Electrochimica Acta 149 (2014) 86–93

Improvement of electrochemical performance of LiNi0.8Co0.1Mn0.1O2

cathode material by graphene nanosheets modification

S.Savut Jan a,b, S. Nurgul c, Xiaoqin Shi a, Hui Xia a,b,*, Huan Pang d,*a School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, ChinabHerbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, ChinacDepartment of Physics and Institute of Biophysics, Central China Normal University, Wuhan 430079, ChinadKey Laboratory for Clearer Energy and Functional Materials of Henan Province, Colleage of Chemistry and Chemical Engineering, Anyang Normal University,Anyang 455000, China

A R T I C L E I N F O

Article history:Received 16 September 2014Received in revised form 18 October 2014Accepted 21 October 2014Available online 23 October 2014

Keywords:LiNi0.8Co0.1Mn0.1O2

lithium-ion batteriesgraphenecomposite

A B S T R A C T

Layered LiNi0.8Co0.1Mn0.1O2-graphene composite is synthesized by a facile chemical approach and usedas the cathode material for lithium-ion batteries. The structural and morphological features of as-prepared LiNi0.8Co0.1Mn0.1O2-graphene composite are investigated with powder X-ray diffraction, Ramanspectroscopy, scanning electron microscopy, and transmission electron microscopy. The characterizationresults indicate that the LiNi0.8Co0.1Mn0.1O2 particles maintain their structural integrity and crystalfeatures after being enwrapped with graphene nanosheets. The graphene-modified LiNi0.8Co0.1Mn0.1O2

comoposite exhibits superior electrochemical performance compared to the pristine LiNi0.8Co0.1Mn0.1O2

powders. The LiNi0.8Co0.1Mn0.1O2-graphene composite shows a large initial discharge capacity up to212.9 mAh g�1 as well as good cycling performance and good rate capability. The outstandingelectrochemical performance of the graphene-modified LiNi0.8Co0.1Mn0.1O2 composite can be attributedto the improved electrical conductivity and structural stability due to the highly conductive graphenematrix.

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1. Introduction

During the past few decades, lithium-ion batteries with highenergy density, light weight, long cycle life, and environmentalfriendliness have been widely used as power sources for variousportable electronic devices such as cellular phones, laptopcomputers, digital cameras and etc. [1–4]. Most recently, theyhave attracted growing attention as power supplies for hybrid,plug-in hybrid, and all electrical vehicles (EVs). However,commercialization of these batteries for the automotive industriesdemands further improvement in energy density and safety forbatteries. The energy densities of current lithium-ion batteries aremuch lower than that of the fuel engine as revealed by the limiteddriving range of current EVs. One of the key factors that decide thebattery's energy density is the specific capacity of the cathodematerial. Compared to the commercialized cathode materialLiCoO2, the Ni-rich layered lithium transition-metal oxides Li[Ni1-xMx]O2 (M = Co, Al, Mn, etc.) have been considered as

* Corresponding author. Tel.: +86 25 84303408, Fax: +86 25 84303408.E-mail addresses: xiahui@njust.edu.cn (H. Xia), huanpangchem@hotmail.com

(H. Pang).

http://dx.doi.org/10.1016/j.electacta.2014.10.0930013-4686/ã 2014 Elsevier Ltd. All rights reserved.

promising cathode material due to their relatively low cost andhigh reversible capacity [5,6]. Among the Ni-rich layered cathodematerials, LiNi0.8Co0.1Mn0.1O2 represents a large capacity cathodematerial with a high reversible capacity of �200 mAh g�1, which ismuch higher than that of LiCoO2 (� 140 mAh g�1) in the samepotential range of 2.8–4.3 V [7,8]. However, several substantialissues such as poor rate capabilitiy and bad cyclabiliity havehindered the LiNi0.8Co0.1Mn0.1O2 cathode material from beingcommercialized for lithium-ion batteries. It has been reported thatthe materials have a problem with rapid moisture uptake, thusforming LiOH/Li2CO3 impurity phases on the particle surface,which is responsible for the capacity loss and poor kinetics of Ni-rich cathode materials [9,10]. Concurrently, the reactive andunstable Ni4+ ions in the delithiated Ni-rich cathode are easilyreduced to form more stable compounds of LixNi1-xO, thus leadingto the increase of interfacial impedance and decrease of the cyclinglife [11,12]. Moreover, the poor electrical conductivity of LiNi0.8-Co0.1Mn0.1O2 further deteriorates its rate performance. To improvethe electrochemical performance of LiNi0.8Co0.1Mn0.1O2, twostrategies are often employed which include: (1) partiallysubstituting LiNi0.8Co0.1Mn0.1O2 with foreign atoms, such as Cr,Mg, Al, and F, in an attempt to provide enhanced structural stability[13,14], and (2) surface coating with a thin metal oxide layer to

