J O U R N A L O F C H E M I S T R Y Materials Synthesis and electrochemical characteristics of spinel phase LiMn2O4-based cathode materials for lithium polymer batteries Yang-Kook Sun* and Sung-Ho Jin Polymer Materials Laboratory, Chemical Sector, Samsung Advanced Institute of Technology, 103-12, Moonji-Dong, Yusong-Gu, Daejon, Korea, 305-380. E-mail: yksun@saitgw.sait.samsung.co.kr Received 15th June 1998, Accepted 11th August 1998 Spinel LiMn2O4 and LiMn1.95Ni0.05O4 powders have been synthesized by a sol–gel method using an aqueous solution of metal acetates containing glycine.The dependence of the physicochemical properties of the spinel LiMn2O4 powders on the calcination temperature and glycine quantity have been extensively investigated. The porous LiMn2O4 and LiMn1.95N0.05O4 electrodes were electrochemically characterized by using charge/discharge experiments along with ac impedance spectroscopy.The LiMn1.95Ni0.05O4 electrode exhibited improved cycling performance in comparison with the stoichiometric LiMn2O4 one in spite of a small reduction in the initial capacity. The good capacity retention of the LiMn1.95Ni0.05O4 electrode is attributed to stabilization of the spinel structure by Ni doping for Mn ion sites and higher chemical diVusivity of lithium ions with cycling.In this work, LiMn2O4 and LiMn1.95Ni0.05O4 powders with Introduction uniform submicron-sized particles were synthesized by a The spinel LiMn2O4 has been extensively studied as the most sol–gel method using glycine as a chelating agent at considerpromising cathode material for lithium secondary batteries ably lower temperatures and shorter times compared with with high energy density.This material oVers several distinct solid-state reaction. Also, the origin of capacity fading on advantages; it is easier to prepare, less expensive and less toxic cycling was investigated in terms of interfacial characteristics than layered oxides such as LiCoO2 and LiNiO2.1,2 However, by using charge/discharge experiments complementary with ac LiMn2O4 has problems related to capacity fading and limited impedance measurements for Li/polymer electrolyte/LiMn2O4 cyclability in the 4 VLi/Li+ region in comparison with the and LiMn1.95Ni0.05O4 cells.layered oxides. The reason for capacity fading has not been clearly resolved, but some possible factors have been proposed. 3–5 In order to improve capacity fading and cyclability Experimental of the LiMn2O4 powders in the 4 VLi/Li+ region, the eVects of A stoichiometric amount of Li, Mn, and Ni acetates (Acros adding excess lithium to the LiMn2O4 spinel,3,6–8 and manga- Co., high purity) with the cationic ratio Li5Mn=152 or nese-substituted spinels LiMxMn2-xO4 (M=Mg, Zn, Co, Cr, Li5Mn5Ni=151.9550.05 were dissolved in doubly distilled Ni, Al, Ti, Fe, Ga)3,9–13 have been studied.water, mixed well with each other and an aqueous solution of The electrochemical properties of LiMn2O4 strongly depend glycine was added (Aldrich, high purity). Glycine was used as on its synthetic method. The LiMn2O4 powders have been a chelating agent to produce gel precursors.Ammonium usually prepared by solid-state reaction which consists of hydroxide was added slowly to this solution with constant extensive mechanical mixing and extended grinding, which is stirring until a pH of 5.0–7.5 was achieved. The resultant detrimental to the quality of the final products. These synthetic solution (0.1 mol aqueous solution of total metal ions) was conditions can result in inhomogeneity, irregular morphology, evaporated at 70–80 °C for 5 h until a transparent sol larger particle size with broad particle size distribution, and was obtained. To remove water from the sol the transparent poor control of stoichiometry. In order to achieve good sol was heated at 70–80 °C while being mechanically stirred eYciency of Li utilization at high current rates and reliability with a magnetic stirrer.As the evaporation of water proceeded, of lithium secondary batteries, a sol–gel method has been the sol turned into a viscous transparent gel. For the prep- introduced, which is a desirable method to obtain cathode aration of the gel precursors with diVerent molar ratios of materials with good homogeneity, uniform morphology and glycine to total metal ions the same procedure was repeated narrow size distribution.14,15 Recently, one of us has reported with the molar ratio of glycine to total metal ions being varied that the spinel LiMn2O4 powders being phase pure and having to 1, 1.5, 2, and 2.551.The gel precursors obtained were excellent rechargeability could be synthesized by the sol–gel decomposed at 250–800 °C for 10 h in air