Microemulsion synthesis and electrochemical properties of LiFePO4-C cathode materials

Materials Chemistry and Physics105 (2007) 80–85Microemulsion synthesis of LiFePO4/C and its electrochemicalproperties as cathode materials for lithium-ion cellsZhihui Xu a,b,Liang Xu a,Qiongyu Lai a,∗,Xiaoyang Ji aa College of Chemistry,Sichuan University,Chengdu Sichuan610064,PR Chinab College of Science,Nanjing Agricultural University,Nanjing Jiangsu210095,PR ChinaReceived26December2005;received in revised form18March2007;accepted8April2007AbstractThe electroactive LiFePO4/C nano-composite powders have been synthesized by a microemulsion method.The material were characterized by X-ray powder diffraction,X-ray photoelectron spectroscopic,scanning electron microscopy and its electrochemical properties were investigated by cyclic voltammetry,AC impedance and charge–discharge tests.The as-prepared LiFePO4/C composite had a high capacity of163mAh g−1at the C/10rate,i.e.95.9%of the theoretical capacity.The composite also display a good rate capability and stable cycle-life.The improved electrode performance originates mainly from thefine particle of nanometric dimension and the uniform size distribution in the product as well as the increase in electronic conductivity by the carbon coating.© 2007 Elsevier B.V. All rights reserved.Keywords:Lithium iron phosphate;Microemulsion synthesis;Cathode material;Lithium-ion cells;Electrochemical performance1.IntroductionThe olivine LiFePO4,which has a large theoretical capacity of170mAh g−1,represents a prospective lithium-intercalating cathode material for lithium-ion batteries.LiFePO4offers bene-fits such as inexpensive,natural abundance and environmentally benign,thermal stable in the fully charged state and good cycle stability.Reversible electrochemical extraction of lithium ions from LiFePO4proceeds at about3.4V versus Li+/Li.The volt-age is not so high as to decompose the electrolyte and is not so low as to sacrifice energy density.However,it was reported that this cathode has very low electronic conductivity and diffusion-controlled kinetics associated with the two-phase character of the insertion/extraction process[1],which decreases utilization in the charge–discharge capacity of active material.Recently, the synthesis of a LiFePO4/electronic conductor composite com-pound[2–10]and doping[11–13]have been used to increase the electronic conductivity.Thereinto,coating carbon on the surface of LiFePO4is proved to be a simple and maneuverable method [2,4,6–10]to increase the bulk conductivity.Li+ions diffusion is always associated with electrons transport.Ideally,both the ∗Corresponding author.Tel.:+862885416969;fax:+862885416969.E-mail address:laiqy5@(i).Li+ions and the electrons are available at the same spot of the active material surface.In the presence of carbon coating,Li+ ions diffusion will not be limited by electrons but more likely by the material ionic conductivity,which depends on the Debye length(λ)[14].As pointed out by Huang et al.[15],poor Li+ ions diffusion can be overcome by decreasing the particle size of the active material.Therefore,interests have been led to the production carbon-containing LiFePO4with a smaller particle size,which will be preferred to improve the electrochemical per-formance of the material and thus make it feasible as a cathode for lithium-ion batteries.The electrochemical behavior of LiFePO4is strongly influ-enced by the method of preparation,as well as the precursors and heat treatment protocols.Synthesis methods determine the crystallinity,phase purity,particle morphology,grain size,and surface area,all of which can impact on the electrochemi-cal performance of the olivine LiFePO4[16].LiFePO4has conventionally been synthesized by diffusion-limited solid-state reactions with their associated repeated grinding and a longer period of high temperature operations.Because of the several disadvantages of this method such as inhomogene-ity,larger particle size and broader particle size distribution, numerous synthetic approaches were adopted for preparation of the title compound in its pure and conductive form.Such an effort includes microwave synthesis[17,18],hydrothermal0254-0584/$–see front matter© 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.04.039Z.Xu et al./Materials Chemistry and Physics 105 (2007) 80–8581method[19–22],spray produced LiFePO4[6,12,23],emul-sion dry approach[24],co-precipitation technique[21,25],soft chemistry route[26,27],mechanochemical activation means [20,28,29],sol–gel method[7,30,31],carbothermal reduction method[10,32],pulsed laser deposition technique[33,34], wherein addition of carbon[32],sucrose[6,8,23],Cu/Ag[3] powder has been implemented to improve the conductivity of native LiFePO4through any one the mentioned methods.In this paper,wefirstly selected an n-octane/n-butyl/ cetyltrimethyl ammonium bromide microemulsion system to synthesize successfully a LiFePO4/C composite with nano-metric dimension and uniform size distribution.The prepared sample displayed a good rate capability and stable cycle-life.2.Experimental2.1.Preparation microemulsionMicroemulsion A:50ml n-octane,20ml n-butyl alcohol,16g cetyltrimethyl ammonium bromide(CTAB),2ml5wt.