S.S. Jan et al. / Electrochimica Acta 149 (2014) 86–93 87

suppress the side reactions. Recently, enwrapping cathodematerials into a conductive network has become a favorablestrategy to achieve better cycling performance and rate capability[15–17]. As discussed by Jiang et al., the improved electrodeconductivities as well as the decrease of polarization will be helpfulto depress the side reactions of the electrode, thus enhancing bothcycling performance and rate capability [18].

Due to the high electronic conductivity, stable chemicalproperty, and large surface area, graphene has emerged as aconductive medium for constructing hybrid cathode materials forlithium-ion batteries [19–21]. In a previous work, Guo et al.developed a 3D conducting network for LiNi1/3Mn1/3Co1/3O2

cathode material by using reduced graphene oxide and foundthe introduction of graphene greatly improved the electrodekinetics and electrochemical performance [22]. Inspired by such acomposite electrode design, in the present work, a simple chemicalstrategy was developed to prepare the composite of graphenenanosheets wrapped LiNi0.8Co0.1Mn0.1O2 particles to boost theelectrochemical performance for the high capacity Ni-rich cathodematerial. The electrochemical results revealed that the LiNi0.8-Co0.1Mn0.1O2-graphene composites are promising cathode materi-al, exhibiting a high reversible capacity, excellent cycling stabilityand improved rate capability in comparison with the pristineLiNi0.8Co0.1Mn0.1O2 powders.

2. Experimental

2.1. Preparation of pristine LiNi0.8Co0.1Mn0.1O2

The layered oxide LiNi0.8Co0.1Mn0.1O2 powders were synthe-sized by a rapid co-precipitation method. An aqueous solution of2 mol dm�3 NiSO4�6H2O, CoSO4�7H2O, and MnSO4�H2O was pouredinto a continuously stirring tank reactor under an Ar atmosphere.At the same time, a 2 mol dm�3 NaOH solution and a desiredamount of NH3.H2O solution were quickly fed into the reactor. PHwas controlled and maintained at levels between 11 and 12. Afterbeing vigorously stirred for 5 min, the solution was filtered and theprecipitated powder was washed with distilled water for severaltimes and dried at 80 �C in a vacuum oven for 24 h. To synthesizethe LiNi0.8Co0.1Mn0.1O2 powder, a mixture of dehydrated [Ni0.8C-o0.1Mn0.1](OH)2 and LiOH.H2O was preheated at 480 �C for 5 h andthen heated at 750 �C for 15 h under flowing oxygen.

2.2. Preparation of LiNi0.8Co0.1Mn0.1O2-graphene composite

Graphene nanosheets were prepared from graphite powder in atwo-step process, involving the oxidation and/or exfoliation of

Fig. 1. Schematic illustration of the preparation of

graphite to graphite oxide by Hummer’s method and chemicalreduction of graphite oxide to graphene according to literature[23]. The preparation procedure of the LiNi0.8Co0.1Mn0.1O2-graphene composite is illustrated in Fig. 1. Firstly, 0.05 g ofgraphene and 1 g of LiNi0.8Co0.1Mn0.1O2 powders were ground for0.5 h in a mortar and the obtained mixture was dispersed in 30 mlethanol by ultrasonication. Then, the solution was vigorouslystirred at 50 �C for 8 h. Finally, the obtained mixture was dried in anoven at 80 �C overnight to obtain the LiNi0.8Co0.1Mn0.1O2-graphenecomposite.

2.3. Materials Characterization

Crystallographic information of all as-prepared samples wereinvestigated with X-ray powder diffraction (XRD, Shimadzu XRD-6000) at a scanning rate of 1 �C min�1 in the 2u range of 10–80�.The morphology features of the as-prepared samples werecharacterized with field-emission scanning electron microscopy(FESEM, JSM-6700F 15 kV), transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM, JEOL 2010 200 kV). To determine the graphene contentin the composite, thermogravimetric analysis (TGA) was carriedout in the air at a heating rate of 10 �C min�1 from 30 �C to 850 �Cusing a DTG-60H Shimadzu thermal analyzer. Micro-Ramanscattering measurements were performed using a DXR RamanMicroscope (Thermao Fisher Scientific Inc.) The excitation sourceused was an argon-ion laser operating at 514.5 nm, and measure-ments were performed in a laser incident power of 0.3 mW.