to obtain phase- method using glycolic acid and poly(acrylic acid) (PAA) as pure polycrystalline LiMn2O4 powders.chelating agents.8,16 Powder X-ray diVraction (Rint-2000, Rigaku) using Cu-Ka Lithium polymer batteries are now being studied extensively radiation was used to identify the crystalline phase of the as promising candidates for electric vehicles and portable materials calcined at various temperatures. Rietveld refinement electric equipment.The use of a polymer electrolyte would was then performed on the X-ray diVraction data to obtain make the batteries highly safe, flexible, light, and thin. One of lattice constants. The change in the particle morphology was the problems with the lithium polymer batteries is a progressive observed using a field emission scanning electron microscope capacity fading on repeated cycling.When a polymer electro- (TOPCON, ABT-150F). lyte is used, the establishment of a proper interfacial contact The electrochemical properties of LiMn2-xNixO4 powders between polymer electrolyte and electrode can be an issue of were determined in the Li/polymer electrolyte/LiMn2-xNixO4 major concern and essential to guarantee acceptable performcells. The polymer electrolyte was made from polyacrylonitrile ance and cycle life.A few studies have been carried out in (PAN), plasticized by a solution of LiClO4 in a 151 mixture order to investigate interfacial characteristics between polymer electrolyte and electrode.17–21 of ethylene carbonate (EC) and propylene carbonate (PC).J. Mater. Chem., 1998, 8(11), 2399–2404 2399A typical polymer electrolyte composition was PAN 12 wt.%–EC40 wt.%–PC 40 wt.%–LiClO4 8 wt.%. The ionic conductivity of the polymer electrolyte was 2×10-3 V-1 cm-1 at room temperature. The composite cathode was made from the as-synthesized LiMn2-xNixO4 spinel powders (89.5 wt.%.), acetylene black (Super-P, conducting agent; 5.5 wt.%), and PAN binder (5 wt.%).The LiMn2-xNixO4 spinel powders and acetylene black were added to PAN solution in dimethyl sulfoxide (DMSO) as a solvent. The slurry was spread onto an aluminum foil current collector, and dried at 110 °C in air. Dried composite cathode was then compressed with a roll presser and further dried under vacuum for 10 h at 110 °C.A three-electrode cell was used for the electrochemical measurements. The reference and counter electrodes consisted of 50 mm thick Li foil (Cyprus Foote Mineral Co.) pressed onto a Cu current collector. A rechargeable lithium polymer cell was assembled by sandwiching the polymer electrolyte between the lithium anode and the composite cathode. A lithium electrode contacting polymer electrolytes in proximity to the working electrode served as the reference electrode.The cell was then Fig. 2 EVect of the calcination temperature on the lattice constant of enclosed in a metallized plastic bag and vacuum sealed. All the LiMn2O4 powders ($: LiMn1.95Ni0.05O4 powders calcined at assemblies of the cell were carried out in a dry box filled with 800 °C) when the molar ratio of glycine to total metal ions was 1.051.argon gas. The cells were usually cycled between cut-oV voltages of 3.4 and 4.3 VLi/Li+ at a constant current density of 0.15 mA cm-2, unless otherwise noted. The cells were activated peaks are much sharper, the widths of the peaks are much narrower, and the positions of the diVraction lines shift to the during the first cycle at a constant current density of 0.1 mA cm-2.ac impedance measurements were performed low angle side in the XRD pattern with the increase of the calcination temperature, which indicates an increase in crystal- using a Zahner Elektrik IM6 impedance analyzer over the frequency range of 1 mHz–100 kHz with an amplitude of linity and a gradual growth of average size particles.Similar results had already been reported whereby the LiMn2O4 5 mV. Each sample was allowed to equilibrate for 30 min at each cycle before measurement at the fully charged state. powders were synthesized by the sol–gel method using glycolic acid and poly(acrylic acid) as chelating agents.8,16 Fig. 2 shows the eVect of the calcination temperature on the Results and discussion lattice constant a of the same powders as shown in Fig. 1, obtained from the Rietveld refinement on the XRD data in Fig. 1 shows the X-ray diVraction (XRD) patterns of the LiMn2O4 powders calcined at various temperatures and the cubic unit cell of the LiMn2O4 powders. It is seen from the figure that the lattice constant increases almost linearly LiMn1.95Ni0.05O4 powders calcined at 800 °C for 10 h in air, where the molar ratio of glycine to total metal ions was 1.051.from 8.1992 to 8.2260 A° with increasing