%poly(ethylene glycol)(PEG,mean molecular weight is about4000),4ml0.4mol l−1(NH4)2Fe(SO4)2,4ml 0.4mol l−1H3PO4.Microemulsion B:50ml n-octane,20ml n-butyl alcohol,16g CTAB,2ml 5wt.%PEG,8ml0.6mol l−1LiOH.Homogeneous water-in-oil(W/O)type microemulsion was obtained when the aqueous solution was vigorously mixed with the oily phase.2.2.Synthesis procedureMicroemulsion B was added drop by drop to microemulsion A while stirring with blowing nitrogen gas into the solution.Thefinal solution,which was grayish blue in color,was matured a few hours at60◦C and separated centrifugally.A measured amount of sugar was dissolved in a small amount of water beforehand. Then the obtained powders were ground with the sugar solution slowly by hand. The mixture was heated at different temperature inflowing nitrogen for24h.2.3.AnalysisThe as-prepared composite was characterized by X-ray powder diffraction analysis(XRD,D/max-rA)using Cu K␣radiation(λ=0.154nm)and X-ray photoelectron spectroscopic(XPS,XSAM800).The morphology of the sam-ple was observed by a scanning electron microscope(SEM,JSM-5900LV)and transmission electron microscope(TEM,JEM-2100).A mixture of15wt.%ethyne black,80wt.%the prepared LiFePO4/C com-posite and5wt.%poly(vinylidene difluoride)binder(PVDF)was mixed together in N-methylpyrrolidinone(NMP).This mixed slurry was heated at70◦C in vac-uum for at least12h.The lithium foils were used both as anode and reference electrodes.The cells werefilled with a1mol dm−3solution of LiPF6in ethylene carbonate/dimethyl carbonate(EC/DMC1:1v/v).All cells were assembled in an argon-filled glove box and tested at room temperature.The cell was cycled galvanostatically between2.0V and4.0V(versus Li)at different rates.The cyclic voltammogram(CV)tests were carried out by electrochemical stations (CHI660b)at0.2mV s−1between2.5V and4.3V(versus Li).AC impedance measurements were carried out by the electrochemical stations(IM6)with the frequency range from8M to100mHz.3.Results and discussionFig.1shows the XRD patterns of the samples synthesized at different temperature.Despite LiFePO4phase began to appear at500◦C,both Li3PO4and Fe3(PO4)2can be observed in the prepared sample,demonstrating that synthesis reaction is not complete at this temperature.However,Li3PO4and Fe3(PO4)2Fig.1.XRD patterns of samples synthesized at different temperature:(a) 500◦C;(b)600◦C;(c)700◦C( :Fe3(PO4)2; :Li3PO4).were not detected in the samples synthesized at600◦C and 700◦C,respectively.The crystalline intensity of the sample prepared at700◦C is a little developed than that of the sam-ple prepared at600◦C.The samples,synthesized at600◦C and700◦C,with an ordered olivine(orthorhombic)structure comprise PO4tetrahedra and distorted FeO6octahedra,which produce a two-dimensional pathway for lithium-ion diffusion [1].There were3.2wt.%and2.9wt.%carbon,measured by TGA,in the600◦C sample and the700◦C sample.The high chemical activity of carbon,generated from the pyrolysis of sugar,ensures in situ coating carbon on the surface of fresh formed LiFePO4particles.It was expected the carbon coating on the LiFePO4crystals would control the oxidation of Fe2+in thefinal product,dramatically increase the electronic conduc-tivity of the electrodes,decrease the charge-transfer resistance and make possible the achievement of good capacities even at room temperature and relative high rates.However,we cannot see any diffraction peak attributed to carbon from Fig.1,which is due probably to the amorphous or low crystalline carbon in thefinal product.Fig.2shows the XPS results for the Fe2p binding energy of the sample synthesized at600◦C.Sample was sputtered three times to eliminate detection error from electrons adsorbed on the surface of the sample.Also,information was collected at a depth of4–5nm from the surface.The shift Fe2p3/2binding energy would be related only to the difference in oxidation state of Fe.The observed shift Fe2p binding energy agrees with the82Z.Xu et al./Materials Chemistry andPhysics 105 (2007) 80–85Fig.2.XPS narrow spectra of the Fe2p of the sample