2.4. Electrochemical Measurements

To investigate the electrochemical properties, the as-preparedsamples were assembled into coin cells (CR-2032) in an Ar-filledglove box with moisture and oxygen below 1 ppm. To prepare theworking electrodes, slurries were prepared by mixing 80 wt% of theactive material, 10 wt% carbon black and 10 wt% polyvinylidenedifluoride (PVDF) in N-methylpyrrolidone (NMP). The slurry waspasted on the Al foil and dried in a vacuum oven at 120 �C for 12 h toremove the solvent. The electrodes were then pressed under15 MPa of pressure with a typical active material loading of 4–5 mg cm�2. Half-cells were assembled using Li foil as both counterand reference electrodes. 1 M LiPF6 in ethylene carbonate anddiethyl carbonate (EC/DEC, v/v = 1:1) solution was used as theelectrolyte and Celgard 2400 was used as the separator.Galvanostatic charge and discharge measurements were carriedout in the voltage range between 2.7 and 4.3 V (vs Li/Li+) atdifferent current densities (1C is 200 mAh g�1 for

the LiNi0.8Co0.1Mn0.1O2-graphene composite.

Fig. 3. Raman spectra of the pristine LiNi0.8Co0.1Mn0.1O2 powders and theLiNi0.8Co0.1Mn0.1O2-graphene composite.

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LiNi0.8Co0.1Mn0.1O2) using the LAND CT2001A electrochemicalworkstation at room temperature. Cyclic voltammogram (CV) andelectrochemical impedance spectroscopy (EIS) measurementswere performed using a CHI660D electrochemical workstation.CVs were measured between 2.7 and 4.5 V (vs Li/Li+) at a scan rateof 0.2 mV s�1. EIS measurements were carried out in the frequencyrange between 100 kHz to 0.01 Hz with an AC amplitude of 5 mV atan open circuit potential.

3. Results and Discussion

Fig. 2 shows the XRD patterns of the pristine graphenenanosheets, LiNi0.8Co0.1Mn0.1O2 powders, and the LiNi0.8-Co0.1Mn0.1O2-graphene composite. The XRD pattern of the pristinegraphene shows a small hump at about 26�, which can beattributed to the (002) reflection of graphite. The XRD patterns forboth the LiNi0.8Co0.1Mn0.1O2 powders and the LiNi0.8Co0.1Mn0.1O2-graphene composite can be indexed as a well-defined layeredphase based on a hexagonal a-NaFeO2 structure with an R-3 mspace group. No impurity phases are detected in the XRD patternsfrom both samples. No diffraction peaks of graphene can beobserved in the XRD pattern of the LiNi0.8Co0.1Mn0.1O2-graphenecomposite, which is probably due to the strong diffraction peaksfrom the highly crystalline LiNi0.8Co0.1Mn0.1O2 and the nanoscalesize feature of low content graphene. The calculated latticeconstants (a and c) for both LiNi0.8Co0.1Mn0.1O2 (a = 2.867 Å andc = 14.192 Å) and LiNi0.8Co0.1Mn0.1O2-graphene composite(a = 2.871 Å and c = 14.196 Å) are congruent with literature values[24,25]. The clear splitting of doublets of (006)/(102) and (108)/(110) observed for both samples indicates the highly orderedlayered structure of LiNi0.8Co0.1Mn0.1O2 [26–28].

