Nano Res

1

From graphite to porous graphene-like nanosheets for

high rate lithium-ion batteries

Dongdong Zhao, Lei Wang, Peng Yu, Lu Zhao, Chungui Tian, Wei Zhou, Lei Zhang, and Honggang Fu ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0805-z

http://www.thenanoresearch.com on April 27, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0805-z

1

TABLE OF CONTENTS (TOC)

From graphite to porous graphene-like nanosheets for

high rate lithium-ion batteries

Dongdong Zhao, Lei Wang, Peng Yu, Lu Zhao, Chungui

Tian, Wei Zhou, Lei Zhang, and Honggang Fu*

Heilongjiang University, China

Page Numbers. The font is

ArialMT 16 (automatically

inserted by the publisher)

A low-cost and effective route has been employed to synthesize 2D

porous graphene-like nanosheets (PGS) based on the simple and

subsequent intercalation of H3PO4 and ZnCl2 into the interlayer of

expandable graphite, respectively. The PGS material presents a

remarkable Li storage performance, capacity retention, rate capability

and cyclic lifetime as for material for LIBs anode.

2

From graphite to porous graphene-like nanosheets for high rate lithium-ion batteries

Dongdong Zhao, Lei Wang, Peng Yu, Lu Zhao, Chungui Tian, Wei Zhou, Lei Zhang, and Honggang Fu* ()

Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang

University, Harbin 150080, P. R. China, E-mail: fuhg@vip.sina.comKey

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2013

ABSTRACT Graphene nanosheets are regarded as potential electrodes in Li-ion batteries (LIBs), therefore, the

development of the synthetic approaches for these materials with both low-cost and productivity are

essential. Herein, porous graphene-like nanosheets (PGS) have been synthesized from expandable graphite

(EG) by firstly intercalating phosphoric acid, followed by an annealed process to enlarge the interlayer

distance of EG, which is favorable for subsequent intercalation of zinc chloride. Then, the followed pyrolysis

of zinc chloride in the interlayer of EG could form the porous structure of PGS, meanwhile the gas produced

during the formation of porous structures could exfoliate the EG to graphene-like nanosheets. The synthetic

PGS as LIBs anode exhibited superior Li+ storage performance, including remarkably discharge capacity of

830.4 mAh g−1 at 100 mA g−1, excellent rate capacity of 211.6 mAh g−1 at 20000 mA g−1 and excellent cycle

performance (a near 100 % capacity retention after 10000 cycles). The well rate performance is attributed to

Li+ ions rapid transport of porous structures and the high electrical conductivity of graphene-like nanosheets.

It is expected that PGS may be widely used as anode materials for high-rate LIBs via the present facile and

low-cost route by employing EG as the raw material.

KEYWORDS porous nanosheets, graphene-like, graphite, high-rate, Li-ion battery

1 Introduction

Increasing interest has recently been devoted to developing electrochemical energy storage devices with high power density, well cycling stability and low-cost due to the rapidly growth and development of electric equipments in the modern society. Nowadays, lithium ion batteries (LIBs) [1-6]

have occupied the main body of energy storage devices market and become the most competitive member in the field of chemical power sources, such as electronics, vehicles, large-scale power-grid storage and other applications [7-11]. Therefore, the new electrode material combines high energy density, low-cost and facile producibility for room-temperature rechargeable LIBs is concerned.

Nano Res DOI (automatically inserted by the publisher)

Research Article

————————————

Address correspondence to H. Fu, fuhg@vip.sina.com

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Although single- or multiple-metal compounds (oxides, sulfides and nitrides) generally exhibit high specific capacity [12-14], the poor stability is the great obstacle in the practical application. Graphite with well stability can be used as anode material for commercial LIBs since the pioneering work by SONY Co. in 1990 [15-17]. Although graphite anode

with well conductivity and perfect stability, the

theoretical specific capacity is as low as 372 mAh g−1

(calculating based on the first-stage graphite

intercalation compound of LiC6) [18-20], so it cannot

meet the worlds’ growing demand for next-generation

LIBs with high energy density and high rate capability. It is highly desirable to fabricate anode materials with a breakthrough of the theoretical specific capacity. Over the years, development and research efforts have been attempted to address these problems by designing novel materials, such as graphene, carbon nanotubes (CNTs) and carbon nanofibers (CNFs) with hollow and defect structures [23-28], graphite microspheres, and mesoporous carbon with non-metal elements doping (B, N, P, S) [29-31]. Although various nanostructured carbonaceous materials with high specific capacity have been successfully prepared, the durability and rate performance of electrodes should be solved.

