Characterization and Electrochemical Behavior of Graphene-based Anode for Li-ion Batteries

In this study, we investigate the characteristics and electrochemical properties of graphene nanosheets derived from chemical-thermal exfoliation processes of SFG44 synthetic graphite (SFG44-GNS). The characterizations and electrochemical measurements were carried out by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry, BET, Raman, rate capability as well as cycling tests and AC impedance. The as-synthesized SFG44-GNS with larger d-spacing of 0.3407 nm exhibits reversible capacity of 626 mAh/g and good rate capability of ~300 mAh/g at 2C rate, which are superior to those of graphite anode. The enhanced electrochemical performance of GNS anode was resulted from larger d-spacing, lower impedance in the interface and enhanced pore volume. The results indicate that graphene-based material is a good candidate for HEV/EV application.


INTRODUCTION
Graphene, one atom thick two dimensional layers of sp 2bonded carbon, has been emerging as a fascinating material with many unusual physical, chemical and mechanical properties.Thus, much attention has been drawn in technology applications, such as solar cells, hydrogen storage, supercapacitors and batteries [1][2][3][4][5].Among these energy storage systems, lithium-ion batteries (LIB) are currently the predominant power sources for portable electronic devices and electrical/hybrid vehicles.Graphite is widely used as an anode material for commercial rechargeable LIBs due to its excellent cycle life.However, lower theoretical capacity of 372 mAh g -1 and poor rate capability limit its application on electrical/hybrid vehicles.Recently, a new kind of carbon material, graphene (or graphene nanosheets, namely GNS) obtained from exfoliation of graphite by chemical method, might be a potential alterative anode materials for LIB applications.Such as Paek et al., [6] and Yao's group [7] reported SnO 2 /graphene composites delivered a reversible capacity of 810 mAh g -1 and 765 mAh g -1 , respectively.Yoo et al., [8] studied the electrochemical performance of graphene/CNT and graphene/C 60 composites, of which the capacities were 730 mAh g -1 and 780 mAh g -1 , respectively.Wang et al., discussed the application of GNS as an anode material, giving a reversible capacity of 650 mAh g -1 in the first cycle [9].Chou et al., demonstrated nanosize silicon/graphene composite anode with 1:1 wt.ratio showed a high capacity of 2158 mAh/g in the first cycle and 1168 mAh g -1 after 30 cycles [10].
Here, we report the rate capability of graphene nanosheets synthesized by modified Hummer's method.Their corresponding crystal structure, surface morphology, cycling tests, as well as AC impedance were discussed in this study.

Samples Preparation
Graphene nanosheet (SFG44-GNS) derived from SFG44 synthetic graphite powders (TIMCAL ) were synthesized by the modified Hummers' method 11 described in detail as follows: 8g graphite powder and 4 g NaNO 3 were put into 560 ml concentrated H 2 SO 4 solution with stirring for 2 hours.Then 24g KMnO 4 was gradually added into the flask with ice bath for 2 hours.The mixture was diluted by 800 ml de-ionized water (DI water).After that, 5% H 2 O 2 was added into the solution until the color of the mixture changed to brown, indicating fully oxidized graphite was obtained.The as-obtained graphite oxide slurry was re-dispersed in DI water.Then, the mixture was washed by 0.1M HCl solution to remove SO 4 2-ions.Finally, the product was washed with DI water to remove the residual acid until the pH was reached to ~7.By cleaning with DI water, filtrating and drying in vacuum for 24 h (the obtaining powders were graphene oxide, namely SFG44-GO).The GO powder was subsequently reduced at 300 o C for 2 hours under a reducing atmosphere of 15%H 2 /85%N 2 , namely SFG44-GNS-300.

