A facile method of preparing mixed conducting LiFePO 4/graphene composites for lithium-ion batteriesLi Wang a ,b ,1,Haibo Wang b ,1,Zhihong Liu b ,Chen Xiao b ,Shanmu Dong b ,Pengxian Han b ,Zhongyi Zhang b ,Xiaoying Zhang b ,Caifeng Bi a ,⁎,Guanglei Cui b ,⁎a Ocean University of China,Qingdao,266003,PR ChinabQingdao Institute of Bioenergy and Bioprocess Technology,Chinese Academy of Sciences,Qingdao,266101,PR Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 23April 2010Received in revised form 25September 2010Accepted 28September 2010Keywords:LiFePO 4GrapheneMixed conducting NetworkLithium-ion batteriesIn this work,mixed (electron and ion)conducting LiFePO 4/graphene composites have been prepared through a facile hydrothermal route followed by heat treatment.It was found that the LiFePO 4particles adhered to the surface of graphene and/or embedded in the graphene nanosheets.As a result,an effective three-dimensional conducting network was formed by bridging graphene nanosheets,which can facilitate electron transport effectively and thus improve the kinetics and rate performance of LiFePO 4.The LiFePO 4/graphene (92:8wt.)composites exhibited a discharge capacity of 160.3mAh g −1at 0.1C and 81.5mAh g −1at 10C,respectively.The comparative investigation on LiFePO 4/graphene composites and LiFePO 4proved that establishing a mixed conducting network with graphene has a potential to enhance the speci fic capacity and rate capability of LiFePO 4.©2010Elsevier B.V.All rights reserved.1.IntroductionIn recent years,LiFePO 4has been extensively studied as the cathode-active material for lithium-ion batteries because of low-cost,low toxicity and relatively high theoretical speci fic capacity of 170mAh g −1[1].However,one of major problem to limit the commercialization of LiFePO 4is poor rate capability which can be attributed to its intrinsically low electronic conductivity (10−9S cm −2)[2]and lithium-ion diffusivity (10−14–10−16cm 2s −1)[3].Considerable efforts have been made to solve this problem,which include:i)surface coating or admixing with electronically conductive materials [4–10],ii)doping LiFePO 4with foreign atoms [11]and iii)decreasing the particle size [12].A concept of establishing a mixed conducting network formed by carbonaceous materials has been demonstrated to improve the rate performance of lithium intercalation materials effectively,because the carbon network provides pathway for electron transfer,and lithium-ion diffusion,resulting in an improvement of the conductivity and electrochemical properties [13–17].Graphene,with one-atom thick layer 2D structure [18],is emerging as a novel nanostructured carbon material with a potential for electrochemical energy storage device applications due to its unique characteristics of high electrical conductivity,large surface area and chemical stability.Recently,graphene was used as support materials to improve the electrochemical performance of anodematerials in lithium-ion batteries [19–22].More recently,Ding et al.reported LiFePO 4/graphene composites with excellent electrochem-ical properties prepared through a co-precipitation method [23].The hydrothermal technique is widely and facilely used to explore advanced materials,especially lithium-ion intercalation materials [24].In this work,we report preparation of the LiFePO 4/graphene mixed conducting network through a hydrothermal route followed by heat treatment.This composite showed an enhanced electrochemical performance as cathode materials in lithium-ion batteries when compared with pristine LiFePO 4.2.Experimental2.1.Preparation of samplesGraphite oxide (GO)was prepared according to the method reported by Hummers [25]from graphite powder (Aldrich,powder,b 20μm,synthetic).LiFePO 4/graphene (8wt.