应用溶胶-凝胶法合成LiNi(0.75-x)Co0.25TixO2(x=0,0.1,0.25)系列正极材料,其结构、形貌、粒度、电化学性能由TG、XRD、SEM和电池充放电测试表征研究表明,材料的电化学性能与钛掺杂量密切相关.在钴含量不变的情况下,随着Ti含量(x)的增加,材料由六方层状结构逐渐向立方结构转变,x=0.25时,出现了立方相与六方相共存.根据实验和理论计算结果简要讨论了钛掺杂对正极材料LiNi0.75Co0.25O2结构和电化学性能的影响.In this study,a series of Co-doped compounds with the formula of LiNi_((0.75-x))Co_(0.25)Ti_(x)O_(2)(x=0,(0.1),0.25)was synthesized by using sol-gel method.Their structure,particle size,electrochemical properties were studied by TG,XRD,SEM and electrochemical tests.It was shown that the structure of materials changed from a hexagonal layered structure to a cubic structure as the x value increased.In particular,a mixture of hexagonal and cubic structure was presented at x =0.25.The electrochemical performance of the materials depended strongly on the amount of Ti doping.Possible effects on the structure and performance of LiNi_((0.75-x))Co_(0.25)Ti_(x)O_(2) due to Ti doping are briefly discussed based upon the experiment and computational results.作者联系地址：厦门大学化学系材料科学与工程系固体表面物理化学国家重点实验室,厦门大学化学系材料科学与工程系固体表面物理化学国家重点实验室,厦门大学化学系材料科学与工程系固体表面物理化学国家重点实验室,厦门大学化学系材料科学与工程系固体表面物理化学国家重点实验室 福建厦门361005,福建厦门361005,福建厦门361005,福建厦门361005Author's Address: *,WANG Shu-fenDepartment of Chemistry,Department of Materials Science & Engineering,State Key Laboratory for Physical Chemistry of Solid Surfaces,Xiamen University,Xiamen 361005,Fujian,Chin

