Fabrication of high-pore volume carbon nanosheets with ...anhuilu.dlut.edu.cn/ky/article/158.pdf · vacuum oven at 100 °C for 12 h. The electrode area was 1.13 cm2, and the loading

Fabrication of high-pore volume carbon nanosheets with uniform arrangement of mesopores

Shuai Wang, Fei Cheng, Peng Zhang, Wen-Cui Li, and An-Hui Lu ()

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

Received: 18 August 2016

Revised: 17 November 2016

Accepted: 30 November 2016

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2016

KEYWORDS

mesoporous carbon,

nanosheet,

energy storage,

battery,

LiFePO4

ABSTRACT

Carbon nanosheets with a tunable mesopore size, large pore volume, and good

electronic conductivity are synthesized via a solution-chemistry approach.

In this synthesis, diaminohexane and graphene oxide (GO) are used as the

structural directing agents, and a silica colloid is used as a mesopores template.

Diaminohexane plays a crucial role in bridging silica colloid particles and GO,

as well as initiating the polymerization of benzoxazine on the surfaces of both

the GO and silica, resulting in the formation of a hybrid nanosheet polymer. The

carbon nanosheets have graphene embedded in them and have several spherical

mesopores with a pore volume up to 3.5 cm3·g–1 on their surfaces. These nuerous

accessible mesopores in the carbon layers can act as reservoirs to host a high

loading of active charge-storage materials with good dispersion and a uniform

particle size. Compared with active materials with wide particle-size distributions,

the unique proposed configuration with confined and uniform particles exhibits

superior electrochemical performance during lithiation and delithiation, especially

during long cycles and at high rates.

1 Introduction

In recent years, high-performance Li-ion batteries

(LIBs) have captured a large share of the rechargeable-

battery market because of their great potential as power

sources for electric vehicles and hybrid electric vehicles.

The electrode material, which is an indispensable part

of LIBs, has attracted considerable interest. Significant

research achievements have been made concerning

electrochemically active materials with a superior

capacity for Li storage, including various cathode and

anode materials such as LiFePO4, sulfur, SnO2, and iron

oxides [1–5]. However, the poor intrinsic electronic

conductivity of these materials seriously limits their

rate capacity and is a common problem.

Porous carbon materials have the advantages of good

chemical and thermal stability, and their nanopores

can provide confinement for active materials [6–9].

Thus, porous carbon materials are effective for the

preparation of high-rate Li-ion batteries [10–11]. To

enhance the electrical conductivity, graphene has recently

been used instead of porous carbon to host active

Nano Research 2017, 10(6): 2106–2116

DOI 10.1007/s12274-016-1399-9

Address correspondence to [email protected]

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2107 Nano Res. 2017, 10(6): 2106–2116

nanomaterials, owing to its superior conductivity,

flexibility, and large surface area. Conventionally,

as-synthesized graphene consists of crystallites with

different numbers of layers, resulting in typical surface

areas that are significantly lower than the theoretical

limit of 2,630 m2·g–1 [12–14]. Although the graphene

can be etched by KOH activation to form pores, such

pores on a flat surface hardly provide confinement

for active materials [13, 15]. Moreover, because of the

weak bonding of active nanoparticles to the graphene

support, the aggregation of active nanoparticles on the

graphene surface may result in capacity fading and

poor cycle performance [16–21]. It is thus necessary

to modify the chemical or physical interactions of the

nanoparticles with graphene in order to stabilize the

structure for real applications [22].

We recently demonstrated the synthesis of

microporous carbon nanosheets with a controllable

thickness on the nanoscale via solution chemistry

using a very small amount of graphene oxide (GO) as

the sheet-directing agent [23, 24]. Such structures with

internal graphene showed an improved conductivity,

and when used as an electrode, they provided large

electrode-electrolyte interfaces for rapid charge-transfer

reactions and enabled a high degree of utilization of

the overall porosity and surface area [24]. To exploit

these nanosheet structures and use them as substrates

to host active materials, mesopores must ideally be

created in a carbon nanosheet. Although there are

several reports on the synthesis of carbon nanosheets,

it is rare that the distribution of mesopores is uniform

and the pore volumes are >1.5 cm3·g–1 [25–29]. A large

pore volume is preferred to host the maximum amount

of active material, allowing a high tap density. Recently,

GO-based polypyrrole nanosheets with adjustable

mesopores have been reported [30]. For obtaining

a high electrochemical performance, it is important

to design and fabricate the nanostructure of carbon

sheets with a large pore volume and suitable pore size.