prepared at600◦C. bonding energy of the oxidation of Fe2+[24].It is also consistent with the results shown in Fig.1that no Fe3+compounds were detected.SEM images of samples,synthesized at600◦C and700◦C are shown in Fig.3.The600◦C sample and the700◦C sample have average primary crystal size of about90nm and200nm, respectively,and the crystals form loose agglomerates.Obvi-ously,the higher synthesis temperature will result in undesirable particles growth.TEM studies were also conducted(Fig.4)for the two sample.The morphology and grain size distribution show some differences.Global texture is more clearly evidenced in the600◦C sample,while larger secondary particles are vis-ible in the700◦C sample.It should be noted that no obvious carbon particles are observed,indicating that the carbon is not simply dispersed between particles,but most likely coated on the surface of LiFePO4particles.In this microemulsion method,four factors exercise a great influence on obtaining the LiFePO4particles with nano-metric particle size and uniform size distribution,and thus influencing the electrochemical properties of the LiFePO4. Firstly,interaction would occur between CTAB and PEG at oil–water interface.Hydrophilic group in CTAB was bonded with the polar group on PEG chain through ion–dipole action, and thus forming polymeride–surfactant composite.The PEG molecule chain will stretch at oil–water interface because of the repulsion effect of the electriferous groups in CTAB molecule.This polymeride–surfactant composite can increase Gibbs Marangoni effect,Gibbs elasticity or interface viscosity [35],which decreases water core coalescence,increases the sta-bility of microemulsion nano-reactor and achieves the goal of controlling the particle size and size distribution of thefinal Fig.3.SEM photograph of samples synthesized at different temperature:(a)600◦C;(b)700◦C.Fig.4.TEM photograph of samples synthesized at different temperature:(a)600◦C;(b)700◦C.Z.Xu et al./Materials Chemistry and Physics 105 (2007) 80–8583Fig.5.Cyclic voltammetry curves of the samples synthesized at different tem-perature:(a)600◦C;(b)700◦C.product.Secondly,the polymeride–surfactant composite was adsorbed on the surface of sedimental particles when mixing microemulsion A with B.The polymeride–surfactant composite surrounding sedimental particles provides great spatial hin-drance and helps to inhibit the conglomeration of sedimental particles.Thirdly,carbon generated from the pyrolysis of sugar was coated on the surface of fresh formed LiFePO4particles during the heat treatment process.The in situ coated carbon acts not only as the electronic conductor in thefinal product but also as the obstructerfilm preventing the undesired particle growths. In addition,synthesis temperature also affects the particles size in thefinal product.The LiFePO4particles with nanometric par-ticle size and uniform size distribution would be more beneficial to the full utilization of active materials.The influence of oil to water ratio in volume and concentration in aqueous phase on particle size and distribution is still under investigation.Fig.5shows thefirst cyclic voltammograms of the LiFePO4/C composite synthesized at600◦C and700◦C.Only a couple of anodic and cathodic peaks,which corresponds to the two-phase charge/discharge reaction of the Fe2+/Fe3+ redox couple,were observed at about3.5V[1].The approxi-mately symmetrical peaks suggest the good insertion/extraction reversibility of the microemulsion produced active material.As can be seen in Fig.5,the peak symmetry of the former was superior to that of the latter,meaning that the600◦C sample has the better reversibility for the lithium insertion/extraction,the less ohmic polarization and the more superior electrochemical performance than the700◦C sample.Fig.6shows thefirst discharge capacities of the samples syn-thesized under600◦C and700◦C at the C/10rate.As seen in Fig.6,all the profiles exhibited extremelyflat operating volt-age at about3.5V(versus Li).Aflat charge–discharge profile over a large range indicates that the Fe2+/Fe3+redox reaction of LiFePO4proceeds as a two-phase via afirst order-transition between FePO4and LiFePO4[1]:LiFePO4(S)⇔FePO4(S)+Li++e(1) Because of the increase in electronic conductivity by the carbon coating,the charge and discharge voltage are very close,indicating small electrode polarization during the charge–discharge process.As shown in Fig.6,the600◦Csample Fig.6.First charge–discharge profiles of samples synthesized at different tem-perature:(a)600◦C;(b)700◦C.delivered an initial charge capacity of167mAh g−1and a subse-quent discharge capacity of163mAh