Two-dimensional (2D) graphene nanosheets have been valued for energy devices for its excellent conductivity could facilitate to the rapid transfer of electrons [32-34]. However, the graphene natures of agglomeration and non-porous nanostructures would limit the transfer of Li+ ions and electrons during the lithiation / delithiation process [35,36]. Nanoporous structures can store large quantities of electrolytes and form ion buffering reservoirs. It could enhance diffusion kinetics by shortening the distance of Li+ ions diffusion, and also benefiting the transmission of electrons companying with graphene, leading to a large specific capacity and well rate performance. Combining the advantages of graphene and nanoporous structures, significant progress has been made in designing porous graphene nanostructures for LIBs anodes, such as porous graphene and mesoporous carbon coated graphene nanostructures [37-39]. However, graphene always derived from graphite oxide (GO) by oxidizing graphite with large quantities of strong acid, which caused environmental pollution seriously and limited the productivity [40,41]. Besides, the abundant of oxygenous groups in GO could not be completely reduced, leading to a poor conductivity and a bad performance of the synthetic graphene-based materials. Therefore, exploring and

developing a facile approach to synthesize of 2D porous graphene nanomaterials for high-rate LIBs is urgently required. Natural graphite (NG) widely exists in the world, especially in the northeast of China, it has the well conductivity and layered structure. Expandable graphite (EG) is derived from NG after simple treatment, which has larger interlayer distance than NG, so some functional composites can be introduced into the interlayer of NG [42,43]. Based on these considerations, in previous, we had synthesized of mesoporous carbon/graphite nanosheets composite by the intercalation of resol prepolymer into the interlayers of EG, followed by the exfoliation of EG through in situ polymerization and carbonization processed [44]. In order to simplify the synthesis and further broaden the application of graphite derivate, it is still a major challenge in synthesis of electrode anode material for the next-generation LIBs include the need to enhance their specific capacity, cycling life and rate capability.

Herein, we employ a facile route for synthesis of porous graphene-like nanosheets (PGS) from EG by a simple and effective route. In the synthesis, phosphoric acid firstly intercalates into the layers of EG and could enlarge the layer distance via a thermal treatment process. As compared, the sample derived from EG by direct thermal annealing process was also synthesized. But the corresponding BET surface area of the exfoliated EG is only about 5.2 m2 g-1, which is much lower than that of the sample derived from EG intercalating H3PO4 after annealing (53.4 m2 g−1) the present PGS anodes were synthesized by intercalating of H3PO4. It is further demonstrated that the process of intercalating H3PO4 could enlarge the interlayer distance of EG, which is beneficial to the following intercalation of ZnCl2, resulting in the formation of PGS. Subsequently, zinc chloride was intercalated into the interlayers of EG under vacuum assistance. During the followed heat treatment process, the pyrolysis of zinc chloride in the interlayer of EG could form the porous structure of PGS, meanwhile the gas produced during the formation of porous structures can exfoliate the EG to graphene-like nanosheets, so the PGS could be obtained after acid treatment. The unique structures endow the PGS with well Li storage performance as illustrated in Scheme 1: (1) The existence of nanopores can be used as an electrolyte buffer memory, which can reduce the surface diffusion distance; (2) the porous structures could provide a larger transport / charge storage systems for Li+ ions, leading to a large discharge capacity. The present method, that uses a

4

cheap and plentiful EG as a resource, is simple and low-cost, opening up the effective strategy in the productivity anode materials for high-performance LIBs.

2 Experimental

2.1 Synthesis of PGS

The starting material EG used in the present work is

about 100μm with expansion ratio of 300 mL g-1. In

the typical synthesis, 10 g of EG was firstly dispersed

into 350 mL of 0.5 wt% H3PO4 aqueous solution by

sonication for 2h. After centrifugation and drying,

the black powder was annealed at 800 °C for 2h

under nitrogen ambient, so the expanded EG with

enlarged layer distance was obtained. Then, 2.0 mL

of PEG400MO (Polyethylene Glycol 400 Monooleate)