Carbon nanosheets with mesopores in the range of

2–50 nm can act as a rigid nano-confinement support

that restricts the particle size of electrochemically

active species to the nanoscale, greatly shortens the

diffusion length of Li ions, and improves rate capacity.

The larger pores in the layers can potentially eliminate

the diffusion resistance and the permanent trapping

of Li ions that can occur in micropores [31].

With these considerations, we demonstrate a solution-

chemistry approach for the synthesis of mesoporous

carbon nanosheets (MCSs) with a tunable mesopore

size, large pore volume, and good electronic con-

ductivity. Diaminohexane is used to manipulate

the assembly of negatively charged building blocks,

including a silica colloid and GO, and to initiate the

polymerization of benzoxazine on the surfaces of the

building blocks. Pyrolysis of the hybrid followed by

silica removal results in carbon nanosheets containing

uniform and spherical mesopores with a tunable

mesoporosity and a large pore volume up to 3.5 cm3·g–1.

These numerous accessible mesopores in the carbon

layers can act as reservoirs to host a high loading of

active charge-storage materials with good dispersion

and a uniform particle size. Compared with active

materials having wide particle-size distributions, such

a unique configuration with confined and uniform

particles can yield superior electrochemical performance

during lithiation and delithiation, especially during

long cycles and at high rates.

2 Experimental

2.1 Materials

Resorcinol (99.5%) was purchased from Tianjin Kermel

Chemical Reagent Co., Ltd. A formaldehyde solution

(37 wt.%) and 1,6-diaminohexane (DAH) were supplied

by Sinopharm Chemical Reagent Co., Ltd. Ludox SM-30

and Ludox HS-40 were purchased from Sigma–Aldrich.

All chemicals were used as received.

The GO dispersion used in this work was prepared

using a modified Hummers method. The as-obtained

concentrated GO was dispersed in deionized water

under sonication at a power of 100 W for 4 h (KQ-

100TDB, Kun Shan Ultrasonic Instruments Co., Ltd.,

China). The diluted GO was then centrifuged to

remove the unexfoliated parts, and the supernatant

homogeneous GO dispersion was collected for later

use. Before use, the concentration of the GO dispersion

was determined according to the weight difference

before and after drying.

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2108 Nano Res. 2017, 10(6): 2106–2116

2.2 Synthesis

2.2.1 Synthesis of MCSs

In a typical synthesis procedure, 500 L of DAH

(0.058 g/mL) and 600 L of LUDOX SM-30 were

mixed for 20 min. Then, the mixture was added to the

GO aqueous solution (1.8 mg/mL). Next, 0.187 g of

resorcinol dissolved in 125 L of the formaldehyde

solution (37 wt.%) was quickly injected into this solution,

and the mixture was shaken until the resorcinol was

well dispersed. The homogeneous solution was then

sealed and kept in an oven at 90 °C for 3 days. The

hybrid polymer was dried at 50 °C until there was no

further visible weight loss, followed by pyrolysis at

800 °C for 2 h under a nitrogen flow to obtain carbon-

silica hybrid nanosheets. The MCS was obtained after

the removal of the silica template using an aqueous

NaOH solution (2.5 M).

2.2.2 Synthesis of LiFePO4/C hybrids

LiFePO4/C hybrids were fabricated by using the

synthesized porous carbon sheet as a support. A lithium

iron phosphate solution was prepared using water-

based solution chemistry with a concentration such that

the solution did not precipitate prior to infiltration.

In a typical procedure, 10 mmol of Fe(NO3)3·9H2O,

10 mmol of C2H3O2Li·2H2O, and 10 mmol of H3PO4

were dissolved in a minimal amount of water with

stirring to obtain a total solution of 5 mL. Then, the

transparent solution was added dropwise to 0.1 g of

carbon while stirring, followed by drying at 90 °C. The

amount of the impregnating solution was calibrated

according to the pore volume of the carbon structure.

