Preparation and characterization of carbon-coated LiFePO 4cathode materials for lithium-ion batteries with resorcinol –formaldehyde polymer as carbon precursorYachao Lan,Xiaodong Wang ⁎,Jingwei Zhang,Jiwei Zhang,Zhishen Wu,Zhijun Zhang ⁎Key Laboratory for Special Functional Materials,Henan University,Kaifeng 475004,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 8February 2011Received in revised form 26May 2011Accepted 3June 2011Available online 12June 2011Keywords:Lithium iron phosphateResorcinol –formaldehyde polymer Lithium-ion batteryLiFePO 4/C composites were synthesized by two methods using home-made amorphous nano-FePO 4as the iron precursor and soluble starch,sucrose,citric acid,and resorcinol –formaldehyde (RF)polymer as four carbon precursors,respectively.The crystalline structures,morphologies,compositions,electrochemical performances of the prepared powders were investigated with XRD,TEM,Raman,and cyclic voltammogram method.The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a de ficient carbon coating on the surface of LiFePO 4particle,but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO 4particle,and the corresponding thicknesses of the uniform carbon films are 2.5nm and 4.5nm,respectively.When RF polymer was used as the carbon precursor,the material showed the highest initial discharge capacity (138.4mAh g −1at 0.2C at room temperature)and the best rate performance among the four materials.©2011Elsevier B.V.All rights reserved.1.IntroductionLiFePO 4is an attractive cathode material for lithium-ion batteries because of its high theoretical capacity of 170mAh g −1,environ-mental benign,and high thermal stability.However,its poor electric conductivity of less than 10−13S cm −1limits its battery performance [1],such as the dramatic decrease in power at a high current density,which is the main drawback to commercial use.To overcome the low electric conductivity of LiFePO 4,many effective approaches have been introduced,including metal substitution [2–5],metal powder com-pounding [6],and conductive carbon coating [7–15].Among them,the preparation of LiFePO 4/carbon composite (LiFePO 4/C)is one of the attractive ways to improve the electric conductivity of the final product by forming a good conduction path.Furthermore,carbon can be also used as a reductant,which can reduce Fe 3+ions to Fe 2+ions.It should be noted that many studies involving the synthesis of nano-sized LiFePO 4employ Fe 2+salts as precursors [3,16–20],such as FeC 2O 4·2H 2O and (CH 3COO)2Fe,which are expensive.Therefore,it is necessary to use cheap materials and a convenient method.Here,we report the synthesis,characterization and electrochemical test of LiFePO 4/C composites prepared by two methods using home-made amorphous nano-FePO 4as the iron precursor and various organics as carbon precursors.The two methods using FePO 4as starting material are cheap and environmentally benign for the production of LiFePO 4material.Particularly,we present a novel method to synthesize a uniformcarbon film coated LiFePO 4cathode materials.This method involved an in situ reaction of resorcinol and formaldehyde on the surface of amorphous FePO 4.At room temperature,electrochemical tests showed that this material exhibited an initial discharge capacity of 138.4mAh g −1at 0.2C and a good cycling property at 0.5and 1.0C rate,respectively.2.Experimental2.1.Preparation of amorphous nano-FePO 4Amorphous nano-FePO 4was prepared by spontaneous precipita-tion from aqueous solutions.An equimolar solution of H 3PO 4was added to a solution of Fe(NO 3)3·9H 2O at 60°C under stirring and given amounts of PEG-400as surfactant.Then ammonia water (NH 3·H 2O)was slowly added to the mixed solution under vigorous stirring and a milk-white precipitate formed immediately.The pH of the solution was kept at 2.0.The precipitate was filtered and washed several times with distilled water.After drying in vacuum oven at 120°C for 12h,yellowish-white amorphous FePO 4was obtained.2.2.Preparation of LiFePO 4/CTwo methods were used to prepare