REVIEW

This section of Journal of Materials Research is reserved for papers that are reviews of literature in a given area

Rechargeable MgndashLi hybrid batteries status and challenges

Yingwen Cheng and Hee Jung ChangEnergy Processes amp Materials Division Pacific Northwest National Laboratory Richland WA 99352 USA

Hui DongDepartment of Electrical and Computer Engineering and Materials Science and Engineering Program Universityof Houston Houston TX 77204 USA

Daiwon Choi Vincent L Sprenkle and Jun LiuEnergy Processes amp Materials Division Pacific Northwest National Laboratory Richland WA 99352 USA

Yan Yaob)

Department of Electrical and Computer Engineering and Materials Science and Engineering Program Universityof Houston Houston TX 77204 USA

Guosheng Lia)

Energy Processes amp Materials Division Pacific Northwest National Laboratory Richland WA 99352 USA

(Received 1 July 2016 accepted 25 August 2016)

A magnesiumndashlithium (MgndashLi) hybrid battery consists of an Mg metal anode a Li1 intercalationcathode and a dual-salt electrolyte with both Mg21 and Li1 ions The demonstration of thistechnology has appeared in literature for few years and great advances have been achieved in termsof electrolytes various Li cathodes and cell architectures Despite excellent battery performancesincluding long cycle life fast chargedischarge rate and high Coulombic efficiency the overallresearch of MgndashLi hybrid battery technology is still in its early stage and also raised some debateson its practical applications In this regard we focus on a comprehensive overview of MgndashLi hybridbattery technologies developed in recent years Detailed discussion of MgndashLi hybrid operatingmechanism based on experimental results from literature helps to identify the current status andtechnical challenges for further improving the performance of MgndashLi hybrid batteries Finally aperspective for MgndashLi hybrid battery technologies is presented to address strategic approaches forexisting technical barriers that need to be overcome in future research direction

I INTRODUCTION

Advanced electrochemical energy storage technologies(ie rechargeable batteries) are critical for the futuresociety because they are important for improving theefficiency of electric power grids stimulating growth ofenergy generation from renewable resources (wind andsolar etc) and providing an alternative to fossil fuelsfor the transportation1ndash4 Currently commercialized high-energy-density batteries are based on lithium-ion battery(LIB) technologies integrated with graphite as the anodeand metal oxides or phosphates as the cathode56 Thistechnology can provide high energy density and goodpower density and has been successful in poweringmany types of applications such as mobile devices andelectric vehicles Despite this past success LIBs havetechnical limitations to fulfill future needs in stationary

energy storage applications The reasons are growingconcerns about the safety of LIBs their cost inherentlimitations in the energy density that can be achievedand the scarcity of lithium (Li) resources These con-cerns have motivated intensive research investigationsduring the past decades on advanced battery deviceswith the goal of developing systems that are ableto provide even higher energy and power densitiessignificantly reduced manufacturing and maintenancecost and improved safety and reliability78 Significantprogress has been made and prototype devices withexcellent performance metrics have been documentedin the literature Some examples of technologies thathave been studied are high power supercapacitors9ndash11

redox-flow batteries1213 sodium (Na)-metal halide bat-teries1415 Na-ion batteries16 and high-energy-densitymetal based batteries1718

Metallic anodes with low standard redox potentialssuch as Li Na and magnesium (Mg) are promisingelectrode materials because of their substantially higherenergy densities compared with typical intercalationconversion-type electrode materials (Table I)

Contributing Editor Sung-Yoon ChungAddress all correspondence to these authorsa)e-mail guoshenglipnnlgovb)e-mail yyao4uheduDOI 101557jmr2016331

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Among these metal anodes Li can provide the highestenergy density of 3861 mA hg with the lowest redoxpotential (304 V versus standard hydrogen electrodeTable I) However its use in practical applications ischallenging because of safety and reliability concernsThe electrochemical deposition of Li tends to formunsafe dendrites and it has very limited Coulombicefficiency (CE) which results in poor cycling stabilityespecially when cycled with a high-capacity cathode1920

While exciting progress has been made toward improv-ing the CE of Li through the use of new electrolytematerials electrolyte additives andor new electrodestructures future research still is needed to achieve practicalapplications21ndash24

The use of Mg metal as the anode has uniquetechnological and economic advantages and has receivedincreasing interests recently3235 Mg has appreciableenergy density (gravimetric 2205 mA hg volumetric3832 mA hcm3) lower cost and much safer comparedwith Li metal In addition electrochemically depositedMg21 ions do not form dendritic structures and havenearly 100 CE when operated in recently developedadvanced electrolytes36 These characteristics suggestthat Mg metal holds great promise for use in durableand safe energy storage devices The first rechargeableMg battery prototype was documented in 2000 byAurbach et al using Chevrel-phase Mo6S8 as the cathodematerial37 The practical application of Mg batterieshowever currently faces several issues including twomajor challenges First the reversible deposition andstripping of Mg21 ions require specially synthesizedelectrolytes that do not form surface passivation layerswhich is believed to block Mg21 ion transport Theseelectrolytes are usually corrosive and have much narrowerelectrochemical window compared with LIB electrolytesRecent developments particularly on all-phenyl-complexelectrolytes38 all-inorganic electrolytes3942 and nonha-lide electrolytes4344 have established electrolytes withsignificantly improved voltage windows and activitiesThe second major challenge is associated with the lackof high-voltage cathode materials that can provide goodMg21 insertionextraction kinetics45 Conventional in-tercalation cathode materials developed for Li and Nabatteries were found to react poorly with Mg21 ionsThis is likely due to the much higher charge density

associated with divalent Mg21 ions which have stron-ger coulombic interactions with the host materials andresult in poor ionic transport and difficulties in structuralstabilization304647 Future research is needed to effec-tively resolve these two challenges for practical Mgbatteries

An alternative but very promising approach forbuilding practical batteries with Mg-metal anodes is thedesign of hybrid batteries that use charge carriers otherthan Mg21 ions for the cathode reaction One potentialarchitecture is the use of Li ions in hybrid MgndashLibatteries The fundamental structure of these batteriesis illustrated in Fig 1

This design has an Mg metal anode a Li1 ionintercalation cathode and a dual-salt electrolyte that hasboth Mg21 and Li1 ions solvated in solution The designintegrates the advantages of the Mg metal anode andthe Li1 ion intercalation cathode especially the goodreaction kinetics and excellent safety features in a singledevice so it can provide much better rate capability andcyclic reliability It should be noted that Li1 ion inter-calation cathodes are not the only option future develop-ments of this design could include cathodes with othercations such as Na1 ions dual cation co-intercalation oranion insertions Therefore this design could providesignificant opportunities for practical energy storage appli-cations ranging from stationary devices to transportationequipment In this review article our goal is to summarizewhat has been achieved thus far and provide ourperspectives on future developments particularly thecritical challenges that should be resolved before thistechnology can penetrate practical markets Our paper isorganized in three sections First we provide a briefsummary of the dual-salt electrolytes that have beenused in literature Then we discuss the performancemetrics of prototype devices reported in the literaturewith the discussion based on the voltages of the devicesreported Finally we provide our perspectives and ouranalysis of the critical challenges