was dispersed into 45 mL of distilled water, followed

by 1.5 g of the above expanded EG was added. After

sonication for 30 min, the solution of 65 mL distilled

water containing 3.0 g ZnCl2 was added into the

above system and then stirring for 4 h. Subsequently,

the reaction system was heated under 100 °C until

the solvent evaporated completely and the precursor

could be obtained. Afterwards, the precursor was

calcined in the tube furnace under nitrogen

atmosphere at 1000 °C for 2 h. After treating with 5

wt% hydrochloric acid solution, washing with

distilled water and drying in oven, the porous

graphene-like nanosheets (PGS) material was

synthesized. The synthetic samples were named as

PGS-m-T, where m and T refer to the mass ratio

between ZnCl2 and EG, and the calcined temperature,

respectively, so the above sample was named as

PGS-2-1000. In order to study the influences of the

calcined temperature on the structure and properties

of the samples, the PGS-2-900 and PGS-2-1100 were

also prepared. For the same purpose, the mass ratio

between ZnCl2 and EG were changed as 1.0 and 3.0,

respectively, so the PGS-1-1000 and PGS-3-1000

samples were obtained.

2.2 Characterization

X-ray diffraction (XRD) patterns were obtained on a

Rigaku D/max-IIIB diffractometer using Cu Kα

(λ=1.5406 Å) with the accelerating voltage of 40 kV

and the applied current of 20 mA, respectively.

Meanwhile, Raman measurement was performed

with a Jobin Yvon HR 800 micro-Raman

spectrometer at 457.9 nm with an Ar ion laser beam.

Scanning electron microscopy (SEM) micrographs

were carried out using a Hitachi S-4800 instrument

operating at 5 kV. Transmission electron microscopy

(TEM) images were obtained with a JEM-2100

electron microscope (JEOL) and an acceleration

voltage of 200 kV. AFM images were recorded on a

5100 ALP (prototype Agilent Technologies) in

tapping mode (dynamic force mode). Commercially

available Si cantilevers with a force constant of 20

N/m were used as substrate. X-ray photoelectron

spectroscopy (XPS) analysed the elements content

with Mg KR (1253.6 eV) achromatic X-ray source.

After the samples had been vacuum-dried at 150 °C

over 6 hours, the nitrogen adsorption−desorption

isotherms of the samples were conducted by using a

Micromeritics TriStar II at 77 K. The pore size

distribution was calculated using the

Barrett−Joyner−Halenda (BJH) method.

2.3 Electrochemical Measurements

To prepare working electrodes, the mixture of 80

wt% active material, 10wt% acetylene black, and 10

wt% polyvinylidene fluoride (PVDF) were dispersed

in N-Methyl-2-Pyrrolidone (NMP), followed by

pasting it onto copper foil before assembly. The mass

loading of total active material on the electrode is

about 1 mg. The CR2025-type LIB cells were

assembled in an argon-filled glovebox by using

as-prepared electrodes, pure Li foil as the counter

electrode, separators (Celgard 2400), and electrolyte

(1 M LiPF6 in EC : EMC : DMC = 1 : 1 : 1 organic

solutions). CT2001A Land battery test system was

used to employ charge-discharge test at different

current densities in the voltage window of 0−3.0 V

(vs Li+/Li). Cyclic voltammetry (CV) scanned at a

scan rate of 1 mV s-1 between 0~3.0 V (vs Li+/Li) and

electrochemical impedance spectroscopy (EIS) with

the frequency ranging from 100 kHz to 10 mHz and

an AC signal of 5 mV in amplitude as the

5

perturbation were taken using Princeton Versa STAT

3 electrochemical station.

Scheme 1 Schematic illustrates the possible mechanism for Li+ ions and electrons.

3 Results and Discussion

The morphologies of the synthetic samples were

characterized by scanning electron microscopy

(SEM). The SEM image of original EG was shown in

Fig. 1A, it exhibits the typical graphite morphology

of multilayer structures with close packing. After

intercalating H3PO4 by sonication, the surface of EG

becomes rough and the interlayer distance is slightly

enlarged as can be seen from Fig. 1B. After

experiencing an annealed process, the thin

nanosheets could be obtained (Fig. 1C), implying

that this process could enlarge the interlayer distance

of EG. Followed by an intercalation and a

subsequent pyrolysis of ZnCl2, the PGS sample was

prepared.

Figure 1 SEM images of the EG and PGS sample: (A) original

EG and enlarged part of the selected area (red line); The EG

intercalated with H3PO4 before (B) and after (C) annealing; (D)

The SEM image of PGS-2-1000 sample.