the LiFePO 4/C composites in this study.2.2.1.Method oneA rheological phase method [21]was employed to synthesize LiFePO 4/C composite.Stoichiometric amount of amorphous FePO 4,LiOH·H 2O were used as the starting materials.The carbon precursorsPowder Technology 212(2011)327–331⁎Corresponding authors.Tel./fax:+863783881358.E-mail address:donguser@ (X.Wang).0032-5910/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.powtec.2011.06.005Contents lists available at ScienceDirectPowder Technologyj o u r n a l h o me p a g e :w w w.e l sev i e r.c o m /l oc a t e /pow t e care soluble starch(50.0g/1mol LiOH·H2O),sucrose(35.0g/1mol LiOH·H2O),citric acid monohydrate(21.0g/1mol LiOH·H2O),respec-tively.These carbon precursors were respectively solved in an appropri-ate amount of distilled water under stirring and heating.Then the amorphous FePO4and LiOH·H2O were added under vigorous stirring. Subsequently,the mixtures were respectively dried in an oven at120°C for6h,heated at350°C for1h in argonflow,treated at750°C for12h in argonflow,and ground.Finally,the LiFePO4/C composites were obtained and were denoted as sample A,sample B and sample C,respectively. 2.2.2.Method twoIn a typical synthesis,0.10g of CTAB was solved in30ml of distilled water solution under continuous stirring.Subsequently,1.52g FePO4·3H2O,0.055g resorcinol(R)and0.10ml formaldehyde(F)were successively added.When the temperature of water bath was up to85°C,LiOH·H2O was added.The mixture was kept stirred up in the dark for2h,dried in an oven at120°C for6h,heated at 350°C for1h in argonflow,treated at750°C for12h in argonflow, andfinally ground to obtain the LiFePO4/C composites(denoted as sample D).These four samples and their corresponding parameters are listed in Table1.The carbon contents of the samples were calculated by the loss on ignition of the four LiFePO4/C composites in air.2.3.CharacterizationThermogravimetric(TG)and differential thermal analysis(DTA) analyses were conducted with an EXSTAR6000thermal analysis system at a heating rate of10°C min−1.The powder X-ray diffraction (XRD,X'Pert Pro MPD,Philips)using Cu Ka radiation was employed to identify the crystalline phase of the prepared materials.Raman spectrum was recorded on a Renishaw RM-1000Microscopic Raman spectrometer with457.5nm excitation requiring a10mW power at room temperature.Low-magnification and high-magnification TEM images were taken on a JEM-2010transmission electron microscope (using an accelerating voltage of200kV).Electrodes were fabricated from a mixture of prepared carbon-coated LiFePO4powders(80wt.%),carbon black(12wt.%),and polyvinylidenefluoride in N-methylpyrrolidinon(8wt.%).The slurry was spread onto Al foil and dried in vacuum at120°C for12h.The carbon-coated LiFePO4loading was2mg cm−2in the experimental cells.The cells were assembled in an argon-atmosphere-filled glove box.The electrolyte was1M LiPF6in a mixture of ethylene carbonate (EC)and dimethyl carbonate(DMC)(1:1volume).The cells were galvanostatically charged and discharged at a voltage range of2.5–4.2V with LAND battery testing system at room temperature.Cyclic voltammograms were run on an IM6impedance and electrochemicalmeasurement system(Zahner,Germany)at a scan rate of0.1mV s−1 between2.5and4.0V.3.Results and discussionThe TEM images of the amorphous nano-FePO4were shown in Fig.1.The morphology of the as-prepared FePO4is an irregular particle with an average diameter of30nm.Most of the particles connected to each other because of their high surface energy which results from their small sizes.Fig.2a shows the TG/DTA curves of the FePO4·3H2O powder with a heating rate of10°C/min from room temperature to850°C in air.On the DTA curve near150°C,there is a very strong endothermic peak, associating with the sharp weight loss on the TG curve,which is related to the quick dehydration of FePO4·3H2O.During150–550°C, 26.3%weight loss on the TG curve indicates the slow elimination of residual H2O in FePO4·3H2O,exactly corresponding to the loss of crystalline water of FePO4·3H2O.And one