%)composites (denoted as LFPG-8%)were prepared by a facile hydrothermal process [24].Typically,appropriate quantities of FeSO 4·7H 2O,H 3PO 4and ascorbic acid were dissolved into 35mL of deionized water.Then LiOH solution was added dropwise into the conical flask while stirring.The molar ratio of the Li:Fe:P was 3:1:1.After vigorous magnetic stirring for 5min,GO (the weight ratio of GO to LiFePO 4was 8:92)was added into the mixed solution and kept stirring for several minutes.The mixed solution was transferred into a 50mL Te flon-lined stainless steel autoclave,which was heated at 200°C for 5h.After the solution cooled down to room temperature,the precipitate powder was centrifuged and washed several times withSolid State Ionics 181(2010)1685–1689⁎Corresponding authors.Tel.:+8653280662746;fax:+8653280662744.E-mail address:cuigl@ (G.Cui).1These authors contributed equally to thiswork.0167-2738/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.ssi.2010.09.056Contents lists available at ScienceDirectSolid State Ionicsj ou r n a l h o me pa g e :ww w.e l s ev i e r.c o m/l o c a t e /s s ideionized water and acetone.Then the obtained powder was dried at 60°C for 4h under vacuum oven,followed by sintering at 600°C for 2h under a H 2/Ar (5:95,v/v)atmosphere.As a reference,LiFePO 4(without GO addition)was also prepared following the same procedure.2.2.CharacterizationThe crystal structure of the as-prepared film was examined by X-ray diffraction (XRD)spectrometer instrument.The XRD patterns were recorded in a Bruker-AXS Micro-diffractometer (D8ADVANCE)with Cu K αradiation (λ=1.5406Å)from 10°to 80°at a scanning speed of 0.33°min −1.X-ray tube voltage and current were set at 40kV and 40mA,respectively.Resonance Raman spectra were recorded on a JY HR800Raman spectrophotometer (Horiba Jobin Yvon,France)with 532nm diode laser excitation.The morphologies were observed by transmission electron microscopy (TEM,JEOL 2010F).2.3.Electrochemical measurementsElectrochemical performance was performed using two-electrode Swagelok type cells assembled in an argon-filled glovebox.The working electrodes were prepared by mixing active materials,super Pand poly(vinylidene fluoride)(PVDF)in a weight ratio of 70:20:10and pasted on an aluminum foil then dried in vacuum oven at 120°C for 8h.Lithium foil was used as the counter electrode and separated by a glass-fiber separator.The electrolyte was 1.0mol L −1LiPF 6in a mixture of ethylene carbonate (EC)and dimethyl carbonate (DMC)(1:1,v/v).The cells were charged and discharged over a voltage range of 2.0–4.2V (vs Li +/Li)at different rates and carried out with a LAND battery testing system.Cyclic voltammetry (CV)was con-ducted by using an IM6instrument at a scanning rate of 0.1mV s −1between 2.0and 4.2V.Electrochemical impedance spectroscopy measurements were carried out at a discharge state with a sinusoidal signal of 5mV over a frequency range from 100kHz to 100mHz.10203040506070LFPG-8%2θ/degreeGOLiFePO 425.025.526.026.527.0Fig.1.XRD patterns of GO,LiFePO 4and LFPG-8%composites (the inset is the selected part between 25and 27°).1200130014001500160017001800I n t e n s i t y (a .u .)GRaman Shift (cm -1)DFig.2.Raman spectrum of LFPG-8%composites.Fig.3.TEM images of (a)graphene,(b)LiFePO 4,and (c)LFPG-8%composites.1686L.Wang et al./Solid State Ionics 181(2010)1685–16893.Results and discussionXRD patterns of oxide graphite (GO),LiFePO 4and LFPG-8%are shown in Fig.1(inset is the selected part of LiFePO 4and LFPG-8%between 25and 27°).The strong re flection peaks located at around 11°of GO disappeared in the LFPG-8%composites,which mean that the GO has been reduced.All the Bragg peaks can be indexed as a well-crystallized LiFePO 4phase with an orthorhombic structure with the space group of Pnma [26],which proved that introduction of graphene has no effect on the structure of LiFePO 4.It is also shown from the inset of Fig.1that no diffraction peak (002)of graphite (located at around 24.5°)was observed,which is consistent with previous studies of graphene [27].Therefore,it was concluded that the LiFePO 4particles attached onto these graphene prevented most of the restack of graphene into graphite or graphite nanosheets,which was