Author’s Accepted Manuscript

Employing synergistic interactions between few-layer WS2 and reduced graphene oxide to improvelithium storage, cyclability and rate capability of Li-ion batteries

Konda Shiva, H.S.S. Ramakrishna Matte, H.B.Rajendra, Aninda J. Bhattacharyya, C.N.R. Rao

PII: S2211-2855(13)00029-3DOI: http://dx.doi.org/10.1016/j.nanoen.2013.02.001Reference: NANOEN178

To appear in: Nano Energy

Received date: 23 January 2013Revised date: 7 February 2013Accepted date: 11 February 2013

Cite this article as: Konda Shiva, H.S.S. Ramakrishna Matte, H.B. Rajendra, Aninda J.Bhattacharyya and C.N.R. Rao, Employing synergistic interactions between few-layerWS2 and reduced graphene oxide to improve lithium storage, cyclability and ratecapability of Li-ion batteries, Nano Energy, http://dx.doi.org/10.1016/j.na-noen.2013.02.001

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Employing synergistic interactions between few-layer WS2 and reduced

graphene oxide to improve lithium storage, cyclability and rate capability of

Li-ion batteries

Konda Shivaa, H. S. S. Ramakrishna Matteb, H. B. Rajendraa, Aninda J. Bhattacharyyaa,∗, C.N.R.

Raoa,b

[*] a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India E-mail: aninda_jb@sscu.iisc.ernet.in; Fax: +91 8023601310; Tel: +91 8022932616 b Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India Abstract

The aim of the contribution is to introduce a high performance anode alternative to graphite for

lithium-ion batteries (LiBs). A simple process was employed to synthesize uniform graphene-

like few-layer tungsten sulfide (WS2) supported on reduced graphene oxide (RGO) through a

hydrothermal synthesis route. The WS2-RGO (80:20 and 70:30) composites exhibited good

enhanced electrochemical performance and excellent rate capability performance when used as

anode materials for lithium-ion batteries. The specific capacity of the WS2-RGO composite

delivered a capacity of 400-450 mAh g-1 after 50 cycles when cycled at a current density of 100

mA g-1. At 4000 mA g-1, the composites showed a stable capacity of approximately 180-240

mAh g-1 respectively. The noteworthy electrochemical performance of the composite is not

additive, rather it is synergistic in the sense that the electrochemical performance is much

superior compared to both WS2 and RGO. As the observed lithiation/delithiation for WS2-RGO

is at a voltage ≈ 1.0 V (≈ 0.1 V for graphite, Li+/Li), the lithium-ion battery with WS2-RGO is

∗ Fax: (+91)8023601310, E-mail: aninda_jb@sscu.iisc.ernet.in

expected to possess high interface stability, safety and management of electrical energy is

expected to be more efficient and economic.

Keywords: chalcogenides, reduced graphene oxide, lithium-ion batteries, cyclability and rate

capability, percolation

1. Introduction

Recent research has demonstrated several transition metal oxides and lithium-binary alloys (e.g.

LixSi, LixSn) among some of the notable alternatives to graphite for LiBs. Exceptionally high

reversible lithium storage (≥ 1000 mAh g-1 as opposed to 372 mAh g-1 for graphite)[1] at ≈ 0.5-

1.5 V is obtained via conventional intercalation and alternative storage mechanisms. However,

these materials display poor intrinsic physical properties predominantly with regard to ion

transport and structural stability. Some are poor electronic conductors as well. Improvement in

electrochemical performance is achieved via complex synthesis procedures involving

optimization of active particle chemical composition, size, morphology and usage of suitable

amounts of electronic conducting materials e.g. carbon.[2-7] The carbon inclusions are expected

to act as electronic conduits and make electron transport efficient without interfering into the

electrochemical storage mechanism of the active material. Recent research has also focused on

layered transition metal dichalcogenides e.g. MoS2 mainly due to their higher intrinsic fast ionic

conductivity[8] (than oxides) and higher theoretical capacity (than graphite).[9] The underlying

crystal structure of transition metal chalcogenides (MoS2, WS2) is composed of three stacked