Electrodes Preparations
The composite electrode consists of 89 wt.% SFG44-GNS-300, 1 wt.%Super-P (40nm, TIMCAL) as conductive additive and 10 wt.% water-based-binders of SBR and CMC [12].The mixed-slurry was coated on Cu-foil with a final film thickness of ~ 50 μm.CR2032 coin cells were fabricated from the electrode for electrochemical characterizations.The bare SFG44 graphite electrode was prepared in the same way.The counter electrode is lithium foil.The electrolyte is 1M LiPF 6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (1:2 vol.%) from Mitsubishi.All the potentials reported herein are referenced to Li.The current densities refer to masses excluding binder and conductive additives.

Characterizations
Cyclic voltammetry (CV) analysis was performed between 0.05 V to 3.0 V at 5 mV/min.The charge/discharge (C/D) tests were carried out with a constant-current-constant voltage (CC-CV) mode within the voltage range from 0.001 to 3.5 V.The CC process employed a current of 0.1 mA/mg, while the CP process was fixed to 0.001V with a cut-off current of 0.03 mA/mg.The morphologies of the synthesized graphene material were examined by scanning electron microscopy (SEM; LEO1530).Transmission electron microscopy (TEM) analysiswas conducted with a fieldemission transmission electron microscope (FE-TEM, JEOL JEM-2100F).X-ray diffraction (XRD) was carried out on a Mac-Science/MXP diffractometer with CuK radiation.Thermal gravimetrical analysis _TGA_ was performed by using an Ulvac TGD-7000 at a scanning rate of 10°C min -1 .

Thermogravimetric Analysis and Crystal Structure of Graphene-Based Materials
Fig. (1) shows the TGA and its derivative curves of SFG44-GO with a heating rate of 10 o C/min in inert atmosphere.Two apparent weight loss at 33 o C and 216 o C were observed in SFG44-GO, which might be attributed to the desorption of adsorbed water and decomposition of functional groups including hydroxyl and carboxylic groups, respectively.The result indicates that the functional groups derived from oxidative exfoliation process could be almost eliminated by post heating process as long as the temperature is above 300 o C.   2) displays the XRD patterns of bare SFG44, SFG44-GO and SFG44-GNS-300, respectively.Before exfoliation treatment, highly crystalline SFG44 give a sharp diffraction peaks, of which crystallite size exceeding 30 nm determined by Scherrer equation.The d-spacing of SFG44 could be calculated to be 0.335 nm according to (002) reflection.After exfoliation and oxidation by modified Hummer's method, the obtained SFG44-GO showed a typical diffraction peak at 12 o (d 002 ~ 7.4 Å).SFG44G, being reduced with hydrazine, almost showed amorphous nature, of which average d-spacing could be roughly determined as 0.375 nm.; in the meanwhile, 300 o C calcined sample (SFG44-GNS-300) showed a larger d-spacing to 0.3407 nm and amorphous nature.These properties are similar to that of hard carbon, which may good for reversible capacity and rate capability.

Surface Morphologies and Cross-Section Images of Graphene-Based Materials
The SEM micrographs of SFG44 and SFG44-GNS-300 are shown in Fig. (3A, B), respectively.SFG44 graphite is a flake-like structure with very dense nature on the surface.SFG44-GNS-300, on the other hand, displayed a flower-like morphology with porous structure, in which the penetration of both electrolyte and Li-ion would be much easier.The porous structure SFG44-GNS-300 with fewer layers could provide not only additional cites for lithium ions, but also to occupy and facilitate the kinetics of lithium for insertion/extraction. Fig. (4A, B) illustrates the TEM micrographs of SFG44 and SFG44-GNS-300.The d 002 and graphene layers of SFG44 were ~0.332 nm and ~100 layers, respectively.After exfoliation process, d-spacing and thickness of SFG44-GNS-300 were observed to be 0.352 nm ~ 0.511 nm and 10~40 layers, respectively.The inset in Fig. (4B) displays the SEM micrograph giving a soft and flexible nature due to the thickness of graphite could be efficiently reduced by exfoliation process.