corroborated by the diffraction pattern.Further information for the structure of LFPG-8%can be supported by the Raman spectra (Fig.2).The peaks at about 1350cm −1(D band)and 1595cm −1(G band)are observed in the composites.The G line is assigned to the E 2g phonon of Csp 2atoms,while the D line is a breathing mode of k-point phonons of A1g symmetry [28].The broadening of D and G bands with a strong D line indicates a localized in-plane sp 2domains and disordered graphitic crystal stacking of graphene nanosheets [29].Therefore,the Raman results consistent with the XRD results further demon-strated the formation of graphene nanosheet composites.Some representative transmission electron micrographs of gra-phene,LiFePO 4and LFPG-8%composites are shown in Fig.3.The typical morphology of agglomerated graphene layers was observed.From Fig.3,it can be seen that the LiFePO 4particles are homogenously adhered to the surface of the graphene surface and/or embedded in the graphene nanosheets.Scheme 1represents the structure of mixed conducting LFPG composites,in which the bridging graphene nanosheets can form an effective conducting network.At the same time,a porous structure between LiFePO 4and graphene nanosheets was formed by the random hybrid composite,which can facilitate the penetration of the electrolyte to the surface of active materials,resulting in the superior rate capability and higher reversible capacities in comparison with the LiFePO 4.Cyclic voltammetry was performed in order to investigate the effect of graphene on the electrochemical properties of LiFePO 4by using a scanning rate of 0.1mV s −1(Fig.4).The CV of LiFePO 4/graphene composites show more symmetrical and sharper shape of the anodic/cathodic peaks,which indicates the better electrochemical activity [30].Furthermore the potential separation between anodic and cathodic peaks is 0.191V,whereas that of the LiFePO 4was 0.341V.The well-de fined peaks and smaller peak potential separation indicate the higher electrochemical reactivity and lower ohmic resistance of LFPG-8%composites.Typical charge/discharge pro files of LiFePO 4and LFPG-8%compo-sites at a current rate of C/10(one lithium per formula unit in 10h)are shown in Fig.5.As can be seen,the LFPG-8%composites exhibit a remarkable improved capacity of 160.3mAh g −1,while that of LiFePO 4is only 123.7mAh g −1.Furthermore,the polarization between the charge and discharge plateaus is reduced from 82mV to 39mV for the LFPG-8%composites,indicating that the kinetics of the LiFePO 4is indeed improved by graphene addition.A possible reason is that graphene nanosheets act as a conducting pathway between the LiFePO 4particles.AC impedance measurements are performed on both the samples at the discharged state and the Nyquist plots are shown in Fig.6.The plots were a combination of a depressed semicircle in the high frequency region and a spike in the low frequency region.Previous explanation on the impedance spectra is that the high frequency semicircle is related to the migration of the Li +ions at the electrode/electrolyte interface and charge transfer process.The spike is attributed to the Warburg impedance of long-range Li-ion diffusion [31].Recently,J.Jamnik et al.gave a new explanation on the semicircle that the high frequency impedance semicircle is related to the contact impedance between the metal current collector and the electrode material according to their experimental results andtheoreticalScheme 1.Schematic illustration of the structure of the LFPG mixed conducting network [32].C u r r e n t (m A /g )Voltage (V vs Li +/Li)Fig.4.Cyclic voltammogram of LFPG-8%(solid line)and LiFePO 4(dash line)electrodes at a scan rate of 0.1mV/s in a potential window from 2.0to 4.2V in the EC/DMC solution containing 1M LiPF 6.V l o t a g e (V v s L i +/L i )Specific capacity (mAh/g)Fig.5.Typical charge and discharge pro files of LFPG-8%(□)and LiFePO 4(○)electrodes at a current density of C/10between the voltage limits of 2and 