Synthesis and Electrochemical Performance of

Graphene/Metal Oxide Nanocomposite

Electrodes in Lithium Secondary Batteries and

Supercapacitors

Kwang SunRyu University ofUIsan/Chemistry, Uisan, Korea

ryuks@ulsan.ac.kr

Abstract- A graphenelNiO and graphenelNiO-Mn02

nano-composites were prepared by simple chemical

precipitation followed by thermal annealing and using a

chelating agent and Ni and Mn hydroxides on graphene.

The graphene/NiO-MnOz was pretreated by

ultrasonication followed by thermal annealing at 300°C for 2 h. Graphene/MoOz-MoS2 composites were

prepared by a hydrothermal reaction of

molybdenum(IV) oxide with sodium sulfide and

subsequent ball-milling with graphene. The results of

XRD, FE-SEM, and FE-TEM analyses confirmed the

presence of NiO, Mn02, Mo02-MoS2 nanoparticles on

the graphene surface. In the properties of anodes in

lithium-ion batteries, the discharge capacities of

graphene, graphenelNiO (59 wt.%) are about 302 and

856 mAh g-1 at 5000 rnA g-1 (5 C), respectively. The cells

containing 59 wt.% NiO show the best performance, and

the graphene nanocomposite materials have high rate

properties that are comparable to some of the good

results reported in the literature using NiO. The mole

ratios of Mo02 and MoS2 in the synthesized Mo02-MoS2

powder were found to be approximately 62.3:37.7. The

graphene/Mo02-MoS2 composite anodes showed a high

discharge capacity (-974 mAh g-l) after 20 cycles and

superior high-rate capability. In the properties of

electrode in supercapacitor, the maximum specific

capacitance of the graphenelNiO-Mn02 electrode was

242.15 F g -1 by cyclic voltammetry at a scan rate of 0.2 m V s -l, which was significantly higher than that of a

graphene electrode.

1. INTRODUCTION

Lithium-ion batteries (LIB) have attracted special attention in the scientific and industrial fields due to their high electromotive force and high energy density. For the anode material in LIB, graphite is usually employed as a standard electrode because it can be reversibly charged and discharged under intercalation potentials with reasonable specific capacity [1, 2]. However, to meet the increasing demand for batteries with higher energy density, much research has explored new electrode materials and designed novel nanostructures of electrode materials [3].

Supercapacitors have attracted great interest since they combine the advantages of the high specific

power of dielectric capacitors and the high specific energy of rechargeable batteries. The two types of supercapacitors are electric double-layer capacitors (EDLC) and pseudocapacitors. EDLCs have a fast charge/discharge ability and a long cycle life but a lower capacitance than pseudocapacitors. The fast faradic reactions of a pseudocapacitor take place at the electrode materials.

Graphene has been found to exist as a free­standing form exhibiting many intriguing physical properties [4, 5]. Its advantages include its high surface area, electrical conductivity, high flexibility, mechanical strength, and lightweight properties. Graphene is an ideal single-atom thick substrate for the growth of functional nanomaterials to render them electrochemically active and electrically conductive to outside current collectors.

Reports on graphene composites with Si [6], Ti02 [7], C0304 [8], Fe304 [9], Sn02 [10], CU20 [11], and CuO [12] claim that a uniform distribution of metal oxide on graphene sheets can eliminate restacking of the sheets during the synthesis and that it stabilizes the volume changes in the metal/metal oxide during charge-discharge cycling. However, the performance at high current densities such as 100-5000 mA/g has not been investigated. Mo02 has been widely studied for use as an anode material in lithium-ion batteries due to its low electrical resistivity, high chemical stability, and high electrochemical activity. Recently, carbon-coated Mo02 [13] or Mo02 composite with graphene have also been studied so as to examine the effects of graphene incorporation into the nanostructure on the electrochemical performance. A small amount of metal oxide is dispersed uniformly over a conducting and porous carbonaceous material with a very high surface area due to the increased electrical conductivity. Through electrochemical utilization of the metal oxide and ionic transport throughout the internal volume of the electrode, the electrochemical properties are increased.

