-ostructures with a graphene-covered Cu foil as the substrateand template (Scheme 11).[60] In the CVD process, the structureof TMDs is diffusion controlled at different lateral distances

from upstream. For example, Zou et al. prepared three types ofmicrostructures of dendritic WSe2 on carbon nanofiber mats(CFM) by diffusion-controlled CVD using (NH4)6H2W12O40 and

PAN (polyacrylonitrile) as the precursors for W and

CFM and Se power as the Se source as illustrated inFigure 2.[113] When the location is near upstream, the

Se vapor concentration is believed to be highly su-

persaturated while a smaller concentration is ob-served at a faraway distance. Therefore, the structure

of WSe2 can be diffusion controlled at different lateraldistances. Consequently, position A generates den-

dritic WSe2 on CFM, which is defined as d-WSe2/CFM,smooth leaves of WSe2 on CFM with a vivid rose-like

shape (defined as r-WSe2/CFM) are produced at posi-

tion C, and the structure generated at position V ex-hibits a transition morphology (Figure 2). Highly crys-

talline MoS2 nanosheets with few layers (about5 layers) anchored on the 3D porous carbon nano-

sheet networks (3D FL-MoS2@PCNNs) are producedwith cubic NaCl particles as the template

(Scheme 12).[114] The 3D NaCl self-assembly not only

serves as the template to grow the porous 3Dcarbon nanosheet networks but also offers a 2D-con-

Scheme 11. Parallel CVD growth of MoS2 nanosheets on the graphene-cov-ered Cu foil. Reproduced with permission from Ref. [60] . Copyright 2012American Chemical Society.

Figure 2.WSe2 growth at different positions in a quartz furnace tube, where A, B and Crepresent different thermal treatment positions for (NH4)6H2W12O40 and PAN precursor.Reproduced with permission from Ref. [113]. Copyright 2015 Royal Society of Chemistry.

Scheme 12. Fabrication procedure of 3D FL-MoS2@PCNNs using NaCl as template. Reproduced with permission from Ref. [114]. Copyright 2015 AmericanChemical Society.

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fined space for in situ construction of few-layer MoS2 nano-sheets on the carbon nanosheet walls.[114] Beside single-layer or

few-layer 2D TMDs, 2D large-area (> tens of micrometers) WS2-MoS2 hetero-nanostructures were prepared by stacking 2D WS2and MoS2 in the CVD process (Figure 3A–C). The heterojunc-tion of WS2 and MoS2 exhibits a type-II band alignment (Fig-

ure 3D), which efficiently separates charge carriers or rectifiescharge flow.[115] Besides direct growth on a substrate, 3D TMDarchitectures can be obtained from 3D metal oxide structures

by reacting with chalcogen vapors. Recently, a quasi-2D core–shell nanostructure of MoO2/MoSe2 nanosheet arrays was fabri-cated by separating the vertical MoO2 nanosheet arrays oncarbon cloth in Se vapor (Scheme 13).[116] Moreover, the combi-

nation of the quasi 2D core–shell nanostructure and vertical

array microstructure produces synergistic properties to boostthe hydrogen evolution reaction performance.[116]

2.4. Electrospinning

Electrospinning is a versatile synthesis method of 1D nanofib-ers (NFs) and applied in the fabrication of 2D TMDs/C hybrid

fibers.[117] Typically, the TMDs or the precursors are mixed with

organic precursors to form a homogeneous mixture, which un-dergoes electrospinning at a specific voltage to form 1D

fibers.[118] Zhu et al. fabricated composites of single-layeredMoS2 nanoplates (thickness of about 0.4 nm and lateral dimen-

sion of 4.0 nm) embedded in carbon nanofibers (CNFs, ca.50 nm) by electrospinning and subsequent annealing (Fig-

ure 4A).[74] The layers of MoS2 are uniformly embedded in the

carbon fibers to form hybrid NFs (Figure 4B). This unique struc-ture not only favors electron and ion transport but also accom-

modates large volume changes during alkali ion insertion/ex-traction, thus giving rise to outstanding Li+- and Na+-storage

properties. Zhou et al. adopted similar strategies to synthesizea WS2/CNFs composite, with the annealing temperature influ-

encing the structure of WS2 and alkali ion diffusion in the

TMDs (Scheme 14).[119] Ryu et al. synthesized mixed sulfidestructures composed of amorphous WS3 and crystalline WS2

Figure 3. A) Optical image taken from a monolayer WS2/MoS2 heterostruc-ture prepared by CVD WS2 on the MoS2 monolayer. B,C) Raman scatteringmapping of the A’1 mode, 408 cm@1 (MoS2) and 421 cm@1 (WS2) ; D) bandalignment and charge transfer in the heterostructure (upper) and schematicof the heterostructure displaying strongly coupled and weakly coupled re-gions with an interlayer spacing of dA and dB, respectively. Here deq,dA<dB

where the deq is the interlayer distance in equilibrium. The band alignmentis at the K-point of the WS2/MoS2 heterostructure in Region B and at the G-(VBM; valence band maximum)and K- (CBM; conduction band minimum)points in Region A. Reproduced with permission from Ref. [115] Copyright2014 American Chemical Society.

Scheme 13. Formation of vertical core–shell MoO2/MoSe2 nanosheet arrays.Reproduced with permission from Ref. [116] Copyright 2016 Wiley.

Figure 4. A) Schematic illustration of the electrospinning process to preparesingle-layered MoS2–CNF composites. B) Schematic representation based onTEM modeling to demonstrate the unique morphology of the composites:single-layered MoS2 nanoplate embedded in a thin CNF. The large blackspheres, small black spheres, and white spheres correspond to Mo, C, and Satoms, respectively. Reproduced with permission from Ref. [74] Copyright2014 Wiley.

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phases by electrospinning of (NH4)2WS4/SAN (poly(styrene-acrylonitrile)) fibers and calcination. The heterogeneous WSx/

WO3 core–shell hybrid with a thornbush-like NF structure deliv-ered an enhanced Na-ion storage performance.[120] By perform-

ing ex situ XRD on the disassembled Na electrode, modifica-

tion of the metal sulfide electrode with an oxide passivationsurface layer (Figure 5) was shown to alleviate S dissolution to

achieve a high capacity and good cycling performance.

3. 2D TMDs-Based Electrode Materials for Elec-trochemical Energy Storage

Layered TMDs possess a structure analogous to graphite buthave larger interlayer spacing (e.g. , 0.63 nm of MoS2 vs.

0.33 nm of graphite) and weaker van der Waals interactions be-tween the layers, consequently allowing intercalation of Li+

and larger ions of Na+ , K+ , even Mg2+ .[36] Therefore TMDs

have attracted considerable attention as versatile electrodematerials for alkali metal ion storage.[121] Various reports have

shown that 2D TMDs with atomic or molecular thicknesses andinfinite planar lengths decrease the ion-diffusion barriers and

increase the adsorption energy compared to the bulk counter-parts, resulting in rapid ion intercalation and better cycling.[122]

Although there are some excellent reviews on 2DTMDs for energy storage and conversion,[51,123,124]

there have been few systematic reviews on the appli-cation of 2D TMDs as electrode materials in alkali

metal-ion batteries, especially SIBs, PIBs, AICs as wellas high-energy Li–S and Li–O2 batteries. Herein, weprovide a summary on the current status of 2D TMDsfor rechargeable LIBs, SIBs, PIB, and emerging AICs,Li–S and Li–O2 batteries.

3.1. 2D TMDs for LIBs

The anodes in commercial LIBs are made of graphite,which has a theoretical specific capacity of

372 mAhg@1, forming the Li-intercalated graphitecompound LiC6.

[8] 2D TMDs have a layered structure

similar to graphite but with larger interspacing andcapacity. Lithiation/delithiation of layered 2D TMDs is

based on intercalation and ensuing conversion reac-tions to deliver a higher capacity than graphite.[11,125] The large

interspacing and surface area of 2D TMDs offer more active

sites for Li+ storage, which effectively reduces Li+ diffusionand electron-transport distance, resulting in large pseudocapa-

citance and good rate performance.[126] 2H-phase MoS2 hasbeen widely studied as a high-capacity electrode in LIBs due

to the large theoretical specific capacity of 669 mAhg@1 basedon the following conversion reaction: MoS2+4Li++

4e@QMo+2Li2S.[127,128] During discharging, Li ions intercalate

in the MoS2 layers to form LixMoS2, which is converted into Li2Sand metallic Mo through a reversible conversion.[129] Shu et al.

studied the thermodynamic phase diagrams and lithiation dy-namics of MoS2-based nanostructures using first-principles cal-

culation and ab initio molecular dynamics simulation.[130] Con-tinuous intercalation of Li ions induces structural destruction

from 2H to 1T and then layer-by-layer dissociation due to the

release of S atoms. The in situ-precipitated S atoms are respon-sible for the high capacity of MoS2 nanomaterials in the first

discharge. However, the conversion between Li2S and S duringcharging/discharging causes large irreversibility in the follow-ing cycles due to the formation of polysulfide intermediatesthat dissolve in the electrolyte and produce the shuttle effect

between the anode and cathode.[58] Moreover, the low conduc-tivity and large volume expansion of TMDs arising from the

conversion mechanism during Li+ intercalation lead to largeelectrochemical polarization, poor cycling stability, and inferiorrate performance.[11]

Integration of 2D TMDs nanosheets with conductive gra-phene sheets,[131–134] CNTs,[135,136] CNFs,[137] and carbon

foam[138,139] is an effective method to enhance the electrochem-ical properties of semiconducting 2D TMDs such as MoS2 and

WS2. In particular, composites consisting of 2D TMDs and gra-

phene have been investigated as anode materials in LIBs. Forexample, our group demonstrated that MoS2 nanoflakes verti-

cally grown on N-doped carbon nanosheets (MoS2/NC) couldserve as promising anode material for LIBs.[140] MoS2 nanoflakes

vertically aligned on the NC nanosheets, produced by hydro-thermal treatment of dodecylamine (DDA) intercalated MoO3

Scheme 14. Change in the crystallinity of WS2 with thermal treatment and lithiation pat-terns of WS2/CNFs-500 (I), WS2/CNFs-700 (II) and WS2/CNFs-900 (III), respectively. Repro-duced with permission from Ref. [119]. Copyright 2016 Elsevier.

Figure 5. Schematic illustration of the reaction mechanism during cell opera-tion and photos of the Na counter electrodes. Reproduced with permissionfrom Ref. [120] Copyright 2016 American Chemical Society.

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nanosheets in a thiourea solution and subsequent annealingunder Ar, provide sufficient active sites for fast Li+ diffusion

and storage whereas NC provides a high conductivity for elec-tron transfer. Consequently, the MoS2/NC electrode shows a re-

markable capacity of 803 mAhg@1 at a current density of100 mAg@1, high rate capability of 554 mAhg@1 at

2000 mAg@1, and excellent cycling stability.[140] Wang et al.[133]

prepared MoS2 nanosheets on electrochemically exfoliated gra-phene (EG). EG with low thickness, low oxidation degree, and

high electrical conductivity functions as both the lightweightsubstrate for MoS2 deposition and current collector during bat-tery operation (Figures 6A,B). The close contact between theMoS2 nanosheets and EG prevents structure deterioration and

provides good electron transport. As anode materials in LIBs,EG-MoS2 with 95 wt% MoS2 has a high specific capacity of

1250 mAhg@1 after 150 cycles at 1 Ag@1, average CE of 99.2%,

and excellent rate performance (970 mAhg@1 at 5 Ag@1). Incontrast, a MoS2 flower electrode prepared under similar con-

ditions initially shows stable cycling but fast capacity fading to425 mAhg@1 after 150 cycles (Figure 6C).[133]

Teng et al.[132] synthesized 2D MoS2/graphene hybrids inwhich individual 2D MoS2 layers are vertically aligned on the

basal plane of graphene through strong C@O@Mo bonds (Fig-

ures 7A,B). The graphene sheets improve the electrical con-ductivity of the composite and at the same time act as a sub-

strate to disperse active MoS2 nanosheets homogeneously aswell as a buffer to accommodate the volume changes during

cycling. The interfacial interaction of C@O@Mo bonds furtherenhances electron transport and structural stability of the

MoS2/G electrode. As anode materials in LIBs, the 2D MoS2/gra-

phene electrode (Figure 7C) shows a stable capacity of1077 mAhg@1 at 100 mAg@1 after 150 cycles, excellent rate ca-

pability, and long cycle life (907 mAhg@1 at 1000 mAg@1 after

400 cycles).[132] Shen and co-workers[141] synthesized honey-comb-like MoS2 nanoarchitectures on 3D graphene foam (HC-

MoS2@GF) by a P123 (PEO–PPO–PEO, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide))-assisted hydro-

thermal process, in which MoS2 nanosheets with a thickness of3 nm form honeycomb-like blocks on 3D GF (Figure 8A,B). The

3D GF provides pathways for efficient electron and Li-ion trans-portation during the charging/discharging cycles and serves as

the current collector without conductive additives. The HC-

MoS2@GF electrode exhibits good cycling stability and deliversa capacity of 1100 mAhg@1 with a capacity retention of 99%after 40 cycles at 200 mAg@1 (Figure 8C) as anode materials inLIBs due to the large surface area, porous hierarchical struc-

ture, conductive substrate as current collector, and 3D inter-penetrating structure.[141] In contrast, MoS2 hollow spheres (HS-

MoS2) prepared under the same conditions but without GF ex-

hibit lower capacity and inferior cycling stability. In addition tographene and reduced graphene oxide (rGO), 1D CNTs and

CNFs can also be used as substrates to integrate with 2DTMDs due to the high electrical conductivity and process sim-

plicity.[74,142] Different types of 2D TMDs hybrids such as 2DMoS2/CNTs in 3D networked structures,[143] cylindrical 2D MoS2/

CNTs,[144] hierarchically integrated 2D MoSx/MWNTs (multi-

walled carbon nanotubes),[145] and 2D VS2/CNTs[146] were pre-

pared as promising anode materials in LIBs. For example, Chen

et al. designed hierarchical MoS2 nanosheets assembled tubu-lar structures that were internally wired by CNTs. The material

shows a high specific capacity of 1320 mAhg@1 at a currentdensity of 0.1 Ag@1, good rate capability, and cycling lifetime

up to 1000 cycles.[92]

Besides 2D MoS2, other TMDs such as WS2,[119] VS2,

[146]

VSe2,[147] and MoSe2

[148] were investigated as anode materials in

LIBs. For example, Wang et al. synthesized a hybrid nanoarchi-

Figure 6. A) Schematic illustration of the preparation procedure and structure design strategy of vertical growth of EG-MoS2. B) Dark-field TEM images of EG-MoS2. C) Cycling performance of EG-MoS2 and MoS2 at a current density of 1 Ag@1. Reproduced with permission from Ref. [133] Copyright 2018 Wiley.

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tecture aerogel composed of WS2 nanosheets and CNT-rGOwith an ordered microchannel 3D scaffold structure by a

simple solvothermal method followed by freeze drying andpost annealing. The WS2/CNT-rGO aerogel nanostructure shows

a specific capacity of 749 mAhg@1 at 100 mAg@1 and an ICE of53.4%.[149] Ming et al. synthesized VSe2 nanosheet assemblies

by a solvothermal process using NMP as the solvent to limitcrystal growth along the c axis. The carbon-coated VSe2 nano-

sheets show a capacity of 768 mAhg@1 for Li+ storage and ex-cellent rate performance and cycling stability.[147]

2D TMDs with expanded interlayer spacings show excellentelectrochemical properties as electrodes in LIBs because the in-

Figure 7. A) schematic illustration showing the Li-ion diffusion pathways in the MoS2/G composite electrode. B) SEM image of MoS2/G. C) Cycling performanceof the MoS2/G, MoS2, and rGO electrodes at a current density of 100 mAg@1 for 150 cycles. Reproduced with permission from Ref. [132] Copyright 2016 Amer-ican Chemical Society.