II DUAL-SALT MgndashLi ELECTROLYTE THE PATHTO SUCCESSFUL HYBRID BATTERIES

The electrolyte is a key component that plays a pivotalrole in rechargeable battery performance It provides

TABLE I Comparison of key performance parameters for lithium (Li) sodium (Na) and magnesium (Mg)25ndash31

Li Na Mg

Gravimetric capacity (mA hg) 3861 (Li metal) 372 (graphite) 1165 (Na metal) 300 (hard carbon) 2205 (Mg metal)Volumetric capacity (mA hcm3) 2066 (Li metal) 837 (graphite) 1127 (Na metal) 450 (hard carbon) 3833 (Mg metal)Potential (V versus NHE) 304 (Li metal) 29 (graphite) 149 (Li4Ti5O12) 271 (Na metal) 261 (hard carbon) 237 (Mg metal)Global production (kgyear) 25 107 (very low) 1011 (very high) 63 109 (high)Ionic radius (Aring) 068 095 065Polarization strength (105pm2) 216 111 473

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electrochemically active species and determines electrodendashelectrolyte interfaces for reactions at both the cathode andanode Development of Mg battery electrolytes had beenchallenging because simple electrolytes prepared bydissolving a Mg salt in aprotic solvents could not toproduce reversible Mg depositions likely because ofthe formation of surface passivation layers48 Howeverthere have been successful demonstrations of severalfamilies of electrolytes that are capable of reversible Mgdeposition Gregory et al initially developed etherealsolutions containing Mg organoaluminates or organo-borates that obtained reversible Mg deposition49 Aurbachet al later produced electrolytes with an improved anodicstability of 25 V by reacting Grignard reagents(MgR2 R 5 ethyl andor butyl) with Lewis acids(AlEtCl2 or AlCl3)

37 These were the first prototypesof rechargeable Mg batteries Since then this type ofelectrolyte with the combination of Grignard reagentsand Lewis acids has been intensively studied and itsproperties have been further optimized50 For examplecombining phenyl magnesium chloride and AlCl3 intetrahydrofuran (THF) results in all-phenyl-complex (APC)electrolytes with electrochemical windows that exceed3 V38 Because of the inherent safety concerns related toGrignard reagents other Mg compounds such as MgCl2ROMgCl and hexamethyldissilazide magnesium chlo-ride (HDMS-MgCl) also have been used as the Mg saltsand electrolytes with excellent electrochemical propertiesfor Mg batteries have been demonstrated39405152

These prior works have provided important bases forformulating dual-salt electrolytes that can be used in the

design of hybrid MgndashLi batteries In fact a generalapproach for preparing dual-salt electrolytes based onthis class of electrolyte involves addition of a Li salt(such as LiCl or LiBF4)

5354 A typical and probably themost widely used thus far combination is LiCl dis-solved in APC electrolyte (see discussions below)54ndash57

Figure 2(a) shows typical cyclic voltammetry (CV)results for the APC electrolyte with and without additionof LiCl58 It can be seen that adding LiCl obviouslyreduced the Mg deposition over-potential and increasedthe current while maintaining similar anodic stability[the inset in Fig 2(a)] Therefore the addition of LiClnot only provides Li1 ions but also improves theelectro-activity for Mg deposition In the followingsections we will provide several examples of the use ofthis electrolyte in prototype hybrid batteries

Mg(BH4)2 and LiBH4 dissolved in ethereal solutionsis another family of electrolytes that has been exploredfor use as dual-salt electrolytes59 Figure 2(b) shows theCV of 01 M Mg(BH4)2 dissolved in diglyme with dif-ferent concentrations of LiBH4 The electrochemistry ofthe electrolyte depends strongly on the ratio of these twosalts With increasing LiBH4 concentrations the Mgdepositionstripping kinetics were obviously enhancedThe current density also increased and reached a maximumvalue with 15 M LiBH4 Comparison of the solvent effects(THF DME and diglyme) and LiBH4 concentration onthe CE of Mg strippingdeposition was also studied byShao et al The best CE values were observed withdigylme as the solvent it reached nearly 100 when theLiBH4 concentration was 06 M The excellent

FIG 1 Schematic illustration of the hybrid MgndashLi battery design The potential of combining the advantages of Mg metal anode and Li-ioncathode make this design extremely attractive for high rate and reliable batteries41

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electrochemical activity of this type electrolyte togetherwith remarkable stability of diglyme as the solvent(boiling point 162 degC) make them an excellent choicefor prototype hybrid batteries with low-voltage cathodematerials even though the electrochemical stabilitywindow is relatively narrow (20 V)

Recently Cheng et al identified a new dimagnesium-dichloro dimer [DMDC Mg2(l-Cl)2(DME)4] cationcomplex for electrolyte solutions formulated in dime-thoxyethane (DME)41 This type of electrolyte can besynthesized with a wide range of salt combinations [egMgCl2ndashAlEtCl2 MgCl2ndashAlCl3 MgCl2ndashMg(TFSI)2] andcan produce excellent Mg depositionstripping kineticswith a wide stability window (34 V versus Mg) Thiselectrolyte material brings exciting opportunities for thedesign of high-voltage Mg-metal-based batteries39ndash41

Similar to the APC-based electrolytes dual-salt electrolytesbased on DMDC cation complex also can be synthesizedby dissolving Li salts Cheng et al examined the solubilityof different Li salts in the electrolyte prepared by reacting04 M MgCl2 with 04 M AlCl3 in DME60 The solubilityof the Li salts was found to be very different and for thisparticular electrolyte the highest solubility was observedwith LiTFSI [TFSI 5 bis(trifluoromethane)sulfonamide]that can reach 20 M compared to LiAlCl4 LiCl andLiPF6 The typical electrochemical performance of this

type of electrolyte in the presence or absence of Li salt isshown in Fig 2(c) It is evident that all of these electrolyteshave voltage windows that exceed 34 V versus Mg andthe reversible deposition and stripping properties of Mgare not affected by the addition of Li salt In fact similarenhancements were observed when LiCl was added toAPC solution The high-voltage stability and good Mgelectrochemical properties of this electrolyte familymake them suitable for studying high-voltage hybridbatteries Later in this paper we present some of therecent exciting results