Tn this paper, we report a simple strategy for synthesizing such NiO composites anchored on conducting graphene as an advanced anode material for high performance LIB. This grapheneiNiO (59 wt.%) nanocomposite exhibits superior LIB

perfonnance with large reversible capacity, high coulombic efficiency, good cyclic perfonnance, and excellent rate capability, highlighting the importance of the anchoring of nanoplate structure NiO on graphene sheets. The nanostructured anode material consisting of MoOrMoSz/graphene is prepared by a facile hydrothermal reaction of MoOrMoSz and subsequent blending with graphene. The effects of graphene in the composite are investigated in terms of the anode properties. We employed a simple chemical precipitation method to prepare morphologically uniform graphenelNiO-MnOz composites and characterized their electrochemical behaviors in supercapacitor. The capacitive properties of the composites were investigated by cyclic voltammetry and a galvanostatic charge/discharge method.

11. EXPERIMENTAL Graphene was purchased from N-baroTech (specific

surface area of about 1800 mZ g.l) and then prepared with the Hummers method. The graphenelNiO composite was prepared by simple chemical precipitation followed by thermal armealing. The graphene was pretreated by ultrasonication for 1 h in 70 ml of distilled water (32 ml) and ethanol (48 ml) mixed solution. Urea (3M or 6M) was added to the graphene suspension. Then graphene treated with urea was dried for 24 h at 60 °C. Ni(N03)z·6HzO (0.1 M) was dissolved in 200 ml of a mixed solution of propanol and distilled water. The dried graphene/urea (3M or 6M) powder was addition into the nikel salt

solution at 95°C. Graphene can be considered as a macromolecule that contains epoxyl and hydroxyl moieties on the basal plane and a carboxylic acid group on the edge sites. When graphene and Ni

z+

cations were mixed, the N?+ cations were attracted and anchored to those functional groups. The precipitated Ni hydroxides were fonned on the graphene composite, collected by centrifugation, and washed several times with distilled water and ethanol.

For the graph ene/ MoOrMoSz, 0.72 g of molybdenum (IV) oxide (99.5%, Aldrich) and 3.6 g of sodium sulfide non-hydrate (> 98%, Aldrich) were added to 200 mL of a 0.4 M HCI (35%, Daejung Chemicals) aqueous solution. The mixed solution was moved to a titanium-taped autoclave and then hydrothermally reacted at 225°C for 12 h. The resulting black precipitates were washed with deionized water several times and dried at 80°C to yield MoOrMoSz powder. Graphene powder was then blended with the MoOrMoSz powder in a 2:5 weight ratio and ball-milled in a zirconium jar (50 mL) at a speed of 100 rpm for 2 h to give a MoOr MoSz/graphene composite powder.

The graphenelNiO-MnOz composite was prepared by simple chemical precipitation followed by thermal annealing. The graphene (0.27 g) was pretreated by ultrasonication for 4 h in 200 ml of a distilled water solution. A stoichiometric amount of Ni and Mn sulfate salts (atomic ratio of Ni:Mn = 1:1) was dissolved in distilled water at a concentration of 0.04 M. An ethylenediamine solution (0.008 M) was added

to the suspension, which was maintained at 60°C for 6 h. Then, a sodium hydroxide solution (0.08 M) was added to the suspension dropwise under vigorous magnetic stirring until the pH reached 11. Precipitated Ni and Mn hydroxides were formed on the graphene composite, collected by centrifugation, and washed several times with distilled water and ethanol.

The electrodes were prepared by casting and pressing a mixture of 80 wt% of the obtained material, 10 wt% of Solef 5130 binder (polyvinylidene difluoride: PVDF), and 10 wt% of carbon black (Super P) in N-methyl pyrrolidinone (NMP) solvent on copper foil followed by drying for 24 h at 70°C. The coin­type cells (CR 2032) were made in an argon-filled glove box with a 1.15 M LiPF6 electrolyte in an EC­DEC-EMC (3:2:5 volume ratio) solution. The negative electrode and the lithium metal electrode were separated from a polypropylene (PP) micro-porous film. A viscous slurry was prepared by mixing the MoOrMoSz/graphene powder (70 wt.%) as an active material, carbon black (Super P, Timcal, 20 wt.%) as a conductive agent, and poly(vinylidene fluoride) (Aldrich, 10 wt.%) as a polymer binder in NMP.

The dried mixture was converted to a rubber-like paste by pressing with isopropyl alcohol for using as the electrode in supercapacitor. The electrode was used as both the cathode and anode was pressed onto a nickel mesh current collector (2 x 2 cm) and wound with a separator. The supercapacitor cell was assembled by superimposing the separator/cathode/separator/anode/ separator sheets with a 6 M KOH electrolyte and vacuum sealing within an aluminum pouch for.