4.2V in the EC/DMC solution containing 1M LiPF 6(the inset shows the flat region magni fied).1687L.Wang et al./Solid State Ionics 181(2010)1685–1689analysis [32,33].Compared with that of LiFePO 4,a smaller depressed semicircle has been shown for LFPG-8%composites,indicating much lower interface impedance between the metal current collector and electrode materials,which may be bene ficial to the rate performance of LFPG-8%composites.Comparison of the rate performance of LiFePO 4and LFPG-8%composites are shown in Fig.7.After the cells have been cycled at a rate of C/10for 10cycles,the current densities are increased stepwise to 10C.Obviously,the LFPG-8%composite exhibits better electro-chemical performance.The highly stable reversible capacity of 81.5mAh g −1has been obtained at the highest current densities of 10C,while only a value of 30.6mAh g −1for LiFePO 4.It is incomprehensible that a very slow increase in the capacity difference was observed (40mAh/g for C/10and 45mAh/g for 10C,respective-ly),which indicates that there are still some other reasons such as interface,polarization and contact in fluencing the electrochemical performance besides the improved ionic/electronic networks [32].In order to get full evaluation on the role of graphene in the composites,the amount of carbon black in the electrode and the graphene ratio in the LiFePO 4/graphene composites were reduced for comparison,respectively.As shown in Fig.8for LFPG-8%,it can be found that when the weight ratio of carbon black in the electrode reduced from 20to 10wt.%,only very slight inferior but fair capacityand rate performance were observed.When the weight ratio of graphene in the composite was further reduced to 3%(denoted as LFPG-3%),the electrochemical performance of the LFPG-3%electrode is inferior to that of LFPG-8%,but is still much better than that of the pristine LiFePO 4electrode.However,there are some anomalies for LFPG-3%especially at a current density of 10C.It is highly possible that LFPG-3%could not build a stable and effective mixed (electron and ion)transportation network at a less amount of graphene (3%).These results clearly show that graphene nanosheets play an important role in improving the speci fic capacity and rate perfor-mance of LiFePO 4.4.ConclusionsIn summary,mixed conducting LiFePO 4/graphene composites have been prepared through a facile hydrothermal route followed by heat treatment.By a comparative study on the electrochemical performance of LiFePO 4and LiFePO 4/graphene composites,we have demonstrated that the speci fic capacity and rate capability of the composites are signi ficantly improved due to the effective conducing network formed by bridging graphene nanosheets.The LFPG-8%composite cathode exhibited excellent electrochemical performances with capacities of 160.3and 81.5mAh g −1at C/10and 10C rates,respectively.This research is of potential interest to employ graphene nanosheets as an electronic conducting network in cathode preparation and to develop high-power lithium-ion batteries for electric vehicles.AcknowledgmentsWe appreciate the support of “100Talents ”program of the Chinese Academy of Sciences,and National Natural Science Foundation of China (Grant Nos.20901044,20971077,and 20902052).References[1] A.K.Padhi,K.S.Nanjundaswamy,J.B.Goodenough,J.Electrochem.Soc.144(1997)1188.[2]S.Y.Chung,Y.M.Chiang,Solid State Lett.6(2003)A278.[3]P.P.Prosini,M.Lisi,D.Zane,M.Pasquali,Solid State Ionics 148(2002)45.[4]H.Huang,S.C.Yin,L.F.Nazar,Electrochem.Solid-State Lett.4(2001)A170.[5]Z.Chen,J.R.Dahn,J.Electrochem.Soc.149(2002)A1184.[6]Y.S.Hu,Y.G.Guo,R.Dominko,M.N.Gaberscek,J.Jamnik,J.Maier,Adv.Mater.19(1963).-Z "(o h m )Z'(ohm)Fig.6.Nyquist plots of LFPG-8%(■)composites and LiFePO 4(●)electrodes.S p e c i f ic c a p a c i t y (m A h /g )Cycle numberFig.7.Cycling and rate performance (herein refers to discharge capacity)of (■)LFPG-8%composites and (●)LiFePO 4electrodes cycled 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