Finally we include a brief discussion of the electro-chemically stable cathode current collectors that aresuitable for Mg batteries The current collector is anintegral part and it must be stable across the voltagewindow of the battery Conventional current collectorssuch as aluminum copper stainless steel and nickelwere found to have poor compatibilities with currenthalide-based high-voltage Mg battery electrolytes62

Therefore the use of this family of electrolytes requiresalternative current collectors Cheng et al recentlyidentified that both molybdenum (Mo) and tungstenmetals have good electrochemical stabilities and theirbehaviors were comparable with inert materials includingplatinum and carbon61 Figure 2(d) compares the CVresults of Mo and stainless steel in 02 M DMDC

FIG 2 (andashc) Representative electrolytes that have been used to hybrid batteries (a) Comparison of CV of 025 M APC electrolyte with andwithout 05 M LiCl with platinum as the working electrode and the scan rate was 25 mVs58 (b) CV of 01 M Mg(BH4)2 dissolved in diglyme withdifferent concentrations of LiBH4 (20 mVs with platinum electrode) These results are adapted from the work of Shao et al59 (c) CV of Mgelectrolyte and MgndashLi dual electrolytes (with either LiAlCl4 or 10 M LiTFSI) in dimethyl ether (DME) These results are adapted from the work ofCheng et al60 (d) Current collectors stability in Mg electrolyte the CV of molybdenum (Mo) and stainless steel acquired in 02 M shows Mo hasremarkable anodic stability that make it suitable for using as the current collector61

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electrolyte It is evident that the anodic stability of Mo iscomparable with platinum and is much better thanstainless steel (around 2 V) The stability mechanismwas identified through formation of surface passivationlayers Later we will discuss examples in which Mo isintegrated for high-voltage devices

A Low-voltage (lt20 V) hybrid batteries showremarkable potentials

The challenges of the limited anodic stabilities of Mgbattery electrolytes developed thus far were discussedabove and the electrolytes are in general much less stablewhen compared with conventional electrolytes for Li-ionbatteries and Na-ion batteries As examples the APCelectrolyte has stability at 30 V versus Mg and theDMDC electrolyte has stability at 34 V versus MgThese stability characteristics limit the voltage win-dows of the hybrid MgndashLi batteries and introduce sig-nificant challenges in developing high-voltage devicesTherefore it is obvious that low-voltage cathode materials(20 V) must be the starting point for examining thepotential of the hybrid cell design The use of low-voltagematerials eliminates side-electrode reactions associatedwith electrolyte decomposition and facilitates selectionsfor both current collectors (ie stainless steel) and cell

cases (ie coin cells) which can further yield substantialbenefits in designing battery architecture

Chevrel phases which are good choices for cathodematerials have been studied by several researchgroups5455 Earlier electrochemical studies by Aurbachet al clearly demonstrated the preferred intercalationof Li1 over Mg21 ions and documented much betterintercalation kinetics with Li1 ions63 Cheng et alrecently included Mo6S8 as a cathode material in thebattery design with a dual-salt electrolyte (04 M APC)and (10 M LiCl) Figure 3(a) shows two well-definedLi1 intercalation discharge plateaus at 166 and 129 Vand a specific capacity of 126 mA hg at 01C which isabout the same as the theoretical capacity of Mo6S8(1288 mA hg)54 The electrode reactions under theparticular conditions (with high Li1 intercalationkinetics) are

(i) Anode 2Mg harr 2Mg21 1 4e(ii) Cathode Mo6S8 1 4Li1 1 4e harr Li4Mo6S8(iii) Overall reaction 2Mg 1 Mo6S8 1 4Li1 harr

Li4Mo6S8 1 2Mg21A stable cycling performance for over 3000 cycles was

observed as shown in Fig 3(b) Post-cycling analysisshows that the main reasons for the capacity decay after3000 cycles were electrolyte evaporation and corrosion ofthe current collector These results indicate a potential for

FIG 3 MgndashLi hybrid batteries with Mo6S8 cathode with electrolytes of APC and LiCl dissolved in THF (a) Typical chargendashdischarge profiles atdifferent C-rates with 04 M APC and 10 M LiCl that shows excellent rate performance of hybrid cells (b) Cyclic stability profile tested at 10C for3000 cycles (c) Typical scanning electron microscope image of the Mg anode showing that no obvious dendritic structures were formedThese results are adapted from the work of Cheng et al54 (d) Lithiation and magnesiation potential profiles at different Li1 activities (aLi1)determined by combining discrete Fourier transform (DFT) energies with Nernst equation in Mo6S8 Possible Li

1 and Mg21 mixed insertion pathsinto the Mo6S8 are shown as dashed lines These results are from the work of Cho et al55 (e) Discharge profiles at varying LiCl concentrations thatshows the Li-ion concentration plays important role in discharge profiles and capacities

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even longer cycling More importantly the battery showsoutstanding rate capability and a specific capacity of105 mA hg was achieved at 10C with only 5 capacityfade Also the scanning electron microscope image[Fig 3(c)] confirmed that the surface of the Mg anoderemained dendrite-free after long-term cycling

The results of DFT analysis [Fig 3(d)] show energet-ically preferable occupation sites for the first ion (eitherMg21 or Li1) in the primitive Mo6S8 crystal structure atmultiple stage of discharge55 Based on the first ionoccupied into the structure various paths of Mg21 andLi1 can be predicted Moreover combining the Nernstequation with the DFT energies determined a thresholdLi1 activity (aLi1) value in the electrolyte for lithiationand magnesiation in the Mo6S8 cathode Cho et al reportedthat a dominant reaction in the cathode was governed byMg21 insertion rather than Li1 insertion at very low initialLi1 activities46 In contrast increasing in Li1 activity leadsthermodynamically more favorable lithiation in Mo6S8rather than magnesiation Figure 3(e) clearly showsdependence of Li1 concentrations on cell performancesin the hybrid system By controlling the LiCl concen-tration in the APC electrolyte they were able to achieve936 and 875 of theoretical capacity in the theory-aided design of the hybrid cell systems at the C-rate ashigh as 20 and 30C respectively

In addition to Mo6S8 several other cathode materialshave been studied Recently Hus et al demonstratedexperimentally that molybdenum sulfide (MoS2) couldbe a suitable cathode material for Mg rechargeablebatteries64 Figure 4(a) shows the MoS2-based cell hashigh power capability at various current densities (up to1000 mAg) Beside a primitive MoS2 two more MoS2materials have been examined such as a MoS2 in-corporated with carbon natotubes (MoS2CNTs) andMoS2 incorporated with graphene and nano-sheets(MoS2GNSs) It is found that the MoS2GNS batterycan deliver the highest capacity of 225 mA hg among tothe other MoS2 cathodes and it shows stable cyclingperformance in a 05 M Li1 containing APC electrolyteover 200 cycles as shown in Fig 4(b)