X-ray diffraction (XRD, Rigaku ultra-X, Rigaku Co., CuKu radiation, 40 kV, 120 rnA) patterns were measured at a step scan rate of 0.02° sec-1 in the 28 range (10-80°) to identity the crystalline phase of samples. The morphologies were observed by field emission-scanning electron microscopy (FE-SEM, Supra 40, Carl Zeiss Co., Ltd.) and field emission transmission electron microscopy (FE-TEM, JEM 2100F, JEOL Ltd., 200 kV). The cells were charged and discharged galvanostatically using a battery tester (WonA Tech) for lithium secondary batteries with a potential range between 0.01 and 3.0 V (graphene/NiO) and in the range of 3-0.01 V at a current rate of 100 rnA g-l(graphenelMoOrMoSz). Galvanostatic charge/discharge cycling in the potential range from 0 V to 1.0 V for supercapacitor was perfonned at a constant current density of 50 rnA g-l.

TTT. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffraction patterns of

graph ene, graphene/Ni(OH)z, graphene/NiO, MoOr MoSz, graphene/MoOrMoSz, and grapheneINiO­MnOz. Both graphene and the graphenelNiO composites show the typical 2 8 peak for graphene at about 24° (002), corresponding to d-spacing of about 0.37 llill. The formation of a-Ni(OH)z·xHzO hexagonal nanoplates structure from nickel nitrate precursor are con finned by XRD peaks as shown in Fig. 1. These

XRD patterns exhibit the characteristic peaks of NiO -

(space group R3m) rock salt as shown in Fig. 1, such as 2 e = 37.1 (111), 43.10 (200), and 62.9° (220), which are the characteristic peaks of graphene and NiO. The graphene exhibits amorphous phase except for a C(002) reflection, whereas the MoOz and MoSz show diffraction peaks that are characteristic of their respective crystal structures in the MoOrMoSz composite powder. In particular, the MoOz has a monoclinic crystal structure with space group P2/c and thus, reflections from the (110), (020), (220), (031), and (231) planes are observed, according to the JCPDS no. 65-5787 reference spectrum [6]. Meanwhile, MoSz shows good crystallinity with peaks corresponding to (002) and (100) planes according to the JCPDS no. 37-1492 reference spectrum. Fig. 1 shows two characteristic peaks at 37.1 ° and 66.3° indicating the presence of MnOz. It can be seen that these MnOz peaks are broad and unclear, which indicates the amorphous nature of the products. The XRD patterns for MnOz correspond to crystalline u­MnOz and are in accordance with the standard spectrum (JCPDS, Card No. 44-0141). These XRD patterns exhibit the characteristic peaks of NiO rock salt at 2 e = 36.7 (111), 43.8 (200), and 63.3° (220), which are in accordance with the standard spectrum (JCPDS Card No. 4-0835).

Fig. 2 shows the FE-SEM and FE-TEM images of grapheneiNiO, graphene, MoOz-MoSz, and graphenel NiO-MnOz. Fig. 2 shows Ni(OH)z hexagonal nanoplates grown on graphene. The morphology of NiO-grown graphene is shown in Fig. 2, too. It is composed of slabs with sizes of 90-120 nm . Tn the graphene/NiO composite, NiO with a uniform size of � 90-120 nm are selectively and directly grown on the graphene. NiO nanoparticles with plate structure act as spacers on the surface of graphene sheets, efficiently preventing the close restacking of sheets, avoiding the loss of their highly active surface. As shown in Fig. 2, the morphology of the synthesized MoOrMoSz powder clearly exhibits a good combination of each phase distributed such that an adequate boundary is established between the components. That is, each MoOz and MoSz phase forms on the surface of the MoOrMoSz composite individually during the hydrothermal reaction at 225°C.

.-::::J � �

UJ C Q)

"E

8 I

:i §

- GntpltcncINiO - - - - QraphcnoiN,(OH12 ....... Or phenc

-

; : t. � § f'I == � �-- ''--_./'-''------��--=-----------

§

10 :w )0 40 SO 00 '" .0

2 Theta (degree)

. � "MoO. g 0 **MoSz e.

10 20 30 40 50 60 70 80

29 (deg)

GriJphene/NiO-Mn02

Graphene

10 20 30 40 50 60 70 80 2 Theta (degn,e)

Fig. 1. XRD patterns of graphene, graphenelNi(OH)2, grapheneiNiO, MoOrMoS2, grapheneiMoOrMoS2, and

grapheneiNiO-Mn02.