atom layers (S-Mo-S) held together by van der Waals forces and allows easy Li+ ion insertion/de-

insertion.[10-14] However, the electronic conductivity of MoS2 is still lower compared to

graphite/graphene and this in principle could limit the operation of pure dichalcogenides at

higher currents. This detrimental intrinsic property can be preferably reduced to a great extent or

even nullified via incorporation of suitable amounts of electronic conducting materials such as

extended carbon structures[15, 16] e.g. graphene, carbon nanotubes. In the context of

dichalcogenides too, several reports[10, 17-20] have showed that incorporation of extended

carbon structures in MoS2 can result in substantial improvement in the cycling stability and rate

capabilities of the material. We therefore considered it worthwhile to examine whether the

electrochemical activity and reversible lithium specific capacity could be enhanced for graphene-

like few layer tungsten sulfide, WS2.[21-25] The few-layer WS2-reduced graphene oxide (RGO)

composite was synthesized using a simple solution-phase method. We report here the reversible

lithium storage into few-layer (4-6 layers) WS2 supported on reduced graphene oxide at widely

varying currents (0.1-4 Ag-1). To our knowledge, there has been no reports to-date on WS2 sheets

supported on graphene as the anode material for LiBs. As will be shown, the electrochemical

performance of the WS2-RGO composite can be improved only using amounts of RGO

(equivalent to a threshold concentration) which optimizes the electrochemical storage in WS2 to

the maximum.

2. Experimental: Materials and Methods 2.1 Synthesis of WS2-RGO composite: Graphite oxide (GO) was synthesized by oxidizing natural

graphite powder using modified Hummers method.[26] The GO prepared as such was dispersed

in deionized water and then exfoliated by ultrasonication to get graphene oxide sheets. Few-

layers of WS2 are prepared by heating tungstic acid with an excess of thiourea (1:48) at 773 K

under nitrogen atmosphere for 3 h (solid state synthesis). After 3 h, the sample was cooled down

to room temperature under nitrogen atmosphere. The black product obtained was used as such

for further analysis.[13] RGO and WS2 in requisite amounts (WS2: RGO = 80:20, 70:30) were

dispersed in 5 ml of water. The dispersions were sonicated for 30 mins and then refluxed with 3

ml of hydrazine hydrate for 12 h. The product obtained was washed with ethanol and dried under

vacuum at 60 °C.

2.2 Structural characterization: X-ray diffraction (XRD) patterns (recorded using Bruker D8

Discover diffractometer, Cu-Kα radiation) were used to determine the phase composition and

crystallinity. The composition and microstructure of the samples were also investigated by

Raman spectroscopy (Jobin Yvon LabRam HR; 514.5 nm Ar laser) and HRTEM (TEM; JEOL

JEM 3010; operated at accelerating voltage 300 kV) measurements.

2.3 Electrochemical characterization: Electrochemical stability and lithium battery performance

studies were done using in laboratory SwagelokTM half-cells with lithium foil (Aldrich) as a

counter and reference electrode, Whatman glass fiber as separator and 1 M LiPF6 in ethylene

carbonate (EC, Aldrich) and dimethyl carbonate (DMC, Aldrich) (1:1 w/w) as electrolyte. For

electrochemical measurements, slurry of active material (WS2, WS2-RGO) was prepared with

super P carbon black (Alfa Aesar) and polyvinylidene fluoride (PVDF, Kynarflex) in a weight

ratio of 70:20:10, 85:05:10 and 90:00:10 for bare WS2, 80:20 and 70:30 respectively, in N-

methyl-pyrrolidone to form a homogeneous slurry. This was then cast on a copper foil and dried

in vacuum at 120 °C for 12 h. The cyclic voltammograms (CH Instruments, CH608C; voltage

range= (0.01-3) V; scan rate= 0.25 mV s-1) and galvanostatic charge/discharge cycling (Arbin

Instruments, MSTAT) were performed at different current rates (0.1-4 Ag-1) in the voltage range

of 0.01 and 3 V (versus Li+/Li). All cell assembly was done at 25 °C in a glovebox (MBraun)

under argon (H2O: < 0.1 ppm).