CHENG Xuan

HE Hui

ZHANG Ying

张颖

王淑芬

程璇

贺慧

Xiamen University Institutional Repository

第 12卷 　第 3期2006年 8月电化学ELECTROCHEM ISTRYVol. 12　No. 3Aug. 2006Acticle ID: 100623471 (2006) 03202432007Received: 2006203231　3 Corresponding author, Tel: (862592) 2180999, Email: yzh@ xmu. edu. cnSupported by the National Natural Science Foundation of China (No. 10472098)Effect of TiDoping on the Structure and Performanceof L iNi0. 75 Co0. 25 O2HE Hui, CHENG Xuan, ZHANG Ying3 , WANG Shu2fen(D epartm en t of Chem istry, D epartm en t of M ateria ls Science & Engineering, S ta te Key L abora tory forPhysica l Chem istry of Solid Su rfaces, X iam en U niversity, X iam en 361005, Fu jian, China)Ab s trac t: In this study, a series of Co2doped compounds with the formula of L iN i(0. 752x) Co0. 25 Tix O2 ( x = 0,0. 1, 0. 25) was synthesized by using sol2gel method. Their structure, particle size, electrochem ical p ropertieswere studied by TG, XRD, SEM and electrochem ical tests. It was shown that the structure of materials changedfrom a hexagonal layered structure to a cubic structure as the x value increased. In particular, a m ixture of hexa2gonal and cubic structure was p resented at x = 0. 25. The electrochem ical performance of the materials dependedstrongly on the amount of Ti dop ing. Possible effects on the structure and performance of L iN i(0. 75 - x) Co0. 25 Tix O2due to Ti dop ing are briefly discussed based upon the experiment and computational results.Key wo rds: L ithium ion batteries, Cathode materials, Ti dop ing, Electrochem ical performanceCLC N um be r: TM 912. 9　　　　　　　Do cum en t Code: A1　 In troductionL ithium cobalt oxide has been widely used as acathode ( positive electrode) material for commercialsecondary lithium 2ion batteries due to its advantagesof easy p reparation, high voltage, good reversibility,and high theoretical specific capacity. A L iCoO2 cath2ode with a carbon anode to make the first successfulL i2ION battery [ 122 ] , which now dom inates the lithiumbattery market. There is a lim ited availability of co2balt, which causes it to have a high p rice and also co2balt brings the environment pollution[ 2 ] . In order totackle the p roblem s associated with the high cost andsystem instability of L iCoO2 , a method for the p repa2ration of a version of the compound which hasimp roved electrochem ical characteristics and cationicsubstitutions on the cobalt sites has been soughtextensively. Of these substituted compounds,L iN ix Co1 - x O2 has been identified as one of the mostattractive materials. The layered L iN ix Co1 - x O2 (0 < x< 1 ) compounds have been studied extensively ascathode materials for lithium batteries[ 629 ]. The poorthermal stability and cyclability of L iN ix Co1 - x O2 (0 <x < 1) demand for dop ing another element, such asTi, A l, Mg, Fe, Y, Sr, etc.Among those dop ing elements tried, Ti is re2sourceful in the earth and it has small structure chan2ges when lithium ion intercalation and deintercala2tion. Previous studies[ 10213 ]confirmed that doped Tican make the structure more stable, hence, imp rovethe thermal stability and cyclability. However, p revi2ouswork only investigated doped Tiwith small amount( < 10% ). In order to further study the effect of theamount of Ti dop ing on the structure and performanceand understand the role of Ti p layed in the cathodematerial of L iN ix Co1 - x O2 , we used sol2gel method tosynthesize a series of L iN i0. 75 - x Co0. 25 Tix O2 ( x = 0,0. 1, 0. 25 ). The structure, morphology, electro2chem ical performance and electric state of the materi2als at different x values both experimentally and bycalculation using VASP were exam ined.2　Exper im en ta l2. 1　Syn thesis of M a ter ia lsThree types of L iN i0. 75 - x Co0. 25 Tix O2 ( x = 0, 0. 1,0. 25 ) were p repared by sol2gel p retreatment usingcitric acid as a chelating agent and solid2phase forma2tion. A stoichiometric amount of lithium nitrate(L iNO3 ) , nickel nitrate (N iNO3 ·6H2 O) and cobaltnitrate ( CoNO3 ·6H2 O ) was dissolved in absoluteethyl alcohol and m ixed with aqueous solution of citricacid. And then a stoichiometric amount of tetrabutyltitanate (C16 H36 O4 Ti) was added. The resulting solu2tion was stirred at 80℃ for more than 12 h to obtain aclear viscous gel. The gel was dried at an oven at 120℃ for 12 h. L iN i0. 75 - x Co0. 