Raman and BET Analyses of Graphene-Based Materials
Fig. (5) shows Raman spectra of bare SFG44, SFG44-GO and SFG44-GNS-300.Raman spectroscopy is a direct and non-destructive measurement for characterization of disorder structure of graphitic materials.Two characteristic peaks generally named by D (~1355 cm -1 ) and G (~1580 cm -1 ) bands were observed in graphite-based materials.The G band represents the first order scattering of the E 2g photons observed for sp 2 carbon domain, while the D band corresponds to breathing mode or j-point photons of A 1g symmetry associated with disorder band of structural defect, amorphous carbon or edge.The intensity ratio of D band to G band (I D /I G ) is usually used as a measure of disorder.In comparison to bare graphite, both SFG44-GO and SFG44-GNS-300 showed broader G band accompanying with board D band, indicative of a highly disorder in graphitic structure.It shows that even most functional groups were removed by the reduction process, the sp 3 could not be restored to original sp 2 graphitic structure.In addition, there is no significant decrease in I D /I G ratio for graphene nanosheets after high temperature reduction, even re-crystallization process was observed in XRD patterns.It is indicative of the functional groups derived from oxidation process could be removed but sp 3 dangling bond could not be restored to original sp 2 graphene structure.6A) shows isotherms and pore size distributions of SFG44 and SFG44-GNS-300.The BET surface area of SFG44 graphite was 2.61 m 2 g -1 .By exfoliation process, the BET of SFG44-GNS-300 could be dramatically enhanced by 171 m 2 g -1 .The value of BET for SFG44-GNS-300 is much smaller than theoretical value of graphene of 2620 m 2 g -1 , indicative of SFG44-GNS-300 sample was not fully exfoliated to single layer graphene instead of few-layers graphene.The BJH pore size distributions were shown in Fig. (6B).The main pore size belongs to meso-pore, where the micro pore portion was not dominant.The larger BET and pore volume could not only provide additional cites for Li-ion to occupy, but also facilitate the kinetics of Li intercalation/extraction processes.

Electrochemical Tests of Graphene-Based Materials
Fig. (7A, B) show the CV curves of SFG44 and SFG44-GNS-300 at the first second cycles.The CV curves of SFG44 exhibited a typical sharp reduction/oxidation peaks at ~0.15 V and 0.23 V, respectively.After exfoliation, the electrochemical behavior definitely differed from that of graphite.The reduction/oxidation peaks become much broaden in the first two cycles in Fig. (6B).The lithiumintercalation reactive potential increased from ~0.15 V to ~0.6 V, which indicates the kinetics of lithium and graphene was higher than that of graphite.In order words, the energy barriers of diffusion and reaction of Li/C were lower for graphene anodes compared to graphite.For de-intercalation process, two broad hums peaking between 0.01 V to 3.0 V were observed in Fig. (7B).The higher potential of deintercalation could be responsible for some lithium ions were tightly trapped in the structure defects due to the amorphous nature of graphene.Fig. (8A, B) shows the C/D curves of SFG44 and SFG44-GNS-300 in the first two cycles.The 1 st and 2 nd reversible capacity of SFG44 were 334 mAh g -1 and 327 mAh g -1 , respectively: in the meanwhile, the irreversibility of the 1 st cycle was 21.7%.The discharging plateau (lithiuminsertion) of SFG44 was observed to be ~0.05V,which is graphite-like behavior.On the other hand, SFG44-GNS-300 showed the extremely high reversible capacities, 626 mAh g - 1 and 561 mAh g -1 for the 1 st and 2 nd cycle, and irreversibility of the 1 st cycle is about 51.9%.Yoo et al., demonstrate that the layer of graphene is a function of d-spacing as well as charge capacity [8].In our case, the average d-spacing of assynthesized graphene is ~ 0.375 nm estimated by XRD.According to fitting data of Yoo's group, the reversible charge capacity of as-synthesized graphene is around 600 mAh g -1 , which is in consistent with our results.Nevertheless, the large irreversible capacity loss of graphene-based anode in the first cycle was also reported by many groups in the range of 50~80%.The possible reasons could be ascribed to (i) larger specific surface area; (ii) structure defects; (iii) function groups and (iv) SEI (solid electrolyte interface).The reducing of irreversible capacity loss for graphene-based anode materials would be the most important issue for their application on LIBs.In the present results, the irreversible capacity loss of graphene was mainly attributed to higher surface area, structure defects and SEI.Which is the main factor is worthy to discuss in the future work.Fig. (9) shows the cycling tests of SFG44 and SFG44-GNS-300 for 30 cycles with charging/discharge currents of 0.2C.SFG44 graphite gived a reversible capacity of ~334 mAh g -1 with fair cyclability.The SFG44-GNS-300 sample, on the other hand, exhibited ~600 mAh g -1 in the first cycyle and retained 280 mAh g -1 after 30 cycles.The inset of Fig. (9) displays the charge capacity of SFG-44 and SFG44-GNS-300 with different C rates.With 2C discharge, the capacity of SFG44-GNS-300 is as high as 300 mAh g -1 (~ 50% of 0.2C).The capacity of SFG44 graphite is only 75 mAh g -1 at 2C (~22% of 0.2C).The results surprisingly showed that the SFG44-GNS-300 exhibited better rate capability than that of graphite sample.The impedance data of these two samples in discharging state of 0.01V at the 4 th cycle are shown in Fig. (10).The result future demonstrated that the charge transfer resistance of SFG44-GNS-300 was lower than that of SFG44 graphite, which indicates that the good rate capability could be explained by the lower impedance for SFG44-GNS-300.Thus, the graphene-based anode materials might be good anode materials for EV or HEV applications due to their good performance with high rate charge/discharge tests.