Fig. 1D shows the SEM image of PGS-2-1000

sample synthesized from 3.0 g ZnCl2 and calcined at

1000 °C. It can be seen that PGS-2-1000 exhibits 2D

graphene-like as a gauze, indicating the successfully

exfoliation of EG to nanosheets. More importantly,

the nanosheet is completely separated but not

aggregated, which would benefit to the transport

resistance for Li+ ions and electrons, resulting in a

well rate performance of LIBs. For comparison, the

PGS samples derived from different ZnCl2 usage

(PGS-1-1000 and PGS-3-1000) and different calcined

temperatures (PGS-2-900 and PGS-2-1100) were also

studied. As evidenced by the SEM images in Fig. S2

and Fig. S3, the exfoliation of EG could also be

achieved by changing the usage of ZnCl2 and the

calcined temperature.

Figure 2 TEM images of the synthetic PGS-2-1000 sample: (A)

the low-resolution images of PGS-2-1000; (B,C) the enlarged

structures of the selected area in (A); (D) is the enlarged image

of the selected area in (C); (E,F) HRTEM images of the selected

area in (D) with different magnified times.

The nanostructure was further characterized by

Transmission electron microscopy (TEM). The

low-resolution TEM images (Fig. 2A-C) show that

the PGS-2-1000 was composed of nanosheets with

thin edge structure. The mesopore size around 2 nm

could be observed by the enlarged TEM images in

Fig. 2D and Fig. 2E, which is a favorable size for the

rapid transport of Li+ ions. HRTEM image in Fig. 2F

shows the distinct lattice distance of graphite (002),

implying the well crystallinity, which is beneficial for

electron transport in the charging/discharging

process of LIBs. The detail thickness of PGS-2-1000 is

also clearly verified by Atomic Force Microscope

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(AFM) (Fig. S1), further declaring the sample has

thin layer structure with an average thickness of 4.2

nm. X-ray diffraction (XRD) pattern was used to

characterize the crystalline structure and chemical

composition of the synthetic samples. As shown in

Fig. 3A, the synthetic PGS-2-1000 sample exhibits the

similar graphite (002) diffraction peak as that of the

EG at 2θ=26.5°, demonstrating the well crystallinity

of PGS-2-1000. The corresponding XRD patterns of

the PGS samples synthesized from different ZnCl2

usage and different calcined temperature were also

provided, and all of them have the similar peak as

that of PGS-2-1000. Notably, the intensity of the main

peak for all the PGS samples is weaker than that of

the original EG, implying the successful exfoliation

from EG to graphene-like nanosheets.

Figure 3 (A) XRD patterns and (B) Raman spectra of PGS samples synthesized under different reaction conditions. The corresponding data about EG were also prepared for comparison.

Raman spectroscopy is a useful tool for analyzing

the crystalline degree of carbon materials. As listed

in Fig. 3B, the synthetic PGS-2-1000 sample exhibits

the three typical bands, that is, the G band (1584

cm−1), D band (1370 cm−1) and 2D band (2748 cm−1).

The G band is a characteristic feature of the graphitic

layers and corresponds to the tangential vibration of

the sp2 carbon atoms, while the D band corresponds

to disordered carbon or defective graphitic structures

[45]. Moreover, the 2D band is assigned to C-H

stretching vibration of methylene, which is a special

symbol of graphitic carbon with high crystallinity or

2D graphene-like nanostructures [46]. The intensity

ratio of these two peaks is related to the crystalline

degree of carbon materials. As shown in Table S1,

PGS-2-1000 sample demonstrated a high IG/ID value

of 2.6, implying a well crystallinity, which could

facilitate to electron transport during

charging/discharging processes of LIBs. Additionally,

the samples derived from different ZnCl2 usage and

different calcined temperatures were also performed

for comparison. It can be seen that all of the PGS

samples exhibited narrow D and G bands, and

intense 2D bands, symbolic of graphene-like

nanostructures. The value for IG/ID increased with

increasing calcined temperature and decreased with

the increasing of ZnCl2 dosage. Obviously, the

original EG exhibits the best crystallinity with an

IG/ID value of 16.8, Meanwhile, the peaks of the

synthetic PGS samples are similar to these of EG,

indicating the good crystallinity of the synthetic

samples, which is consistent with the above XRD

analyses and our previous study [41]. The tapping

density for all the PGS samples is about 1.075g cm−3.

The electrical conductivity values of the as-prepared

samples are shown in table S-2. Besides, it can be

seen that the electrical conductivity values of the

as-prepared samples are in the range of 167~270 S

cm-1, which is much higher than these of the reported

carbon nanostructures [47,48]. X-ray photoelectron

spectroscopy (XPS) is an useful spectroscopic

technique for measuring the elemental composition

and content in a material. As the wide XPS spectrum

is shown in Fig. S4, the PGS-2-1000 sample

composed with 96.53 at. % of C and 3.47 at. % of O,

indicating no Zn and P existed in the resultant

sample.