Fig. 2. FE-SEM and FE-TEM images of grapheneiNiO, graphene, MoOrMoS], and grapheneiNiO-MnO].

The Mo02 phase (see Fig. 2) consisted of nanorods with an average aspect ratio (LID) of 80 (=400 nm/50 nm). The nanorod phase of Mo02 is very similar to that obtained from a low-temperature hydrothermal synthesis procedure where ethylene diamine was employed as a surfactant. Typical FE-SEM and FE­TEM images of the graphenelNiO-Mn02 composite, which is a NiO-Mn02 nanocomposite grown on graphene, are shown in Fig. 2. In the graphene composite material, nanocrystallines of NiO-Mn02 were selectively and directly grown on highly conducting graphene. It is believed that both chemisorption and van der Waals interactions between NiO-Mn02 and graphene exist at oxygen-containing defect sites and the pristine regions of graphene, respectively.

The galvanostatic charge-discharge behavior of the graphene, graphene/NiO (59 wt.%) composites are shown in Fig. 3. The charge-discharge current density provides 300 mAh/g (0.3 C), which is denoted as a rate of 1000 mAh/g (1 C) for each half cell. Both the first discharge/charge voltage profiles for graphene/NiO (59 wt.%) is shown in Fig. 3. There are two sloping potential ranges (l.67-l.l7, and l.l0-0.85 V), which are consistent with the lithium reaction and the graphenelNiO composite during the first discharge, but no obvious voltage plateau is observed for graphene. The initial discharge capacity is � 2087 and 1360 mAh g -\ for the graphene, graphenelNiO (59 wt.%) electrode, respectively. The introduction of the NiO nanoparticle shows a strong synergistic effect in the composite for improving the reversible capacity. The good cycle performance and rate capability of graphene/NiO (59 wt.%) are indicated in Fig. 3. The reversible capacity of the graphene/NiO (59 wt.%) electrode is as high as 716 mAh il after 40 cycles at a current density of 300 rnA g-\ whereas that of the graphene electrode rapidly decays to 56l.2 rnA il. The NiO nanoparticles are homogeneously dispersed int% nto the graphene. The compact graphene sheets significantly decrease the contact resistance of active particles in the composites and provide high electrical conductivity for the electrode; secondly, both graphene and NiO are electroactive components for Li storage and contribute to the overall capacity of the electrode.

� ..... �

;:>-'" -� 1 "..

0

� ID 0> � 0

>

0

3.0

2.5

2_0

L5

LO

05

a..qro c...-, 0.3 C (300 mA/g) D�ro c� 0.3 C (3oo-.AJg)

.'"

200 400 lion 800 ICXlO 1200 1400 1600 JROO 20ID 2200

�y(mAhlJ;}

10

M002-MoS/graphene

0_0 0�� --4:-:'OO--�-8::<: 00

=-=---1-2-'- 0 -0--==-1

--l600

Specific capacity (mAh g-1)

1.0

.O!l "5 0.5

&

0.0 �--�--�--�--�--�--�--�--� 10000 20000 30000 40000

Time (s)

Fig. 3. Charge and discharge curves of grapheneiNiO, graphene/ MoOe-MoS2 in lithium secondary batteries and

grapheneiNiO-MnO] in supercapacitors.

The galvnostatic charge-discharge profiles obtained for MoOrMoS2/graphene composite electrode at a low current rate of 100 rnA il (see Fig. 3). Shoulders are observed in the charging curves of the MoOr MoS2/graphene composite electrode at l.8 V. These features correspond to the oxidation peak with a higher intensity in the cyclic voltammogram. Additional shoulders with lower intensity were detected in the charging curves at 1.5 and 2.25 V, which correspond to the oxidation peaks of Mo02 and MoS2, respectively. The shoulders near 3 V in the discharging curves also correspond to reduction peaks that overlap with the lithiation effects of Mo02 and MoS2. Meanwhile, the charging-discharging profiles of the graphene anode shows monotonic behavior, indicating the absence of a particular lithiation-delithiation mechanism or the simple transport of lithium ions in and out of the graphene matrix. Thus, the initial irreversible capacity of graphene becomes greater than

that of the MoOrMoS2/graphene anode, which can allow for the more effective electrochemical storage of the lithium ions.