TiS2 was introduced as a cathode material for recharge-able magnesium batteries by several groups5665 Gao et alreported a hybrid MgLi battery using a Mg anode a TiS2cathode and an APC-LiCl electrolyte which is stablewithin the operating voltage window of TiS2 (10ndash16 Vversus MgMg21)65 Figure 4(c) shows the results ofgalvanostatic tests of TiS2 cathodes in three different cellsTiS2jLi1jLi TiS2jMg21jMg and TiS2jLi1 Mg21jMgThe chargendashdischarge profiles demonstrated that a revers-ible Li1 intercalation into TiS2 in the Li1 Mg21 dual-saltelectrolyte takes place in the same manner of that in a Li1

FIG 4 Other typical metal sulfides cathodes examined for hybrid batteries (andashb) Chargendashdischarge profiles of MoS2 electrodes at different rates in05 M Li1 containing APC electrolyte The data are adapted from Hsu et al64 Battery performance of a titanium disulfide (TiS2) cathode (c) Theelectrolyte was 04 M APC-LiCl The data are adapted from Gao et al65 (d) Voltage profile of TiS2 cathode and Mg anode at various current densitiesin a hybrid system (e) Cycling performance of TiS2 cycled at 1C for 2000 cycles The data are adapted from Yoo et al56

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electrolyte Excellent cycling stability was observedover 400 cycles with no capacity fading (995 CE)In addition these batteries can deliver a specific capacityof 161 mA hg which is 32 higher than general Mg-ionbatteries

Yoo et al also reported a high capacity for hybridbatteries using a TiS2 cathode56 They have testedthe TiS2 cathode against different anode materialssuch as a metallic Li and Mg anode to study cyclingstability and the electrodeposition behavior of theanodes during cycling The rate performance of TiS2[Fig 4(d)] shows a high specific capacity of 220 mA hgat 01C This is by far the highest value reported amongconventional MgLi hybrid batteries Figure 4(e) showsthe galvanostatic performance of TiS2 TiS2 in dual-saltelectrolytes cycled at 1C had a stale cyclability of over

2000 with 996 of the CE Also the TiS2-based cell ismore stable with the Mg anode while the cell with the Lianode eventually resulted in a cell failure due to thedendrite formation

Titanium dioxide (TiO2) is found to be a possiblecandidate for rechargeable Mg batteries because of itshigh capacity and suitable working voltage (09 Vversus Mg) that matches the electrochemical windowof the dual-salt electrolytes system66 Figure 5(a) showsa chargendashdischarge voltage profile of a commercialTiO2jMg cycled at 02C in a dual-salt electrolyte It isshown that the TiO2 can deliver a high capacity of140 mA hg Su et al reported that a MgLi hybridbattery with 1D mesoporous TiO2(B) nanoflakes as acathode and Mg anode in 05 M Mg(BH4) and 15 MLiBH4 dissolved in tetraglyme (TG) also can be

FIG 5 Typical oxide cathodes examined for hybrid MgndashLi batteries Electrochemical and battery performance data of several other types of cathodematerials for hybrid MgndashLi batteries where significantly increased capacity rate capability and cyclic stability have been observed with the hybriddesign Chargedischarge profile of (a) commercial TiO2 and (b c) one-dimensional mesoporous TiO2 nanoflakes in 05 M Mg(BH4)2 and 15 MLiBH4 in tetraglyme The data are adapted from Su et al (d) Chargedischarge profile and (e) cyclic stability profile of Li4Ti5O12 cathode in theelectrolyte of 04 M APC1 15 M LiBH4 The data are adapted from Miao et al68 (f) Chargedischarge profile of MoO2 in dual-salt electrolyte and theimprovement of its activity through using new structures of hollow microspheres69 (gndashi) are adapted from Wu et al70

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a possible combination that could deliver high capac-ity with excellent electrochemical performance67

Figures 5(b) and 5(c) show that a TiO2(B) cathode candeliver a charge capacity of 130 mA hg at 1C and hasexceptional long-term (up to 6000 cycles) stability even athigh rates (up to 2C) They proposed that the high capacityis based on pseudocapacitive reactions dominated by asynergic interaction between Mg21 and Li1 ions

Li4Ti5O12 (lithium titanium oxide LTO) has been studiedas an anode material for Li-ion batteries because its latticedimensions do not change upon lithiationdelithiationAlthough LTO has known to be a zero-strain for Mg-ioninsertion Mg21 electrochemical insertion into LTO is akinetically sluggish reaction and has strong size depen-dence Miao et al reported an effect of dual-salt electrolyteson an MgLi hybrid battery consisting of an LTO cathodeand Mg anode68 Two different LTO cathodes (made fromcommercially available LTO and simple ball-milled LTOwith graphene 5 LTOG) against Li or Mg anodes in twodifferent electrolytes [Mg(BH4)2-based mixed electrolyteand APC-based mixed electrolyte] were tested LTO inthe APC-based electrolyte showed slightly better cy-cling performance than the LTO in the Mg(BH4)2-basedelectrolyte because of higher electronic conductivity andthe low over-potential of the APC-based electrolyte forMg dissolutiondeposition than Mg(BH4)2-based mixedelectrolyte as shown in Figs 5(d) and 5(e) In additionthe LTOG batteries show higher capacities with bettercycling stability because of higher specific surface areaof graphene which facilitates fast electron transport inthe LTO structure

Figures 5(g) and 5(h) show voltage profiles and rateperformances of LTO at various Li1 concentrations indual-salt electrolyte70 The data shows the electrochemicalperformance of the LTO electrode is highly dependent onthe Li1 concentration and can be improved significantlyby increasing the Li1 ion concentration in the hybridsystem Moreover a theoretical DFT calculation suggeststhat co-insertion of Mg21 and Li1 ions into the LTO ispossible at the optimized Li1 ion concentration which isup to 05 M in the experimental condition High-angleannular dark field and annular bright field scanning trans-mission electron microscopy images confirmed the co-existence Mg21 and Li1 phases [Fig 5(i)] in LTO70

Because of its stable chemical and thermal propertiesmolybdenum dioxide (MoO2) has been introduced asan attractive cathode material for Li-ion batteries71

Pan et al synthesized hollow microspheres consistingof MoO2 nanoparticles (denoted as MoO2-HMS) andthe combination of Mg anode with MoO2 cathode wasevaluated in a dual-salt electrolyte (04 M APC and1 M LiCl dissolved in THF)69 They found that co-insertion of Mg21 and Li1 into MoO2-HMS occurs in theintercalation reaction Figure 5(f) shows MoO2-HMSsignificantly improved electrochemical performance with