calcination temperature from 250 to 800 °C. It is speculated that the value of the The X-ray diVraction patterns for the powders calcined at 250 °C represent the slow appearance of low crystalline average oxidation state of manganese in the spinel phase is closely related to the lattice constant of the cubic unit cell.A LiMn2O4 spinel. Impurity peaks corresponding to Li2CO3 and MnCO3 are not observed, but are often found in other low lower calcination temperature results in a more oxidized manganese cation because manganese ions are stable preferen- temperature techniques. It was confirmed from the XRD patterns that the well-defined spinel LiMn2O4 phase was tially as Mn4+ at lower temperatures.22 For example, MnO2 with all Mn4+ transforms progressively into Mn2O3 with all formed over the whole calcination temperature range.For all powders, there is no (220) diVraction line (2h=30.4°) which Mn3+ for the binary manganese oxide system as the temperature increases. The atomic radius of Mn4+ (0.67 A° ) is smaller is generated by Li ions at 8a sites in the LiMn2O4 host, because the scattering factor of Li ions is very small.The diVraction than that of Mn3+ (0.72 A° ) and thus the lattice constant of the cubic unit cell of the spinel LiMn2O4 calcined at higher temperatures is larger than that of the spinel LiMn2O4 calcined at lower temperatures. The lattice constant of LiMn1.95Ni0.05O4 powders calcined at 800 °C is 8.2236 A° which is in agreement with the literature value of 8.228 A° for LiNi0.04Mn1.96O4.15 The substitution of manganese with divalent nickel increases the average oxidation state of manganese above 3.5 to keep electrical neutrality in the spinel structure and thus there are many Mn4+ cations, which decrease the lattice constant of the LiMn1.95Ni0.05O4 host structure.Fig. 3 shows scanning electron micrographs (SEM) of the powders prepared from the gel precursors having the molar ratio of glycine to metal ions of 1.051 as a function of temperature. The presence of loosely agglomerated spherical particles with average grain size 70 nm was observed from the powders calcined at 220 °C. For the powders calcined at 650 °C, the particle size increased to 100 nm.As calcination temperature was increased, growth kinetics were favored and thus agglomerated spherical particles changed to larger par- Fig. 1 X-Ray diVraction patterns of the LiMn2O4 powders calcined ticulates. When the gel precursors were heated at 800 °C, the at various temperatures and LiMn1.95Ni0.05O4 powders calcined at particle size increased to about 600 nm with a fairly narrow 800 °C for 10 h in air when the molar ratio of glycine to total metal ions was 1.051.particle-size distribution. 2400 J. Mater. Chem., 1998, 8(11), 2399–2404Fig. 5 X-Ray diVraction patterns of the LiMn2O4 powders prepared from the gel precursors having various molar ratios of glycine to total metal ions and calcined at 700 °C; (a) 1.0, (b) 1.5, (c) 2.0, and (d) 2.551.could be formed regardless of the molar ratio of glycine to Fig. 3 Scanning electron micrographs of the LiMn2O4 powders total metal ions tested. A close look at Fig. 5 reveals that the calcined at (a) 220 °C, (b) 500 °C, (c) 650 °C, and (d) 800 °C. diVraction peaks are sharper and that their intensity is increased with increasing glycine quantity, which indicates an Fig. 4 shows the X-ray diVraction patterns for the LiMn2O4 increase in the crystallinity of the spinel phase. In order to powders prepared from gel precursors having the molar ratio investigate the structural diVerences in the spinel phase at the of glycine to metal ions of 0.5 and 1.551. Both the LiMn2O4 various molar ratios of glycine to total metal ions, the Rietveld powders were calcined at 250 and 400 °C for 10 h in air.refinement was performed on the XRD data to obtain lattice Whereas the X-ray diVraction pattern for the powders preconstants. Fig. 6 shows the eVect of the molar ratio of glycine pared by the molar ratio of glycine to metal ions of 1.551 and to total metal ions on the lattice constant of the same powders calcined at 250 °C presents no diVraction peaks indicating an as shown in Fig. 5. With increasing glycine quantity the lattice amorphous phase, the powders prepared by the molar ratio constant and thus the crystallinity of the LiMn2O4 powders of glycine to metal ions of 0.551 and calcined at 250 °C shows increases linearly, although the extent of increase is not as impurity phases such as b-MnO2, Mn2O3, and Li2CO3 apart much as the case of increasing calcination temperature as from the LiMn2O4 spinel phase.For the powders prepared by shown in Fig. 1. the molar ratio of glycine to metal ions of 