Figure 8. A) Schematics of the fabrication of the honeycomb-like MoS2 nanoarchitecture@GF. B) Field-emission (FE)-SEM image of the honeycomb-like MoS2nanoarchitecture@GF. C) Cycling performances of the HC-MoS2@GF, HS-MoS2@GF, and HS-MoS2 electrodes at a current density of 200 mAg@1. Reproducedwith permission from Ref. [141] Copyright 2014 John Wiley and Sons.

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creased interlayer spacing enables effective accommodation ofLi+ in the interlayer gaps without significant volume variation

during charging/discharging cycles, resulting in enhanced Li-ion storage capacity, rate retention capability, and cycling sta-

bility.[64] Wen and co-workers[150] described a polyvinyl pyrroli-done (PVP)-assisted hydrothermal method to synthesize 3D ra-

dially oriented MoS2 nanospheres (Figure 9A). The absorbedPVP surfactant protects the 2D MoS2 nanosheets from restack-ing. The nanosheets in the MoS2 nanospheres are less than fivelayers thick and show an expanded (002) plane from 0.63 to0.71 nm (Figure 9B–E), which facilitates the storage and trans-port of Li ions. The interlayer-expanded MoS2 nanosphere elec-trode shows a large specific capacity of 1095.7 mAhg@1 at

100 mAg@1 after 110 cycles (Figure 9F), which is much higherthan that of PVP-free MoS2 nanosheets with an interlayer spac-

ing of 0.63 nm.[150] Huang et al. synthesized interlayer-expand-

ed WS2 nanosheets on 3D graphene (3DG).[151] The WS2 nano-sheets have an expanded interlayer spacing of 9.58 a due to

oxygen incorporation to provide more space for Li ion interca-lation. As LIB anode materials, the WS2/3DG composite shows

a high capacity of 766 mAhg@1 at 100 mAg@1 after 100 cycleswith 98% capacity retention, whereas the annealed MoS2nanosheets shows continuous and progressive capacity fading,yielding a capacity of only 416 mAhg@1 after 100 cycles. The

combination of interlayer-expanded MX2 nanostructures andconductive carbon intercalation offers an effective approach to

prepare high-performance anode materials for LIBs. The en-larged interlayer distance of MX2 provides large space for fastion intercalation and diffusion and the intercalated carbon ma-

terials supply enough conduction channels for electron trans-port and also prevent MX2 nanosheets from aggregation.[36]

Jiang et al.[152] prepared 2D MoS2-mesoporous carbon inter-overlapped nanosheets (MoS2/m-C) by amination of oleic acid

(OA)-protected single-layer MoS2 nanosheets with dopamine,self-polymerization of dopamine in the interlayer of MoS2, and

finally annealing of MoS2/polydopamine (MoS2/PDA) at 850 8Cfor 2 h (Figure 10A). In the MoS2/m-C superstructure, thesingle-layer MoS2 and mesoporous carbon (m-C) are stacked al-

ternately and the MoS2/m-C hybrid nanosheets with alternat-ing single layers provide large interspacing in MoS2 (0.98 nm)

Figure 9. A) TEM images of the 3D radially oriented MoS2 nanospheres. B) High-magnification TEM image of the MoS2 nanospheres. C,D) HRTEM images corre-sponding to area 1 and area 2 in (B), respectively. E) Structural model of ultrathin MoS2 viewed from the [110] and [001] directions. The Mo and S atoms arepurple and green, respectively. F) Cycling performance of the MoS2 nanospheres and MoS2 nanosheets and CE of the MoS2 nanospheres in the voltage rangeof 0.01–3.0 V vs. Li/Li+ at a current density of 100 mAg@1. Reproduced with permission from Ref. [150] Copyright 2015 American Chemical Society.

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and maximum atomic interface contact/interaction between

the single-layer MoS2 and the carbon nanosheet (Fig-ure 10B,C). Electrochemical experiments revealed that theMoS2/m-C nanosheets have large capacities of 1183, 1113,1088, 1035, 990, and 943 mAhg@1 at current densities of 200,400, 800, 1600, 3200, and 6400 mAg@1 (Figure 10D), respec-tively, which are higher than those of the MoS2/graphene com-

posite, exfoliated graphene, and annealed MoS2 nano-sheets.[152]

2D/2D heterostructured architectures prepared by stacking

different 2D materials provide an opportunity to constructhigh-performance electrodes for LIBs because they combine

the advantages of individual 2D building blocks while eliminat-ing the associated shortcomings.[153] The 2D MXene phases

with high electron conductivity and mechanical strength are

promising candidates for Li-ion storage because of the low Li+

diffusion barriers on the MXenes surface and excellent conduc-

tivity.[154] Chen et al.[153] prepared 2D MoS2-on-MXene hetero-structures by in situ sulfidation of Mo2TiC2Tx MXene (Fig-ure 11A). Two layers of MoS2 are in close contact with theMo2TiC2Tx layers, forming MoS2-on-MXene heterostructure con-

firmed by transmission electron microscopy (TEM)images of MoS2/Mo2TiC2Tx-500 shown in (Figure 11B).First-principles calculations predicted that the hetero-structure is metallic because of the presence of the

highly conductive MXene. As anode materials in LIBs,the MoS2-on-MXene heterostructure has high specific

capacities and CE, promising rate capability, and ex-cellent cycling stability compared to pristineMo2TiC2Tx (Figure 11C). The MoS2/Mo2TiC2Tx-500

structure shows initial charging and discharging ca-pacities of 554 and 646 mAhg@1 at 100 mAg@1, whichare 4.1 and 2.4 times that of pure Mo2TiC2Tx (134 and268 mAhg@1), respectively. Strong Li adsorption on

the 2D MoS2-on-MXene heterostructure leads to en-hanced intercalation, stable Li polysulfide adsorption,

as well as improved CE and cycling performance.[153]

Different from Mo- and W-based dichalcogenideswith semiconducting properties, V- and Nb-based

TMDs such as VS2 and NbS2 possess intrinsic metallicconductivity.[155] Jing et al. investigated adsorption

and diffusion of Li ion at VS2 monolayer, MoS2 mono-layer, and graphite. Being intrinsically metallic, the

VS2 monolayer exhibits a higher theoretical capacity

(466 mAhg@1) and lower or similar Li diffusion barrierthan MoS2 and graphite.[156] However, VS2 generally

exhibits poor stability due to large Peierls distortionduring cycling.[157] Cao et al. synthesized TiO2-B@VS2heterogenous nanowire (TVNAs) arrays on Ti foilthrough a facile two-step hydrothermal process for

LIBs anode.[158] This heterogeneous nanostructure

could effectively integrate the good electrochemicalfunctionalities of the individual components, includ-

ing good cycling stability of the TNAs and high ca-pacity and conductivity of VS2. As a consequence, the

anode could deliver a reversible capacity of365.4 mAhg@1 after 500 cycles at 1C (335 mAg@1),

being significantly higher than that of the pure TiO2-

B nanowire array (TNA) electrode (192.7 mAhg@1).[158]

In addition to serving as anode material in LIBs, the applica-

tion of TMDs as cathodes was reported. Li et al.[159] synthesizedTiS2-coated VS2 flakes on a CNT current collector substrate byCVD and a conformal TiS2 coating is deposited on the VS2 pla-telets by atomic layer deposition (ALD) (Figure 12A,B). TiS2 on

the surface prevents delamination and breakup of VS2 duringthe lithiation/delithiation process. Therefore, the stability of thebattery is drastically improved after the VS2 core is covered

with a thin(&2.5 nm) TiS2 layer. As cathode materials in LIBs, the VS2–TiS2electrode has high specific capacity(&180 mAhg@1 at 200 mAg@1) and rate capability

(&70 mAhg@1 at 1000 mAg@1), whereas the capacity retention

of the VS2 electrode is only approximately 40% of the value ofVS2–TiS2 under the same testing conditions (Figure 12C).[159]

These results suggest that the rational design of conductive2D TMD materials or 2D/2D heterostructured architectures

could enhance the electrochemical properties of LIBs. Fangand co-workers[97] reported a facile one-step hydrothermal syn-

Figure 10. A) Schematic illustration of the synthesis process of the MoS2/m-C nanosheetsuperstructure. B) SEM image of the annealed MoS2 nanosheets. C) High-magnificationTEM image of the MoS2/m-C nanosheet superstructure. D) Capacity retention of theMoS2/m-C nanosheet superstructure, MoS2/graphene composites, exfoliated graphene,and annealed MoS2 nanosheets at current densities from 200 to 6400 mAg@1. Repro-duced with permission from Ref. [152] Copyright 2015 John Wiley and Sons.

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thesis of VS2/graphene nanosheets (VS2/GNS) as cathode mate-rial in LIBs. The hierarchical architecture of VS2 sheets support-

ed on graphene and layered structure effectively facilitate Li-

ion extraction and insertion and ensures the electrode integrityduring cycle. Therefore, the VS2/GNS electrode maintains a dis-

charge capacity of 160.9 mAhg@1. Even at a high current densi-ty of 20C, this electrode still can deliver discharge capacities of

114.2 mAhg@1, which is higher than that of pristine VS2 (only90 mAhg@1).[97]

Recent progress on 2D TMDs as electrode materials for Li-ion storage are summarized in Table 1, suggesting promising

applications in high-performance LIBs.

3.2. 2D TMDs for SIBs

SIBs have emerged as an appealing alternative to LIBs, espe-

cially for large-scale energy storage systems, because of thehigh abundance and low cost of Na. Although SIBs share the

Figure 11. A) Schematic illustration of the preparation of MoS2/MXene hybrids. B) Cross-sectional TEM image of the MoS2-on-MXene heterostructure. C) Cy-cling stability and CE of the pristine Mo2TiC2Tx, MoS2/Mo2TiC2Tx-500 and MoS2/Mo2TiC2Tx-700 at 100 mAg@1, where 500 and 700 represent the calcination tem-perature of Mo2TiC2Tx. Reproduced with permission from Ref [153] Copyright 2018 Wiley.

Figure 12. A) Schematic of the procedure to fabricate the VS2 and VS2-TiS2 composites. B) HRTEM image (scale bar =2 nm, inset shows the fast Fourier trans-form (FFT) pattern (scale bar=2 nm@1) of the region in the green box) of a typical VS2-TiS2 hybrid flake, where the thickness of TiS2 layer is &2.5 nm. C) Ca-pacity retention of the VS2 electrode and VS2-TiS2 electrode at 1000 mAg@1. Reproduced with permission from Ref. [159] Copyright 2019 Springer Nature.

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Table

1.Su

mmaryoftheproperties

of2D

TMDsforhigh-perform

ance

LIBs.

Material

Synthesismethod

Structure

Electrolyte

[a]

ICE[%

]Cap

acity[m

Ahg@1]/

cyclingnumber

Ratecapab

ility

[mAhg@1]

Voltag

erange[V]

Ref.

MoS 2

electrospinning

single-layered

MoS 2

embed

in1D

NF

1m

LiPF 6

inEC

/DEC

(1:1)

74@0.1Ag@1

661/10

00th@10

Ag@1

374@

50Ag@1

0.00

5–3.0

[74]

MoS 2

CVD

mesoporousfilms

1m

LiClO

4in

PC

–&15

0/10

000t

h@23

C[b]

–1.0–

3.0

[227

]MoS 2

CVD

vertical

growth

nan

obelts

1m

LiPF 6

inEC

/DMC(1:1)

–84

0/10

0th@1Ag@1

&48

0@20

Ag@1

0.00

5–3.0

[276

]MoS 2

hyd

rothermal

3Dsphere

1m

LiPF 6

inEC

/DEC

(1:1)

&78

@0.1Ag@1

1009

.2/500

th@0.5Ag@1

353@

2Ag@1

0.01

–3.0

[150

]PEO

/MoS 2

exfoliation

few-layer

nan

oshee

t1m

LiPF 6

inEM

C/EC(7:3)

–95

3/50

th@0.05

Ag@1

–0.01

–3.0

[127

]CNFs@MoS 2

hyd

rothermal

vertical

growth

1m

LiPF 6

inEC

/DMC/D

EC(1:1:1)

66@0.1Ag@1

688/30

0th@1Ag@1

864@

5Ag@1

0.01

–3.0

[137

]MoS 2/G

hyd

rothermal

heterostructure

1m

LiPF 6

inEC

/DMC/EMC(1:1:1)

77.2@0.1Ag@1

900/40

0th@1Ag@1

890@

1Ag@1

0.01

–3.0

[132

]MoS 2/C

annealed

few

layer

1m

LiPF 6

inEC

/DMC(1:1)

68@0.1Ag@1

400/50

0th@1.6Ag@1

252.2@

6.4Ag@1

0.01

–3.0

[145

]MoS 2/C

hyd

rothermal/annealed

layerbylayer

1m

LiPF 6

inEC

/DMC(1:1)

76@0.2Ag@1

1023

/500

th@0.4Ag@1

943@

6.4Ag@1

0.01

–3.0

[152

]MoS 2@N-Doped

carbon

hyd

rothermal

vertical

growth

1m

LiPF 6

inEC

/PC(1:1)

77.5@0.1Ag@1

722/10

0th@0.1Ag@1

554@

2Ag@1

0.01

–3.0

[140

]MoS 2/CNT

hyd

rothermal/annealed

hierarchical

tubularstructures

1m

LiPF 6

inEC

/DEC

50:50(w/w

)–

&1100

/200

th@0.5Ag@1

670@

10Ag@1

0.01

–3.0

[92]

MoS 2/CNT

hyd

rothermal

vertical

growth

1m

LiPF 6

inEC

/DMC50

:50(w/w

)75

.8@0.2Ag@1

1200

/200

th@0.4Ag@1

888@

3.2Ag@1

0.01

–3.0

[143

]MoS 2

HNS

hyd

rothermal

hollo

wsphere

1m

LiPF 6

inEC

/DEC

50:50(w/w

)83

@0.5Ag@1

1100

/100

th@0.5Ag@1

576@

5Ag@1

0.01

–3.0

[277

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hyd

rothermal

3dhoneycomb-likenetwork

structures

1m

LiPF 6

inEC

/DMC(1:1)

73@0.1Ag@1

753/10

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747@

5Ag@1

0.01

–3.0

[134

]MoS 2-SWNT

liquid-phaseexfoliation

films

1m

LiPF 6

inEC

/DMC50

:50(w/w

)–

992/10

0@0.1Ag@1

670@

3.2Ag@1

0.01

–3.0

[136

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MoS 2@C

CVD

morinanep

alen

sis-like

1m

LiPF 6

inEC

/DEC

/DMC(1:1:1)

76.3@0.1Ag@1

905/50

0th@1Ag@1

707@

2.5Ag@1

0.01

–3.0

[135

]MoS 2

NSs

hyd

rothermal

heterostructures

1m

LiPF 6

inEC

/DEC

(1:1)

–95

%/200

th@2Ag@1

780@

2Ag@1

0.01

–3.0

[131

]Mesoporous-carbon/M

oS 2

hyd

rothermal

heterostructures

1m

LiPF 6

inEC

/DMC/EMC(1:1:1)

–1130

/100

th@0.1Ag@1

400@

10Ag@1

0.01

–3.0

[278

]3D

FL-M

oS 2@PCNNs

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3dporousnetwork

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inEC

/DMC/D

EC(1:1:1)

70@0.1Ag@1

1127

/200

th@0.1Ag@1

250@

10Ag@1

0.01

–3.0

[114

]HC-M

oS 2@GF

hyd

rothermal

honeycomb-like

1m

LiPF 6

inEC

/DEC

(1:1)

65@0.1Ag@1

&1100

/40t

h@0.1Ag@1

&80

0@5Ag@1

0.01

–3.0

[141

]MoS 2/CNT

liquid

phaseexfoliation

films

1m

LiPF 6

inEC

/DC(1:1)