Raman spectra of the pristine LiNi0.8Co0.1Mn0.1O2 and theLiNi0.8Co0.1Mn0.1O2-graphene composite are shown in Fig. 3.According to the theoretical factor-group analysis, the layeredmetal oxides with rhombohedral space group R-3 m symmetryhave two Raman active modes of A1g and Eg [29,30]. The Ramanbands are related to the M-O stretching and O-M-O bending as thecontribution to the Raman modes is only from the motion ofoxygen atoms [31,32]. The Raman spectrum of the pristineLiNi0.8Co0.1Mn0.1O2 powders displays two bands at about 480and 600 cm�1, which can be assigned to the Eg and A1g Raman-active modes, respectively. The A1g mode has greater oscillationstrength and therefore exhibits higher peak intensity [33,34]. Inaddition to the two Raman bands ascribed to layered LiNi0.8-Co0.1Mn0.1O2, the Raman spectrum of the LiNi0.8Co0.1Mn0.1O2-graphene composite also shows two bands at about 1345 and1592 cm�1, which can be assigned to the D and G bands ofgraphene, respectively. The D band arises because of the disorderor the defect induced in sp2-bonded carbon, whereas the sharp Gband arises from the in-plane vibrational mode that involves sp2

Fig. 2. XRD patterns of the pristine graphene nanosheets, LiNi0.8Co0.1Mn0.1O2

powders and the LiNi0.8Co0.1Mn0.1O2-graphene composite.

hybridized carbon atoms that comprises the graphene sheet. Asmall 2D band observed at about 2696 cm�1 is the charactersticband of graphene and is usually used to differentiate the layerthickness of graphene in sample [34]. TGA was employed todetermine the amount of graphene present in the LiNi0.8-Co0.1Mn0.1O2-graphene composite (Fig. 4). Based on the weightloss of the composite in the temperature range of 200–500 �C, thegrahene content was determined to be about 5.5 wt%.

The FESEM images of the pristine LiNi0.8Co0.1Mn0.1O2 powder,graphene nanosheets and the LiNi0.8Co0.1Mn0.1O2-graphene com-posite are shown in Fig. 5. As shown in Fig. 5a, agglomeratedLiNi0.8Co0.1Mn0.1O2 particles are generally in a uniform polyhedralshape with an average particle size of 100–200 nm. Micron-sizewrinkled and transparent graphene nanosheets can be clearly seenfrom Fig. 5b. Fig. 5c and 5d show the FESEM images of theLiNi0.8Co0.1Mn0.1O2-graphene composite with various magnifica-tions, revealing that the LiNi0.8Co0.1Mn0.1O2 particles are homo-geneously dispersed on the graphene nanosheets without severeagglomeration. The uniform LiNi0.8Co0.1Mn0.1O2-graphene hetero-structures can be attributed to the facile chemical mixing method.Initially, the milling in the mortar can effectively separate theLiNi0.8Co0.1Mn0.1O2 particles from forming the aggregated clusters.Afterward, the ultrasonication and the continuous stirring processcan be helpful for the uniform particle distribution on thegraphene nanosheets, resulting in a good dispersion of the oxideparticles over the conductive matrix.

To further investigate the microstructure of the composite, TEMwas used to characterize the sample and the obtained results areshown in Fig. 6. Fig. 6a and 6b show the TEM images of theLiNi0.8Co0.1Mn0.1O2-graphene composite with various magnifica-tions. It can be seen that the LiNi0.8Co0.1Mn0.1O2 particles areenwrapped by the transparent graphene nanosheets, agreeing wellwith the FESEM observation. Fig. 6c shows the HRTEM image fromthe circle area in Fig. 6b, revealing a clear interface between theLiNi0.8Co0.1Mn0.1O2 particle and the graphene nanosheet. Furtherenlargement of the interface area (Fig. 6d) clearly shows the latticefringes with an interplanar spacing of about 0.20 nm, which is

Fig. 4. Thermogravimetric analysis of the LiNi0.8Co0.1Mn0.1O2-graphene composite.

Fig. 5. (a) FESEM image of the LiNi0.8Co0.1Mn0.1O2 particles. (b) FESEM image of the graphene nanosheets. (c,d) FESEM images of the LiNi0.8Co0.1Mn0.1O2-graphene composite.

Fig. 6. (a,b)TEM images of the LiNi0.8Co0.1Mn0.1O2-graphene composite. (c) HRTEM image obtained from the circle area in (b). (d) HRTEM of the LiNi0.8Co0.1Mn0.1O2-grapheneinterface area.

S.S. Jan et al. / Electrochimica Acta 149 (2014) 86–93 89

Fig. 7. CV curves of (a) the pristine LiNi0.8Co0.1Mn0.1O2 electrode and (b) theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode between 2.7 and 4.5 V at a scanrate of 0.2 mV s�1.