N2 adsorption−desorption isotherms were

employed to determine the specific surface area (SSA)

using the Brunauer-Emmett-Teller (BET) method at

77 K in the P/Po range of 0~1.0. The corresponding

results for PGS samples were listed in Fig. 4 and

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Table 1. The PGS-2-1000 exhibits a SSA of 330.3 m2 g−1,

which is about 254 times as that of the original EG

(only 1.3 m2 g−1 as shown in Fig. S5A). The

adsorption isotherms were used to calculate the pore

size distribution by adopting nonlocal density

functional theory (NLDFT) method. The pore size

distribution of PGS-2-1000 is about 3.89 nm, which is

consistent with the above TEM analyses. Moreover,

the SAA values for the samples synthesized from

different ZnCl2 usage and different calcined

temperatures were in the range of 142.9~438.8 m2 g−1.

It could also be deduced that the SSA value

decreased consistently with increasing ZnCl2 usage

and calcined temperature, so the SSA value

decreased with increasing crystallinity in the PGS

samples. Notable, the SSA for the sample derived

from EG intercalating H3PO4 after annealing is only

53.4 m2 g−1 (Fig. S5B), which is in the middle of EG

and PGS samples. It is further demonstrated that the

process of intercalating H3PO4 could enlarge the

interlayer distance of EG, which is beneficial to the

following intercalation of ZnCl2, resulting in the

formation of PGS.

Figure 4 (A) N2 adsorption−desorption curves and (B) the corresponding pore size plots of the synthetic PGS samples.

Table 1. Adsorption parameters calculated from N2 adsorption

isotherms at –196 °C.

Table 1. Adsorption parameters calculated from N2 adsorption

isotherms at –196 °C.

Samples AAS [m2 g−1][a] Vt [cm3

g−1][b]

Pore size

[nm][c]

PGS-2-900 438.8 0.382 3.90

PGS-2-1000 330.3 0.338 3.89

PGS-2-1100 142.9 0.139 3.89

PGS-1-1000 339.9 0.257 3.88

PGS-3-1000 223.8 0.197 3.86

EG 1.3

[a] AAS calculated from the BET equation. [b] Vt is the

single-point pore volume from adsorption isotherms. [c] Pore

size is the micropore diameter at the maximum of the PSD

curves, calculated according to the adsorption branch of the

isotherm by using the NLDFT method.

To study the formation mechanism of PGS, the

samples synthesized without intercalating of H3PO4

and without modifying by PEG400MO were also

studied, respectively. As the SEM shown in Fig. S6,

both of the synthetic samples are composed of thick

nanosheets, so the intercalating of H3PO4 and

modifying by PEG400MO are essential for the

formation of PGS. The existence of surfactant

PEG400MO could help EG to well disperse in water,

which would ensure the easily contact between EG

and ZnCl2. Based on the above analyses, the

supposed formation mechanism of PGS is

illustrated in Scheme 2. Firstly, H3PO4 molecules

intercalated into the interlayers of EG under

sonication, and then annealed for enlarging the

layer distance of EG (the sample is denoted as

expanded EG), which benefit to the followed

intercalation of ZnCl2 molecules into the interlayer

of EG. Subsequently, the obtained expanded EG

was modified with PEG400MO and then well

dispersed in deionized water. Afterwards, zinc

chloride molecules intercalated into the interlayers

of the modified EG under vacuum assistance,

8

followed by a thermal treatment process in nitrogen

ambient. In this synthesis, the pyrolysis of zinc

chloride in the interlayer of EG could form the

porous structure of PGS, meanwhile the gas

produced during the formation of porous structures

could further exfoliate the EG to 2D graphene-like

nanosheets. Therefore, the zinc chloride used in

here exhibits bi-functional effect, namely not only

plays as the porogent but also plays as the stripping

agent for EG. Finally, the PGS could be obtained

after a hydrochloric acid treatment.

Scheme 2 The supposed formation mechanism of PGS.

Due to the large surface area, porous texture and

high conductivity of porous 2D graphene-like

materials, it has been increasingly used in LIBs as a

flexible confinement structure to provide a

buffering capability for reducing electrode

pulverization [49]. Hence, the electrochemical

properties of the synthetic PGS samples as anodes

for a coin-type half-cell LIBs were investigated.