–&10

37/500

th@2Ag@1

580@

20Ag@1

0.01

–3.0

[279

]MoS 2/Graphen

ehyd

rothermal

nan

oshee

ts1m

LiPF 6

inEC

/DMC(1:1)

65@0.1Ag@1

1187

/100

th@0.1Ag@1

&90

0@1Ag@1

0.01

–3.0

[96]

MoS 2@Sn

O2-Sn

S/C

hyd

rothermal

microstructures

1m

LiPF 6

inEC

/DMC(1:1)

70.9@0.1Ag@1

637/50

0th@1Ag@1

613@

5Ag@1

0.01

–3.0

[280

]Sn

S/MoS 2-C

hyd

rothermal

nan

oshee

ts1m

LiPF 6

inEC

/DMC(1:1)

81@0.2Ag@1

718/70

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0.01

–3.0

[281

]Co9S

8/MoS 2

hyd

rothermal

yolk–shell

1m

LiPF 6

inEC

/DMC(1:1)

66.7@0.2Ag@1

732/20

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Ag@1

562@

5Ag@1

0.01

–3.0

[93]

TiO

2@NC@MoS 2

hyd

rothermal

tubularnan

ostructures

1m

LiPF 6

inEC

/DEC

50:50(w/w

)59

.4@0.2Ag@1

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2Ag@1

0.01

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[282

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2CVD

ordered

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1m

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/DMC(1:1)

79@0.05

C[c]

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2@2C

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2/C

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nan

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/DMC(1:1)

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]MoO

2@MoSe

2hyd

rothermal

heterostructures

1m

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inEC

/DMC/EMC(1:1:1)

67@0.1Ag@1

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o2TiC

2Tx

chem

ical

reaction

layerbylayer

1m

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/DMC(1:1)

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2Ag@1

0.01

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hyd

rothermal

3Dfram

eworks

1m

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inEC

/DMC/EMC(1:1:1)

&67

@0.03

5Ag@1

766/10

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519@

1Ag@1

0.01

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[151

]WS 2@N-doped

CNT

electrospinning

unifo

rmhyb

ridization

1m

LiPF 6

inEC

/DEC

(1:1)

84@0.1Ag@1

712/12

0th@0.1Ag@1

423@

1Ag@1

0.01

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[118

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electrospinning

few-/single-layer

unifo

rmly

embed

ded

1m

LiPF 6

inEC

/DMC/EMC(1:1:1)

80.3@0.1Ag@1

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0th@1Ag@1

495@

1Ag@1

0.05

–3.0

[119

]

WS 2/CNT-rGO

solvothermal

hyb

ridnan

oarchitecture

aerogel

1m

LiPF 6

inEC

/DEC

(1:1)

53.4@0.2Ag@1

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337@

10Ag@1

0.01

–3.0

[149

]VS 2/Graphen

ehyd

rothermal

nan

oshee

ts1m

LiPF 6

inEC

/DMC(1:1)

–52

8/10

0th@0.2Ag@1

–0.05

–3.0

[97]

VS 2@CNTs

CVD

core/branch

structure

1m

LiPF 6

inEC

/DEC

(1:1)

88.1@0.2Ag@1

&90

0/10

0th@0.2Ag@1

696@

10Ag@1

0.01

–3.0

[146

]VSe

2NSA

/Csolvothermal

ultrathin

nan

oshee

t1m

LiPF 6

inEC

/DMC(1:1)

75.9@0.1Ag@1

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th@0.1Ag@1

671@

2Ag@1

0.01

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[147

]2H

-NbS 2

solid

-state

reaction

nan

oshee

ts1m

LiPF 6

inEC

/DEC

(1:1)

–12

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3.0

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]3R

-NbS 2

solid

-state

reaction

nan

oshee

ts1m

LiPF 6

inEC

/DEC

(1:1)

–12

0.1/20

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0.3Nb0.7S

1.6Se 0

.4oil-phasesyntheticprocess

nan

oshee

ts1m

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/DEC

(1:1)

65.4@0.1Ag

@144

4/15

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461.3@

10Ag@1

0.00

5–3.0

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]NbSe

2@G

wet

milling

few-layer

nan

oshee

ts1m

LiPF 6

inEC

/DEC

(1:1)

44.9@0.1Ag@1

&70

0/10

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373@

10Ag@1

0.01

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[285

]

[a]In

whichEC

isethylen

ecarbonate;DEC

isdiethyl

carbonate;DMCisdim

ethyl

carbonate;PCispolycarbonate;EM

Cisethyl

methyl

carbonate;theproportionin

thebracketsisthevo

lumeratioofmix

solu-

tion.[b]Where1Ciscurren

tden

sity

of0.67

Ag@1

(whichisbased

onthetheo

reticalcapacityofMoS 2).[c]Where1Cisthecurren

tden

sity

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(whichisbased

onthetheo

reticalcapacityofMoSe

2).

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1129

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same reaction mechanism with LIBs, the larger size of Na+

(1.02 a) than Li+ (0.76 a) causes sluggish ion diffusion, slow

electrochemical reaction kinetics, and larger volume changesupon Na+/Na insertion/extraction, meaning that some elec-

trode materials used in LIBs may not be suitable for SIBs.[26] Forexample, graphite, the most commonly used anode materials

in LIBs, shows poor reaction kinetics and low capacity whenused in SIBs.[22] 2D TMDs with atomic or molecular thicknessand infinite planar lengths are promising host materials for the

reversible sodiation/desodiation process in SIBs.[14]

Li et al.[160] investigated the electrochemical properties of aseries of MX2 (M=Mo, W, Nb, Ta; X=S, Se) during Li/Na inter-calation by first-principles calculation, indicating that the char-

acterizing voltages of MX2 during Li/Na intercalation process isdominated by charge transfer from Li/Na to MX2 layers. MoX2

and WX2 have lower intercalation voltages, and WX2 has the

lowest energy barriers for Na-ion migration. Yang et al. ex-plored the Na adsorption and diffusion characteristics of mon-

olayered MX2, including TiS2, VS2, CrS2, CoTe2, NiTe2, ZrS2, NbS2,and MoS2 by first-principles calculation.[161] In terms of average

voltage and capacity, monolayered TiS2, ZrS2, NbS2, and MoS2are suitable anodes for SIBs with voltages of 0.49–0.95 V and

theoretical capacities of 260–339 mAhg@1. In particular, mono-

layered TiS2 and NbS2 have low Na ion migration barriers of0.22 and 0.07 eV, respectively.[161]

Na-ion storage in MoS2 involves intercalation at higher po-tential windows and conversion reaction at lower potential

windows as shown in Equations (1) and (2), respectively :[10]

MoS2 þ x Na Ð NaxMoS2 ð> 0:4 V, x < 1:5Þ ð1Þ

NaxMoS2 þ ð4@xÞNa Ð Moþ 2Na2Sð< 0:4 VÞ ð2Þ

By controlling the cut-off voltage, the layer structure between

MoS2 and NaxMoS2 (x<1.5) through the intercalation reaction

[Eq. (1)] can be preserved to achieve high cycling reversibility.For example, Bang et al. showed that layered MoS2 nanosheets

display excellent cycling stability with nearly no capacity lossfor over 100 cycles by limiting the terminal voltage to 0.4 V.[162]

Hu et al. showed that MoS2 nanoflowers with expanded inter-layers have excellent cycling stability by adjusting the cut-off

voltage to 3–0.4 V and Na-ion-storage capacities of300 mAhg@1 at 1 Ag@1 and 195 mAhg@1at 10 Ag@1 after

1500 cycles through intercalation-type reactions.[18] When thevoltage window is expanded to 0.01–3.0 V, MoS2 follows an in-tercalation–conversion mechanism for Na+ storage and MoS2is fully converted to Mo metal and Na2S [Eq. (2)] . In the charg-ing process, Mo nanoparticles remain electrochemically inert

and Na2S is oxidized to S. Similar to Li–S batteries, this processproduces soluble polysulfide intermediates that can dissolve in

the electrolyte, resulting in poor cycling stability and capacity

fading due to irreversible reactions and shuttle effects of poly-sulfides.[57] Moreover, the poor electron/ion conductivity be-

tween adjacent S@Mo@S sheets hinders the application as elec-trode materials in SIBs. Integration of MoS2 with conductive

carbon to form MoS2/C hybrids can improve the Na-ion-stor-age properties due to the enhanced conductivity; furthermore,

C in the MoS2/C hybrid inhibits aggregation of the 2D MoS2sheets and restrains release and shuttling of polysulfides to

maintain a long cycling lifetime with good capacity retention.David et al.[163] prepared MoS2/graphene paper as anodes in

SIBs, with MoS2 reacting with Na ions by successive intercala-tion and conversion processes during sodiation. Shi et al.[138]

synthesized 1D MoS2/CNTs consisting of 2D-monolayer MoS2and carbon (MoS2 :C) inter-overlapping nanosheets by a one-pot solvothermal synthesis in conjunction with in situ carbona-

tion of the organic solvent (Figure 13A–C). The nanotubeswith one closed-tip, ideal MoS2/monolayer carbon heterointer-face contact, and enlarged (002) interlayer spacing of 0.98 nmshow an initial discharge capacity of 620 mAhg@1 and maintain

a reversible discharge capacity of 477 mAhg@1 at 200 mAg@1

after 200 cycles (Figure 13D). At a high current density of1000 mAg@1, the discharge capacity is maintained at

415 mAhg@1 after 200 cycles.[138] Wang et al. prepared MoS2nanosheets vertically aligned on carbon paper derived from

paper towels. The MoS2@C paper composites as freestandingelectrodes show a large reversible capacity of 446 mAhg@1 at

20 mAg@1, high rate capability of 205 mAhg@1 at 1000 mAg@1,and good cycling stability, suggesting promising application of

MoS2@C in reversible SIBs.[103] MoSe2 possesses a similar struc-

ture as MoS2 but has a larger interlayer spacing of 0.646 nmand higher electron conductivity, enabling faster charge trans-

fer and improved Na-ion storage properties.[165] Chen and co-workers embedded graphene (G) nanosheets in carbon-coated

MoSe2 (MoSe2@C@G) hybrid NFs by electrospinning, and thematerials show a specific capacity of 366.9 mAhg@1 after

200 cycles at 0.2 Ag@1.[166] Niu et al. synthesized MoSe2 nano-

sheets on N,P-co-doped carbon nanosheets; the compositehas a capacity of 378 mAhg@1 at 0.5 Ag@1 after 1000 cycles and

capacity retention of 87% relative to the 2nd discharge capaci-ty.[167]

Sun et al. synthesized interlayer-expanded few-layer MoSe2nanosheets confined in hollow carbon nanospheres (Mo-

Se2@HCNS). The MoSe2 exhibits few-layer crystal fringes of less

than three layers, and the interlayer spacing is expanded to1.02 nm. As an anode in SIBs, MoSe2@HCNS shows prolonged

cycling lifetime with discharge capacities of 502 and471 mAhg@1 after 1000 cycles at 1 and 3 Ag@1, respectively.[165]

The good electrochemical properties of MoSe2@HCNS can beattributed to the 2D few-layer nanosheets with expanded (002)planes and unique conductive hybrid shell structure, which im-

proves electron/ion transfer, buffers the volume change, andpromotes the reversible sodiation/desodiation reactions. Xieet al.[164] synthesized self-supported sandwiched vertical gra-phene (VG)/MoSe2/NC arrays (Figure 14A) through a combined

hydrothermal and polymerization method. The N-C shell onthe VG/MoSe2 improves charge transfer at the electrode–elec-

trolyte interface and reduces the inner resistance in the elec-

trode, resulting in enhanced reaction kinetics and rate capabili-ty. Upon cycling at current densities of 0.2, 0.5, 1.0, and

2.0 Ag@1, the VG/MoSe2/N-C electrode exhibits average dis-charge capacities of 538, 470, 395, and 300 mAhg@1, respec-

tively, while the VG/MoSe2 electrode has a low capacity of76 mAhg@1 at 2 Ag@1 (Figure 14B). Moreover, the VG/MoSe2/N-

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Reviews

C core–shell arrays show excellent long-term cycling stability,

with the capacity stabilizing at 398 mAhg@1 at 1.0 Ag@1 and298 mAhg@1 at 2.0 Ag@1 from the 2nd to 1000th cycles (Fig-

ure 14C). The excellent properties of the sandwiched core–

shell structure are attributed to dual conductive carbon net-works and outer N-C shell protection, which facilitate fast elec-

Figure 13. A) Structural models of the pristine 2H-MoS2 and the MoS2 :C interoverlapped superstructure with expanded interlayer spacing and schematic illus-tration of the design of MoS2 :C inter-overlapped superstructure with discontinuous carbon monolayers beneficial for Na+ ion insertion/extraction and growthsteps of the MoS2 :C superstructure nanosheets. B,C) SEM and TEM images of the MoS2 :C nanotubes, respectively. D) Cycling performance of the annealedMoS2 :C nanotube electrode. Reproduced with permission from Ref. [138] Copyright 2016 Elsevier.

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tron transfer and suppress diffusion of soluble polyselenidesand aggregation of active materials.

Na ion storage in WS2 and WSe2 is similar to that in MoS2and MoSe2 because they have a similar lamellar structure, large

interlayer spacings (0.62 nm for WS2 and 0.65 nm for WSe2),and weak van der Waals interactions between layers.[168] Incor-poration of WS2 or WSe2 with conductive substrates such as or-

dered mesoporous carbon,[169] CNTs,[149] and graphene[170] pro-duce improved Na-ion-storage properties. Yang and co-work-

ers[171] prepared cubic WS2@NC hybrid nanostructure by solvo-thermal growth of 2D WS2 nanosheets on Prussian blue (PB)-

derived NC nanocubic framework (Figure 15A). As anode mate-

rial in SIBs, the WS2@NC has a higher rate capability of 384,360, 336, 302, 236 and 151 mAhg@1 at 100, 200, 500, 1000,

2000 and 5000 mAg@1 respectively, while the pure WS2 elec-trode shows 244, 157, 130, 103, 65 and 50 mAhg@1 at the

same current densities (Figure 15B). Furthermore, the specificcapacity of WS2@NC is maintained at 320 mAhg@1 after

200 cycles, implying better cycling stability (Figure 15C).[171] Bycontrolling the cut-off potential, WS2 exhibits better cycling

stability through Na-ion-intercalation reactions. Yu and co-workers[173] adopted a one-step in situ solid-state hybridization

strategy to construct WS2 nanowall arrays on N-doped carbonlayers (NCLs). The lamellar hybrid architecture shows improved

Na-storage performance as SIB anode material with a stable re-versible capacity of 180 mAhg@1 at the potential range of 0.4–3.0 V vs. Na+/Na after 1200 cycles at 1.0 Ag@1. However, at a

lower terminal potential, the reaction between Na ions andWS2 involves the intercalation of Na+ in the beginning and asubsequent conversion reaction (WS2+4Na++4e@!2Na2S+W). During this process, soluble polysulfide intermediates

during Na2S formation/decomposition causes loss of S compo-nents and capacity reduction.[173] The introduction of a protec-

tive layer on the surface improves the cyclability by preventing

the dissolution of the intermediates and accumulation of thesulfur phase on the Na counter electrode. Ryu et al.[120] synthe-

sized heterogeneous tungsten sulfide/oxide core–shell nano-fibers (WSx/WO3 NFs) with vertically and randomly aligned

thornbush features by electrospinning (Figure 5). This hetero-geneous WSx/WO3 thornbush NF electrodes have a high

second discharge capacity of 791 mAhg@1 and improved cy-

cling performance for 100 cycles compared to the pristine WSxNF (74% of the second discharge capacity). The ex situ X-ray

diffraction (XRD) patterns of the Na counter electrode obtainedby disassembly of a Na-ion cell comprising WSx and WSx/WO3

NF electrodes verify that S deposition is reduced by protectingthe WO3 layer.