exothermic peak is displayed at a higher temperature of590°C,which is not accompa-nied by appreciable weight loss in the TG curve,indicating the transformation of the amorphous FePO4to hexagonal FePO4crystal. The XRD patterns of FePO4·3H2O before and after calcination have been investigated in Fig.2b.As illustrated in pattern A,it can be seen that there is no evidence of diffraction peaks before calcination, indicating the synthesized FePO4·3H2O is just amorphous.While for the calcinated FePO4·3H2O at600°C for6h in air,it exhibits strong and narrow peaks revealing a well-crystallized material in pattern B. All of the diffraction peaks of the prepared FePO4are indexed to a single-phase hexagonal structure with a P3121space group and without any impurities,which is in good agreement with the standard card(JCPDS card no:72–2124).Table1Carbon precursors and residual carbon content of samples A,B,C and D.Samples A B C DCarbon precursor Starch Sucrose Citric acid RF polymer Final carbon content(wt.%) 5.48.5 4.35.1Fig.1.TEM images of the prepared amorphous nano-FePO4.n et al./Powder Technology212(2011)327–331The XRD diffraction patterns of LiFePO 4/C powders prepared with different carbon precursors were shown in Fig.3.All peaks can be indexed as a single phase with an ordered olivine structure indexed to the orthorhombic space group,Pnmb (JCPDS card no.83–2092).The obtained lattice parameters are sample A:a=10.2956Å,b=6.0367Å,and c =4.7001Å,sample B:a =10.1992Å,b =6.0483Å,andc=4.6971Å,sample C:a=10.2472Å,b=6.0208Å,and c=4.6882Åand sample D:a=10.3372Å,b=5.9993Å,and c=4.6932Å,respec-tively.There is no evidence of diffraction peaks for carbon,though some amorphous masses and films attached to the LiFePO 4particles were observed from TEM images (see Fig.4).This indicates the carbon contents are very low.Morphologies of these LiFePO 4/C composites were shown in Fig.4.It is obvious that the samples show different carbon distribution on LiFePO 4particle surface.From Fig.4a,c,e and g,we observed that the samples were composed of agglomerated particles whose sizes range from 50to 300nm.From Fig.4b and d,there is not enough carbon coating to spread throughout the substrate particles.In contrast to sample A and sample B,there are uniform carbon thin films on the grain surfaces of sample C and sample D,and the thickness of the carbon films are about 2.5nm (Fig.4f)and 4.5nm (Fig.4h),respectively.The reason may lie in that different carbon precursors have different adsorbabilities on the surface of FePO 4·3H 2O particles,resulting in different carbon distribution on the surface of LiFePO 4particle after the post treatment.Soluble starch and sucrose possess plentiful hydroxyl groups,by which soluble starch and sucrose molecules could probably weakly adsorb on the surface of FePO 4·3-H 2O particles in the hydrogen bonding.In the post treatment process,part of soluble starch and sucrose molecules desorbed from the surface of FePO 4·3H 2O particles,resulting in the de ficient carbon coating.But citric acid possesses carboxyl groups,which may be partially esteri fied by hydroxyl groups on the FePO 4·3H 2O particles,forming a tight connection.This results in more complete carbon coating after the post treatment.For sample D,we suppose that,in the present synthetic system,the surfactant CTAB may con fine the resorcinol –formaldehyde (RF)polymer molecules and FePO 4·3H 2O particles in plenty of tiny spaces,so the RF polymer molecules were tightly attached to FePO 4·3H 2O particles.After the post treatment,the RF polymer was transformed into the carbon film which tightly stuck on the surface of LiFePO 4particle.In addition,from the HRTEM image of sample D (shown in Fig.4h),the d-spacing of 0.294nm corresponds to the (211)plane of LiFePO 4.As an important aid investigating the structure of the carbon,the Raman measurement was adopted,and the results were shown in Fig.5.Every Raman spectrum consists of a small band at 940cm −1,which corresponds to the symmetric PO 4stretching vibration in LiFePO 4.The intense broad