0.551 and calcined In order to investigate the morphological features of the at 400 °C, Mn2O3 peaks were still observed, though the pro- LiMn2O4 powders having diVerent molar ratio of glycine to portion of the LiMn2O4 spinel phase is increased.On the total metal ions, scanning electron microscopy (SEM) was contrary, the gel precursors prepared by the molar ratio of used for the powders prepared from the gel precursors having glycine to metal ions of 1.551 and calcined at 400 °C crysmolar ratios of glycine to metal ions of 0.5 and 2.051, and tallized into a phase-pure LiMn2O4 spinel phase without any calcined at 700 °C for 10 h in air as shown in Fig. 7. The development of impurity phases. surface of the powders with a molar ratio of glycine to total Fig. 5 demonstrates the X-ray diVraction patterns for the metal ions of 0.551 contained monodispersed spherical fine powders prepared by the molar ratio of glycine to metal ions particulates with an average particle size of about 200 nm.For of 1.0, 1.5, 2.0, and 2.551 calcined at 700 °C for 10 h in air. It the powders with a molar ratio of glycine to total metal ions was confirmed from the XRD patterns that the spinel phase Fig. 4 X-Ray diVraction patterns of the LiMn2O4 powders prepared from the gel precursors having the molar ratio of glycine to metal Fig. 6 EVect of the molar ratio of glycine to total metal ions on the ions of (a) 0.551 and calcined at 250 °C, (b) 1.551 and calcined at 250 °C, (c) 1.551 and calcined at 400 °C, and (d) 1.551 and calcined lattice constant of the LiMn2O4 powders calcined at 700 °C for 10 h in air.at 400 °C for 10 h in air. J. Mater. Chem., 1998, 8(11), 2399–2404 2401Li/polymer electrolyte/LiMn1.95Ni0.05O4 cell at the current densities of 0.15–2.0 mA cm-2.In this cell, the composite cathode was prepared from the LiMn1.95Ni0.05O4 powders calcined at 800 °C. The discharge capacity of the cell decreased very slowly with an increase in current density. For example, the cell delivered a capacity of 126, 122, and 116 mA h g-1 at current densities of 0.15, 0.5, and 1 mA cm-2 respectively. The cell showed an attractive capacity of 105 mA h g-1 at 1.5 mA cm-2 or 1.7 C rate.However, the discharge capacity of the cell abruptly decreased to 57 mA h g-1 at a current density of 2 mA cm-2 or 2.2 C rate. This may be due to the low conductivity of the polymer electrolyte compared to the Fig. 7 Scanning electron micrographs of the LiMn2O4 powders liquid electrolyte. When the current densities were lowered calcined at 700 °C when the molar ratio of glycine to metal ions was to 0.15, 0.5, and 1.0 mA cm-2 at the 61, 71, 81st cycle, (a) 0.5 and (b) 2.051. respectively, the discharge capacities increased to the original value.The observed cycling stability of the spinel of 2.051, it was observed that the particle size of the powders LiMn1.95Ni0.05O4 could be due to a small structural transition was 100 nm.The particle size of the former was two times in the powders, and good contacts among the composite smaller than that of the latter at the same calcination cathode constituents. temperature. Fig. 9(a) and (b) show the charge/discharge curves with the Increase of the crystallinity and decrease of the particle size number of cycles for the Li/polymer electrolyte/LiMn2O4 of the LiMn2O4 powders with the quantity of glycine used in and LiMn1.95Ni0.05O4 cells using the LiMn2O4 and preparing gel precursors can be explained as follows; the less LiMn1.95Ni0.05O4 powders calcined at 800 °C.The Li/polymer glycine used in preparing gel precursors, the shorter is the electrolyte/LiMn2O4 cell showed two-stage reduction and oxidistance between the Li and Mn cations, and thus the higher dation processes which are characteristic of the manganese is the probability of crystallization between the cations.oxide spinel structure.23,24 For the Li/polymer electrolyte/ However, the amount of heterogeneously distributed cations LiMn1.95Ni0.05O4 cell, the two-stage reduction and oxidation in the calcined powders increased as shown in Fig. 4. Therefore, processes became less distinct. This behavior suggests that the bigger particles with a low crystallinity are produced at lower glycine quantity. On the contrary, when the quantity of glycine is increased, the highly cross-linked gel precursors suppress the cation mobility and eVectively prevent the cations from contacting each other. Thus, the degree of segregation of the cations occurring during calcination is decreased, and the homogeneously distributed cations crystallize into the spinel phase.It has been reported that the combustion heat from the chelating agents increases the crystallinity