[120]

In the practical application of portable devices, the volumet-

ric energy density is very important and thick-film electrodeswith low tortuosity and high energy density are desirable. Li

et al.[172] prepared monolithic electrodes composed of 40 mm-thick anisotropic WS2 vertical arrays on an Al foil with a mass

loading of 17.5 mgcm@2 by physical vapor deposition (PVD)(Figure 16A). To enhance the conductivity of the monolithicelectrodes, a thin carbon layer (less than 10 nm) was deposited

on the monolithic WS2 at 2 mm intervals. The monolithicanode shows a high Na-ion diffusivity of 2.05V10@10 cm2 s@1,

which is two orders of magnitude larger than that of theanode prepared with a slurry paste. In monolithic WS2, the ver-tically aligned WS2 arrays provide unblocked Na-ion diffusionchannels throughout the electrodes even for commercial elec-

trode thicknesses of 40 mm and under high current densities.However, the randomly dispersed particles (active materials,carbon agents, and binders) lead to high tortuosity for charge-carrier transport in the pasted electrode (Figure 16B,C), whichis clearly shown in the rate and cycling data. Owing to the

carbon layer improving electronic conductivity, the verticalWS2/C electrode shows good rate capacities of 352, 320, 289,

252, 158, and 70 mAhg@1 at 20, 100, 200, 500, 1000 and

2000 mAg@1 (corresponding to approximately 0.05C, 0.25C,0.5C, 1C, 2C, and 5C of the theoretical capacity of WS2) (Fig-

ure 16D). The vertically aligned architecture enhances the reac-tion kinetics, giving rise to areal and volumetric capacities for

the monolithic WS2/C electrode of 5.57 mAhcm@2 and1.39 Ahcm@3, respectively, at a current density of 100 mAg@1

Figure 14. A) Schematic illustration of the synthesis steps of the sandwichedVG/MoSe2/NC core–shell arrays with the SEM images corresponding to VG,VG/MoSe2, and VG/MoSe2/NC, respectively; B) rate capability of the VG/MoSe2/NC electrodes and VG/MoSe2, respectively; C) cycling stability of VG/MoSe2/NC at 1 and 2 Ag@1 for 1000 cycles. Reproduced with permissionfrom Ref. [164] Copyright 2016 Wiley.

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Reviews

with less than 20% decay during 300 cycles. In comparison,the pasted WS2 shows considerable capacity decay within

50 cycles (Figure 16E).[172]

V and Nb are group-V transition metals, and their chalcoge-

nide compounds such as VS2, VSe2, NbS2, NbSe2 exhibit metal-

lic conductivity, which is very important for achieving high ratecapability for Na-ion storage. First-principles calculations re-

vealed that NbS2 nanosheets have the lowest Na+ migrationbarrier of 0.07 eV whereas those of MoS2 and TiS2 are 0.25 and

0.22 eV, respectively.[161] Moreover, NbS2 does not suffer fromunwanted phase transformations and maintains the crystal

structure during soidation/desodiation.[161] Thesemerits suggest that NbS2 nanosheets are promising

anode materials for SIBs with high rate capability andlong cycle life. Ou et al. synthesized 2D NbS2 nano-

sheets by chemical exfoliation.[174] The chemically ex-foliated NbS2 (ce-NbS2) nanosheets have a high Na-

ion storage capacity of 205 mAhg@1 at 100 mAg@1,high rate performance, and excellent cycling stability.In situ XRD data demonstrates that the ce-NbS2 nano-

sheets do not undergo unwanted phase transforma-tion during soidation/desodiation, which makes thempromising high-capacity and long-cycle-life anodematerials in SIBs.[174] Zhang et al. synthesized NbS2nanosheets co-doped with M/Se (M=Fe, Co, Ni) ; theFe0.3Nb0.7S1.6Se0.4 nanosheets have excellent rate capa-

bilities with specific capacities of 136 mAhg@1 at

5 Ag@1 for Na-ion storage after 5 cycles.[175] Xu et al.prepared layered NbSe2 sheets with an interlayer

spacing of 6.30 a by solid-state vacuum sintering.The NbSe2 sample shows excellent cycling stability

and rate performance for Na storage with an initialreversible capacity of 116.6 mAhg@1 and retained ca-

pacity of 98.1 mAhg@1 after 100 cycles at

100 mAg@1.[176] Moreover, a capacity of 78.6 mAhg@1

was observed at a high current density of

4000 mAg@1, suggesting excellent rate capability. Luoet al[177] demonstrated that NbSe2 has a closed hexag-

onal and layered structure, in which each Nb atomiclayer is sandwiched between two Se atomic layers

and bonded by van der Waals forces between the

layers (Figure 17A). The as-synthesized layered NbSe2has an almost regular hexagon morphology with a

thickness of 500 nm. The high-resolution (HR)-TEMimage shows a 0.314 nm lattice spacing correspond-

ing to the (004) plane of NbSe2 (inset in Figure 17B).The NbSe2 flake electrode has a discharge capacity of

115 mAhg@1, and the capacity retention is 91% from

the 2nd to the 50th cycle (Figure 17C).[177] Ex situXRD measurements show that NbSe2, Na0.5NbSe2, and

NaNbSe2 retain the same P63/mmc space groupduring Na+ intercalation, indicating that NbSe2 pos-sesses a favorable host structure for Na+ storage.[17]

Layered VS2 has an intrinsic metallic conductivity

with large interlayer distances (5.76 a).[179] Putunganet al. investigated the potential of single-layer VS2polytypes as SIB anode materials using DFT calcula-

tions and demonstrated that sodiation tends to inhibit the 1H-to-1T structural phase transition, which is different from the

lithiation-induced phase transition in 2D MoS2.[122] Li et al. syn-

thesized 3D hierarchical VS2 microrods with nanosheets com-

prised of small VS2 nanograins by in situ chemical etching. As

an anode in SIBs, it has high capacities of 255 and 230 mAhg@1

at high rates of 1.0 and 2.0 Ag@1 and reversible capacity of

350 mAhg@1 after 200 cycles at 0.2 Ag@1.[180] Mai et al. fabricat-ed layer-by-layer VS2 stacked nanosheets (VS2-SNSs) using a

one-step PVP-assisted assembly method.[181] With a highlystable framework and mesoporous structure, the VS2-SNSs

Figure 15. A) Schematic of the WS2@NC synthesis process derived from PB nanocubesand corresponding SEM images. The insets show the SEM images at lower magnification.B) Galvanostatic rate capabilities of the WS2@NC and pure WS2 electrodes at differentcurrent densities. C) Cycling performance of the WS2@NC and pure WS2 electrodes mea-sured at 200 mAg@1 and coulombic efficiency of WS2@NC. Reproduced with permissionfrom Ref. [171]. Copyright 2017 Royal Society of Chemistry.

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1133

Reviews

electrode delivers excellent electrochemical performance inSIBs with a reversible discharge capacity of 250 mAhg@1 at

0.2 Ag@1 and high rate capability of 150 mAhg@1 at 20 Ag@1.The energy storage depends on the pseudocapacitive interca-

lation mechanism with a high capacitive contribution up to69% of the total capacity at 1 mVs@1.[181] Yu et al.[178] synthe-

sized hierarchical flower-like VS2 nanosheets by a solvothermal

method. The 3D hierarchical architecture consists of dozens of2D VS2 nanosheets with a smooth surface and uniformity (Fig-

ures 18A,B). The flower-like VS2 nanosheets have a high rever-sible capacity of around 600 mAhg@1 at 0.1 Ag@1 and excellent

cycling stability, with 83% and 87% of its initial capacitiesbeing retained after 700 cycles at 2 and 5 Ag@1, which was

achieved by controlling the cut-off voltage (3.0–0.3 V) in anelectrolyte of 1.0m NaCF3SO3 in diglyme (diethylene glycol di-

methyl ether) (Figure 18C). Ex situ Raman scattering was em-ployed to examine the vibration changes during cycling, and

HR-TEM and selected-area electron diffraction (SAED) experi-ments were used to study the phase formation at the end of

initial discharging and charging (Figure 18D). The intercala-

tion/de-intercalation process is shown in Equation (3):

VS2 þ 2Naþ þ 2 e@ Ð Na2VS2 ð3Þ

Quantitative kinetic analysis reveals that the contributionfrom pseudocapacitance is above 75% and it increases rapidly

Figure 16. A) Illustration of the WS2/C monolithic electrode fabrication process; B) Schematic diagram of Na ion transport behavior in a traditional slurry-pasted WS2 electrode and PVD-deposited WS2/C monolithic electrode. C) Top-view SEM image of the WS2/C monolithic electrode; D) rate-performance com-parison of the three types of electrodes (vertical WS2/C, vertical WS2 and pasted WS2) at different current densities from 20 to 2000 mAg@1. E) Areal capacityand CE of the three types of electrodes during cycling. Reproduced with permission from Ref. [172] Copyright 2019 Royal Society of Chemistry.

Figure 17. A) Projections of the NbSe2 structure. B) SEM picture of NbSe2 with the inset showing the HRTEM image of NbSe2. C) Cycling performance at a cur-rent density of 25 mAg@1. Reproduced with permission from Ref. [177] Copyright 2018 John Wiley and Sons.

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1134

Reviews

to 93% at higher current rates. In addition, the excellent ca-

pacity retention of 87% after 700 cycles at 5 Ag@1 suggestspromising application of the flower-like VS2 as anode in

SIBs.[178] Wang et al.[182] prepared hierarchical VS2 spheres as-sembled with porous VS2 nanosheets through a one-step and

additive-free solvothermal route. As an anode in SIBs, the VS2spheres show a large capacity of 720 mAhg@1 at 0.2 Ag@1 after

100 cycles and good rate performance of 565 and 479 mAhg@1

at 2 and 5 Ag@1, respectively. The excellent electrochemicalcharacteristics of the VS2 nanosheet-assembled spheres can be

ascribed to the metallic nature of VS2 nanosheets and hierarch-ical spherical structure based as shown by DFT calculations.[182]

Table 2 presents the recent progress of 2D TMDs as elec-trode materials for high-performance SIBs.

Figure 18. A,B) Morphological features of the flower-like VS2. C) Long-term cycling performance for the VS2 electrode at 2 and 5 Ag@1. D) Discharge/chargecurve of the first cycle, where the green section is the HRTEM images and SAED pattern of the VS2 electrode charged to 3.0 V and the purple section showsdischarging to 0.3 V. Reproduced with permission from Ref. [178] Copyright 2018 Elsevier.

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1135

Reviews

Table

2.Su

mmaryofproperties

forthe2D

TMDsforSIBs.

Materials

Synthesismethod

Structure

Electrolyte

[a]

ICE[%

]Cap

acity[m

Ahg@1]/

cyclenumber

Ratecapab

ility

[mAhg@1]

Voltag

erange[V]Ref.

MoS 2

CVD

vertical

growth

nan

obelts

1m

NaC

lO4in

PC/FEC

(1:0.05)

70@0.1Ag@1

520/10

0th@1Ag@1

380@

20Ag@1

0.00

5–3.0

[276

][276

]

MoS 2

liquid-phaseexfoliation

nan

oshee

ts1m

NaC

lO4in

PC/FEC

64.6@0.02

Ag@1

161/50

th@0.02

Ag@1

–0.4–

2.6

[162

][162

]

MoS 2

exfoliation

few-layer

nan

oshee

ts1m

NaC

lO4in

EC/PC(1:1)

–38

6/10

0th@0.04

Ag@1

251@

0.32

Ag-1

0.01

–3.0

[286

][286

]

MoS 2

CVD

mesoporousfilms

1m

NaC

lO4in

PC

–118/100

0th@10

mVs@

1–

1.0–

3.0

[227

][227

]

MoS 2

electrospinning

single-layered

MoS 2

embed

in1D

NF

1m

NaC

lO4in

PC

52@0.1Ag@1

253/10

0th@10

Ag@1

75@50

Ag@1

0.00

5–3.0

[74]

[74]

F-MoS 2/NG

hyd

rothermal

nan

oflo

werswithexpan

ded

interlayer

dis-

tance

1m

NaP

F 6in

EC/D

MC(1:1)

65.7@0.1Ag@1

1060

/150

th@0.1Ag@1

516@

2Ag@1

0.00

1–3.0

[129

][129

]

MoS 2/G

exfoliation

heterostructures

1m

NaC

lO4in

DMC/EC(1:1)

—95

%/250

th@0.3Ag@1

201@

50Ag@1

0.01

–2.7

[287

][287

]

MoS 2:C

hyd

rothermal

hierarchical

nan

otubes

1m

NaC

lO4in

PPC/FEC

(1:0.05)

84@0.2Ag@1

415/20

0th@1Ag@1

187@

20Ag@1

0.01

–3.0

[138

][138

]

US-MoS 2@NG

hyd

rothermal

nan

ostructures

1m

NaC

lO4in

EC/D

EC/FEC

(1:1:0.1)

–19

8/10

00th@1Ag@1

141@

12.8Ag@1

0.4–

3.0

[288

][288

]

EG-M

oS 2

hyd

rothermal

vertical

1m

NaP

F 6in

DEG

DME

96@0.1Ag@1

509/25

0th@1Ag@1

420@

2Ag@1

0.01

–3.0

[133

][133

]

CC@CN@MoS 2

hyd

rothermal

nan

oshee

ts1m

NaC

lO4in

EC/D

MC/FEC

(1:1:0.05[

b] )

52@0.2Ag@1

619.2/10

0th@0.2Ag@1

235@

2Ag@1

0.01

–3.0

[139

][139

]

MoS 2-rGO/HCS.

hyd

rothermal

3Dhoneycomb-likenetwork

structures

1m

NaP

F 6in

EC/D

EC/FEC

(1:1:0.05)

68@0.1Ag@1

443/50

0th@1Ag@1

364@

5Ag@1

0.01

–3.0

[134

][134

]

MoS 2/RGO

hyd

rothermal

heterostructure

1m

NaC

lO4in

EC/PC(1:1)

–22

7/30

0th@0.32

Ag@1

352@

0.64

Ag@1

0.01

–3.0

[289

][289

]

MoS 2/C

hyd

rothermal

nan

ostructured

1m

NaP

F 6in

EC/D

MC(1:1)

–40

0/30

0th@1C

390@

2C

0.00

5–2.5

[290

][290

]

FG-M

oS 2

hyd

rothermal

nan

oflo

wers

1m

NaSO

3CF 3

inDEG

DME

–29

5/30

0th@0.2Ag@1

175@

10Ag@1

0.4–

3.0

[18]

[18]

FG-M

oS 2

hyd

rothermal

nan

oflo

wers

1m

NaSO

3CF 3

inDGM

–29

5/30

0th@0.2Ag@1

195@

10Ag@1

0.01

–3.0

[18]

[18]

MoS 2@C

hyd

rothermal

nan

oflo

wers

1m

NaP

F 6in

EC/PC/FEC

(1:1:0.05)

79.5@0.08

Ag@1

286/10

0th@0.08

Ag@1

205@

1Ag@1

0.01

–3.0

[103

][103

]

Co9S

8/MoS 2

hyd

rothermal

yolk–shell

1m

NaC

lO4in

PC/FEC

(1:0.05)

50.3@0.3Ag@1

300/12

00th@2.0Ag@1

403@

2Ag@1

0.01

–3.0

[93]

[93]

VO-M

oS 2/N-rGO

solvothermal

vertically

orien

teddistributed

1m

NaC

lO4in

DMC/EC/EMC(1:1:0.05)

–25

515

00th@1Ag@1

86@50

Ag@1

0.01

–3.0

[291

][291

]

MoSe

2collo

idal

nan

ospheres

1m

NaC

lO4in

PC/D

MC(1:1)

82.7@0.04

22Ag@1

345/

200t

h@42

.2mAg@1

212@

4.22

3Ag@1

0.1–

3.0

[292

][292

]

C-M

oSe

2/rGO

hyd

rothermal

hierarchical

porousstructure

1m

NaC

lO4in

DMC/EC/FEC

(1:1:0.05)