Cyclic voltammetry (CV) is generally used to

analyze the charge storage behavior. Fig. 5A shows

typical CVs for PGS-2-1000 and EG at a scan rate of

1 mV s−1. The first few cycles of PGS-2-1000 were

shown in Fig. S8. In the cathodic scan of first cycle,

a pronounced reduction peak between 1.5 and 0.6 V

could be observed, which is attributed to the

electrolyte decomposition and concomitant SEI

layer formation. Thus this peak contributes to the

loss of some irreversible lithium storage sites

during the initial discharge process. In the anodic

scan of first cycle, the two broad oxidation peaks

are located at about 0.2 V and 1.2 V, respectively,

which could be ascribed to the lithium extraction

and the delithiation of defective sites within the

PGS [50].

Specific capacity is the intuitive evidence of the

performance for LIBs, and galvanic charge /

discharge test is an effective and direct way for

evaluating the specific capacity. Fig. 5B presents the

discharge/charge voltage profile in 0−3.0 V (vs Li+/Li)

for the 1st cycle at a current rate of 100 mA g−1. The

PGS-2-1000 delivered an initial discharge capacity

of 1146.6 mAh g−1 and a continuous charge capacity

of 832.8 mAh g−1, providing an irreversible capacity

loss of 27.4 % and a low Coulombic efficiency of

72.6 %. The initial irreversible consumption of Li+

ions and the inevitable formation of a solid

electrolyte interface (SEI) layer are responsible for

the first-cycle losses of specific capacity and

Coulombic efficiency [51,52]. The existence of O

atoms from the XPS results and the obvious D-band

peak in Raman spectra of the synthetic PGS sample

indicate the disorder and defects exist in PGS.

However, the reaction of Li with the active defects

during discharge processes would occur at low

voltages, and the break of the relatively strong

bonds of Li with the defects during the charge

processes requires higher voltages, thus resulting in

the large voltage hysteresis was observed [53].

Although this is a problem for the commercial

batteries, it could not be avoided. At a current rate

of 100 mA g−1, the reversible specific capacity values

for PGS-1-1000, PGS-3-1000, PGS-2-900 and

PGS-2-1100 are about 974.1, 555, 581.6 and 832.4

mAh g−1, respectively, which are much lower than

that of the PGS-2-1000 electrode of 1146.6 mAh g−1.

Notable, all the PGS electrodes exhibit a much

larger reversible specific capacity than EG, further

demonstrating the advantages of the special

structures. The rate capability of the PGS electrodes

was tested at various rates of 100~1000 mA g−1 for

each of the 5 cycles, and the obtained results are

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shown in Fig. 5C. It can be observed that the

PGS-2-1000 electrode exhibits a stable capacity of

830.4 mAh g−1 at 100 mA g−1 and still exhibits the

highest reversible capacity of 676.8 mAh g−1 at

current rate of 1000 mA g−1.

Figure 5 Electrochemical performance and structural characterization of PGS electrode for LIBs, (A) CVs of PGS-2-1000 electrode scanned between 0~3.0 V (vs Li+/Li) at a scan rate of 1 mV s−1 room temperature; (B) First charge-discharge curves at 0~3.0V; (C) rate performance at different current densities (mA g−1) (discharge capacity is presented) and (D) Nyquist plots of PGS-2-1000 and original EG electrodes obtained with an amplitude of 5.0 mV in the frequency range from 100 kHz to 10 mHz. The EIS in Figure 5D is tested under an open circuit voltage by two-electrode cell.

Subsequently, the current rate was brought back

to an initial value of 100 mA g−1 at which the specific

capacity reached a value of 726.1 mAh g−1, this

value is about 87.4 % of the stable capacity for the

first 5 cycles at this rate. It is worth mentioning that

the specific capacity of PGS-2-1000 electrode could

rapidly return to the initial value after undergoing a

large charge-discharge current rate, indicating the

excellent rate capability of the sample. The

discharged specific capacities for all the PGS

electrodes have no decay after 50 cycles at current

rate of 100 mA g−1, and the PGS-2-1000 electrode

still keep the largest specific value (Fig. S7),

indicating the excellent performance of PGS-2-1000

electrode. To illustrate the capacity devotion of

different potential window, the charge-discharge

curves with the potential window of 0~1.0 V at a

current density of 100 mA g−1 were also provided as

shown in Fig. S9. It can be seen that the initial

discharge potential is above 2.4 V, which is

consistence with the corresponding value tested in

the potential window of 0~3V (see Fig.5B). Owing

to the initial discharge potential is the nature

characteristic of an electrode material, so it is not

limited by the test potential range. The voltage

plateau at about 0.7 V can be attributed to the

electrolyte decomposition and the solid electrolyte

interface (SEI) film formation on the electrode

surface. In each charge process, a long voltage

plateau at about 0.2 V is attributed to the formation

of LiC6. The PGS-2-1000 exhibits a first discharge

capacity an initial capacity of 1156.9 mAh g−1 and a

continuous charge capacity of 805.7 mAh g−1, which

is similar to the capacity value tested in the

potential range of 0~3.0 V. It is demonstrated that

the potential window in 1~3V for carbon anode

indeed has little contribution to the discharge

capacity of LIB.