bands at 1350and 1590cm −1can be attributed to the characteristic Raman spectra of carbon.The bands at 1590cm −1mainly correspond to graphitized structured carbon of G band,while that at 1350cm −1corresponds to disordered structured carbon of D band [22,23].The graphitized carbon contains sp 2hybrid bonding,which is positively correlated with the electronic conduc-tivity of carbon,and the disordered carbon mainly corresponds to sp 3hybrid bonding.As shown in Fig.5,the integrated intensity ratios of sp 2/sp 3of the LiFePO 4/C composites synthesized with different carbon precursors are 0.865(curve A),0.857(curve B),0.856(curve C)and 0.860(curve D),respectively.So the similar sp 2/sp 3ratios of the four samples give us few clues to explain the difference in their electrochemical performances.Fig.6shows the cycling performance curves of all the samples at different rates.As shown in Fig.6,the initial discharge capacities of sample A,sample B,sample C and sample D at room temperature at 0.2C rate are 110.4,118.8,137.7and 138.4mAh g −1,respectively.The capacity of sample D gradually increases in the initial cycles.This may be due to the incomplete dispersion of the electrolyte into the electrode material at the beginning.Moreover,the capacity of sample D is highest among the four samples at 0.5C and 1.0C,indicating that method two is better than method one.The lower capacities of sample A and sample B must be due to the incomplete carbon coating on the LiFePO 4particles.The higher capacity of sample D than that of sample C may be attributed to the thicker carbon film of sample D keeping the crystal structure of LiFePO 4morestable.Fig.2.(a)TG/DTA curves of the FePO 4·3H 2O.(b)XRD patterns of the FePO 4samples before (A)and after (B)calcination inair.Fig. 3.XRD patterns of LiFePO 4/C composites synthesized with different carbon precursors.329n et al./Powder Technology 212(2011)327–331In order to further understand the electrochemical properties of the four samples,the cyclic voltammogram (CV)curves were performed at a scan rate of 0.1mV s −1at room temperature (as shown in Fig.7).Each of the CV curves consists of an oxidation peak and a reduction peak,corresponding to the charge reaction and discharge reaction of the Fe 2+/Fe 3+redox couple.In the CV pro files of the LiFePO 4cathode material,the smaller voltage difference between the charge and discharge plateaus and the higher peak current means better electrode reaction kinetics,and consequently better rate performance.Sample A and sample B electrodes have broad peaks in CV curves.In contrast,sample C and sample D electrodes demonstrate sharp redox peaks,which indicate an improvement in the kinetics of the lithium intercalation/de-intercalation at the electrode/electrolyte interface.The voltage difference of sample D is smaller than that of sample C,so sample D demonstrates a better rate performance.4.ConclusionsLiFePO 4/C composites were synthesized by two methods using home-made amorphous nano-FePO 4as the iron precursor and various organics as carbon precursors.It was found that employing soluble starch and sucrose as the carbon precursors resulted in a de ficient carbon coating on the surface of LiFePO 4particle,but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO 4particle.Particularly,when RF polymer was used as the carbon precursor,the carbon film is thicker,and the material showed the highest initial discharge capacity (138.4mAh g −1at 0.2C at room temperature)and the best rate performance among the four materials.The intensities of redox peak and the voltage differences in the CV curves of the four samples are consistent with their rateperformance.Fig.4.TEM images of synthesized LiFePO 4/C composite synthesized with different carbon precursors.(a)and (b)sample A,(c)and (d)sample B,(e)and (f)sample C,(g)and (h)sampleD.Fig. 5.Raman shift of LiFePO 4/C composites synthesized with different carbonprecursors.Fig.6.The cycling performance curves of the samples with different carbon precursors at various discharge rates.n et al./Powder Technology 212(2011)327–331References[1] A.K.Padhi,K.S.Nanjundaswamy,J.B.Goodenough,Phospho-olivines as positive-electrode materials for rechargeable lithium batteries,J.Electrochem.Soc.144(1997)1188–1194.