62.4@0.2Ag@1

445/35

0th@0.2Ag@1

228@

4Ag@1

0.01

–3.0

[293

][293

]

MoSe

2@HCNS

hyd

rothermal

hollo

wcore–shell

1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.05)

75.3@1Ag@1

501/10

00th@1Ag@1

382@

10Ag@1

0.01

–3.0

[165

][165

]

MoSe

2/N,P-rGO

hyd

rothermal

S1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.05)

70.4@0.5Ag@1

378/10

00th@0.5Ag@1

240@

10Ag@1

0.01

–3.0

[167

][167

]

MoSe

2-C

hyd

rothermal

scale-likeyo

lk–shell

1m

NaC

lO4in

EC/D

EC(1:1)

–37

8/10

00th@3Ag@1

308.6@

10Ag@1

0.01

–3.0

[294

][294

]

MoO

2/MoSe

2-G

hyd

rothermal

heterostructures

1m

NaC

lO4in

EC/D

EC(1:1)

–34

0/30

0th@0.5Ag@1

301@

3.2Ag@1

0.01

–3.0

[295

][295

]

C@MoTe

2spraypyrolysis

core–shell

1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.05)

81@1.0Ag@1

286/20

0th@1.0Ag@1

209@

5Ag@1

0.00

1–3.0

[296

][296

]

VS 2

solvothermal

hierarchical

flower

1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.06)

86@0.1Ag@1

&70

0/10

0th@0.1Ag-

400@

1Ag@1

0.00

1–3.0

[94]

[94]

VS 2

hyd

rothermal

ultrathin

nan

oshee

ts1m

NaSO

3CF 3

inDGM

–56

5/12

00th@2.0Ag@1

533@

4Ag@1

0.3–

3.0

[182

][182

]

VS 2

hyd

rothermal

hierarchical

flower

1m

NaSO

3CF 3

inDGM

94@0.1Ag@1

87%/700

th@5Ag@1

277@

20Ag@1

0.3–

3.0

[178

][178

]

VS 2

insitu

chem

ical

etching

3Dhierarchical

microrods

1m

NaSO

3CF 3

inDGM

&87

.3@0.1Ag@1

350/20

0th@0.2Ag@1

165@

5Ag@1

0.5–

3.0

[180

][180

]

NTO

-VS 2

hyd

rothermal

yolk–shell

1m

NaC

lO4in

EC/PC/FEC

(1:1:0.03)

–20

3/10

0th@0.2Ag@1

75@4Ag@1

0.01

–2.5

[297

][297

]

c-VS 2@VOOH

hyd

rothermal

nan

oflo

wer

1m

NaSO

3CF 3

inDGM

&89

.3@0.1Ag@1

330/15

0th@0.2Ag@1

102@

5Ag@1

0.5–

3.0

[106

][106

]

VS 2-SNSs

hyd

rothermal

layer-by-layer

1m

NaSO

3CF 3

inDGM

85@0.2Ag@1

204/60

0th@5Ag@1

180@

10Ag@1

0.4–

2.2

[181

][181

]

VS 4/rGO

hyd

rothermal

linearchainsstructure

1m

NaC

lO4in

DMC/EC(1:1)

75.1@0.1Ag@1

240.8/50

th@0.1Ag@1

192.1@

0.8Ag@1

0.01

–2.2

[298

][298

]

VSe

2NSA

/Csolvothermal

ultrathin

nan

oshee

ts1m

NaC

lO4in

EC/D

MC(1:1)

81.5@0.1Ag@1

766/50

th@0.1Ag@1

–0.01

–2.5

[147

][147

]

WS 2@NC

hyd

rothermal

cubic-shap

edstructure

1m

NaP

F 6in

EC/D

EC(1:1)

79.8@0.2Ag@1

78/500

th/5

Ag@1

151@

5Ag@1

0.01

–3.0

[171

][171

]

WS 2/CNT-rGO

solvothermal

hyb

ridnan

oarchitecture

aerogel

1m

NaP

F 6in

EC/D

EC/FEC

(1:1:0.02)

33.9@0.2Ag@1

252.9/10

0th@0.2Ag@1

47.2@10

Ag@1

0.01

–3.0

[149

][149

]

WS x/W

O3

electrospinning

thornbush

nan

ofib

er1m

NaC

lO4in

PC

–58

5/10

0th@0.1Ag@1

538@

3.2Ag@1

0.01

–3.0

[120

][120

]

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1136

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3.3 TMDs for PIBs

K (2.09 wt% abundance) ismuch more abundant than

Li (0.0017 wt%) in theearth crust and can be ex-

tracted at a lowercost.[183,184] The standardpotential of K+/K is@2.97 V versus the stan-dard hydrogen electrode(SHE) is 220 mV less thanthat of Na (@2.71 V versusSHE), meaning a theoreti-cally wider electrochemical

voltage window and

higher energy density forPIBs.[183] Despite the larger

atomic number, the radiusof solvated K cations is

smaller than those of Liand Na in aqueous solu-

tions and its kinetic re-

sponse is better with ahigher diffusion rate com-

pared to SIBs.[184] More-over, the current collector

in the cell could consist ofAl instead of Cu due to

the inertness between Al

and K at low potentials. Allof these intrinsic advantag-

es of PIBs bode well forthe energy-storage field,

but the development ofPIBs electrode materials is

challenging because of the

larger radius of K+ (1.38 a)and high reactivity.[185,186]

Research on PIB electrodematerials has mainly fo-

cused on carbonaceousmaterials,[187,188] transition-

metal oxides,[189,190] andTMDs;[191,192] among them,few-layered 2D TMDs are

promising anode materialsbecause they have larger

interlayer spacings to ac-commodate large K ions.

Tian et al.[191] investigat-

ed the electrochemicalproperties, phase transfor-

mation, and stability of K-intercalated TiS2 (KxTiS2,

0,x,0.88). In situ XRDreveals that TiS2 undergoes

phase transformation upon potassiation. The phase transitionsin the various intercalated stages impedes the kinetics of dis-charging/charging in the bulk TiS2 hosts. However, the phasetransitions can be bypassed and better ion insertion kinetics

can be accomplished by chemically pre-potassiation of bulkTiS2 (K0.25TiS2) to reduce the domain size of the crystal, resultingin improved CE, rate capability, and cycling stability of PIBs.[191]

TiSe2 was also investigated as an intercalating-type electrodefor K-ion storage. Li et al. demonstrated that TiSe2 has three

phases with partially reversible formation of K0.25TiSe2,K0.58TiSe2, and KxTiSe2 (x&0.7) during K+ intercalation/deinter-calation[186] and Shu et al. demonstrated that commercial TiSe2can reversibly store potassium ions with a capacity of

72.6 mAhg@1 as an anode in PIBs. However, it has a low CE inthe first cycle because of cracks in the TiSe2 host along with

phase transitions.[193]

MoS2 and MoSe2 have been widely investigated as electrodematerials in LIBs and SIBs.[18,127–141,148,163,165] However, the large

volume change during charging/discharging in PIBs and lowintrinsic conductivity cause severe capacity decay and poor

rate capability during long-term cycling. Hybridizing with aconducting carbon matrix, creating space between active ma-

terials, and enlarging the interlayer distance of MoS2 and

MoSe2 are effective approaches to improve the electrochemicalproperties. For example, Jia et al. fabricated a mesoporous

MoS2-monolayer/carbon composite for K-ion intercalation anddemonstrated a high specific capacity of 323 mAhg@1 at

100 mAg@1.[194] Wang et al. prepared a pistachio-shuck-likeMoSe2/C core–shell nanostructure by a simple one-pot colloi-

dal technique, and the materials show a high reversible K ion

storage capacity of 322 mAhg@1 at 200 mAg@1 over100 cycles.[197] Zhou et al. prepared tubular and interlayer-ex-

panded MoS2-N/O-doped carbon composites (E-MoS2/NOC TC)by a hydrothermal treatment and subsequent carbonization.

The expanded interlayer spacing (0.92 nm) of the ultrathinMoS2/C nanosheets facilitates insertion/extraction of K+ , result-

ing in faster K+ diffusion, and the N/O-doped carbon skeleton

and tubular structure improve structural stability, buffer thevolume change, and enhance electron transfer during cycling.Therefore, E-MoS2/NOC TC has excellent K+ storage character-istics with a high reversible capacity of 455 mAhg@1 at50 mAg@1 and cycling stability of 220 mAhg@1 after 300 cyclesat 250 mAg@1.[198] Xie et al. synthesized MoS2 roses on rGO

sheets (MoS2@rGO) by a two-step solvothermal route (Fig-ure 19A).[195] The uniformly distributed petal-like MoS2 nano-sheets with expanded interlayers (0.93 nm) were prepared on

rGO with strong chemical bonding to facilitate fast ion diffu-sion and electron transport (Figure 19B). The MoS2@rGO anode

has the highest specific capacity of 679 mAhg@1 at 20 mAg@1

among reported data so far for PIB anode materials, and the

capacity is maintained at over 380 mAhg@1 after 100 cycles at

100 mAg@1 (Figure 19C). The K+-storage mechanism in the lay-ered metal dichalcogenides of MoS2 can be expressed by com-

bining K+ intercalation and conversion reactions [Eqs. (4) and(5)] as follows:

MoS2 þ x Kþ þ x e@ Ð KxMoS2 ðabove 0:54 V vs: K=KþÞ ð4ÞTable

2.(Continued

)

Materials

Synthesismethod

Structure

Electrolyte

[a]

ICE[%

]Cap

acity[m

Ahg@1]/

cyclenumber

Ratecapab

ility

[mAhg@1]

Voltag

erange[V]Ref.

WTe

2hyd

rothermal/CVD

nan

orods

1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.05)

71@0.1Ag@1

221/10

0th@0.1Ag@1

143@

1Ag@1

0.01

–3.0

[299

][299

]

WTe

2hyd

rothermal/CVD

nan

oflo

wers

1m

NaC

lO4in

DEC

/EC/FEC

(1:1:0.05)

78.4@0.1Ag@1

260/40

th@0.1Ag@1

191@

1Ag@1

0.01

–3.0

[299

][299

]

NbS 2

chem

ical

exfoliation

nan

oshee

ts1m

NaP

F 6in

EC/D

MC(1:1)

63@0.1Ag@1

157/10

0th@0.5Ag@1

93@5Ag@1

0.01

–3.0

[174

][174

]

NbSe

2solid

-state

sintering

nan

oshee

ts1m

NaC

lO4in

DEC

/EC(1:1)

86.6@0.1Ag@1

98.1/100

th@0.1Ag@1

78.6@4Ag@1

0.01

–3.0

[176

][176

]

Fe0.3Nb0.7S

1.6Se 0

.4oil-phasesyntheticpro-

cess

2Dnan

oshee

ts1m

NaC

lO4in

EC/D

EC(1:1)[c

]47

@0.1A@g

260/75

0th@1Ag@1

136@

5Ag@1

0.01

–3.0

[175

][175

]

[a]DEG

DME=diethylen

eglycoldim

ethyl

ether,FEC=flu

oroethylen

ecarbonate,

PPC=propylen

ecarbonate,

andDGM=diglyme.

[b]T

heratiotypeofEC

/DMC/FEC

ismass/mass/vo

lume.

[c]With5wt%

FEC.

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Reviews

KxMoS2 þ ð4@xÞ Kþ þ ð4@xÞ e@ Ð 2 K2SþMo

ðbelow 0:54 V vs: K=KþÞ ð5Þ

S or Se vacancies in TMDs have attracted attention for opti-

mization of energy-storage performance due to the unique

properties of vacancies, that is, manipulation of the electronicstructure and active sites. Guo et al. fabricated MoS2(1@x)Se2xalloys (x=1, 0.75, 0.5,0.25, and 0) by partial substitution of Satoms with Se atoms in the MoSe2 nanoplates using an alloy-

ing technique (Figure 20A).[196] The alloying process generatesanion vacancies in situ in the crystal structure, the vacancy

Figure 19. A) Illustration of the synthesis strategy of uniform MoS2 roses on the rGO sheet by a two-step solvothermal method. B) Schematic illustration show-ing the paths of K-ion diffusion and electron conduction in the MoS2@rGO composites. C) Cycling performances of the MoS2@rGO, MoS2, and rGO anodes at100 mAg@1. Reproduced with permission from Ref. [195] Copyright 2017 Wiley.

Figure 20. A) Electron paramagnetic resonance results of MoSe2, MoSSe, and MoS2 with density of states and PDOS (partial density of states) of MoSSe show-ing one S vacancy and one Se vacancy. B) HRSTEM image. C) Cycling performance of MoSe2, MoSSe, and MoS2 at 2000 mAg@1 over 1000 cycles. Reproducedwith permission from Ref. [196] Copyright 2019 American Chemical Society.

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Reviews

concentration can be adjusted by tuning the substitution con-centration of S. Experiments and DFT calculations suggest that

the anion vacancies in MoS2(1@x)Se2x increase the electron con-ductivity, create more active sites, accelerate K+ diffusion, and

alleviate the structural variation in the alloys during potassiumstorage. The synthesized MoS2(1@x)Se2x exhibits a nanoplatemorphology with a lateral size of several dozens of nanome-ters and interlayer spacing of about 0.64 nm (Figure 20B),which is in between those of MoSe2 (0.66 nm) and MoS2(0.62 nm). The optimized MoSSe electrode has a high reversi-ble charge capacity of 220.5 mAhg@1 after 1000 cycles andgood cyclability, whereas those of the MoS2 and MoSe2 electro-des are relatively poor with only 18.3 and 114.3 mAhg@1 re-

tained after 1000 cycles (Figure 20C).Similar to other 2D materials, MoSe2 nanosheets tend to ag-

glomerate and undergo large volume change during K-ion re-

action, giving rise to rapid capacity fading due to the high sur-face energy. To address these issues, Huang et al. used conduc-

tive MXene flakes to form vertical MoSe2 nanosheets hydro-thermally and coated the MoSe2/MXene hybrid nanosheets

with a polydopamine (PDA)-derived carbon layer (denoted asMoSe2/MXene@C).[192] The MoSe2 nanosheets are anchored on

the MXene substrate vertically, forming a hierarchical 2D nano-

sheets structure that prevents self-aggregation of the MoSe2nanosheets and improves the conductivity. The strong chemi-

cal interactions at the interface of the MoSe2 nanosheets andMXene flakes further promote the charge-transfer kinetics and

improve the structural stability. As an anode in PIBs, MoSe2/MXene@C shows stable cycling with a high reversible capacity

of 355 mAhg@1 at 200 mAg@1 after 100 cycles and rate per-

formance with 183 mAhg@1 at 10.0 Ag@1.Compared to semiconducting MoS2 and MoSe2, 2D VS2 and

VSe2 nanosheets are metallic and so no conductive additive is

needed in the electrode structure, permitting a higher energydensity for K-ion storage. Previous studies demonstrated that

hierarchical VS2 nanosheets possess superior K ions storage ca-pability compared to that of MoS2 and MoSe2, indicating that

2D layered VS2 is a good candidate as PIB anode.[199] However,VS2 is metastable and preparation of 2D ultrathin VS2 nano-

sheets or hierarchical assemblies is challenging. Zhou et al. pre-pared hierarchical VS2 NSA composed of aligned ultrathinnanosheets.[94] The microsized NSA has a flower-like morpholo-

gy of curved nanosheets oriented at different angles with athickness of ten atomic layers (Figure 21A–C). Electrochemicalresults show that the VS2 nanosheets enable rapid and durablestorage of Li+ , Na+ , or K+ . PIB measurements of VS2 NSA are

carried out using 0.5m KPF6 in EC/DEC as the electrolyte. At100 mAg@1, the specific capacity is 380 mAhg@1 and it increas-

es to 410 mAhg@1 after 60 cycles at 100 mAg@1 with the corre-

sponding CEs being close to unity. When cycling the VS2 NSAat 500 mAg@1, the capacity of VS2 NSA increases initially from

290 to 360 mAhg@1 at the 100th cycle probably due to in-creased wetting of the electrode film by the electrolyte (Fig-

ure 21D). VSe2 is a metallic member of the TMD family, andthe interlayer distance of VSe2 is 0.611 nm. Yang et al. prepared

ultrathin VSe2 nanosheets as anode materials in PIBs.[16] The

crumpled VSe2 nanosheets have a smooth surface with lateraldimensions ranging from 500 nm to micrometers and a thick-

ness of 3.6 nm (Figure 22A). Benefiting from the unique 2Dnanostructure, high electron per K+ and small capacity decay

of 0.025% per cycle over 500 cycles can be obtained (Fig-ure 22B). Ex situ XRD and HRTEM measurements revealed that

K-ion storage in the VSe2 nanosheets proceeds with K-ion in-

tercalation in the VSe2 layers followed by the conversion reac-tion to metallic V and K2Se as shown in Figure 22C.