25 Tix O2 ( x = 0, 0. 1,0. 25) were calcined at 725 ℃ for 2 h after p recalcin2ing the obtained p recursor at 380 ℃. During heatingand cooling, the variation of the temperature wasfixed at 1 ℃ /m in.2. 2　Character iza tion of M a ter ia lsThe thermal analysis was carried out on aNetzsch STA 400 analyzer with 50 mL /m in of flowingair and a heating rate of 10 ℃ /m in in the temperaturerange of 323～1273 K used p recursors of dry gels.The structures of the materials p repared were charac2terized by powder X2ray diffraction ( XRD ) , usingPhilip s Panalytical X’pert diffractometer with CuKαradiation operated at 40 kV and 30 mA. The datawere collected in the 2θ range of 10°～90°using astep size of 0. 0167°and a counting time of 10 s perstep. The morphologies of the materials were obtainedby using a scanning electron m icroscope (LEO 1530field em ission SEM , Oxford Instruments) which wasoperated at 15 kV. The samp les were coated by Aubefore the SEM observation.Electrochem ical characterization was performedwith coin2type cells. The cathode was p repared bym ixing an 85∶5∶10 (w /w ) ratio of active material,carbon black, and polyvinylidene fluoride binder, re2spectively, in N 2methyl pyrrolidinone. L ithium metalwas used as an anode and a polyp ropylene separatorwas used to separate the anode and the cathode; 1. 0mol/L L iPF6 dissolved in a 1∶1 m ixture of ethylenecarbonate / diethyl carbonate was used as an electro2lyte. The charge and discharge cycles were carriedout at a 0. 1 C2rate over a potential range between 2. 7and 4. 4V.2. 3　Ca lcula tionA ll calculations were performed in the general2ized gradient app roximation ( GGA ) to density func2tional theory as imp lemented in the V ienna Ab InitioSimulation Package (VASP). A p lane2wave basis setwith a kinetic2energy cutoff of 500 eV was used. Therecip rocal space samp ling was done with a 4 ×4 ×4 k2point grid for structures containing 4 grids.3　Results and D iscussionFigure 1 shows the TGA2DTG curves of the dry2gel p recursors p repared for the p recalcination and cal2cination p rocesses. The symbols C1、C2 and C3 in thefigure rep resent L iN i0. 75 Co0. 25 O2、L iN i0. 65 Co0. 25 Ti0. 1 O2and L iN i0. 5 Co0. 25 Ti0. 25 O2 , respectively. Below 200℃, all three types of the materials had small peaks,and this weight loss of the p recursors could be due tothe desorp tion of superficial and structural water. Inthe temperature range of 200～300 ℃, a peak oc2curred due to the decomposition of nitrate and /or thedehydration of metal citrate to aconitate[ 14 ] . Then asharp drop in weight appeared between 350 ℃ and400 ℃, which is attributed to the combustion of acon2itate and its comp lex. There were small changes inthe temperature range of 400 ～600 ℃. W hen thetemperature was higher than 700 ℃, a small peak ap2peared and could be related to the decomposition ofcarbonate. It is evident that temperature of phasetransformation for C1 was above 650 ℃, while thetemperature for the phase transformation of C2 and C3were about 600 ℃ and 500 ℃, respectively. There2by, the thermal stability can be imp roved by increas2ing the amount of Ti dop ing.Based on the TGA /DTA results given in Fig. 1,eight characteristic temperatures, namely, 350 ℃,·442· 　　　　　　　　　　　　电 　化 　学 2006年Fig. 1　TG2DTG curves of the dry2gel p recursor of three differentamounts of Ti doped materials380 ℃, 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃,650 ℃ and 725 ℃, were selected and a series ofXRD measurements at each temperature kep t for 2 hwas made ex2situ. Figure 2 shows the XRD patters ofC1 , C2 , and C3 at eight characteristic temperaturesraised consequently from 350 ℃ to 725 ℃. The p re2cursors were first heated up to each of the selectedtemperatures and kep t constant in the oven for 2 h,and then were analyzed by XRD at room temperature.The XRD patterns of C1、C2 and C3 kep t at 725 ℃ arethose of finished p roduct. It can be seen from Fig. 2aFig. 2　XRD patters of three different amount of Ti doped materi2als on synthesized at for 2 hoursthat C1 at 725 ℃ had the typ ically hexagonal structurewith the space group of R23m. For hexagonal struc2ture, the relative intensities of XRD lines I(003) / I(104)or I(101) / I(006, 102) are considered to be indicators of theordering of lithium and other transition metal cations(N i and /or Co) [ 15 ] , and the ordering of the structurecan also be evaluated from the XRD spectra with thedegree of either ( 108 ) / ( 110 ) or ( 006 ) / ( 102 )peak sp litting. A ll the peaks belong to those of hexa2gonal structure and no other phases were p resented.The value of I(003) / I(104) is 1. 