3. Results and Discussion

Figure 1a shows the XRD patterns of bare WS2 and WS2-RGO composites. The reflections

observed for WS2 (2θ = 13.8º, 33.4º, 59.4º, 69.4º), WS2:RGO= 80:20 (2θ = 13.7º, 32.8º, 58.5º)

and WS2:RGO= 70:30 (13.7º, 32.8º) can be indexed on the hexagonal WS2 structure. The

presence of (002), (100) and (110) reflections strongly suggests a few-layered structure for

WS2.[27] Both 80:20 and 70:30 WS2-RGO composites contain reflections, also suggesting the

presence of few layers of WS2 and RGO. As observed in Figure 1a, the bare WS2 exhibits higher

degree of crystallinity than the composites, the crystallinity decreasing with increasing RGO

concentration.

Micro Raman spectroscopy was carried out to confirm formation of the composites. Figure 1b

shows the Raman spectra of few layer bare WS2 and WS2-RGO composites. The characteristic

bands of WS2 observed at 350 cm-1 and 415 cm-1 correspond to the E2g and A1g modes

respectively. The Raman spectrum of RGO sheets show two major bands i.e. G-band at 1585 cm-

1 corresponding to the E2g phonon of C sp2 atoms and D-band at 1350 cm-1 assigned to defects in

graphitic carbons. Figures 1b(c) and 1b (d) show the composite peaks at 1345 cm-1 and 1580 cm-

1, which are assigned to the D and G peaks of the graphene and 350 cm-1 and 415 cm-1

corresponds to the WS2 respectively, thus confirming the presence of the graphene and WS2 in

the composite and complete correspondence with the findings from the XRD diffraction studies.

The shift in the D and G bands may be attributed to several reasons. It can be either due to the

charge-transfer interactions between RGO and WS2 or it can be due to different extent of

reduction of GO during the synthesis. We do not want to speculate on this matter any further as

this requires additional studies aimed at studying at probing the microstructure. Detailed

microstructural study (including Raman) is out of scope for this communication paper and will

be reported in the near future in a detailed article.

Morphology of RGO, bare WS2 and WS2-RGO composites were investigated by transmission

electron microscopy (TEM). Typical morphology of RGO is characterized by TEM. Figure 2a

presents the representative TEM image of RGO sheets, showing the layered platelets. Figure 2b

shows a representative high magnification TEM image of bare WS2. The d-spacing between two

WS2 nanosheets estimated from the micrograph is ≈ 0.69 nm. Figures 2c, 2d, 2e and 2f show the

TEM images of the WS2-RGO composites. The images reveal a general trend with the sheets of

WS2 homogeneously embedded in RGO (Figure 2c). In the magnified TEM image of WS2-

graphene composites (Figure 2d), the WS2 (black stripes) layers are visible at micron length

scales and appear to be in intimate contact with the graphene layers. The interlayer spacing

between the WS2 sheets in the composite was estimated to be 0.69 nm (Figures 2e and f) which

correspond to the d-spacing for bare WS2. This confirms the retention of the intrinsic WS2

structure in the composite.

We now discuss the investigations with regard to the electrochemical properties of bare WS2 and

WS2-RGO composites. Figures 3a-c show the cyclic voltammograms of bare WS2 and WS2-

RGO samples for the first 5 cycles. In the 1st cycle for bare WS2, a reduction peak at 1.28 V and

oxidation peak at 2.43 V is attributed to the lithium intercalation/deintercalation according to the

reaction: WS2 + xLi+ + xe- ↔ LixWS2. The reduction peak at 0.49 V is ascribed to the formation

of Li2S resulting from the conversion reaction (WS2 + 4Li+ + 4e- → W + 2Li2S)[18, 24] and

accompanying decomposition of nonaqueous electrolyte. From 2nd cycle onwards the reduction

peak at 0.49 V disappears while new reduction peak appear in the potential range from ≈ 1.6–2.2

V. This change can be accounted by the formation of a gel-like polymer layer formed out of the

dissolution of the Li2S in the electrolyte.[18] It is observed that the intensities of the peaks (say at