Figure 21. A,B) Low- and high-magnification SEM images of VS2 NSA. C) TEM image of VS2 NSA. D) Cycling stability and CE at 100 mAhg@1 and 500 mAg@1.Reproduced with permission from Ref. [94] Copyright 2017 Wiley.

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V5S8 is another member of the 2D layered vanadium sulfidefamily, possessing metallic properties and a much larger inter-

layer spacing of 11.32 a than VS2 (5.76 a), boding well for the

accommodation of the large K ion.[199] Li et al.[199] synthesizedhollow-carbon-templated 2D V5S8 nanosheets (denoted as

V5S8@C) by a hollow carbon template. The hollow carbon pre-vents the particles from aggregating and induces the forma-

tion of VS4 particles, which are then converted into ultrathinfew-layered V5S8 in the following annealing treatment. V5S8@C

has an excellent K-storage capacity of 645 mAhg@1 at

50 mAg@1 and cycling capabilities of 360 and 190 mAhg@1

after 500 and 1000 cycles even at 500 and 2000 mAg@1, re-

spectively. The reaction mechanism of the V5S8@C electrode inPIBs was studied by synchrotron X-ray diffraction, which re-

vealed that the charging of the V5S8@C electrode occursthrough reversibly sequential intercalation (KV5S8) and conver-sion reactions (K2S3) during the potassiation process.[199] The

recent progress of 2D TMDs in high-performance PIBs is pre-sented in Table 3.

3.4 2D TMDs for AICs

Improving the energy densities, power densities, and cycling

stabilities of EES has been the focus of major efforts for their

research and development. Rechargeable batteries and super-capacitors (SCs) are the two main EES systems. Although bat-

teries deliver high energy densities, they generally suffer fromlow power densities.[200] For instance, the commercial LIBs from

Panasonic have a specific energy upward of 200 Whkg@1 butonly a maximum specific power below 350 Wkg@1.[201] In con-

trast, SCs offer a large power densities (as high as 10 kWkg@1)

and good cyclability (>10000 cycles) but low energy densitiesof 5–10 Whkg@1. An objective for advanced electrical energy

storage is to deliver both high energy and high power in asingle system. Hybrid AICs that combine battery-type negative

electrodes and capacitor-type positive electrodes in an organicelectrolyte constitute a new family of EES devices bridging the

gap between batteries and SCs, which, in principle,can produce SC-like power and cyclability and higher

energy density than SCs.[202]

Lithium-ion capacitors (LICs) (also called Li-ion

hybrid supercapacitors) are utilize a high-energy LIBnegative electrode and a high-power SC positive

electrode in a Li-salt-containing organic electrolyte.The first LIC device was demonstrated by Amatucciand co-workers using activated carbon (AC) as the

positive electrode and nanostructured Li4Ti5O12 (LTO)as the negative electrode.[203] The energy density of

the device is 20 Whkg@1, which is about three timesthat of a conventional carbon-based supercapaci-

tor.[204] Subsequently, various LICs based on LTO andother LIB electrodes including insertion, conversion,

and alloy types such as Co3O4,[205] RuOx,

[206] nickel co-

baltite aerogels,[207]TiO2,[208] Li7Ti5O12,

[209] graphene,[210]

graphite,[211] pseudocapacitive T-Nb2O5,[212] V2O5,

[213]

VN,[214] MoS2,[215] VSe2,

[216] and Sn@C[217] have been re-ported. In LICs, carbonaceous materials such as AC,

CNTs, graphene, and metal–organic framework-derived porouscarbons are widely used as the positive electrodes,[204] which

have a high power density due to fast ion absorption/desorp-

tion. However, the Li ion intercalation/deintercalation processin the battery-like electrode is more sluggish and there is a ki-

netics imbalance between the battery-type negative electrodeand a capacitor-type positive electrode. The key challenge to

fabricate a high-performance LIC is to develop suitable nega-tive electrodes with large capacity and superior rate capability

so that the negative electrode and capacitive positive elec-

trode can be effectively coupled in a device to offer simultane-ously a high energy and power density. Among the various

negative electrode materials, 2D TMDs are promising in LICsdue to the large interlayer distances, large surface area, en-

riched active sites, and high intercalation pseudocapacitance.Cook et al. demonstrated that the Li+ intercalation/deintercala-

tion process in MoS2 is pseudocapacitive and the kinetics is

comparable to that of the double-layer electrode.[227] The 2D V-based and Nb-based chalcogenides have metallic conductivityand large pseudocapacitance, which are promising electrodematerials for LICs. Additionally, renewed interest in SIBs and

PIBs has spurred research in Na-ion capacitors (NICs) and K-ioncapacitors (PICs).

Zhang et al. synthesized MoS2–carbon microflowers on rGO(denoted as MoS2-C-rGO) hydrothermally with the addition ofpoly(diallyldimethylammonium chloride) (PDDA) and annealing

(Figure 23A).[225] The graphene sheets serve as the skeleton fordeposition of MoS2 microflowers to form a hierarchical struc-

ture (Figure 23B). By inserting the amorphous carbon layerinto adjacent MoS2 monolayers, the interlayer distance of MoS2was expand preferentially from 0.63 to 0.99 nm, resulting in in-

creased atomic interface contact/interaction. The MoS2-C-rGOnanocomposite has pseudocapacitance-dominated Li ion stor-

age characteristics and shows a reversible specific capacity of759 mAhg@1 at 0.1 Ag@1 and rate capability of 375 mAhg@1 at

10 Ag@1 in a wide working potential range (0.01–3.0 V vs. Li/Li+). A LIC fabricated was with MoS2-C-rGO as the negative

Figure 22. A) SEM image of the ultrathin layered VSe2 nanosheets. B) Cycling stabilityand CE of the VSe2 nanosheet electrode at 200 mAg@1. (C) Schematic illustration of rever-sible K-ion storage in the ultrathin VSe2 nanosheets. Reproduced with permission fromRef. [16] Copyright 2018 Wiley.

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electrode and polyaniline-derived porous carbon (PDPC) as thepositive electrode. During discharging, Li ions are deintercalat-

ed from the MoS2-C-rGO composite while PF6@ desorbs from

the porous carbon (Figure 23C). The LIC shows a large specific

energy density of 188 Whkg@1 at 200 Wkg@1 and 45.3 Whkg@1

at a high power density of 40000 Wkg@1 besides a long cyclinglifetime with &80% capacitance retention after 10000 cyclesat 2 Ag@1 (Figure 23D). The specific energy and power densi-ties of the MoS2-C-rGO//PDPC hybrid LIC are higher than thoseof other reported hybrid LICs such as graphite//AC,[218] Fe3O4-rGO//AC,[219] Li4Ti5O12-graphene//AC,

[220] Nb2O5-C//AC,[221] VN-

rGO//AC,[214] TiO2-graphene//AC,[222] H2Ti6O13//CMK-3,[223] and

TiN//AC[224] (Figure 23E). Lee and co-workers[226] synthesized a

3D porous MoS2@3D graphene (MoS2@3DG) composite withuniformly incorporated MoS2 flocculent nanostructure on 3D

graphene as an anode in LICs. The porous hierarchical

MoS2@3DG was prepared by solution processing, and the highdensity of flocculent MoS2 nanostructures with a main diame-

ter of 300 nm are uniformly embedded into the 3D porous gra-phene hydrogel to form the MoS2@3 DG hybrid (Figure 24A,B).

The MoS2@3DG composite shows a high rate capability of688 mAhg@1 at a current density of 8 Ag@1 and stable cycling

(997 mAhg@1 after 700 cycles at 2 Ag@1) due to the synergistic

effects of the conductive graphene sheets and interconnectedporous structure. The prelithiated MoS2@3DG as a negative

electrode is coupled with a N-doped porous carbon (NPC) pos-itive electrode to produce the LIC denoted as MoS2@3DG//NPC

(Figure 24C). The MoS2@3DG//NPC HSC show stable cycling forover 2000 cycles with energy density retention of 78% from

97.2 Whkg@1 to 75.4 Whkg@1 at 1.0 Ag@1 (Figure 24D). The CE

is above 90% during the cycling measurement. In contrast, theMoS2//NPC HSC with the pure MoS2 anode shows severe re-

duction in energy density after only 100 cycles, thus demon-strating the important function of the 3D conductive and

porous structure of the MoS2@3DG anode.[226]

2D TiS2 was also investigated as promising insertion anode

materials in LICs. Chaturvedi et al. showed that a LIC full cell

with TiS2 as the negative electrode and AC as the positive elec-trodes shows a maximum energy density of 49 Whkg@1 and

76% energy-density retention after 2000 cycles.[228] Moreover,the energy density increases slightly to 50 Whkg@1 by replac-ing some of the S atoms in TiS2 with Se to form TiSe0.4S1.6. TheAC//TiSe0.6S1.4-based LIC shows an energy density of 50 Whkg@1

and good cyclability for 5000 cycles.[229]

Inspired by the high energy/power density of LICs, SICs andPICs are gaining increasing attention as a promising alternative

to LICs due to the abundance and low cost of Na and K resour-ces. The development of high-performance SIB and PIB anodes

with low Na+ or K+ migration barrier, high rate capability, andlarge pseudocapacitance is crucial for the development SICs

and PICs. Compared to MoS2, MoSe2 has a larger interlayer dis-

tance, facilitating the insertion/extraction of Na+ . Moreover,the band gap of MoSe2 is about 1.1 eV narrower than that of

MoS2 (&1.7 eV), indicating higher electronic conductivity,which is beneficial for a high rate capability.[104] Zhao et al. pre-

pared oriented, interlayer-expanded MoSe2 nanosheets on gra-phene (MoS2/G) with Mo@C bonding by a surfactant-directedTa

ble

3.Su

mmaryoftheproperties

of2D

TMDsin

PIBs.

Materials

Synthesismethod

Structure

Electrolyte

[a]

ICE[%

]Cap

acity[m

Ahg@1]/

cyclenumber

Ratecapab

ility

[mAhg@1

]

Voltag

erange[V]

Ref.

MoS 2/NC

solvothermal

bam

boo-likehollo

wtubes

0.8m

KPF 6

inEC

/DEC

(1:1)

–33

0/50

th@0.05

Ag@1

131@

2.0Ag@1

0.01

–2.5

[17]

MoS 2/C@NDG

hyd

rothermal

ultrafin

ecarbon-coated

nan

oshee

ts0.8m

KPF 6

inEC

/DEC

(1:1)

59.3@0.1Ag@1

220.7/15

0th@1.0Ag@1

176.6@

2.0Ag@1

0.01

–3.0

[300

]Mos 2-M

onolayers/C

solvothermal

mesoporouscomposite

0.8m

KPF 6

inEC

/DEC

(1:1)

–18

0/24

0th@0.5Ag@1

164@

2.0Ag@1

0.01

–2.5

[194

]MoS 2@rGO

hyd

rothermal

nan

orose-like

1m

KPF 6

inEC

/DEC

/PC(1:1:1)

72.2@0.1Ag@1

416/20

0th@0.1Ag@1

196.8@

2Ag@1

0.01

–3.0

[301

]MoS 2@rGO

solvothermal

ultrathin

andultraunifo

rmrose-like

0.8m

KPF 6

inEC

/DEC

(1:1)[b

]–

380/10

0th@0.1Ag@1

178@

0.5Ag@1

0.01

–3.0

[302

]Co9S

8/NSC

@MoS 2@NSC

hyd

rothermal

3Dhierarchical

nan

oboxes

0.8m

KPF 6

inEC

/DEC

(1:1)

65.9@0.1Ag@1

403/10

0th@0.1Ag@1

141@

3Ag@1

0.01

–2.6

[303

]MoS 2@Sn

O2@

Chyd

rothermal

sandwich-like

0.8m

KPF 6

inEC

/DEC

(1:1)

73@0.05

Ag@1

312/25

th@0.05

Ag@1

86@0.8Ag@1

0.01

–3.0

[105

]E-MoS 2/NOCTC

hyd

rothermal/annealin

gtubularcomposite

structure

0.8m

KPF 6

inEC

/DEC

(1:1)

44@0.25

Ag@1

220/30

0th@0.25

Ag@1

247@

1Ag@1

0.01

–3.0

[198

]MoSe

2/N-Doped

Carbon

solvothermal

carbon-coated

nan

oshee

ts1m

KFSIin

EMC

79.9@0.1Ag@1

258.02

/300

th@0.1Ag@1

178@

2.0Ag@1

0.01

–3.0

[15]

MoSe

2/C

annealin

gpistachio-shuck-likecore–shell

0.8m

KPF 6

inEC

/DEC

(1:1)

63.4@0.2Ag@1

226/10

00th@1.0Ag@1

224@

2.0Ag@1

0.01

–2.5

[197

]MoSe

2/C-700

electrospinning/selen

ization

nan

oshee

ts0.8m

KPF 6

inEC

/DEC

(1:1)

50.2@0.1Ag@1

316/10

0th@0.1Ag@1

133@

3A@g

0.01

–3.0

[304

]C-coated

MoSe

2/MXen

ehyd

rothermal/annealin

ghierarchical

3Dstructure

1m

KFSIin

EC/D

EC(1:1)

54.2@0.2Ag@1

317/30

0th@1Ag@1

183@

10.0Ag@1

0.01

–3.0

[192

]FeMoSe

4@N-doped

carbon

annealin

gcore–shell

0.8m

KPF 6

inEC

/DEC

(1:1)[c

]–

298/10

0th@0.2Ag@1

178@

1Ag@1

0.01

–2.5

[305

]TiNb2O

6@MoS 2/C

solvothermal/annealin

gthree-layeredheterostructure

0.8m

KPF 6

inEC

/DEC

(1:1)

50.45@

0.1Ag@1

424/50

th@0.1Ag@1

233@

1Ag@1

0.01

–2.5

[306

]V5S

8@C

solvothermal

core–shell

1m

KFSIin

EC/PC(1:1)

64@0.05

Ag@1

360/50

0th@0.5Ag@1

274@

2.0Ag@1

0.01

–3.0

[199

]VS 2

solvothermal

hierarchical

nan

oshee

ts0.5m

KPF 6

inEC

/DEC

(1:1)

–41

0/60

th@0.1Ag@1

100@

2.0Ag@1

0.01

–3.0

[94]

VSe

2chem

ical

reaction

metallic

graphen

e-likeultrathin

nan

oshee

ts0.8m

KPF 6

inEC

/DEC

(1:1)

69.1@0.1Ag@1

335/20

0th@0.2Ag@1

169@

2.0Ag@1

0.01

–2.6

[16]

NbSe

2solid

-state

sintering

nan

oshee

ts0.8m

KPF 6

inEC

/DEC

(1:1)

72.5@0.05

Ag@1

26/60t

h@0.05

Ag@1

24.1@0.5Ag@1

0.01

–3.0

[176

]

[a]In

whichKFSIispotassium

bis(fluorosulfo

nyl)im

ide.