0027 and (108) / (110)·542·第 3期 　　 贺 　慧等 :钛掺杂对 L iN i0. 75 Co0. 25O2结构与性能的影响and (006) / (102) are sp lited, indicating an orderedand layered structure. A s evident in Fig. 2b, C2which calcined at 725 ℃ also had hexagonal structurewith I(003) / I(104) being 1. 027 and the degree of thepeak sp liting of ( 006 ) / ( 012 ) and ( 018 ) / ( 110 )was better than that of C1. That is to say doped Ti canincrease the ordering of lithium and other transitionmetal cations (N i and /or Co) and can increase theordering of structure. However, for C3 at 725 ℃( Fig. 2c) , two phases, cubic structure and hexagonalstructure co2existed.Fig. 3　M icrographs of three types of cathode materials syn2thesized at 725 ℃ for 2 hours　　From the whole XRD patterns of C1 , it can beseen the courses of phase transformation from cubicstructure to hexagon structure. In the temperaturerange of 350～600 ℃, the XRD patters were sim ilar,and all the peaks were characteristic of the cubicstructure, the development of ( 003 ) and ( 104 )peaks with the increase of temperature became appar2ent. The phase transformation took p lace above 650℃and the hexagonal structure rep laced the cubic struc2ture. W ith the increase of temperature, the intensityFig. 4 　Cyclic discharge curves of the first two cycles ofthree different amounts of Ti doped materialsof characteristic peaks became stronger and well2de2fined, the degree of the peak sp litting in ( 006 ) /( 012) and (018) / (110) became significant. For C2the hexagonal structure was obtained at above 600℃, both agreed well with the TGA results. In thecase of C3 , two phases were p resented through thewhole temperature rang. The results in Fig. 2 sugges2ted the better defined hexagonal and layered structurecan be obtained at x = 0. 1. W hile two phases arep resented with x = 0. 25.·642· 　　　　　　　　　　　　电 　化 　学 2006年Tab. 1　Summary of cycling performance from Fig. 4MaterialC1 C2 C3Charge capacity /mAh·g- 1 193 201 170First D ischarge capacity /mAh·g- 1 172 190 140Loss( % ) 12 6 18SecondCharge capacity /mAh·g- 1D ischarge capacity /mAh·g- 1178166181179123115Fig. 5　DOS of three different amount of Ti doped materials　a) ～c) lithiation, d) ～f) delithializationFigure 3 is the SEM m icrographs of C1 , C2 , andC3which calcined at 725 ℃ for 2 h after p recalciningthe obtained p recursor at 380 ℃. It can be seen thatthe shape of the particles for C1 and C2 was regular·742·第 3期 　　 贺 　慧等 :钛掺杂对 L iN i0. 75 Co0. 25O2结构与性能的影响with clear edges ( Fig. 3a and 3b) , indicating goodcrystal structure. The particle sizes for C2 are smallerand more uniform than those of C1. The morphologyrevealed C3 indistinct edges of particles and roughsurfaces, hence poor structure. The SEM observationis consistent with the XRD results discussed before.Figure 4 compares the first charge and dischargecurves of these three materials. For C1 and C2 , theyboth had p lateaus at about 3. 6 V , but for C3 , therewas no obvious p lateaus. The charge /discharge capal2ities and there losses are summarized in Table 1. Itcan be seen that for C1 the first charge capacity was193 mAh /g and the first cycle loss was 12%. Thefirst charge capacity for C2 reached 201 mAh /g whilewhich is higher than that of C1 , the first cycle lossmaintained at 6% , suggesting that the cyclability ofC2 is better than that of C1. The first charge capacityfor C3 was 23mAh /g lower and the first cycle loss was6% higher than those of C1 , suggesting poor electro2chem ical performance.The density of states for L iy N i0. 75 - x Co0. 25 Tix O2 ( x= 0, 0. 1, 0. 25) when y = 1 and y = 0 were calculat2ed and the results are p rovided in Fig. 5. W hen thelithium ion deintercalation occerred, the energy ofFerm i ( EF ) was lower and the N i2eg orbit moved to ahigher lever of EF , indicating the degree of polariza2tion on N i2O and Co2O increased, hence, the valenceof N i ion changed from +N i(Ⅲ) to + N i(Ⅳ). Thedop ing with small amount of Ti ( x = 0. 1) increasedthe DOS of N i2eg ( Fig. 5b) , imp lying that the va2lences of N i are + N i(Ⅱ) and + N i(Ⅲ) with lessN i3 +and more N i2 +. The p resence of N i2 +made thestructure more stable. However, when x = 0. 