g−1.However,the700◦C sample delivered an initial charge capacity of164mAh g−1and a subsequent discharge capacity of159mAh g−1.According to the results obtained from Figs.3,5and6,the sample had the smaller particles and also had the better electrochemical properties.Fig.7shows thefirst discharge capacities of samples syn-thesized under600◦C and700◦C at the C/10,C/2,C,2C rate, respectively.The discharge capacities of samples decreased with the speed up of the discharge rate from C/10to2C.It is shown that the electrochemical property of the700◦C sample worsened more dramatically at high rate than that of the600◦C sample. This phenomenon may also be ascribed to the different particles size of the two samples.The larger particles give rise to transport limitation both for lithium ions and electrons diffusion,which results in capacity loss in utilization,especially at the higher current.Furthermore,the less the particles size is,the more eas-ily electrolyte penetrate across the whole active material.In this way,lithium ions and electrons are available everywhere on the particles surface,and thus can greatly improve the electrochem-ical reaction(1).The rate capability difference between the600◦C sample and the700◦C sample can also be explained from their Nyquist plots (Fig.8).The spectra shows an intercept at high frequency,fol-lowed by a depressed semicircle in the high-middle frequency region,and a straight line in the low frequency region.The inter-cept impedance on the real resistance axis represents the ohmic resistance,which consists of the resistance of the electrolyte and electrode.The high frequency region of the semicirclerep-Fig.7.Rate capability of samples synthesized at different temperature.84Z.Xu et al./Materials Chemistry andPhysics 105 (2007) 80–85Fig.8.ac impedance of samples synthesized at different temperature.resents the migration of the Li +ions at the electrode/electrolyte interface through the SEI layer,whereas,the middle frequency range of the semicircle corresponds to the charge-transfer.The low frequency region of the straight line is attributed to the dif-fusion of the lithium ions into the bulk of the electrode material or so-called Warburg diffusion.As can be seen from Fig.8,the 600◦C sample exhibited lower charge-transfer resistance (R ct )than the 700◦C sample.At a low discharge rate,the effect of a large R ct can be neglected,but for higher discharge rates the R ct is mainly responsible for the voltage drop causing a sudden decrease in the electrochemical performance.To some extent,the difference of charge-transfer resistance between the 600◦C sample and the 700◦C sample is related to the differ-ence of particle size between the two samples.A decrease of the LiFePO 4particles’size will decrease the polarization asso-ciated with electronic and/or ionic resistance at the boundary of crystallites within each polycrystalline particle of active mate-rial,electronic and/or ionic resistance through LiFePO 4/FePO 4interface within each crystallite,and thus improve the reversible capacity of LiFePO 4material.The cycle-life at different discharge rate of the sample,syn-thesized at 600◦C,is shown in Fig.9.Slow capacities decrease is observed and stable cycle-life is present at the C/10,C/2and C rate.Carbon coating was used to increase the bulk electronic conductivity.However,the intrinsic electronic con-ductivity remained unchanged.Under high-rate performance,intrinsic electronic conductivity should become more important [36],which demonstrates that the sample has better cycling sta-bility below C rate.After 40cycles,the discharge capacity of the sample is still about 95.2%of its first dischargecapacitiesFig.9.Cycling performance of the sample synthesized at 600◦C.at the 2C rate.The good cycling behavior is mainly attributed to the nanometric particle size,uniform size distribution and enhancement of the electronic conductivity by the carbon coat-ing.Besides,the high stability of the olivine structure and the minor lattice adjustments [16]are favorable for the cycle stabil-ity.4.ConclusionA microemulsion method is used to manufacture LiFePO 4/C composite with uniform and fine particles.The main advan-tage of this synthesis route is that the reactants are mixed on more homogeneous level by this oil-based solution approach,and that grains are effectively inhibited from coalescence dur-ing the synthetic process.The in situ coated carbon,grains size of nanometric dimension and uniform size distribution as well as phase purity brought about the superior capability and the high capacity retention upon cycling.The 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