Theproportionin

thebracketsisthevo

lumeratioofmixsolution.[b]With2wt%

FEC.[c]

With3vo

l%FEC.d

ChemSusChem 2020, 13, 1114 – 1154 www.chemsuschem.org T 2020 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1141

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hydrothermal reaction.[104] DFT calculations demonstrated

charge accumulation at the interface of MoS2/G and improvedNa+ transport through the interface. A SIC was assembled

using MoS2/G as the negative electrode and AC as the positive

electrode (Figure 25A).[230] The crimped, thin MoSe2 nanosheetscover the graphene nanosheets to form a porous honeycomb

Figure 24. A) Schematic of the fabrication of the MoS2@3DG hybrid structure. B) SEM image of the MoS2@3DG composite (inset: optical photograph ofMoS2@3DG bulk sample, scale bar: 1 cm). C) Schematic illustration of the mechanism of the MoS2@3DG//NPC HSC. D) Cycling performance of theMoS2@3DG//NPC HSC and MoS2//NPC HSC at a current density of 1.0 Ag@1 for 2000 cycles. Reproduced with permission from Ref. [226] Copyright 2016 Amer-ican Chemical Society.

Figure 23. A) Schematic illustration of the synthesis of MoS2-C-rGO via a hydrothermal reaction followed by annealing. B) ADF-TEM image of MoS2-C-rGO.C) Schematic illustration of the hybrid LIC at the discharging state. Li+ and PF@6 can be intercalated/deintercalated in or out of the MoS2-C-rGO anode andPDPC cathode. D) Cycling stability at a current density of 2 Ag@1 with a high CE close to 100%. E) RAgone plots of MoS2-C-rGO//PDPC hybrid LIC as well asPDDC//PDDC symmetric supercapacitor. The specific energy and power densities of MoS2-C-rGO//PDPC hybrid LIC are compared with those of other reportedhybrid LICs: graphite//AC,[218] Fe3O4-RGO//AC,

[219] Li4Ti5O12-graphene//AC,[220] Nb2O5-C//AC,

[221] VN-RGO//AC,[214] TiO2-graphene//AC,[222] H2Ti6O13//CMK-3,[223] and

TiN//AC.[224] Reproduced with permission from Ref. [225] Copyright 2017 Elsevier.

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hierarchical structure (Figure 25B). MoSe2 nanosheets on the

conductive graphene substrate with M@C bonding at the inter-face avoids undesired restacking of both components. The

few-layer MoSe2 nanosheets with rich edge defects and en-

larged interlayer spacing enable the storage of more Na+ atthe surface and facilitate fast Na+ diffusion in the interlayers.

Consequently, superior pseudocapacitive Na-ion storage prop-erties and 94.2% capacity retention after 50 cycles at 0.4 Ag@1

were accomplished. The SIC with the MoSe2/G negative elec-trode and AC positive electrode shows high energy and powerdensities of 82 Whkg@1 and 10752 Wkg@1, respectively. Even at

a very high power output of 6688 Wkg@1, the SIC shows an im-pressive energy density of 43 Whkg@1.[230] The energy density

and power output of the MoSe2/G//AC SIC are superior tothose of other SIC systems such as Na2Ti3O7-carbon cloth,[231]

V2O5-CNT,[232] single-crystal TiO2-graphene,

[233] and 2D Nb2O5[234]

(Figure 25C). Moreover, the MoSe2/G//AC SIC exhibits excellent

cycling stability with retention of 81% after 5000 charging/dis-charging cycles at a current density of 5 Ag@1 as well as highCE of nearly 100% at room temperature (Figure 25D). Zhao

et al.[235] and Cui et al.[236] reported metal oxide/2D TMD com-posites as promising anodes in SICs. SnO2 nanoparticles grown

on MoSe2 nanosheets with Se@O bonding enhance chargetransport and prevent the restacking of MoSe2. A SIC based on

the SnO2/MoSe2 anode and AC cathode has a high energy den-

sity of 70 Whkg@1 at a power output of 62 Wkg@1 and excel-lent cycling stability as exemplified by a capacitance retention

rate of 94% for 6000 cycles at 5 Ag@1.[235] MoO2 on the MoSe2nanosheets promote the reversible conversion of MoSe2, and

the MoO2/MoSe2-G//AC device delivers a maximum energydensity and power output of 71 Whkg@1 and 14316 Wkg@1, re-

spectively, with only 8% capacitance loss after 7000 cycles at

6 Ag@1.[236]

VSe2 has a similar layered structure as MoSe2 but metallic

characteristics favorable for Na ion storage. Wu et al.[216] pre-

pared rough endoplasmic reticulum-like 3D VSe2/rGO aerogelnanocomposites as the anode in SICs. VSe2 in the 3D VSe2/rGO

aerogel is uniformly dispersed on both sides of rGO to formthe sandwiched nanosheets (Figures 26A,B). The biomimetic

porous 3D rGO skeleton provides a stable host to anchor VSe2,which accelerates diffusion and adsorption of the electrolyte inthe electrode and buffers the volume expansion of VSe2 during

sodiation/desodiation (Figure 26C). The 3D VSe2/rGO aerogelelectrode has a high specific capacity of 196.5 mAhg@1 at

1.0 Ag@1 after 5000 cycles, and the hybrid SIC with a VSe2/rGOanode and AC cathode shows large energy and power densi-

ties of 106 and 68 Whkg@1 under 125 and 5000 Wkg@1, respec-tively. Moreover, it shows good cycling stability with

30 Whkg@1 retention after 1000 cycles (Figure 26D) bodingwell for next-generation high-energy and high-power devi-ces.[216]

Recently, Yi et al. fabricated N-doped MoSe2/graphene (N-MoSe2/G) composites with favorable pseudocapacitive K+ stor-

age for KICs using a diatomite-templated synthetic strategy(Figure 27A).[237] The biomorphic N-MoSe2/G possesses holey

and crumpled architectures consisting of curly MoSe2 and gra-

phene nanosheets (Figure 27B). The interlayer spacing of0.71 nm is slightly larger than that of the (002) plane (0.64 nm)

in bulk MoSe2 (Figure 27C). The N-MoSe2/G composite has re-markable K-ion storage properties as exemplified by a high

rate capability of 155 mAhg@1 at 2.0 Ag@1. A KIC was assem-bled using the bio-templated N-MoSe2/G anode, bioderived AC

Figure 25. A) Schematic illustration of the charge-storage mechanism of the MoSe2/G//AC SICs. B) SEM image of MoSe2/G. C) Ragone plot of the asymmetricMoSe2/G//AC SIC with the data calculated from the total mass weight of both electrodes. D) Cycling performance of MoSe2/G//AC SIC at 5 Ag@1, demonstrat-ing retention of 81% and CE of 100% after 5000 cycles. Reproduced with permission from Ref. [230]. Copyright 2018 Elsevier.

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cathode, and potassium salt-based (KPF6) electrolyte. Uponcharging, the PF6

@ anions in the electrolyte adsorbs onto the

AC cathode while K+ ions in the electrolyte intercalate into theN-MoSe2/G anode; during discharging the processes are re-

versed (Figure 27D). The hybrid capacitor shows a largeenergy density of 119 Whkg@1 at a power density of39.6 Wkg@1, 29 Whkg@1 at a high power density of 7212 Wkg@1

as well as a long cycling life of over 3000 cycles at 1.0 Ag@1.

The high energy/power characteristics compare favorably withthose of other SIC and LIC devices, which are superior to thoseof recently reported KICs such as Ca0.5Ti2(PO4)3//AC,

[238] soft

carbon//AC,[239] K2TP//AC,[240] and K2Ti6O13/N-doped graphitic

carbon (KTO//NGC)[241] (Figure 27E). The recent progress of 2D

TMDs in AICs including LICs, SICs, and PICs is summarized andpresented in Table 4.

3.5 2D TMDs for high-energy Li–S and Li-O2 batteries

The ever-increasing demand for large-scale energy storage,portable electronics, and electric vehicles today has triggered

the need for alternative battery technologies with muchhigher specific energies. Li–S batteries are considered as the

next-generation rechargeable batteries to succeed LIBs due to

its high theoretical capacity (1675 mA g@1) and energy density

(2600 Whkg@1).[242–244] Moreover, S is inexpensive, naturallyabundant, and environmentally benign. The electrochemicalreactions of Li–S batteries involve reversible conversions be-tween sulfur (S8) and Li sulfides (Li2S and Li2S2) [Eq. (6)] .

[245]

S8 þ 16 Liþ þ 16 e@ Ð 8 Li2S ð6Þ

During the discharge process, the S8 species at the cathodeare electrochemically reduced to Li polysulfides (LiPSs, Li2Sn,

n=4–8) and finally to solid Li2S. During the charging process, areversible reaction takes place and solid Li2S is first converted

into Li2S2, then into soluble long-chain LiPSs, and lastly intosolid S8. The uncontrolled dissolution and loss of intermediate

LiPSs into the electrolyte, leading to the effusion of active S

from the cathode and “LiPSs shuttling”. Moreover, the Li–S bat-teries are also plagued by the low electronic conductivity of

both S and Li2S and the large volumetric expansion of S(&80%) upon lithiation. These problems inevitably result in

low S utilization, poor cycling lifetime and low CE, impedingthe practical implementation of Li–S batteries. Recently, signifi-

Figure 26. A) Schematic of the microscopic and working mechanisms of VSe2/rGO. B) FE-SEM image of VSe2/rGO. C) Schematic illustration of the VSe2/rGO//ACSIC. D) Long-term cyclic performance of the VSe2/rGO//AC SIC. Reproduced with permission from Ref. [216] Copyright 2019 Royal Society of Chemistry.

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cant progress has been achieved in the design of new electro-lytes (additives), novel Li anodes, and functional materials for S

hosts, modified separator, or interlayers.[246]

The poor performance of Li–S batteries is unquestionably

dominated by the effusion and shuttling of soluble LiPSs. In-corporation of active S into porous carbon materials formingS/carbon composites can alleviate polysulfide shuttling by

physical confinement and improve the electrical conductivityof the cathodes.[251] Various carbon matrices such as porous

carbon,[252] carbon spheres,[253] CNTs,[254] graphene[255,256] andtheir hybrids[257,258] have been explored as S host materials,which lead to reduced shuttle effect and improved cycle stabil-ity. However, the weak interaction between nonpolar carbon

and polar LiPSs inevitably causes effusion and easy detach-ment of Li2Sn from the carbon surface, especially at a high Smass loading. Compared with physical confinement, polar host

materials can anchor the LiPS species effectively, which aremore promising for Li–S batteries to achieve long-term cy-

cling.[249,259,260] Recently, 2D TMDs have been explored as effi-cient host materials for S cathodes in Li–S batteries by chemi-

cal anchoring of LiPSs on the cathodes utilizing moderate

binding strength between TMDs and polysulfides, which arethe best choices for battery electrodes (Table 5).[247] Moreover,

some of TMDs can effectively catalyze the polysulfide redox-re-action kinetics, leading to considerably enhanced Li–S battery

performance. Lei et al.[249] reported polar WS2 nanosheet-de-posited CNFs (C@WS2) as a host material for Li–S batteries. The

C@WS2/S composite electrode was prepared by directly grow-ing 2D WS2 nanosheets on a carbon cloth through a simple hy-

drothermal method followed by sulfur loading via annealing Sand C@WS2 at 155 8C, as illustrated in Figure 28A. In the flake-

structured C@WS2 composite electrode, dense WS2 nanosheetsare wrapped and coated on CNFs (Figure 28B). These thin WS2nanosheets possess a high active surface and low contact re-

sistance, which could provide a high specific surface area for Sloading and fast electron transfer. The nanoscale S is firmly ab-sorbed on the surface of CNFs not only through physical vander Waals forces but also via the WS2 polar functional groups.Thus, LiPSs are chemically anchored on the cathode and thedissolution of LiPSs intermediates is significantly inhibited (Fig-

ure 28C). Due to the polar adsorption of LiPSs on the WS2nanosheets and the excellent electronic transport of the 3Dstructure of the CNFs, the C@WS2/S cathode delivers an initial

reversible capacity of 1180 mAhg@1 at 0.5C (1C=1675 mAhg@1), and with 995 mAhg@1 after 500 cycles (Fig-

ure 28D). Moreover, the battery still maintained about 90% ofits specific capacity after 1500 cycles at 2C.[249] Other 2D TMDs

such as MoS2,[259, 260] VS2,

[28,261,262] and NbS2[263,264] have been ex-

plored as S host materials for Li–S batteries. However, the in-trinsic low electronic conductivity of TMDs results in sluggish

redox reaction kinetics in Li–S batteries.TMDs also can be applied as a separator modifier to sup-

press the shuttling of LiPSs between cathode and anode. Forexample, Tang et al. reported a Li-conducting MoS2/Celgard

Figure 27. A) Schematic illustration of the synthetic procedure of N-MoSe2/G. B, C) TEM images of the N-MoSe2/G interface with scale bars of 50 and 3 nm, re-spectively. D) Schematic of the N-MoSe2/G//AC KIC device. E) Ragone plots of LICs, SICs, and KICs reported in literature. Reproduced with permission fromRef. [237] Copyright 2019 John Wiley and Sons.

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composite separator as an efficient polysulfide barrier in Li–S

batteries. The MoS2/Celgard composite separator was con-structed by simply filtering exfoliated MoS2 solution through

the Celgard substrate. Due to the high density of Li ions on

the MoS2 surface, this composite separator exhibits high Li-ionconductivity, fast Li-ion diffusion, and facile transference across

the separator.[265] When used in Li–S batteries, the separator isproven to be highly efficient for depressing polysulfide shut-

tling. The device with MoS2/Celgard separator delivers an initialcapacity of 808 mAhg@1 and a substantial capacity of

401 mAhg@1 after 600 cycles, corresponding to only 0.083%

capacity decay per cycle.[265]

During battery discharge and charge, S, Li2S2, and Li2S are in-

soluble, the conversion between S and its end products (Li2S2and Li2S) has to occur via LiPSs as the intermediate products,

which are soluble in most Li–S battery electrolytes. In this con-text, accelerating the rates of conversion of soluble LiPSs (to S,

Li2S2, or Li2S) can reduce the presence of polysulfides in the

electrolyte, and hence improve both the sulfur utilization andthe battery cycling stability. Lee et al. show that MoS2@x/rGO

can be used to catalyze the polysulfide reactions to improvethe battery performance; S deficiencies in the MoS2 nanoflakes

were found to be the catalytic centers (Figure 29A). The fastconversion of soluble polysulfides can lower their accumula-tion in the cathode and hence the loss from the cathode by ef-

fusion. When a small amount of MoS2@x/rGO (4 wt%) wasadded to the sulfur cathode, high rate (8C) performance of theS cathode was improved from a capacity of 161.1 mAhg@1 to826.5 mAhg@1. Moreover, MoS2@x/rGO also showed enhancedcycling stability of the S cathode with a capacity fade rate of0.373% per cycle (over 150 cycles) decreasing to 0.083% per

cycle (over 600 cycles) at 0.5C, which is much better comparedto rGO/S and MoS2/rGO/S (Figure 29B,C).[266]

Li–O2 batteries have attracted tremendous attention over

the past few years due to the highest theoretical energy densi-ties (3500 Whkg@1).[267] Generally, Li–O2 batteries are divided

into two types: aprotic and aqueous systems.[268] For the aprot-ic systems, the battery is composed of a Li-metal anode, aprot-

ic electrolyte with dissolved Li salt, and a porous air cathode

with an electrocatalyst.[269] The electrochemical reaction be-tween anode and cathode is based on the reversible redox re-

action between Li metal and O2 [Eq. (7)] .[269]

2 Liþ þ 2 e@ þ O2 Ð Li2O2 ð7ÞTable

4.Su

mmaryoftheproperties

of2D

TMDsin

AICs.

Materials

Cap

acitor[a

]Synthesismethod

Structure

Electrolyte

Cap

acity[

b]

[mAhg@1]

Max.e

nergy

den

sity

[Whkg

@1]

Max.p

ower

den

sity

[kWkg

@1]

Voltag

erange[

c][V]

Ref.