100 CE in comparison with the commercial MoO2Also the morphological feature of MoO2-HMS positivelyenhances performance

B Conversion-type cathodes

The intercalation compounds have relatively lowspecific capacity as cathode materials for examplethe Chevrel-phase Mo6S8 cathode discussed above has atheoretical capacity of only 122 mA hg FeS2 and FeStwo typical resource-abundant materials with theoreticalcapacities of 894 and 609 mA hg respectively havebeen investigated as a conversion-type electrode in Mgbased batteries72 Although FeS2 has been tested as a Libattery cathode in the past its high capacity is seriouslycounteracted by the dissolution of polysulfide (PS)intermediates as well as Li-dendrite growth resultingin a fast capacity-fade during cycling Compared toFeS2 FeS conversion is less complex with expectedbetter in common non-aqueous electrolytes owing to theabsence of anionic redox process By displacing metallicLi by Mg anode and using dual-salt electrolytes withoptimized Mg21 and Li1 concentration high reversibleLi-driven conversion and Mg platingstripping withoutany cathode decoration and unsafe Li-dendrite forma-tion are expected

With optimized Li salt concentrations in dual-saltelectrolytes Zhang et al demonstrated better revers-ibility from MgFeS2 and MgFeS chemistries thanLiFeSx

72 The MgFeSx materials delivered maximumreversible capacities of 600 and 520 mA hg at 005C[Figs 6(a) and 6(b)] respectively with in situ formationof solid electrolyte interphases on both the sulfide andMg surfaces which effectively mitigate PS dissolutionshuttle phenomenon and anode passivation Betweentwo common dual-salt electrolytes APC coupled withLiCl and Mg(BH4)2 with LiBH4 borohydride-basedelectrolyte showed better capacity retention of MgFeSxbatteries than chloride-based electrolyte Cycling perform-ances of MgFeSx batteries using borohydride-based elec-trolytes with 15 M LiBH4 with a cutoff voltage 17 Vthat favor suppression of soluble PSs are shown inFig 6 As shown in Figs 6(c) and 6(d) the reversiblecapacities at 01C lie between 350 and 400 mA hg after50 cycles and are preserved at 200 mA hg after150ndash200 cycles respectively for both the sulfides

Alternatively sulfur as a high-capacity (1675 mA hg)cathode material has attracted great interest in LiS andNaS systems Realization of an MgS battery is also ofgreat interest due to its high theoretical capacity of957 mA hg from a full cell with a voltage of 177 VUnfortunately the magnesium organohaloaluminateelectrolyte that allows reversible Mg deposition issynthesized by an in situ reaction between Lewis acid(AlCl3) and nucleophilic Lewis base (RMgCl) which

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reacts with the sulfur Therefore to circumvent suchincompatibility Kim et al proposed a new electrolytesystem using non-nucleophilic hexamethyldisilazidemagnesium chloride (HMDSMgCl)52 That systemwas the first proof-of-concept MgS battery but it lost70 of its storage capacity at the second dischargeMore recently Zhao-Karger et al developed a novelnon-nucleophilic electrolyte based on magnesium-bis(hexamethyldisilazide) [(HMDS)2Mg] however de-spite the two discharge plateaus at 15 and 07 V duringinitial sulfur reduction no plateau was observed in sub-sequent cycles74 Consequently no rechargeable MgSbattery has been demonstrated yet mainly due to theelectrochemical inactivity of the formed lower-orderPSs (Mg-PS) toward oxidation To enhance the revers-ibility of MgS reaction Gao et al used non-nucleophilicMg electrolytes with a LiTFSI additive that enabledconjugation of a reversible PS redox reaction on thecathode with Mg depositionstripping on the anode73

The electrolyte cycling stability with and withoutLiTFSI additive is shown in Fig 6(e) where the sulfurcathode shows a rapid capacity-drop in the Mg-only

electrolyte while the presence of Li1 dramaticallyimproves the reversibility with a stable capacity of1000 mA hg over 30 cycles with specific capacitycomparable to the LiS system

The effect of Li1 on the anode-side surface chemistryof Mg anodes after cycling in electrolytes with andwithout LiTFSI were analyzed using x-ray photoelectronspectroscopy analysis73 In an Mg-only electrolyteMgS formed from exposure to dissolved sulfur speciesWhen LiTFSI is added the x-ray photoelectron spec-troscopy spectrum indicates the absence of MgS in theelectrolyte From an Mg-metal corrosion experiment toexplore the effect of Li1 on the solubility of short-chain Mg-PS species the surface layer of MgS wasdissolved by the action of Li1 and the Mg surfacecould not be passivated anymore To confirm whetherMgS is indeed dissolved inductively coupled plasmaoptical emission spectroscopy (ICP-OES) analysiswas performed in tetraethylene glycol dimethyl ether(TEGDME) solution after the corrosion experimentThe concentration of Mg in the TEGDME was negli-gible when no Li1 was present indicating negligible

FIG 6 Galvanostatic chargendashdischarge curves of (a) FeS2 and (b) FeS as conversion cathodes by using a borohydride-based electrolyte with 15 MLiBH4 during the first six cycles at 005C Discharge capacities of (c) MgFeS2 and (d) MgFeS batteries as a function of cycling number at 01C byusing a borohydride-based electrolyte with 15 M LiBH4 The cycling stability of discharge capacities of LiMg(BH4)2ndashLiBH4FeSx and LiLiPF6FeSxbatteries is also plotted as a comparison Data adapted from Zhang et al72 (e) Cycling stability of the MgS battery in electrolyte with and withoutLiTFSI and (f) working mechanism of the MgS battery with LiTFSI additive Data adapted from Gao et al73

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

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presence of MgPS in the TEGDME However theconcentration of Mg increased by three orders ofmagnitude in the presence of Li1 after 12 h of LiTFSIaddition As shown in Fig 6(f) two possible mecha-nisms for Li1 enhancement of reversibility are suggested(i) Li1 participates in the cathode reaction to form readilyrechargeable LiPS or incorporates into MgPS to formhybrid MgLiPS during discharge or (ii) the hard Lewisacid Li1 strongly coordinates to the surface S2of lower-order Mg-PS hence enhancing its solubility decreasingits oxidation energy barrier and making it electrochem-ically active Although further studies are needed thenew scientific insights obtained so far will pave the pathfor the realization of practical conversion-type recharge-able MgS battery

C High-voltage (gt20 V) hybrid devices requiredfor practical applications but are facing greatchallenges