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consistent with the (104) planes of layered LiNi0.8Co0.1Mn0.1O2. Forsuch heterostructure, the electron transport between the nano-sized LiNi0.8Co0.1Mn0.1O2 particles and the current collector can beremarkably improved by the graphene conductive matrix, thusendowing the LiNi0.8Co0.1Mn0.1O2-graphene composite with supe-rior electrochemical performance.

Fig. 7a and 7b show the CV curves of the pristine LiNi0.8-Co0.1Mn0.1O2 and LiNi0.8Co0.1Mn0.1O2-graphene composite elctr-odes in the potential range of 2.7–4.5 V (vs Li/Li+) at a scan rate of0.2 mV s�1 for the initial two cycles. The CV curves for both the

Fig. 8. Charge/discharge curves of the pristine LiNi0.8Co0.1Mn0.1O2 electrode (a) and the L0.5C, 1C, and 5C in the potential range between 2.7 and 4.3 V at room temperature.

pristine LiNi0.8Co0.1Mn0.1O2 and LiNi0.8Co0.1Mn0.1O2-graphenecomposite elctrodes show a pair of well-defined redox peaks.The cathodic peak appears in the potential range of 3.9–4.0 V (vs Li/Li+) corresponds to the oxidation of Ni3+ to Ni4+ while the anodicpeak appears in the potential range of 3.4–3.7 V (vs Li/Li+)corresponds to the reduction of Ni4+ to Ni3+. In comparison withthe pristine LiNi0.8Co0.1Mn0.1O2 electrode, the potential differencebetween the oxidation and reduction peaks is much smaller for theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode, indicating theelectrode polarization can be greatly reduced with the introduc-tion of graphene nanosheets.

Fig. 8a and 8b show the charge/discharge curves of the pristineLiNi0.8Co0.1Mn0.1O2 and LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrodes at various current rates of 0.1C, 0.5C, 1C, and 5C in thepotential range between 2.7 and 4.3 V (vs Li/Li+) at roomtemperature. Both electrodes show a potential plateau at about3.6 V (vs Li/Li+) for both charge and discharge curves, correspond-ing to the redox reaction of Ni3+/Ni4+. The initial charge/dischargecapacities obtained for the pristine LiNi0.8Co0.1Mn0.1O2 electrode atvarious current rates of 0.1C, 0.5C, 1C, and 5C are 234.6/194.8,204.1/179.2, 177.2/160.8, and 147.1/132.6 mAh g�1, respectively. Incomparison, the initial charge/discharge capacities of the LiNi0.8-Co0.1Mn0.1O2-graphene composite electrode at various currentrates of 0.1C, 0.5C, 1C, and 5C are 237.6/212.9, 220.4/199.3, 197.5/184.2, and 171.6/163.8 mAh g�1, respectively. It can be seen that theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode can deliverlarger reversible capacity compared to the pristine LiNi0.8-Co0.1Mn0.1O2 at the same C rate. Apart from that, the initialcoulombic efficiency of the LiNi0.8Co0.1Mn0.1O2-graphene compos-ite electrode (89.6%) of the initial cycle at 0.1C is also larger thanthat of the pristine LiNi0.8Co0.1Mn0.1O2 electrode (83.5%). The largerreversible capacity and higher initial coulombic efficiency of theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode can be attrib-uted to its large surface area and surface modification by graphenenanosheets. As nanoparticle agglomeration could be greatlysuppressed by graphene nanosheets, the LiNi0.8Co0.1Mn0.1O2-graphene composite will expose larger surface area to theelectrolyte compared to the pristine LiNi0.8Co0.1Mn0.1O2, thusincreasing the utilization of LiNi0.8Co0.1Mn0.1O2 nanoparticles. In

iNi0.8Co0.1Mn0.1O2-graphene composite electrode (b) at various current rates of 0.1C,

Fig. 9. (a) Rate performance of the pristine LiNi0.8Co0.1Mn0.1O2 electrode and theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode. (b) Nyquist plots of the pristineLiNi0.8Co0.1Mn0.1O2 electrode and the LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrode after five charge/discharge cycles at 0.1 C. The inset figure in (b) showsthe equivalent circuit for the EIS spectra. (c) Cycling performance of the pristineLiNi0.8Co0.1Mn0.1O2 electrode and the LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrode at 1C for 150 cycles.

Table 1Resistance values obtained from equivalent circuit fitting of experimental data forthe pristine LiNi0.8Co0.1Mn0.1O2 and the LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrodes.