[2]T.Nakamura,Y.Miwa,M.Tabuchi,Y.Yamada,Structural and surfacemodi fications of LiFePO 4olivine particles and their electrochemical properties,J.Electrochem.Soc.153(2006)A1108–A1114.[3]S.-Y.Chung,J.T.Bloking,Y.-M.Chiang,Electronically conductive phospho-olivinesas lithium storage electrodes,Nat.Mater.1(2002)123–128.[4]P.S.Herle,B.Ellis,N.Coombs,L.F.Nazar,Nano-network electronic conduction iniron and nickel olivine phosphates,Nat.Mater.3(2004)147–152.[5]G.X.Wang,S.Bewlay,S.A.Needham,H.K.Liu,R.S.Liu,V.A.Drozd,J.-F.Lee,J.M.Chend,Synthesis and characterization of LiFePO 4and LiTi 0.01Fe 0.99PO 4cathode materials,J.Electrochem.Soc.153(2006)A25–A31.[6] F.Croce,A.D'Epifanio,J.Hasson,A.Duptula,T.Olczac,B.Scrosati,A novel conceptfor the synthesis of an improved LiFePO 4lithium battery cathode,Electrochem.Solid-State Lett.5(2002)A47–A50.[7]H.Huang,S.C.Yin,L.F.Nazar,Approaching theoretical capacity of LiFePO 4at roomtemperature at high rates,Electrochem.Solid-State Lett.4(2001)A170–A172.[8]M.Herstedt,M.Stjerndahl,A.Nyten,T.Gustafsson,H.Rensmo,H.Siegbahn,N.Ravert,M.Armand,J.O.Thomas,K.Edstroem,Surface chemistry of carbon-treated LiFePO 4particles for Li-ion battery cathodes studied by PES,Electrochem.Solid-State Lett.6(2003)A202–A206.[9]M.M.Doeff,Y.Hu,F.McLarnon,R.Kostecki,Effect of surface carbon structure onthe electrochemical performance of LiFePO 4,Electrochem.Solid-State Lett.6(2003)A207–A209.[10]Y.Hu,M.M.Doeff,R.Kostecki,R.Finones,Electrochemical performance of sol –gelsynthesized LiFePO 4in lithium batteries,J.Electrochem.Soc.151(2004)A1279–A1285.[11]X.Z.Liao,Z.Ma,L.Wang,X.Zhang,Y.Jiang,Y.He,A novel synthesis route forLiFePO 4/C cathode materials for lithium-ion batteries,Electrochem.Solid-State Lett.7(2004)A522–A525.[12] C.H.Mi,X.B.Zhao,G.S.Cao,J.P.Tu,In situ synthesis and properties of carbon-coated LiFePO 4as Li-ion battery cathodes,J.Electrochem.Soc.152(2005)A483–A487.[13]R.Dominko,M.Bele,M.Gaberscek,M.Remskar,D.Hanzel,S.Pejovnik,J.Jamnik,Impact of the carbon coating thickness on the electrochemical performance of LiFePO 4/C composites,J.Electrochem.Soc.152(2005)A607-A607.[14]K.Zaghib,J.Shim,A.Guer fi,P.Charest,K.A.Striebel,Effect of carbon source asadditives in LiFePO 4as positive electrode for lithium-ion batteries,Electrochem.Solid-State Lett.8(2005)A207–A210.[15]K.S.Park,J.T.Son,H.T.Chung,S.J.Kim,C.H.Lee,K.T.Kang,H.G.Kim,Surfacemodi fication by silver coating for improving electrochemical properties of LiFePO 4,Solid State Commun.129(2004)311–314.[16] A.Yamada,S.C.Chung,K.Hinokuma,Optimized LiFePO 4for lithium batterycathodes,J.Electrochem.Soc.148(2001)A224–A229.[17]N.Meethong,H.-Y.Shadow Huang,W.C.Carter,Y.-M.Chiang,Size-dependentlithium miscibility gap in nanoscale Li 1−x FePO 4,Electrochem.Solid-State Lett.10(2007)A134–A138.[18] C.Delacourt,P.Poizot,S.Levasseur,C.Masquelier,Size effects on carbon-freeLiFePO 4powders,Electrochem.Solid-State Lett.9(2006)A352–A355.[19] D.Choi,P.N.Kumta,Surfactant based sol –gel approach to nanostructured LiFePO 4for high rate Li-ion batteries,J.Power Sources 163(2007)1064–1069.[20]K.Zaghib,A.Mauger,F.Gendron,C.M.Julien,Surface effects on the physical andelectrochemical properties of thin LiFePO 4particles,Chem.Mater.20(2008)462–469.[21]Y.H.Huang,H.B.Ren,S.Y.Yin,Y.H.Wang,Z.H.Peng,Y.H.Zhou,Synthesis ofLiFePO 4/C composite with high-rate performance by starch sol assisted rheolog-ical phase method,J.Power Sources 195(2010)610–613.[22]M.M.Doeff,J.D.Wilcox,R.Kostecki,u,Optimization of carbon coatings onLiFePO 4,J.Power Sources 163(2006)180–184.[23]G.L.Yang,A.F.Jalbout,Y.Xu,H.Y.Yu,X.G.He,H.M.Xie,R.S.Wang,Effect ofpolyacenic semiconductors on the performance of olivine LiFePO 4,Electrochem.Solid-State Lett.11(2008)A125–A128.Fig.7.The CV pro files of the different samples at the scan rate of 0.1mV s −1.331n et al./Powder Technology 212(2011)327–331。