The energy density of a battery is proportional to itsvoltage and therefore one of the most importantapproaches producing high-energy-density devices isthrough the use of high-voltage cathode materialsAs discussed above designing high-voltage hybridMgndashLi batteries faces significant challenges becauseof poor control over the cathode-electrolyte interfaceand the limited stability of the electrolyte These char-acteristics lead to low CE due to decomposition ofthe electrolyte (either solvents or salts or both) poorselectivity of the cathode reactions and sluggish ionde-solvation and transport across the interface These

challenges are shown in Fig 7(a) with the CV profile ofLiFePO4 (LFP) obtained in a THF electrolyte containingAPC and LiBF4

53

Intercalation of Li1 ions was clearly observed and wasthe dominant reaction (after comparing with the nearlyno activity of pure APC electrolyte) Substantial anodiccurrents corresponding to electrolyte decomposition atvoltages beyond 25 V also were observed As a resultprototype batteries based on this system had low efficiencyand limited reversible capacity This is clear evidencethat developing new electrolytes with better stability arenecessary

On the basis of the DMDC electrolyte establishedrecently (see discussions above) Cheng et al examinedthe use of this electrolyte in the design of high-voltagehybrid batteries60 Figures 7(b) and 7(c) shows the CVprofiles of LFP and LiMn2O4 (LMO) in this electrolyteThe LFP exhibited a set of well-defined redox peaks thatare characteristic of Li1 ion intercalation Furthermorethis result also suggests that the electrolyte was stableover the voltage window of LFP and no obviouselectrolyte decomposition was observed Therefore thiselectrolyte has good stability and has advantages overthe APC electrolyte for LFP The behavior of LMO onthe other hand showed two sets of redox peaks thatcorrespond to Li1 ion intercalation However the redoxpotentials of LMO are close to the electrolyte decom-position (as suggested by the sharp increases in anodiccurrent beyond 34 V) hence the efficiency of pro-totype batteries was low Cheng et al demonstrated anassembly of prototype hybrid batteries with the LFP

FIG 7 Realization of high-voltage batteries requires both advanced electrolyte and cell architecture design (a) CVs of LFP in conventional APC-based electrolytes show relatively poor efficiency (results adapted from Yagi et al53) whereas the same material in (b) (advanced electrolyte) showsexcellent efficiency but with even higher voltage cathodes (c) The efficiency for LMO is poor (d) Rate capability of LFP The results are adaptedfrom Cheng et al60

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

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cathode using a Swagelok cell and Mo rods for thecathode current collector60 It is worth noting here thatsuch a design ensures good sealing and electrochemicalstability The LFP cathode studied in this work wasfabricated as a free-standing film using the rollingmethod and polytetrafluoroethylene as the binder Theactive material loading was 10 mgcm2 and sucha high loading is compatible with practical applicationsFigure 7(d) shows a set of chargendashdischarge profiles ofprototype batteries at rates ranging from 01 to 10C(1C 5 170 mAg) The cell exhibited voltage profilessimilar to those of cycled Li cells The electrode wasable to deliver an outstanding capacity of 140 mA hg ata rate of 01C The reversible Li intercalation wasconfirmed with x-ray diffraction analysis In additionthe cell had good rate performance and the capacity at

1C was 120 mA hg The cyclic stability also wasgood with capacity retained90 mA hg after 100 cycles

High-voltage devices can also be designed using a solidelectrolyte separator with different electrolyte chemistriesfor the cathode and anode reactions Figure 8(a) showsa design using Grignard-based electrolyte as the Mganode electrolyte (1 M PhMgBr and 01 M LiBr in THF)and 05 M Li2SO4 aqueous solutions as the cathodeelectrolyte75 Figure 8(b) shows the chargendashdischargeprofile of this type of hybrid battery which demon-strated good reversibility and delivered a capacity of1217 mA hg with an output voltage of 21 V The cyclicstability profile for 20 cycles is shown in Fig 8(c) and thebattery had 10 capacity after 20 cycles The effi-ciency was less than 100 which was due to the lowefficiency for Mg plating-stripping in the Grignard reagent

FIG 8 Approaches for high-voltage hybrid batteries (andashc) New architectures using a solid-state separator and an aqueous electrolyte for cathodereaction (b) Chargedischarge profile (c) Cyclic stability Data are adapted from Cheng et al60 (d) Chargedischarge profiles of LFP cells(as punch cells) with flexible pyrolytic graphite fiber current collector and APC-LiCl as the electrolyte Data from Cheng et al60 (endashf) The use ofPrussian blue analogues (PBA) as cathodes in APC-LiCl electrolyte (e) Charge-discharge profiles of vacuum-dried PBA with differentconcentrations of LiCl (f) Comparison of cyclic stability of PBA prepared as either hydrated or vacuum-dried form Data from Chang et al75

(gndashi) Data from Itchitsubo et al77

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

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In another study Zhang et al assembled a stable high-voltage battery using flexible pyrolytic graphitic film as thecurrent collector and a pouch cell configuration (instead ofa coin cell)76 The electrolyte used in this work was 05 MAPC and 04 M LiCl Figure 8(d) shows typical chargendashdischarge profiles at different C-rates The cell delivered aninitial capacity of 1564 mA hg at 015C and a dischargeplateau of 245 V The discharge capacities at different C-rates were 1441 1232 966 and 688 mA hg at 03 0612 and 30C respectively In addition this work demon-strated the capability of the cell to operate at a lowtemperature of 40 degC The cyclic stability test showsthat this hybrid cell had capacity retention of 98 after200 cycles at 01C

Sun et al examined the use of PBA compounds as thecathodes for hybrid batteries78 They studied the behavior ofhydrated and vacuum-dried PBA (Fe[Fe(CN)6]09523H2Oand Fe[Fe(CN)6]09507H2O respectively) with anAPC-LiCl electrolyte Figure 8(e) shows the chargendashdischarge profile of the vacuum-dried PBA at variedconcentrations of LiCl The specific capacity was foundto depend strongly on the LiCl concentration and themaximum capacity was reached to 125 mA hg with05 M LiCl Similar results were obtained with hydratedPBA Figure 8(f) compares long-term cycling stabilityof both hydrated and vacuum-dried PBA at 200 mAgThe vacuum-dried sample showed a capacity-drop duringthe first 10 cycles and then stabilized at 65 mA hg forup to 300 cycles with 99 CE In contrast the hydratedsample had a faster capacity-decay and only retained55 mA hg after 300 cycles The results demonstrated inthis work are encouraging and could bring excitingopportunities for applying the PBA cathode for hybridcell design