25, theDOS of N i2eg decreased, indicating that the high do2p ing with Ti is detrimental to the structure of materi2al. The calculated results agreed well with the experi2mental results.4　ConclusionThree types of materials L iN i0. 75 - x Co0. 25 Tix O2 ( x= 0, 0. 1, 0. 25 ) were p repared by sol2gel methodand analyzed by TG, XRD and SEM for the structureand morphology information. The electrochem ical per2formance electron structure were also exam ined. Theresults showed that the thermal stability can be im2p roved by dop ing Ti. Better defined hexagonal struc2ture can be obtained at x = 0. 1 with good electro2chem ical performance. W hen x = 0. 25, two phasewere p resented, which resulted in poor electrochem i2cal performance. The calculation in the density ofstates revealed the change of N i ion valence duringdelithializating.R efe rence s:[ 1 ]　Palacin M R, Chabre Y. On the origin of the 3. 3 and4. 5 V step s observed in L iMn2 O4 2based sp inels[ J ]. J.Electrochem. Soc. , 2000, 147: 845～853.[ 2 ]　Peng Z S, W an C R, J iang C Y, et al. Synthesis by solgel p rocess and characterization of L iCoO2 cathode mate2rials [ J ]. J. Power Sources , 1998, 72: 215～220.[ 3 ] 　B roussely M , B iensan P, Simon B. L ithium insertioninto host materials: the key to success for L i ion batter2ies [ J ]. Electrochim. Acta, 1999, 45: 3～22.[ 4 ]　Ohzuku T, Ueda A. Phenomenological exp ression of sol2id2state redox potentials of L iCoO2 , L iCo1 /2N i1 /2O2 , andL iN iO2 insertion electrodes [ J ]. J. Electrochem. Soc. ,1997, 144: 2780～2785.[ 5 ]　Delmas C, MenetrierM , Croguennec L, et al. An over2view of the L i(N i,M ) O2 system s: syntheses, structuresand p roperties[ J ]. Electrochim. Acta, 1999, 45: 243～253.[ 6 ]　Banov B, Bourilkov J, M ladenov M. Cobalt stabilizedlayered lithium2nickel oxides, cathodes in lithium re2chargeable cells[ J ]. J. Power Sources, 1995, 54: 268～270.[ 7 ]　Caurat D, Baffier N, Carcia B. Synthesis by a soft chem2istry route and characterization of L iN ix Co1 - x O2 (0 ≤ x≤ 1) cathode materials[ J ]. Solid State Ionics, 1996,91 : 45～54.[ 8 ]　A lcantara R, Lavela P, Tirado J L. Changes in struc2·842· 　　　　　　　　　　　　电 　化 　学 2006年ture and cathode performance with composition andp reparation temperature of lithium cobalt nickel oxide[ J ]. J. Electrochem. Soc. , 1998, 145: 730～736.[ 9 ]　R itchie A G, Giwa C O, Lee J C. Future cathode mate2rials for lithium rechargeable batteries [ J ]. J. PowerSurces, 1999, 80: 98～102.[ 10 ]　Jae Woo Joeng, Seong2Gu Kang. Structural and elec2trochem ical p roperties of L iN iy Ti1 - y O2 p repared by awet p rocess[ J ]. J. Power Sources, 2003, 123: 75～78.[ 11 ]　Decheng L i, Takahisa Muta, H ideyuki Noguch. Elec2trochem ical characteristics of L iN i0. 5Mn0. 5 - x Tix O2 p re2pared by solid state method [ J ]. J. Power Sources,2004, 135 : 262～266.[ 12 ]　Kim J H, Myung S T, YoonC S, et al. Effect of Tisubstitution forMn on the structure of L iN i0. 5Mn1. 5 x TixO4 and their electrochem ical p roperties as lithiuminsertion material [ J ]. J. Electrochem. Soc. , 2004,151 (11) : A1911～A1918.[ 13 ]　Molenda J, W ilk P, Marzec J. Transport p roperties ofthe L iN i1 - y Coy O2 system [ J ]. Solid State Ionics,1999, 119: 19～22.[ 14 ]　Song M Y, Lee R. Synthesis by sol2gel method andelectrochem ical p roperties of L iNO2 cathod material forlithium secondary battery [ J ]. J. Power Sources,2002, 111: 97～103.[ 15 ]　Morales J , Perez2V icente C, Tirado J L. Cation distri2bution and chem ical deintercalation of L i1 - x N i1 + x O2[ J ]. Mater. Res. Bull. , 1990, 25: 623～630.钛掺杂对 L iNi0. 75 Co0. 25 O2结构与性能的影响贺　慧 ,程　璇 ,张　颖 3 ,王淑芬(厦门大学化学系 ,材料科学与工程系 ,固体表面物理化学国家重点实验室 ,福建 厦门 361005)摘要 : 　应用溶胶 2凝胶法合成 L iN i(0. 75 - x) Co0. 25 TixO2 ( x = 0, 0. 1, 0. 25)系列正极材料 ,其结构、形貌、粒度、电化学性能由 TG、XRD 、SEM和电池充放电测试表征研究表明 ,材料的电化学性能与钛掺杂量密切相关. 在钴含量不变的情况下 ,随着 Ti含量 ( x)的增加 ,材料由六方层状结构逐渐向立方结构转变 , x = 0. 25时 ,出现了立方相与六方相共存. 根据实验和理论计算结果简要讨论了钛掺杂对正极材料 L iN i0. 75 Co0. 25 O2结构和电化学性能的影响.关键词 : 　锂离子电池 ;正极材料 ;钛掺杂 ;电化学性能·942·第 3期 　　 贺 　慧等 :钛掺杂对 L iN i0. 75 Co0. 25O2结构与性能的影响

Effect of Ti Doping on the Structure and Performance of LiNi_(0.75)Co_(0.25)O_(2)

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Effect of Ti Doping on the Structure and Performance of LiNi_(0.75)Co_(0.25)O_(2)

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