MoS 2@3D

GLIC

solvo-/hyd

rothermal

3Dporousstructure

1.0m

LiPF 6

inEC

/EMC/D

MC(1:1:1)[d

]99

7@2Ag@1

156

8.31

40.0–

4.0

[226

]MoS 2-C-RGO

LIC

hyd

rothermal/annealin

ginterlayer-overlap

ped

structure

1.0m

LiPF 6

inEC

/DEC

(1:1)

759@

0.1Ag@1

188

400.0–

4.0

[225

]WSe

2LIC

CVD

onion-like

1.0m

LiPF 6

inEC

/DEC

/DMC(1:1:1)

211@

1.6Ag@1

80.1

14.1

1.0–

3.4

[307

]E-MoS 2/CF

SIC

hyd

rothermal/annealin

gnan

oflo

wers

1MNaC

lO4in

EC/D

EC(1:1)[e

]10

4@20

Ag@1

54.9

11.0–

4.3

[225

]MoSe

2SIC

hyd

rothermal

vertical

growth

1MNaC

lO4in

EC/D

EC/FEC

,(1:1:0.05)

445@

0.1Ag@1

8210

.752

0.5–

3.0

[230

]MoSe

2/Sn

O2

SIC

hyd

rothermal

heterostructure

1MNaC

lO4in

EC/D

EC/FEC

,(1:1:0.05)

249@

10Ag@1

702.30

40.5–

3.0

[235

]MoO

2/MoSe

2SIC

solvothermal

heterostructure

1MNaC

lO4in

EC/D

EC/FEC

(1:1:0.05)

301@

3.2Ag@1

7114

.316

1.0–

3.4

[295

]WSe

2SIC

CVD

onion-like

1MNaC

lO4in

EC/D

EC/FEC

(1:1:0.05)

205@

10Ag@1

123.1

14.1

1.0–

3.4

[307

]VSe

2/rGO

SIC

hyd

rothermal

roughen

doplasm

icreticulum-like

1MNaC

lO4in

EC/D

EC(1:1)[f

]36

5@0.05

Ag@1

106

51.0–

4.0

[216

]MoSe

2/C-700

PIC

electrospinning/selen

ization

nan

oshee

ts0.8m

KPF 6

inEC

/DEC

(1:1)

316@

0.1Ag@1

169

0.58

81.0–

4.0

[304

]N-M

oSe

2/G

PIC

diatomite-templatedsyntheticroute

nan

oarchitectures

0.8m

KPF 6

inEC

/DEC

(1:1)

401@

0.2Ag@1

119

7.21

20.5–

4.0

[237

]

[a]In

whichtheLICsrepresentLithium-ioncapacitor;

SICsrepresentsodium-ioncapacitor;PICsrepresentpotassium-ioncapacitor.[b]In

whichthecapacityis

tested

athalf-cellsystem

forTM

Dsan

ode.

[c]In

whichtheworkingvo

ltag

eisforfull-capacitortestingsystem

.[d]With10

wt%

FEC.[e]

With10

wt%

FEC.[f]With5wt%

FEC.

Table 5. The binding energy (Eb) and bond lengths (dLi@S) for differentlayered materials when Li2S4 and Li2S6 clusters are adsorbed.

Materials Eb with Li2S4 [eV] Eb with Li2S6 [eV] dLi@S [a] Ref.

graphene 0.47 0.602 – [247, 248]MoS2 0.77 – 2.40 [247]WS2 0.80 – – [249]VS2 – 1.04 2.54 [250]NbS2 1.80 – 2.65 [247]TiS2 1.54 – 2.54 [247]

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During discharging, Li metal is converted into Li ions, which

migrate to the cathode side. At the cathode surface, O2 is then

reduced to O2@ to form the discharge product Li2O2, which is

oxygen reduction reaction (ORR). During charging, Li2O2 is de-

Figure 28. A) Schematic illustration for the preparation of WS2 vertically aligned on the CNFs; B) SEM images of the as-prepared free-standing C@WS2 compo-site; C) UV/Vis spectra of the Li2S6 solution with C@WS2/S and C/S (inset photograph of the Li2S6 solution with different anchoring materials) ; D) Long cyclingwith 0.5C. Reproduced with permission from Ref. [249] Copyright 2016 Wiley.

Figure 29. A) Schematic illustration for the synthesis of NbS2@S@IG composite; B) cycling performance of G-S, IG-S, and NbS2@S@IG electrodes at 0.5C; C) cy-cling performance of NbS2@S@IG electrode at 1C with areal sulfur loading of 3.25 mgcm@2. Reproduced with permission from Ref. [266] Copyright 2017 RoyalSociety of Chemistry.

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composed to generate O2 (oxygen evolution reaction, OER).[267]

The gravimetric energy density and volumetric energy density

for this system can reach up to 3505 Whkg@1 and 3436 WhL@1,respectively,[270] much higher than those of other rechargeable

batteries. However, the practical implementation of Li–O2 bat-teries is plagued by several challenges such as high charging

overpotential, poor cyclability, low energy efficiency, and poor

stability.[267, 271,272] Since ORR and OER are the two main process-es during the rechargeable process, a popular solution is using

a catalyst to promote the catalytic activities of electrode mate-rials. Thus, developing effective cathode catalysts (OER andORR) is crucial for the development of rechargeable Li–O2 bat-teries.[268]

Recently, 2D TMDs such as MoS2,[66] WS2.

[273] have been treat-ed as OER and ORR catalysts owing to their stability, low cost,

and good catalytic activity. For instance, Asadi et al. found that

when combining MoS2 nanoflakes with an ionic liquid (IL) to-gether as a cocatalyst, both ORR and OER performance were

superior to the precious-metal catalyst. The [EMIM]+ (1-ethyl-3-methylimidazolium)-covered Mo edge catalytically active sites

could efficiently reduce O2 into O2@ , giving rise to a remarkable

85% round-trip efficiency and reversible 50 cycles .[234] The hy-

bridization of TMDs with carbon materials is an effective strat-

egy to overcome the low conductivity and improve the per-formance of TMDs. Hu et al. synthesized a 3D mesoporous in-

terconnected network composed of ultrathin MoS2 nanosheetsand highly conductive CNTs (MoS2/CNTs) through a simple

one-step hydrothermal method.[274] The 3D-interconnected net-work architecture of MoS2/CNTs efficiently promote the diffu-

sion of O2 and Li ions as well as the impregnation of the elec-

trolyte and provides abundant storage space for the accom-modation of discharge products while the incorporation of uni-

formly dispersed CNTs improves the electronic conductivityand maintains the integrity of the cathode structure. Therefore,

and improved performance with low overpotentials and alarge discharge capacity of 6904 mAhg@1 at a current density

of 200 mAg@1 and excellent cycling stability of 132 cycles was

achieved for the Li–O2 battery based on MoS2/CNTs. Song et al.reported an integrated cathode by depositing a &5 nm-thickamorphous MoS2 layer on CNT forest-covered graphite foam.This integrated 3D cathode exhibits high catalytic activity for

both ORR and OER. The Li–O2 battery exhibit a high energy ef-ficiency of 83% at 250 mAg@1, a long cycle life of 190 cycles,

and a high capacity of 4844 mAhg@1 at 500 mAg@1.[275]

Recently, Zhang et al. demonstrated a facile one-step hydro-thermal synthesis of MoS2 nanosheets decorated with Au

nanoparticles (MoS2/AuNPs) for rechargeable Li–O2 batteries(Figure 30A).[210] Owing to the large specific surface area of

MoS2 and the uniform distribution of AuNPs, the hybrids canshow considerable exposure of active electrochemical sites

(Figure 30B). The combination of MoS2 nanosheets and AuNPs

displays a synergistic catalytic effect, and the cell with MoS2/AuNPs exhibits a discharge and charge overpotential of 0.21

and 1.28 V, which is lower than that of a cell with Super P andpristine MoS2 nanoflowers. This hybrid MoS2/AuNPs cathode

delivers a specific capacity of 4336 mAhg@1, whereas the MoS2nanoflowers and Super P display a specific capacity of 3007

and 2760 mAhg@1, respectively (Figure 30C). Moreover, thehybrid cell can maintain good performance up to 50 cycles at

300 mAg@1 without apparent voltage degradation (Fig-ure 30D), suggesting a potential application of 2D TMDs in Li–

O2 batteries.

4. Summary and Outlook

In this Review, recent progress on the synthesis strategies and

Li-, Na-, and K-ion-storage properties of 2D transition-metal di-chalcogenides (TMD) nanosheets and their 3D hybrid architec-tures with a sandwich-like MX2 structure (M=Mo, V, W, Nb;X=S, Se, Te) is described. Different types of 2D TMDs and com-

posites have been prepared by various methods such as chem-ical exfoliation, hydro(solvo)thermal synthesis, chemical vapordeposition (CVD) and electrospinning. Owing to the large in-terlayer distance and weak van der Waals forces, the TMDnanosheets and composites have great potential as versatile

anode materials with respect to Li-ion storage and even moreso for Na- and K-ion storage in Li-, Si-, and K-ion batteries (LIBs,

SIBs, PIBs) or alkali metal-ion capacitors (AICs). The 2D TMDs

are also promising for Li–S and Li–O2 batteries as host and cat-alytic materials. By integrating TMDs with conductive materials

such as graphene and 3D graphene foam, the electrical con-ductivity of TMD-based electrodes such as MoS2, MoSe2, WS2,

and WSe2 can be improved significantly and restacking of TMDnanosheets can be effectively inhibited. Moreover, plenty of

pores or channels in TMD/C composites effectively reduce the

distances of ion and mass transport. Consequently, 3D TMD/Carchitectures have been demonstrated to deliver better per-

formance in terms of capacity, rate capability, and cycling sta-bility than their bulk counterparts or other types of nanostruc-

tures. Furthermore, the interlayer spacing of MX2 can be fur-ther expanded by controlling synthesis conditions or inserting

carbon between MX2 layers. Carbon intercalation between the

MX2 layer to form carbon–TMDs–carbon sandwiched structureincreases the interlayer distance, which not only facilitates and

accelerates the ion insertion/extraction process but can alsoaccommodate the large volume expansion during cycling to

improve the electrochemical properties, especially for SIBs andPIBs. To minimize the side reactions and irreversible capacity

and maintain the layered structure of 2D TMDs during cycling,the cut-off voltage and electrolyte should be optimized to

avoid the formation of conversion products and intermediatesof soluble polysulfides or polyselenides to yield long-term cy-cling. Although some improvement has been made in the elec-

trochemical properties by means of carbon modification, heter-ostructure design, and cut-off voltage methods, many chal-

lenges remain to be addressed.Firstly, CVD and chemical exfoliation could produce high-

quality 2D TMDs; however, the low yield and quantities couldnot meet the application in electrochemical energy storage,which would require large quantities of 2D TMDs. Solution-

based synthetic methods allow the high-yield and large-scalesynthesis of 2D TMDs nanosheets and composites. However,

the exploration of effective wet-chemical synthesis methodsfor producing ultrathin 2D TMDs and their composites with

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high-yield at relatively low cost is still challenging. Moreover, a

combination of conventional solution-based and hydrothermalsynthesis methods with conducting substrates such as gra-

phene and 3D carbon foam often produce randomly oriented

assemblies of 2D TMDs, leading to uncontrolled interfacialmorphology and degraded materials properties.

Secondly, the Li (Na, K)-ion storage properties of 2D TMDsare highly dependent on their size, shape, thickness, crystal

phase, interface, and doping effect. However, the relationshipbetween the structural characteristics and electrochemical

energy-storage properties of 2D TMDs nanomaterials is still not

clear. Therefore, a better understanding is crucial for the futuredesign and development of 2D TMDs for high-performance

electrochemical energy storage, especially for SIBs and PIBs.Thirdly, the 2D TMDs generally exhibit larger capacity via in-

tercalation and conversion reactions. However, the relativelyhigh electrochemical potentials of TMDs result in a small volt-age and energy density of the full cell utilizing a 2D TMD

anode. Heteroatom-doped TMDs and multinary 2D TMDsshowed some unique advantages compared to the 2D binary.Therefore, the development of multi-metal chalcogenides withlarge interspacing, large capacity, and low reaction potential isdesirable. Moreover, the conversion reaction of 2D TMDs withalkali metal ions will produce soluble intermediates such as Li

polysulfides (LiPSs), leading to low initial columbic efficienciesand poor reversibility. The design and synthesis of high-per-formance 3D core–shell TMDs with strong absorption and cata-lytic conversion of the soluble intermediates are desirable toimprove the cycle reversibility but remain challenging.

Fourthly, a high intercalation capacity with fast kinetics inthe host is desirable and a hybrid ion intercalation-based bat-

tery design by introducing multiple materials seems to be a

promising direction. However, the detailed reaction mecha-nisms for TMDs with alkali ions, especially for large Na+ and

K+ are still unclear and, in fact, there are conflicting ideas. The

mechanism for the intercalation and conversion processesshould be further studied to clarify the transformation behav-

ior. Moreover, the capacity fading mechanism of 2D TMDsduring cycling is still unclear, and the impact of the shuttle

effect of soluble intermediates such as LiPSs on the electro-chemical characteristics need further investigation. In this re-

spect, DFT simulations and advanced in situ characterization

techniques such as in situ Raman scattering, electron microsco-py, X-ray absorption near-edge structure and infrared spectros-

copy should be pursued.Fifthly, most of the current research of 2D TMDs for Li-, Na-,

and K-ion storage are based on a half-cell using Li, Na, or K foilas the counter electrode. Critical parameters such as the tap

density of the electrode materials, mass loading of active ma-terials, as well as electrode thickness have not been providedin detail, thereby making it difficult to compare the alkali-ion

storage properties with the actual energy and power densitiesof these devices. In practical applications, electrodes with a

large thickness and high mass loading are necessary, but it isstill a tough challenge to achieve high cycle life with large

areal and volumetric storage properties using thick electrodes(e.g. , 100–200 mm) by optimizing both ion and electron trans-port.

All in all, although there are still significant challenges facingpractical utilization of advanced energy-storage devices based

on 2D TMDs and composites, it is reasonable to expect that abetter fundamental understanding and more exploration will

Figure 30. A) Schematic presentation of the one-step hydrothermal synthesis of MoS2 nanosheets decorated with AuNPs for rechargeable Li–O2 batteries.(B) HRTEM image of MoS2/AuNPs nanohybrids. C) Discharge/charge profiles of the Li–O2 battery with pristine Super P, MoS2 nanoflowers, and MoS2/AuNPnanohybrids at a current density of 70 mAg@1 (based on the cathode material). D) Curtailing capacity at a current density of 300 mAg@1. Reproduced withpermission from Ref. [210] Copyright 2015 Royal Society of Chemistry.

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lead to exciting achievements paving the way for commercialadoption in high-performance rechargeable batteries and

hybrid supercapacitors.

Acknowledgements

This work was financially supported by National Natural Science

Foundation of China (No. 21875080, 51572100 and 51504171),Major Project of Technology Innovation of Hubei Province

(2018AAA011), Natural Science Fund for Creative Groups of HubeiProvince (2019CFA020), Special Projects for Local Science and

Technology Development Guided by the Chinese Central Govern-ment (2019ZYYD024), HUST Key Interdisciplinary Team Project

(2016JCTD101), City University of Hong Kong Strategic Research

Grant (SRG) No. 7005015, and Hong Kong Research Grants Coun-cil (RGC) General Research Funds (GRF) No. CityU 11205617. The

authors are grateful for the facility support provided by theNanodevices and Characterization Centre of WNLO-HUST and An-alytical and Testing Center of HUST.

Keywords: capacitors · batteries · chalcogenides · transition

metals · Two-dimensional

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Manuscript received: November 26, 2019Revised manuscript received: January 10, 2020Version of record online: March 9, 2020

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