To fully utilize the pores, the impregnation procedure

was repeated once, followed by drying at 90 °C for

24 h. Finally, the powder was heated in a flowing

atmosphere of H2 (5 vol.%)/N2 (95 vol.%) at 3 °C·min–1

to 750 °C and maintained at this temperature for 6 h,

yielding a black powder.

2.3 Characterization Methods

Scanning electron microscopy (SEM) was performed

using a NOVA NanoSEM 450 instrument. Transmission

electron microscopy (TEM) images of the samples

were obtained using an electron microscope (Tecnai

G220S-Twin) equipped with a cold field-emission gun.

The thermal-decomposition behavior of the products

was monitored using a simultaneous thermal analyzer

(Netzsch STA 449 F3) from 40 to 800 °C in nitrogen

with a heating rate of 10 °C/min. The infrared (IR)

spectrum was obtained using a Fourier transform IR

(FTIR) spectrometer (Nicolet 6700). Elemental analysis

was performed using an elemental analyzer (Vario EL

III, Elementar). N2 sorption isotherms were measured

using a Micromeritics ASAP 2020 adsorption analyzer

at 77 K. Prior to each adsorption experiment, the

sample was degassed for 6 h at 200 °C, ensuring that

the residual pressure decreased below 5 10–3 mbar,

and then cooled to the target temperature, followed

by the introduction of pure N2 gas into the system.

The Brunauer–Emmett–Teller method was used to

calculate the specific surface area. The total pore

volumes were calculated using the amount of nitrogen

adsorbed at a relative pressure, P/P0, of 0.99. The

micropore volumes were calculated using the t-plot

method. The pore-size distributions were determined

according to the N2 adsorption branches of the isotherm.

The Raman spectra were collected on a DXR Microscope

Raman Spectrometer using the 532-nm line of a

KIMMON laser. The zeta potentials of the samples

were measured using a Malvern Zetasizer Nano ZS90.

The electrodes were fabricated using a mixture of

the active material (80 wt.%), conductive carbon black

(15 wt.%), and polyvinylidene fluoride (5 wt.%, Aldrich)

in N-methyl-2-pyrrolidon, which formed a slurry.

The slurry was spread on an Al foil and dried in a

vacuum oven at 100 °C for 12 h. The electrode area

was 1.13 cm2, and the loading of the active material

was 2.0 0.5 mg·cm–2. The specific capacity was

calculated according to the weight of LiFePO4, and

the specific power was calculated using the following

formula: specific power = specific energy/discharge

time. Electrochemical experiments were performed

using CR2025 coin-type test cells assembled in an

Ar-filled glove box, with Li metal as the negative

electrode and a Celgard 2400 membrane as the

separator. The electrolyte comprised a solution of 1 M

LiPF6 in dimethyl carbonate, ethyl methyl carbonate,

and ethylene carbonate (1:1:1, v/v/v). A galvanostatic

charge–discharge experiment was performed in the

range of 2.5–4.2 V at room temperature using a Land

CT2001A battery test system. Cyclic-voltammetry (CV)


2109 Nano Res. 2017, 10(6): 2106–2116

measurements were performed using a CHI660D

electrochemical workstation. Electrochemical impedance

spectroscopy was performed using a EG&G model

273 galvanostat/potentiostat equipped with a 5280

two-phase lock-in analyzer by superimposing an

alternating-current (AC) signal with an amplitude of

5 mV on an open-circuit potential over the frequency

range of 100 kHz to 10 mHz.

3 Results and discussion

In a typical synthesis procedure, LUDOX SM-30

(colloidal silica with an average particle size of 7 nm)

mixed with DAH was first added to a GO aqueous

dispersion. Resorcinol and formaldehyde were then

quickly injected into this dispersion, and the mixture

was vigorously shaken to obtain a good dispersion

with molar ratios of SiO2/resorcinol of 2.2:1 (denoted

MCS-1) and 5:1 (denoted MCS-2). After polymerization

at 90 °C for 3 days under ambient conditions, a

polymer-silica-GO hybrid was obtained. Carbon nano-

sheets with a large mesoporosity were then prepared

through the pyrolysis of this hybrid at 800 °C and

the subsequent removal of the silica using a 2.5 M

NaOH solution. Different colloidal silica (Ludox-HS-40)

with silica nanoparticles having an average size of

14 nm were used to synthesize carbon nanosheets

(denoted MCS-3). The morphology and nanostructure

of the resulting carbon hybrid material were inves-

tigated by SEM and TEM. We observed only sheet-like

structures with uniform mesopores (Figs. 1 and 2).