Samples Rsei/V Rct/V

LiNi0.8Co0.1Mn0.1O2 239.1 443.8G-LiNi0.8Co0.1Mn0.1O2 123.1 193.5

S.S. Jan et al. / Electrochimica Acta 149 (2014) 86–93 91

addition, the enwrapping with gaphene nanosheets could alsosuppress the side reactions of LiNi0.8Co0.1Mn0.1O2 particles,resulting in improved coulombic efficiency and larger reversiblecapacity.

Fig. 9a compares the rate performance of the pristineLiNi0.8Co0.1Mn0.1O2 and LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrodes. As the C rate increases, the reversible capacity for bothelectrodes decreases with incrasing polarization between chargeand discharge curves (Fig. 8). It is obvious that the LiNi0.8-Co0.1Mn0.1O2-graphene composite electrode possesses muchbetter rate capability as it can retain more reversible capacity asthe discharge rate increases. Even at 5 C rate, the LiNi0.8-Co0.1Mn0.1O2-graphene composite electrode can still deliver adischarge capacity of about 163.8 mAh g�1, which is much largerthan that of the pristine LiNi0.8Co0.1Mn0.1O2 electrode (132.6 mAhg�1). For this composite electrode design, graphene nanosheetswere introduced into LiNi0.8Co0.1Mn0.1O2 particles with two mainfunctions. The first is to construct a three-dimensional conductivematrix for LiNi0.8Co0.1Mn0.1O2 particles, which could provide fastelectron transfer and improve the electrical conductivity of theelectrode. The second is to suppress the particle aggregation forLiNi0.8Co0.1Mn0.1O2. As can be seen from the FESEM image of thepristine LiNi0.8Co0.1Mn0.1O2 nanoparticles (Fig. 5a), there is severe

particle aggregation, which could block electrolyte penetration,thus prolonging the Li+ ion diffusion paths and slowing down theionic transportation in the electrode. When graphene nanosheetsare introduced into LiNi0.8Co0.1Mn0.1O2 nanoparticles, the particleaggregation can be significantly suppressed with unfirom disper-sion of LiNi0.8Co0.1Mn0.1O2 nanoparticles on graphene nanosheets(Fig. 5 and Fig. 6). The well dispersion of LiNi0.8Co0.1Mn0.1O2

nanoparticles on graphene nanosheets could provide large surfacearea and short Li+ ion diffusion paths, thus endowing the compsiteelectrode with improved ionic transportation and improved rateperformance.

To further understand the beneficial effects of the graphenenanosheets on the electrochemical properties of LiNi0.8-Co0.1Mn0.1O2 in the composite, EIS measurements were carriedout for both the LiNi0.8Co0.1Mn0.1O2 and the LiNi0.8Co0.1Mn0.1O2-graphene composite electrodes after five charge/discharge cyclesat 0.1 C. The obtained Nyquist plots and corresponding equivalentcircuit are shown in Fig. 9b. Both EIS spectra show two semicircles.For the equivalent circuit, Rb is the uncompensated ohmicresistance of the electrodes. The first semicircle at high frequencyrepresented by a resistor Rsei and a constant phase element Cseicorresponds to the lithium ion transfer through the surface layer.The second semicircle at low frequency can be attributed to thecharge transfer reaction represented by charge-transfer resistanceRct and the non-ideal double layer capacitance Cdl [35]. The valuesof parameters Rsei and Rct obtained by the nonlinear least squaresfitting are summarized in Table 1. It can be seen that the addition ofgraphene into LiNi0.8Co0.1Mn0.1O2 can reduce the values of both Rseiand Rct. Rct is remarkably reduced due to the improvement ofelectrical conductivity and enlargement of surface area, whichresult in outstanding improvement in the electrode kinetics andconsequent increase in rate capability.