CONCLUSIONS
In summary, SFG44-GNS-300 is synthesized by modified Hummers' method.The relationship between electrochemical performance, such as reversible capacity, rate capability and irreversible capacity, and structure characterization of BET, pore distribution and surface morphology, are discussed by means of charge/discharge tests, rate capability tests, BET, as well as Raman spectroscopy and XRD measurements.These corresponding results could be the reference for structure design of graphene-based anode for the application of Li-ion batteries in the future.

Fig. (
Fig. (2) displays the XRD patterns of bare SFG44, SFG44-GO and SFG44-GNS-300, respectively.Before exfoliation treatment, highly crystalline SFG44 give a sharp diffraction peaks, of which crystallite size exceeding 30 nm determined by Scherrer equation.The d-spacing of SFG44 could be calculated to be 0.335 nm according to (002) reflection.After exfoliation and oxidation by modified Hummer's method, the obtained SFG44-GO showed a typical diffraction peak at 12 o (d 002 ~ 7.4 Å).SFG44G, being reduced with hydrazine, almost showed amorphous nature, of which average d-spacing could be roughly determined as 0.375 nm.; in the meanwhile, 300 o C calcined sample (SFG44-GNS-300) showed a larger d-spacing to 0.3407 nm and amorphous nature.These properties are similar to that of hard carbon, which may good for reversible capacity and rate capability.

Fig. (
Fig.(6A)shows isotherms and pore size distributions of SFG44 and SFG44-GNS-300.The BET surface area of SFG44 graphite was 2.61 m 2 g -1 .By exfoliation process, the BET of SFG44-GNS-300 could be dramatically enhanced by 171 m 2 g -1 .The value of BET for SFG44-GNS-300 is much smaller than theoretical value of graphene of 2620 m 2 g -1 , indicative of SFG44-GNS-300 sample was not fully exfoliated to single layer graphene instead of few-layers graphene.The BJH pore size distributions were shown in Fig.(6B).The main pore size belongs to meso-pore, where the micro pore portion was not dominant.The larger BET and pore volume could not only provide additional cites for Li-ion to occupy, but also facilitate the kinetics of Li intercalation/extraction processes.