No isolated graphene was observed, indicating that

the graphene was covered with a layer of mesoporous

carbon. The thickness of the mesoporous carbon

MCS-1 was estimated as 20 nm according to the SEM

image shown in Fig. 1(a). Numerous nanosize pores

~7 nm in diameter were uniformly distributed on both

sides of the carbon sheets (Figs. 1(b) and 1(d)). These

spaces were previously occupied by the silica colloidal

nanoparticles. Graphene, as an ideal electron conductor,

can increase the conductivity of the carbon nanosheets

in principle. Indeed, sample MCS-1 had a good

electronic conductivity of 125 S·m–1 even with a low

graphene content of 3.8 wt.%. A control sample

synthesized without GO exhibited an inferior electronic

conductivity of 52 S·m–1.

Figure 1 SEM images of MCS-1 (a) and (b), MCS-2 (c) and (d), and MCS-3 (e) and (f).

TEM images show that the mesopores in these

sheets were closely packed, with an average pore size

of ~8 1 nm (Figs. 2(a)–2(d)). The carbon nanosheets

had ultrathin and partially graphitic pore walls ~3 nm

thick (Fig. 2(b)). Raman spectroscopy was used to

further examine the microstructure of the carbon

nanosheets. As shown in Fig. S1 (in the Electronic

Supplementary Material (ESM)), the Raman spectrum

of the MCSs had strong characteristic bands at

1,350 cm–1 (D-band) and 1,600 cm–1 (G-band). The G

band corresponds to the bond stretching of sp2 carbon,

and the D band reflects the breathing mode of aromatic

rings. The intensity ratio of the D-band to the G-band is

0.99, revealing the presence of graphitized nanodomains

in the nanosheets. The surface characteristics of the

nanosheets were examined via FTIR analysis (Fig. S2

in the ESM). The band around 3,455 cm–1 may be

related to N–H or O–H stretching vibrations or water

molecules. The peaks around 1,640 and 950–650 cm–1

are attributed to N–H in-plane and out-of-plane

deformation vibrations, respectively. A stretching

vibration band of C–N was detected around 1,174 cm–1.


2110 Nano Res. 2017, 10(6): 2106–2116

Figure 2 TEM images of MCS-1 (a) and (b), MCS-2 (c) and (d), and MCS-3 (e) and (f).

N2 sorption isotherms show a typical current–voltage

curve with a hysteresis loop, indicating the existence

of mesopores (Fig. 3(a)). The surface area of the

MCS-1 is determined to be ~814 m2·g–1. The pore-size

distribution curve is bimodal, with maxima at ~2 and

7–9 nm, which are attributed to small pores in the

larger pore walls and the cavity size derived from the

removal of SiO2 nanoparticles. Such small pore sizes

agree well with what is determined by the template,

i.e., silica nanoparticles, and are therefore ascribed to

the DAH triggering the polymerization of benzoxazine

on the surface of the silica particles. The pore volume

is estimated to be ~1.60 cm3·g–1 according to the

abundant mesopores in these nanosheets. To increase

the mesopore volume, which was later done to maximize

the uptake of charge-storage active materials, we

increased the SiO2/resorcinol ratio from 2.2:1 (MCS-1)

to 5:1 (MCS-2). These new carbon nanosheets had a

higher pore volume of 2.58 cm3·g–1 and similar bimodal

pore sizes, i.e., approximately 2 and 9–10 nm (Fig. 3(b)).