To further gain a measure of high rate capabilities

under conditions, where extremely fast electron

transfer and stable formation of SEI should be

essential, the electrochemical performance of the

electrodes was carried out at even higher current

densities of 2000~20000 mA g−1. Notably, PGS-2-1000

electrode exhibits ultrafast lithium storage

properties as illustrated in Fig. 6. Even at a higher

current density of 5000 mA g−1, the PGS-2-1000

electrode still delivers a capacity of as high as 426.4

mAh g−1 after the 50th cycle, which is still higher

than the theoretical capacity of graphite (372 mAh

g−1). Besides, the value is far superior to those of the

reported hollow carbon nanofibers [54-56], hollow

carbon nanospheres [57], graphene nanosheets [58],

amorphous carbon [59] and other carbon-based

anode materials [60-66]. Besides, the Coulombic

efficiency is always 100 % for all the current rates.

At a very high current rate of 20000 mA g−1,

corresponding a charge time of 38 s, the reversible

capacity is still up to 211.6 mAh g−1 for PGS-2-1000

electrode, demonstrating that it is a promising

10

high-energy and high-power electrode material for

LIBs. The reported samples of graphene-like

nanosheets, such as porous graphene and porous

carbon coating on graphene composite [67,68],

exhibit the stabilized capacity of 397 mAh g−1 and

770 mAh g−1 (at a rate of C/5), respectively.

Compared with these samples, the present PGS

derived from expanded EG could inherit the well

conductivity of graphite, which not only could

provide a continuous pathway for electron

transport and Li+ diffusion but also would

effectively stabilize the as-formed SEI films,

resulting in a well Li storage performance.

Figure 6 (A) Rate performance plots of PGS-2-1000 electrode at various current densities from 2000 to 20000 mA g−1, and (B) charge and discharge capacity retention plots and the corresponding Coulombic efficiency of PGS-2-1000 electrode measured at 20000 mA g−1 for 10000 cycles.

Cycling stability is another important criterion for

high rate performances LIBs, so the cycling stability

of the PGS-2-1000 electrode was performed at the

current rate of 20000 mA g−1. The electrode retains

nearly 100 % retention of the initial capacity after

10000 discharge/charge cycles at 20000 mA g−1 (Fig.

6B). Notably, the cyclic stability of Fig. 6B tested by

using the same battery after the test at different

rates, so the first discharge and charge capacity are

nearly the same and an almost 100 % efficiency

could be observed from the first cycle. The superior

performance of the PGS-2-1000 is attributed to the

unique structures, which can be verified by the

followed electrochemical impedance spectroscopy

(EIS) measurements. EIS was used to evaluate the

PGS-2-1000 and EG electrodes after several cycles in

the Nyquist plot. As shown in Fig. 5D, the

semicircle in the medium frequency region is

related to the charge-transfer reaction of Li+ ions

intercalation into electrodes, and the inclined line at

an angle of approximately 45° to the real axis

corresponds to the Li+ diffusion process within the

electrodes [69]. It can be seen that the diameter of

the semicircle in the medium frequency region of

the PGS-2-1000 is much smaller compared to the EG,

indicating a lower charge-transfer resistance in the

PGS-2-1000 electrode, which is attributed to be easy

accessibility and transportation of the electrolyte in

the loose, packed and porous nanosheet

structures.The PGS-2-1000 anode exhibits much

better Li storage performance than that of the

previous studies by using expandable graphite,

natural graphite and graphene as LIBs anodes

[70-72]. The excellent conductivity of PGS-2-1000

can arouse excellent high-rate performance and

superior cycle lifetime.

The excellent electrochemical performance of the

PGS-2-1000 could be attributed to its unique

microstructure has the following advantages: (1) the

porous structure may act as a reservoir for Li+ ions

storage and is good for the easy immersion and

diffusion of the electrolyte, which can shorten the

path lengths with less resistance for both Li+ ions

and electron transport within electrolyte [73]; (2) the

highly conductive of the isolated graphene-like

nanosheets without aggregation not only could

11

provide a continuous pathway for electron

transport and Li+ diffusion but also could effectively

stabilize the as-formed SEI films [74]. All of these

factors are responsible for the excellent Li storage

performance of the PGS-2-1000 electrode.