2.43 V) decrease remarkably with increasing cycle numbers. Similar reduction and oxidation

peaks were also observed with regard to WS2-RGO composites however, their variation with

cycle number was different. The cathodic and anodic peaks of the composites in general show a

much more stable profile and tend to overlap each other in the first five cycles. Figure 3d shows

the first charge and discharge curves of the bare WS2 and WS2-RGO composites at a current

density of 100 mA g-1. The charge/discharge profiles confirm the findings from cyclic

voltammetry. All samples exhibited two potential plateaus in the charge/discharge profiles. Bare

WS2 displayed potential plateaus at ≈ 1.1 V and 0.54 V in the 1st discharge (lithiation) cycle. For

the 80:20 composite plateaus were observed at ≈ 1.5 V and 0.83 V while for the 70:30 composite

potential plateaus were observed at ≈ 1.6 and 0.82 V. Plateaus at 2.4 V and 2.3 V were observed

in the 1st charge (delithiation) cycle for the bare WS2 and composites respectively. In the

subsequent cycles, the potential plateaus at 1.3 and 2.2 V (80:20) and 1.96 V (70:30) were also

observed.

The composites display high specific capacity and most importantly exceptional stability

compared to the bare WS2. Figure 4a shows the discharge specific capacities of bare WS2, RGO,

80:20 and 70:30 at a current density of 100 mA g-1 (voltage range = (0.01- 3) V) and Coulombic

efficiencies of bare WS2, 80:20 and 70:30 for the first 50 cycles. In the 1st cycle, the RGO, bare

WS2, 80:20 and 70:30 electrodes delivered a discharge capacity of approximately equal to 756

mAh g-1, 1149 mAh g-1, 1034 mAh g-1 and 774 mAh g-1 respectively. In the 2nd cycle, the

discharge capacities decreased to 434, 544, 552 and 457 mAh g-1 which are 57%, 47%, 53% and

59% respectively of the 1st cycle capacities. After 50 cycles, the discharge capacity of RGO and

bare WS2 electrode decreased significantly and is equal to 280 and 278 mAh g-1, respectively.

On the contrary, the discharge capacity of 80:20 and 70:30 electrodes delivered as high as 451

and 408 mAh g-1 (82% and 89% with respect to the 2nd cycle), indicating excellent

electrochemical performance. The Coulombic efficiencies of the bare WS2, 80:20 and 70:30

electrodes are initially low with efficiencies being 45%, 48% and 54% respectively. From the 2nd

cycle, the efficiencies for 80:20 and 70:30 electrodes become stable (87% for 80:20 and 84% for

70:30) which is higher than for bare WS2 electrode (82%). It is important to note here that

dispersion of higher amounts of RGO does not necessarily lead to enhancement in reversible

lithium storage and strongly suggests that the enhancement is not a mere additive effect. The

enhancement may be synergistic in nature arising from the electronic interactions involving RGO

and WS2. However, conclusive evidence is required based on combined structural and

electrochemical studies. These will be taken up in future.

From point of view of applicability of lithium-ion batteries as power storage devices, a necessary

criterion of electrode materials is to sustain very high current rates. We now discuss the rate

capability (Figure 4b) of bare WS2 and WS2-RGO which is a necessary criterion of an electrode

material to sustain very high current rates. High reversible capacity and excellent cycling

behavior were observed only for the WS2-RGO composites. Reduced graphene oxide plays a

very important role in tremendously enhancing the rate capability of the WS2-RGO composites.

The WS2-RGO was tested in a wide range of discharge current densities: 0.1, 0.3, 0.5, 0.7, 1, 2,

and 4 A g-1. The specific capacities of 80:20 and 70:30 are 1034 (0.1 Ag-1), 480 (0.3 Ag-1), 414

(0.5 Ag-1), 375 (0.7 Ag-1), 349 (1 Ag-1), 339 (2 Ag-1) and 179 mAh g-1 (4 Ag-1) and 774 (0.1 Ag-

1), 393 (0.3 Ag-1), 347 (0.5 Ag-1), 324 (0.7 Ag-1), 294 (1 Ag-1), 277 (2 Ag-1) and 236 mAh g-1 (4

Ag1) respectively. The discharge capacity of bare WS2 electrode decreases significantly with

increasing current rates. The capacities of bare WS2 are 1035 (0.1 Ag-1), 368 (0.3 Ag-1), 329 (0.5

Ag-1), 99 (0.7 Ag-1), 45 (1 Ag-1) and 8 mAh g-1 (2 Ag-1) respectively. The capacity of bare WS2

fades rapidly and cannot recover to initial specific capacity values (Figure 4b highlighted in box)

when reverting back to lower charge/discharge currents following cycling at high current rates.