In this synthesis method, the mesopore size was further

Figure 3 (a) Nitrogen sorption isotherms and (b) the corres-ponding Barrett–Joyner–Halenda pore-size distributions of the MCS samples. The isotherms of MCS-2 and MCS-3 were offset vertically by 150 and 650 cm3·g–1 (STP), respectively.

tuned by using Ludox-HS-40 with a SiO2/resorcinol

molar ratio of 6.7:1. Consequently, the synthesized

carbon nanosheet MCS-3 had mesopores 2 and ~16 nm

in size on both sides (Figs. 1(c), 1(d), 2(e) and 2(f) and

Table S1 in the ESM).

This hybrid structure was rationally designed and

constructed by nanoengineering. Various oxygen-

containing groups are present in the original GO

basal plane and edges, ensuring the feasibility of the

structural design using GO. A broad band at ~3,400 cm–1

is ascribed to the O–H stretching of C–OH (Fig. 4(a)).

A peak detected at ~1,730 cm–1 is ascribed to the

stretching of the C–O bond of carbonyl or carboxyl

groups. The distinct peaks at approximately 1,051 and

1,225 cm–1 are associated with C–O vibrations of the

epoxy (C–O–C) and C–OH units [32]. However, the

signals characteristic of oxygen-containing functional

groups clearly decrease in intensity or even disappear

after the introduction of DAH to the GO dispersion.


2111 Nano Res. 2017, 10(6): 2106–2116

Figure 4 (a) Zeta potentials of LUDOX SM-30 colloidal silica in the presence of DAH over time, (b) FTIR spectra of GO samples before and after functionalization with DAH.

Furthermore, new bands at 1,560 and 2,922, 2,848 cm–1

are associated with the vibration of N–H and the

symmetric and antisymmetric vibrations of –CH2,

respectively [33]. The FTIR spectra shown in Fig. 4(a)

confirm that the diamine was attached to the surface

of the GO sheets because of the nucleophilic attack of

the amine (–NH2) end groups to the epoxy groups of

the GO [34–36].

Taking the synthesis of MCS-2 as an example, before

the assembly with GO colloids, LUDOX SM-30 was

mixed with DAH to allow the amine groups of the

DAH molecules to anchor onto the surfaces of the

silica nanoparticles through electrostatic attraction and

H bonding [37, 38]. The strength of the interaction

between the silica nanoparticles and the DAH molecules

was a function of their reaction time (Fig. 4(b)), as

discussed in the Supporting Information.

The molecular structure of DAH contains two –NH2

groups at both ends of the aliphatic chain. These are

considered to link the silica colloid and the GO, which

would otherwise experience electrostatic repulsion.

By controlling the reaction time between the colloidal

silica and the DAH, their interactions can be adjusted

to prevent the self-assembly of the silica colloid and

ensure that the SiO2 nanoparticles are monodispersed

and attach to the surface of the GO [38].

As shown in Fig. 5(a) and Fig. S3 (in the ESM), both

sides of the GO sheet surface were uniformly covered

with a layer of closely packed SiO2 nanoparticles, which

were firmly and homogeneously anchored on the GO

nanosheets as a result of the diamine-molecule linkages

between the silica nanoparticles and GO sheets. After

this, resorcinol and formaldehyde were quickly injected

into the aforementioned solution, and the resultant

homogeneous solution was sealed and heated to the

final polymerization temperature of 90 °C. According to

benzoxazine chemistry, polybenzoxazines are generated

by the condensation of phenols, aldehydes and amines

[39–43]. Because of its prior attachment to the silica

nanoparticles and GO sheets, the DAH initiated the

polymerization of benzoxazine only on the surfaces

of the nanoparticles and sheets, resulting in the observed

Figure 5 (a) TEM image of a GO sheet covered with a layer of

patterned SiO2 nanoparticles, with DAH linking the GO and the

colloidal silica. (b) and (c) TEM images of the polymer-silica-GO

hybrid structure of MCS-2 before pyrolysis, where diaminohexane

initiated the polymerization of benzoxazine on the surfaces of the

silica nanoparticles and the graphene sheet. (d) SEM image of

carbon-silica hybrids generated after pyrolysis at 800 °C.