Ichitsubo et al examined the design of ldquorocking-chairtyperdquo hybrid batteries77 They studied the properties ofspinel oxide MgCo2O4 for co-intercalation of Mg21 andLi1 ions77 Figure 8(g) shows the CV with a chronoam-perometry profile for Li insertionextraction processesinto MgCo2O4 They reported that Mg and Li co-insertions can occur in the host MgCo2O4 and thesecations then can be reversibly extracted during a chargeprocess The observed equilibrium redox potentials forthe insertionextraction of Mg and Li cations areestimated to be about 29 V versus Mg21Mg (34 Vversus Li1Li in the reverse extraction) and 31 V versusLi1Li (32 V versus Li1Li in the reverse extraction)respectively Figure 8(h) shows the cell voltage versuscapacity curve obtained for a MgndashLi dual-salt batteryin a three-electrode cell which has Mg49Li51 alloy inatomic ratio as the anode material a ternary ionic liquidof (Li10Mg10Cs80)-TFSI (atomic ratio of cations) forthe electrolyte and a Li reference electrode In this casesurprisingly the anodic dissolution of the MgndashLi alloycan occur at reasonably low potentials between 05 and

06 V versus Li1Li in the reverse extraction (note thatthe anodic dissolution potential is much lower than thepotential (15 V versus Li1Li in the reverse extraction)of the passivated Mg electrode By taking advantage ofan MgndashLi alloy anode and co-intercalation of Mg andLi they proposed a rocking-chair-type MgLi dual-salt battery that does not require accretive electrolytesFigure 8(i) shows predicted dischargendashcharge processesof the rocking-chair-type MgndashLi dual-salt battery

III SUMMARY AND PERSPECTIVES

MgLi hybrid batteries have unique advantages ofcombining the Mg-metal anode and well-studied Li1 ionintercalating cathodes (Table II) As reported in theliterature MgLi hybrid batteries typically present fasterbattery cycling performance (higher C-rate) when com-pared with pure Mg batteries Cheng et al reported thatthe capacity is close to the theoretical value at low C-rates(126 mA hg at 01C) and the high-capacity retentionratios at increased C-rates (102 mA hg at 15C) forMgLiMo6S8 hybrid cells54 In contrast much lower(80 mA hg at 01C) capacity was observed forMgMo6S8 cell Yoo et al also reported the specificcapacity of a TiS2 electrode could be increased to220 mA hg in MgLiTiS2 hybrid cells56 Howeverthe capacity of the TiS2 electrode measured in pure Mgelectrolyte (without Li1 ions in the electrolytes) is lessthan 20 mA hg Nevertheless MgLi hybrid cellsovercame the sluggish kinetics of Mg21 ion diffusion incathode materials which are mainly the result of stronginteractions between Mg21 ion and the cathode hostlattice

High CE and stable cycling performance were typicallyobserved for MgLi hybrid batteries In recent work thecycling stability of MgLiMo6S8 hybrid batteries wasstudied with a 3000-cycle chargendashdischarge test conductedat a high rate of 10C The MgLiMo6S8 hybrid cell wasvery stable with close to 100 CE for each cycle and only5 capacity fading after 3000 cycles54 Yoo et al carriedout more detailed mechanism studies by comparing anMgLiTiS2 hybrid battery to a LiTiS2 battery56 Theyobserved that the Li anode retrieved from a cycled LiTiS2battery was covered with a 100 lm thick mossy layercomposed of a mixture of Li particles and solid electrolyteinterphase In contrast to a LiTiS2 cell the Mg anodeobtained from a cycled MgLiTiS2 hybrid batteryrevealed a single-layer of polyhedral Mg deposits due tothe hexagonal close packed structure of Mg metal Indeedthe MgLiTiS2 hybrid battery showed very stable capac-ity retention and high CE over 300 cycles Taking all ofthese observations into consideration Yoo and co-workersconcluded that the superior cell performance of theMgLiTiS2 hybrid battery versus the LiTiS2 battery isdue to the absence of dendritic growth in the Mg anode at

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

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practical current density (2 mAcm2) and mass loading(1 mA hcm2)

Because of the unique chargedischarge processes ofMgLi hybrid batteries where Mg21 and Li1 ions areused asymmetrically on the Mg anode and Li cathodedual-salt electrolytes must be able to supplyreceivesufficient Mg21 and Li1 ions throughout the cyclingprocesses Depending on the Li cathodes used MgndashLihybrid batteries can be briefly divided into two categories(i) those using lithiated cathode materials such as LFPLMO etc and (ii) those using delithiated cathodematerials such as Mo6S8 TiS2 TiO2 etc In this reviewwe focus our discussion on lithiated cathode materialsWhen using lithiated cathode materials the MgndashLi hybridbattery starts with the charging process because the batteryis assembled in the discharged state During the chargingprocess the dual-salt electrolytes have to supply enoughMg21 ions for depositing on the anode and then acceptLi1 ions de-intercalated from lithiated cathode materialsFor the discharging process dual-salt electrolytesaccepted Mg21 ions stripped from the Mg anode andsupply Li1 ions for intercalating into the de-lithiatedcathode Identifying chargedischarge processes andinvolved charge carriers are critical for calculating thespecific energy density of MgLi hybrid batteriesFor example the active Mg21 species presented in recentwork by Cheng et al on MgLiLFP hybrid batteriesis identical to DMDC and the charging process ofMgLiLFP hybrid batteries can be described as follows60

Anode 1=2 Mg2Cl2frac12 AlCl4frac12 2 thorn 2e0Mgthorn Cl

thorn AlCl4 eth1THORN

Cathode 2Li2FePO402FePO4 thorn 2Lithorn thorn 2e eth2THORNFull reaction for the charging process

2Li2FePO4 thorn 1=2 Mg2Cl2frac12 AlCl4frac12 202FePO4 thorn LiCl

thorn LiAlCl4 thornMg

eth3THORNBased on the full reaction Eqs (1)ndash(3) the specific

energy density of MgLiLFP hybrid batteries alongwith other Li cathode materials are shown in Fig 9

As shown in Fig 9 a MgLiLFP hybrid battery candeliver a theoretical energy density up to 246 Whkgwhich is considerably higher than the energy density(134 Whkg) of the conventional pure Mg battery usingMo6S8 and the energy density (143 Whkg) ofthe LTOLFP system The higher energy density of theMgLiLFP battery leads to a higher output voltage(25 V) which is significantly higher than 12 Voutput voltage of the MgMo6S8 battery and 19 Vof the LTOLFP battery60

Assuming all Mg21 ions are supplied from dual-saltelectrolytes the amount that Mg21 ions in the dual-saltelectrolyte should match the capacity of the Li cathodeThe minimum amount of required dual-salt electrolytecan be determined as

Vh frac14 3600000 CLi

zFCMg eth4THORN

where CLi is the capacity density of Li cathode (mA hcm2)z is the number of charge for Mg21 ion (2 for Mg21) F isthe Faraday constant (96485 Cmol) CMg is the con-centration of Mg21 in the dual-salt electrolyte and Vh is