On the contrary, when the testing currents are regularly returned to a lower rate, say 0.1 Ag-1, the

discharge capacities for 70:30 and 80:20 electrodes could be recovered to approximately the

initial capacity values. The specific capacity of 80:20 composite is higher than the 70:30

composite at all currents except at the highest current (≈ 4 Ag-1). At 4 Ag-1 the specific capacity

for the 70:30 is slightly higher compared to the 80:20 composite samples. This may be attributed

to efficient electron transport leading to higher specific capacity in WS2-RGO (70:30) compared

to the WS2-RGO (80:20) sample over a few cycles. The anomaly could be due to various reasons

including in situ (during battery operation) transformation of the extended carbon structures from

one form to another. This has been recently highlighted in ref [10], where in situ conversion of

multi walled carbon nanotubes to graphitic carbon resulted in superior cyclability and rate

capability of lithium iron phosphate (LiFePO4). The stable cyclability and rate capability of the

WS2-RGO composites are due to several reasons. As RGO is a very good electronic conductor,

it’s presence along with WS2 would result in a percolating particle network which will make

electron transport efficient and fast. Due to close proximity of the RGO to WS2 in the

composites, the electrons of the S atom layer and the π-electrons from the graphene layer have a

high probability to overlap to form an electron rich cloud.[19] The high concentration of

electrons between the WS2 and RGO layer will greatly enhance the electron transport in the

composites during lithium ion intercalation/de-intercalation processes. This is expected to take

place for RGO concentrations at or below a threshold concentration. Increase in RGO

concentration above the threshold, which as per the synthesis protocol employed in the present

work can be approximated to be equal to 20%, does not necessarily lead to improvements in the

electrochemical performance of the WS2-RGO composites. This is especially observed for the

composite with 30% where the rate performance is in general lower compared to the sample with

20% RGO. RGO exceeding ≈ 20 % act as resistance to electron movement leading to decrease in

energy storage and rate capability. Introduction of RGO has an added advantage. Presence of

RGO leads to increase in disorder in the WS2 structure leading to increase in various types of

defects. This is confirmed by lower composite crystallinity compared to the bare WS2 (c/f

Figure 1). The increase in defect concentration provides extra lithium storage sites during

lithiation/delithiation processes leading to enhanced specific capacities in the case of the

composites. Besides this, the RGO stabilizes the intrinsic WS2 structure from unfavorable

reorganizations that may occur during successive cycles. However, here too the presence of

RGO in excess to percolation threshold may lead to trapping of lithium in defect sites and as a

result lead to lowering of capacities. The two effects coupled together will not only enhance the

rate capability of the composite but may also aid increased lithium storage and facile transport.

4. Conclusions

In conclusion, we have convincingly demonstrated few-layer WS2 to be a viable alternative

anode material for use in lithium-ion batteries. WS2 layers supported on requisite amount of

reduced graphene oxide showed excellent cyclability and rate capability. Additional optimization

in composite electrode components and content would lead to further improvement in storage

capacity and rate performance. It is expected that the approach employed here can be utilized to

optimize other materials properties and can also aid in search of better materials for battery and

other electrochemical devices such as solar cells.

Acknowledgements

The authors thank for JNCASR for TEM and Raman spectroscopy facilities. Konda Shiva

acknowledges SSCU, IISc, CSIR for SRF. AJB thanks Department of Science and Technology

(DST)-Nano Mission and Ministry of Communication and Information Technology (MCIT,

Centre of Excellence in Nanoelectronics, Phase II grant) of Government of India for financial

support.

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