2112 Nano Res. 2017, 10(6): 2106–2116

sandwich structure (Figs. 5(b) and 5(c)). Clearly, the GO

sheet was fully covered with a layer of polymer-

containing silica nanoparticles. By pyrolysis at 800 °C,

the polybenzoxazine layers attached to the GO sheets

were converted into porous carbon layers while the

GO sheets were thermally reduced to graphene sheets,

and thus a sandwich carbon hybrid was generated

[44]. During carbonization, the polymer layer shrank

and exposed the upper surface of the embedded silica

nanoparticles, and additional pores were produced

by separation of the exposed silica from the carbon

layer, as shown in Fig. 5(d) and Fig. S4 in the ESM.

An MCS was obtained after the removal of the silica

template using an aqueous NaOH solution (Fig. 1).

The proposed mechanism is summarized in Fig. 6.

We consider that the DAH served as a bridge to direct

the formation of the GO-silica hybrid. An analogous

sample synthesized without DAH showed an irregular

mesoporous carbon matrix or a carbon sheet with

a disordered surface (Fig. S5 in the ESM), resulting

from the lack of a link between the GO and the silica

nanoparticles.

These carbon nanosheets exhibit the following

advantages for electrode materials for a high-

performance Li-ion battery: a continuous conductive

path, a controllable mesopore size, a large pore volume,

and a short diffusion distance. We were therefore

motivated to use the MCS-2 nanosheets to host

LiFePO4 for the fabrication of hybrid electrodes for

Li-ion batteries. The SEM image of the LiFePO4/MCS

hybrid shown in Fig. S6 (in the ESM) indicates that

most of the LiFePO4 was well confined in the nanopores

Figure 6 Diagram of the formation mechanism of the MCSs.

of the carbon sheet. The weight percent of LiFePO4

in the hybrid was determined as 73 wt.% using

thermogravimetric analysis (Fig. S7 in the ESM). The

electrochemical properties of the hybrid (e.g., LiFePO4/

MCS) were tested in CR-2025 coin half-cells at 298 K.

Figure 7(a) shows the CV curves of the LiFePO4/

MCS electrode obtained at a scan rate of 0.2 mV·s–1.

The well-defined sharp redox peaks in the small voltage

range of 3.33–3.55 V are attributed to the Fe2+/Fe3+ redox

couple reaction, which correspond to Li extraction

and insertion in LiFePO4 crystal structure, indicating

fast reaction kinetics. The galvanostatic charge–

discharge curves of LiFePO4/MCS versus Li for the

cell, cycled between 2.5 and 4.2 V at 0.1 C, are shown

in Fig. 7(b). The LiFePO4/MCS hybrid exhibits subs-

tantially lower polarization between the charge and

discharge plateaus and delivers an ultrahigh discharge

capacity of 166 mA·h·g–1, which is very close to the

theoretical capacity of LiFePO4 (170 mA·h·g–1). This is

strong evidence that the kinetics of the LiFePO4/MCS

were improved.

To further clarify the superior electrode performance

of the hybrid, we tested the rate performance of the

LiFePO4/MCS electrode at different rates ranging

from 0.1 to 50 C. As shown in Fig. 7(c)), although

the specific capacity gradually decreased with the

increasing current rate, a high initial capacity of

148 mA·h·g–1 was achieved at 1 C. Even at high current

rates of 10, 20, 30, and 50 C, respective capacities

of 128, 116, 99, and 66 mA·h·g–1 were achieved,

demonstrating that our LiFePO4/MCS hybrid can

endure high-rate charging and discharging. This was

verified by AC impedance spectroscopy (Fig. 7(d)).

The diameter of the semicircle in the high-frequency

range corresponds to the charge-transfer resistance

(Rct), which is related to the electrochemical reaction

at the electrode–electrolyte interface and the particle–

particle contact in the active material. The small-

diameter semicircle for the hybrid reflects the low

charge-transfer impedance due to the good contact

between the nanosize and uniform LiFePO4 particles

and the continuous graphene-containing carbon

framework, which formed an excellent conductive path

allowing efficient charge transport and increasing the

electronic conductivity of the hybrid.