galvanostatic charge/discharge behaviors of the graphene electrodes were investigated by the galvanostatic method from 0 to 1.0 V at 50 rnA g-l. In Fig. 3, it can be seen that the curve is very symmetrical, implying that the graphene electrode has a perfect electrochemical double-layer capacitive behavior. Tn the graphene/NiO-Mn02 electrode, mirror-like EDLC­pseudocapacitor response behavior can be observed in the curve in Fig. 3. This implies that the charge/discharge process of the electrode is reversible. Compared with the graphene electrode in Fig. 3, a much larger capacitance was obtained due to the presence of NiO-Mn02. Moreover, the discharge time of the graphenelNiO-Mn02 electrode was approximately 2.7 times longer than that of the graphene electrode due to the presence of graphene to support the NiO-Mn02. The average specific capacitances for the graphene and graphene/NiO­Mn02 composite electrodes obtained from charge/discharge curves were �77.2 and �208.8 F g-l, respectively. It is likely that the electrochemical characteristics of the composite materials improved as a result of the addition of NiO-Mn02 to the graphene.

ACKNOWLEDGMENT

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy and by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093818).

REFERENCES

[1] Y. P. Wu, E. Rahm, R. Holze, "Carbon anode materials for lithium ion batteries," J. Power Sources, vol. 114, pp. 228-236, March 2003.

[2] H. Buqa, D. Goers, M. Holzapfel, M. E. Spahr, P. Novak, "High Rate Capability of Graphite Negative Electrodes for Lithium-Ion Batteries," J. Electrochem. Soc., vol 152(2), pp. 474-482, February 2005.

[3] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, " Tin-Based Amorphous Oxide: A High­Capacity Lithium-lon-Storage Material," Science, vol. 276(5317), pp. 1395-1397, May 1997.

[4] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonons, 1. V. Grigorieva, A. A. Firsov, " Electric Field Effect in Atomically Thin Carbon Films," Science, vol. 306(5696), pp. 666-669, October 2004.

[5] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. 1. Katsnelson, 1. V. Grigorieva, S. V. Dubonos, A. A. Firsov, "Two-dimensional gas of massless Dirac fermions in graphene," Nature, vol. 438(7065), pp. 197-200, September 2005.

[6] S.-L. Chou, J.-Z. Wang, M. Choucair, H.-K. Liu, J.A. Stride, S.-X. Dou, "Enhanced reversible lithium storage in a nanosize silicon/ graphene composite," Electrochem. Commun., vol. 12(2), pp. 303-306, February 2010.

[7] D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, D. Hu, C Wang, L.V. Saraf, J. Zhang, LA. Aksay, J. Liu, " Self­Assembled Ti02-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion," ACS Nano, vol. 3(4), pp. 907-914, April 2009.

[8] Z.-S. Wu, W. Ren, L. Wen, L. Gao, 1. Zhao, Z. Chen, G. Zhou, F. Li, H.-M. Cheng, " Graphene Anchored with Co304 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance," ACS Nano, vol. 4(6), pp. 3187-3194 , June 2010 .

[9] J.-Z. Wang, C Zhong, D. Wexler, N.H. Idris, Z.-X. Wang, L.-Q. Chen, H.-K. Liu, "Graphene-Encapsulated Fe304 Nanoparticles with 3D Laminated Structure as Superior Anode in Lithium Ion Batteries," Chem. Eur. J., vol. 17(2), pp. 661-667, January 2011.

[10] S.-M. Paek, E. Yoo, 1. Homna, " Enhanced Cyclic Performance and Lithium Storage Capacity of Sn02/Graphene Nanoporous Electrodes with Three­Dimensionally Delaminated Flexible Structure," Nano Lett., vol. 9(1), pp. 72-75 , January 2009.

[11] Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, CD. Gu, J.P. Tu, "CuO/graphene composite as anode materials for lithium-ion batteries," Electrochim. Acta, vol. 56 (1), pp. 2306-2311, February 2011.

[12] Y. Liang, S. Yang, Z. Yi, X. Lei, J. Sun, Y. Zhou, "Low temperature synthesis of a stable Mo02 as suitable anode materials for lithium batteries." Materials Science and Engineering: B, vol. 121(1-2), pp. 152-155, July 2005.

[13] X. Ji, P.S. Herle, Y. Rho, L.F. Nazar, " Carbon/Mo02 Composite Based on Porous Semi-Graphitized Nanorod Assemblies from In Situ Reaction of Tri­Block Polymers," Chem. Mater., vol. 19(3), pp. 374-383, February 2007.