TABLE II Summary of performance metrics of typical prototype hybrid MgLi-ion batteries developed by far

Cathode material ElectrolyteVoltagecapacity

(V versus MgmAg)Columbicefficiency

Rate performance(mA hg)

Cycle(cycle number) Ref

Mo6S8

APCLiCl 13126 ffi100 1932 3000 54APCLiCl 13120 3660 100 55

Mg(BH4)2LiBH4 13995 ffi100 300 59MoO2 APCLiCl ndash2172 88 50 69MoS2 APCLiCl 165225 99 1000 200 64TiO2 Mg(BH4)2LiBH4 091558 336 90 67TiS2 APCLiCl 14160 ffi100 480 400 65TiS2 APCLiCl 14220 ffi100 4800 2000 56LTO APCLiCl 07190 ffi100 300 100 70LTO APCLiBH4 07160 180 100 68LFP APCLiBF4 24124 53LFP APCLiCl 245156 985 510 200 76LFP APCaqueous Li2SO4 211217 90 20 75LFP DMDCLiTFSI 25140 ffi100 170 100 60LMO DMDCLiTFSI 31ndash Low 60S Mg-HMDSLiTFSI 151000 30 73FeSx (x 5 1 or 2) Mg(BH4)2LiBH4 520 (FeS)600 (FeS2) Low Poor 200 72

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

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the volume of the dual-salt electrolyte (lLcm2) Asshown in Fig 10 the minimum volume of dual-saltelectrolyte is determined by the Mg21 concentration ofdual-salt electrolyte and Li cathode capacity Apparentlyhigher concentrations of Mg21 require less volumeof dual-salt electrolyte in the MgLi hybrid batteryFor instance a MgLiLFP hybrid battery with a cathodeloading of 1 mA hcm2 will require 94 47 and 19 lLcm2

of dual-salt electrolyte for 02 04 and 10 M Mg21

concentrations respectively Consequently the energydensity of MgLiLFP hybrid batteries will decrease from246 Whkg (without considering the mass of solvents) to30 Whkg (02 M of Mg21) 50 Whkg (04 M ofMg21) and 100 Whkg (10 M of Mg21) respectivelyif the mass of solvents is considered

Several research directions could be pursued to furtherimprove the performance of MgLi hybrid batteriesAccording to the above discussions the energy density isclosely related to the output voltage and the amount ofdual-salt electrolytes incorporated in hybrid batteriesRecently developed dual-salt electrolytes with widerelectrochemical windows significantly expanded theselection of cathode materials toward higher redox poten-tials The current state-of-the-art dual-salt electrolytemdashDMDCLiTFSIDMEmdashhas an electrochemical windowup to 34 V (versus Mg) that is sufficient for an LFPcathode60 To take further advantages of well-studiedhigher voltage Li cathodes (such as LMO NMC andNCA etc) a new class of novel dual-salt electrolytewhich presents even higher electrochemical windowneeds to be developed For practical applications long-chain glyme-based electrolytes are preferred to replacethe volatile THF solvent due to its higher boiling pointand lower vapor pressure Reducing the amount ofelectrolytes will also help to increase the energy density

of MgLi hybrid batteries As shown in Fig 10increasing the concentration of Mg21 ion in dual-saltelectrolytes will effectively reduce the amount of elec-trolyte needed thus the energy density will be higherRecently reported ldquosolvent-in-saltrdquo type electrolyteswith ultrahigh salt concentrations can be an interestingapproach for making high-concentration dual-salt elec-trolytes79 Further reducing the amount of electrolytescan be achieved by adopting ldquoprecipitation-dissolutionrdquomechanism for charge and discharge processes Duringthe charge process Li salts will be precipitated from theelectrolytes due to the delithiation of the Li cathodeduring the discharge process magnesium salts will beprecipitated The demonstration of a precipitation-dissolution type MgndashLi hybrid battery has not beenreported yet Technically it would be more viable todemonstrate a precipitation-dissolution type MgndashLi hybridbattery in a pouch cell architecture rather than in a coincell in which excessive amounts of electrolytes aretypically added Apparently understanding precipitation-dissolution processes of Mg and Li salts and how it affectcharge and discharge processes could be a critical step fordeveloping practical MgLi hybrid batteries with a mini-mum amount of dual-salt electrolyte

IV CONCLUSIONS

Demonstrations of MgLi hybrid battery technol-ogies have appeared in literature for only a few yearsDuring that brief time great advances have beenachieved in terms of electrolyte materials various Licathode materials and configurations and cell architecturesHowever research focused on MgLi hybrid batterytechnology is still in the early stage Beyond all the

FIG 10 The minimum volume of MgLi dual-salt electrolytesdependence of Mg21 concentration and Li cathode area capacityThe specific capacity of Li cathode is assumed to be 150 mA hg

FIG 9 Specific energy density comparison for MgMo6S8 batteryLIB and MgndashLi hybrid batteries without considering the mass ofsolvents60

Y Cheng et al Rechargeable MgndashLi hybrid batteries status and challenges

J Mater Res Vol 31 No 20 Oct 28 20163138httpdxdoiorg101557jmr2016331Downloaded from httpwwwcambridgeorgcore University of Houston on 24 Nov 2016 at 220226 subject to the Cambridge Core terms of use available at httpwwwcambridgeorgcoreterms

technical challenges identifying practical applicationsis another hurdle that must be addressed Can hybridbatteries be applied in electrical vehicles or other energystorage devices In conducting the review we clearlyrecognize that improving the energy density of currentMgLi hybrid battery technology would be difficult whencompared to state-of-the art LIB However consideringthe longer cycling life higher rate capability and superiorbattery safety which are intrinsic characteristics of MgndashLihybrid batteries we believe that MgndashLi hybrid batteriesand its analogues (eg MgNa hybrid8081) could providemore compelling solutions for stationary energy storageapplications Rational design and development of MgLihybrid battery technologies could be achieved throughfurther understanding and investigating the synergeticeffects of using different charge carriers for cathodes andanodes Therefore we hope the information provided inthis review will be helpful to those engaged in efforts todevelop low-cost and reliable hybrid battery technologiesfor stationary energy storage applications in the future

ACKNOWLEDGMENTS

YWC HJC and HD equally contributed forthis work Financial support was provided by the USDepartment of Energy (DOE) Office of ElectricityDelivery and Energy Reliability under ContractNo 57558 and the Office of Basic Energy SciencesDivision of Materials Sciences and Engineering underAward KC020105-FW P12152 Y Y acknowledgesfinancial support from the Office of Naval Research(No N00014-13-1-0543) PNNL is a multiprogramnational laboratory operated for DOE by Battelle undercontract DE AC05-76RL01830

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