The inclined lines in the low-frequency range in


2113 Nano Res. 2017, 10(6): 2106–2116

Fig. 7(d) are related to the Warburg impedance, which

is associated with the Li-ion diffusion behavior in the

LiFePO4 electrode. Rapid Li-ion diffusion is essential

for achieving a high rate performance. The Li-ion

diffusion coefficient of our LiFePO4/MCS hybrid

calculated from AC impedance spectroscopy was 1.8 ×

10–12 cm2·s–1, which is significantly improved compared

with the value for pure LiFePO4. This is because of

the small LiFePO4 particles confined in the mesoporous

carbon, which significantly shortened the diffusion

path of Li-ions through the nanoparticles and the

thin carbon walls. This result is consistent with the

observed high Li-storage capacity and good rate

performance.

In addition to the good rate performance, the

excellent cycling stability during the Li-ion insertion/

extraction process is a key factor for the industrial

production of a good cathode. Hence, the LiFePO4/

MCS electrode was tested for long-term cycling, and

the results are shown in Fig. 8. Surprisingly, the cell

retained 93% of its initial capacity over 1,000 cycles even

at a high rate of 20 C, and the Coulombic efficiency was

almost 100%. The average specific discharge capacity

Figure 8 Cycling performance and coulombic efficiency of LiFePO4/MCS hybrid at 20 C over 1,000 cycles.

during 1,000 cycles at 20 C was 116 mA·h·g–1. Con-

verting to specific power, this means that the cell

delivered an ultrahigh specific power of 10,590 W·kg–1

during 1,000 cycles. This superior electrochemical

performance is comparable to or better than those of

previously reported LiFePO4/MCS hybrids [45–55].

In summary, such a carbon-sheet structure allows

the easy insertion and extraction of Li ions and

consequently improved kinetics. Furthermore, the large

pore volume in the carbon sheet provides sufficient

space for a high LiFePO4 loading, allowing the

Figure 7 Electrochemical tests of the LiFePO4/MCS hybrid: (a) cyclic voltammograms obtained at a scan rate of 0.2 mV·s–1; (b) third charge–discharge profiles at 0.1 C (0.1 C corresponds to 17 mA·g–1); (c) rate performance at current rates ranging from 0.1 to 50 C and back to 0.1 C for five cycles; (d) AC impedance spectroscopy of the battery after three cycles of the CV test.


2114 Nano Res. 2017, 10(6): 2106–2116

fabrication of hybrid materials with high tap density.

Nanosize, uniform, spherical LiFePO4 particles con-

fined in the carbon sheets simultaneously allow the

achievement of a fast Li-ion diffusion rate, a high

degree of utilization, and a high activity of the active

materials. Additionally, the interconnected carbon

framework allows an effective supply of electrons

to the LiFePO4 particles through the continuous

conductive network during operation, yielding

superior electrochemical performance. Considering

its outstanding rate and cycling performance, our

LiFePO4/MCS hybrid is a promising cathode material

for high-power LIBs. As a universal conductive

substrate, this MCS can be extended to the preparation

of other metal-oxide hybrid materials for energy devices,

such as a high-performance SnO2/MCS anode (Fig. S8

in the ESM).

4 Conclusions

We demonstrated a strategy for creating a uniform

arrangement of mesopores on the surfaces of two-

dimensional carbon nanosheets via a solution-based

self-assembly route, with GO and diaminohexane

as the structure-directing agents and a silica colloid

as the mesopores template. These sheets can be

nanoengineered to have a continuous conductive

path, a controllable mesopore size, a large pore

volume, and a short Li-ion diffusion distance and can

therefore be used as substrates for the fabrication of

high-performance Li-ion batteries. The well-distributed

uniform mesopores serve as reservoirs to support,

disperse, and confine the active charge-storage materials

during lithiation and delithiation, especially during

long cycling and at high rates. This novel structure

may provide a universal substrate for the fabrication

of other high-power cathode and anode materials.

Acknowledgements

The project was supported by National Natural

Science Foundation of China (Nos. 21225312,

21473021 and U1303192).

Electronic Supplementary Material: Supplementary

material (Raman spectra, FTIR spectrum, SEM and

TEM images of MCS, SEM image and TGA curve of

LiFePO4/MCS) is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-016-1399-9.

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Fabrication of high-pore volume carbon nanosheets with ...anhuilu.dlut.edu.cn/ky/article/158.pdf · vacuum oven at 100 °C for 12 h. The electrode area was 1.13 cm2, and the loading