Fig. 9c compares the cycling performance of the pristineLiNi0.8Co0.1Mn0.1O2 and the LiNi0.8Co0.1Mn0.1O2-graphene compos-ite electrodes cycled at 1C for 150 cycles at room temperature.After 150 cycles, the LiNi0.8Co0.1Mn0.1O2-graphene compositeelectrode can still deliver a discharge capacity of 167.5 mAh g�1,corresponding to 92.2% of the initial discharge capacity. Thecapacity retention of the pristine LiNi0.8Co0.1Mn0.1O2 electrode isonly about 76.5%, which is much lower compared to that of theLiNi0.8Co0.1Mn0.1O2-graphene composite electrode. The electro-chemical performance of the LiNi0.8Co0.1Mn0.1O2-graphene com-posite electrode is comparable or even better than that of otherLiNi0.8Co0.1Mn0.1O2 materials modified by doping or surfacecoating reported in the literature [18,19,36], indicating thedesigned graphene wrapped hybrid structure is effective toimprove the electrochemical performance of LiNi0.8Co0.1Mn0.1O2.Besides the improved electrical conductivity of the electrode, thewrapped graphene nanosheets may also act as protection layers forthe particles' surface, suppressing side reactions and the activematerial dissolution in the electrolyte during the cycling process,which leads to the excellent cycling performance and superiorhigh-rate capability.

In previous works [18,22], Jiang and Guo et al. investigated theelectrochemical performance of the graphene-wrapped Li-excesslayered Li(Li0.2Mn0.54Ni0.13Co0.13)O2 composite and LiNi1/3Co1/3Mn1/3O2/grahene composite. In the present work, the

92 S.S. Jan et al. / Electrochimica Acta 149 (2014) 86–93

LiNi0.8Co0.1Mn0.1O2/graphene composite electrode can deliver alarge reversible capacity up to 213 mAh g�1 in the potential rangebetween 2.7 and 4.3 V (vs Li/Li+), which is much larger than those ofthe LiNi1/3Co1/3Mn1/3O2/grahene composite electrode (less than170 mAh g�1,[18]) and Li(Li0.2Mn0.54Ni0.13Co0.13)O2/grahene com-posite electrode in the same potential range. To achieve largerreversible capacity, LiNi1/3Co1/3Mn1/3O2 or Li(Li0.2Mn0.54-

Ni0.13Co0.13)O2 needs to be charged to higher potentials such as4.5 V and 4.8 V (vs Li/Li+), which is not practical for currentcommercialized electrolytes due to their poor stability at highpotentials. Besides large reversible capacity, the LiNi0.8-Co0.1Mn0.1O2/graphene composite electrode also exhibits out-standing cycling stability and rate capability. The capacityretention after 150 cycles for the present LiNi0.8Co0.1Mn0.1O2/graphene composite electrode is 92%, which is larger than that ofthe Li(Li0.2Mn0.54Ni0.13Co0.13)O2/grahene composite electrode (90%after 100 cycles, [22]). The rate capability of the presentLiNi0.8Co0.1Mn0.1O2/graphene composite electrode (163 mAh g�1

at 5C) is better than those of the LiNi1/3Co1/3Mn1/3O2/grahenecomposite electrode (140 mAh g�1 at 3C, [18]) and Li(Li0.2Mn0.54-

Ni0.13Co0.13)O2/grahene composite electrode (125 mAh g�1 at 3C,[22]). Therefore, considering electrochemical performance andpractical application, the LiNi0.8Co0.1Mn0.1O2/graphene compositedeveloped in this work is superior to the previously reportedcomposite cathode materials.

4. Conclusions

In summary, the LiNi0.8Co0.1Mn0.1O2-graphene composite hasbeen successfully prepared through a facile chemical approach. Inthe hybrid cathode material design, the LiNi0.8Co0.1Mn0.1O2 nano-particles are well dispersed on the graphene nanosheets, providingboth large surface area and improved electrical conductivity. Theelectrochemical measurements clearly demonstrated that thehybrid design can significantly improve the specific reversiblecapacity, rate capability, and long term cycling performance. EISresults confirm that the electrode kinetics is substantiallyimproved by the introduction of graphene nanosheets, facilitatingthe transfer of lithium ions across the active material/electrolyteinterface, as well as the transfer of electrons from the currentcollector to the active material. Therefore, the results indicate thatthe LiNi0.8Co0.1Mn0.1O2-graphene composite is a promising cath-ode material candidate for application in high-performancelithium-ion batteries.

Acknowledgements

This work was supported by National Natural Science Founda-tion of China (No. 51102134, 21201010), New Century ExcellentTalents of the University in China (grant no. NCET-13-0645),Natural Science Foundation of Jiangsu Province (No. BK20131349,BK2011709), QingLan Project of Jiangsu Province, China Postdoc-toral Science Foundation (No. 2013M530258), and Jiangsu PlannedProjects for Postdoctoral Research Funds (No. 1202001B).

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