4 Conclusions

A facile and effective route has been proposed to

synthesize 2D porous graphene-like nanosheets (PGS)

by firstly intercalating H3PO4 for exfoliating EG to

thin nanosheets, and then intercalating ZnCl2 for

realizing the formation of pores and further

exfoliation of EG. When evaluated as an anode

material for LIBs, the synthetic PGS-2-1000 material

presents a remarkable Li storage performance,

capacity retention, rate capability and cyclic lifetime.

The outstanding performance is mainly attributed to

the existences of porous structure and graphene-like

structures, which could shorten the pathways for Li+

diffusion and electron transport, respectively. The

excellent Li storage performance demonstrates the

great potential as a promising high rate anode

material in LIBs. The present method adopted the

low-cost and worldwide EG as the raw material,

which is easy to realize the industrial productivity of

LIBs anode.

Acknowledgements

We gratefully acknowledge the support of this

research by the Key Program Projects of the National

Natural Science Foundation of China (21031001), the

National Natural Science Foundation of China

(21401048, 21371053, 21376065), the China

Postdoctoral Science Foundation (2014M551285), the

Cultivation Fund of the Key Scientific and Technical

Innovation Project, the Ministry of Education of

China (708029), Innovative Research Team in

University (IRT-1237), the Postdoctoral Science

Foundation of Heilongjiang Province (LBH-TZ0519),

the Natural Science Foundation of Heilongjiang

Province (QC2014C007), the Heilongjiang University

Youth Foundation (QL201303).

Electronic Supplementary Material: Supplementary

material such as SEM image and additional data, is

available in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

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15

Electronic Supplementary Material

From graphite to porous graphene-like nanosheets for high rate lithium-ion batteries

Dongdong Zhao, Lei Wang, Peng Yu, Lu Zhao, Chungui Tian, Wei Zhou, Lei Zhang, and Honggang Fu* ()

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang

University, Harbin 150080, P. R. China, E-mail: fuhg@vip.sina.com

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Figure S-1 AFM measurement of PGS-2-1000 with average thickness of about 4.2 nm.

16

Figure S-2 The PGS samples synthesized from different ZnCl2 usage: (A) PGS-1-1000 and (B)

PGS-3-1000.

17

Figure S-3 SEM images of the PGS synthesized from different calcined temperatures: (A)

PGS-2-900 and (B) PGS-2-1100.

18

Table S-1. Raman analystic results for all the compared samples.

Samples D-band (cm-1) G-band (cm-1) 2D-band (cm-1) IG/ID

PGS-2-900 1363.9 1576.1 2742.13 1.9

PGS-2-1000 1361.9 1571.9 2727.1 2.6

PGS-2-1100 1361.2 1568.8 2729.5 3.4

PGS-1-1000 1359.2 1571.8 2731.5 2.8

PGS-3-1000 1359.2 1565.8 2725.5 2.5

EG 1372.1 1564.2 2754.6 16.8

19

Table S-2. Electrical conductivity results for all the compared samples.

Samples Electrical conductivity(S cm-1)

PGS-2-900 204

PGS-2-1000 208

PGS-2-1100 196

PGS-1-1000 167

PGS-3-1000 270

20

Figure S-4 The wide survey XPS spectrum of the PGS-2-1000 sample.

21

Figure S-5 N2 adsorption−desorption curves of (A) the original EG and (B) the sample synthesized

after intercalating of H3PO4.

22

Figure S-6 SEM image of the compared samples synthesized without intercalating of H3PO4 (A)

and without using PEG400MO (B), respectively.

23

Figure S-7 Cyclic stability of all the PGS electrodes at 100 mA g−1 for 50 times. The cyclic stability

tested by using the same battery after the test at different rates from 100 to 1000 mA g−1 as shown in

Fig. 5C, so the first discharge displayed in here is not change obviously compared to the followed

50 cycles.

24

Figure S-8. CVs of the first few cycles for PGS-2-1000 electrode scanned between 0~3.0 V (vs

Li+/Li) at a scan rate of 1 mV s−1 room temperature.

25

Figure S-9 Charge-discharge curves of sample PGS-2-1000 with a potential range of 0~1.0V at a

current density of 100 mA g−1.