of the particles, yielding fluVy LiMn2O4 powders which result from large void volumes generated by CO and CO2 during the combustion of the chelating agent.8,16 This can be supported by the observation that the materials become more ‘puVed up’ after calcination in the presence of larger amounts of glycine at the same calcination temperature, which results in a decrease in the particle size of the LiMn2O4 powders. Therefore, it can be concluded that the amount of glycine determines the crystallinity and particle size of the LiMn2O4 powders.Fig. 8 represents the specific discharge capacity of the Fig. 9 Cycling charge/discharge curves with the number of cycles Fig. 8 Variation of specific discharge capacity with number of cycles at various discharge current densities of the Li/polymer electrolyte/ of (a) the Li/polymer electrolyte/LiMn2O4 and (b) the LiMn1.95Ni0.05O4 cells. LiMn1.95Ni0.05O4 cell. 2402 J. Mater. Chem., 1998, 8(11), 2399–2404local distortion of the host structure resulting from the substitution of nickel ions may eliminate the small Li–Li repulsion energy diVerence between the half-filled 8a sites in Li0.5Mn2O4 and the completely filled sites in LiMn2O4. Fig. 10 demonstrates the variation of discharge capacity with the number of cycles for the Li/polymer electrolyte/ LiMn2O4 and LiMn1.95Ni0.05O4 cells.The initial capacity of the LiMn2O4 electrode delivered 145 mA h g-1. To our best knowledge, this is the highest value that has ever been reported in practice. The capacity slowly decreases with cycling and remained at 129 mA h g-1 at the 50th cycle. The discharge capacity of the LiMn1.95Ni0.05O4 electrode decreased more slowly with cycling and remained at 120 mA h g-1 at the 50th cycle.This suggests that the theoretical capacity fading of the Ni-doped spinel phases is attributed to a decrease in the amount of Mn3+, because the deintercalation of Li+ from the spinel structure must be electrically compensated for by the oxidation of Mn3+ to Mn4+ and thus, the capacity of the LiMn1.95Ni0.05O4 electrode was lower than that of the LiMn2O4 one.The cycling stability of the Ni-doped spinel Fig. 11 Ac impedance spectra of the Li/polymer electrolyte/ compared to the stoichiometric LiMn2O4 was due to the LiMn1.95Ni0.05O4 cell in fully charged state as a function of number suppression of the Jahn–Teller distortion in the spinel electrode of cycles. at the end of discharge since the M–O bonds for M=Ni, Co, or Cr are stronger than the Mn–O bond.9 Therefore, the Ni is associated with lithium ion diVusion through the dopant could enhance the stability of the octahedral sites in LiMn1.95Ni0.05O4 particle.24 The high-frequency semicircle is the spinel host structure.It should also be noted from Fig. 2 progressively increased with the increase in number of cycles that the lattice constant of the Ni-doped spinel is smaller than which indicates an increased interfacial resistance between that of the standard LiMn2O4.Another factor of the cycling polymer electrolyte and electrode (Li and oxide electrode). stability of the Ni-doped spinel is attributed to the volume The apparent chemical diVusivity of lithium ions in the porous changes during the intercalation and deintercalation reaction LiMn2O4 and LiMn1.95Ni0.05O4 electrodes with respect to the of lithium ions, which will be less than those of the stoichionumber of cycles was calculated using the relation (1)25 metric LiMn2O4.In order to investigate the capacity fading of Li/polymer electrolyte/LiMn1.95Ni0.05O4 cell with cycling, ac impedence D� Li+= pfTr2 1.94 (1) spectra with respect to the number of cycles were measured.Fig. 11 illustrates typical Nyquist plots obtained from the where fT is the frequency at which the impedance spectrum Li/polymer electrolyte/LiMn1.95Ni0.05O4 cell in a fully charged shows a transition from semi-infinite diVusion behavior to state with respect to the number of cycles. The impedance finite-length diVusion behavior.The average radius r of the spectra consist of one semicircle in the high and intermediate oxide was determined from scanning electron microscopic frequency range, a line inclined at a constant angle to the real observation. axis in the low frequency range of 5 Hz to 10 mHz, and a The calculated chemical diVusivities of lithium ions in the capacitive line due to the accumulation of lithium ions at the LiMn2O4 and LiMn1.95Ni0.05O4 electrodes are plotted against center of the oxide particle in the frequency range below 10 number of cycles in Fig. 12. The chemical diVusivities were mHz. The semicircle in the higher frequency range is related determined to be of orders of 10-10 to 10-11 cm2 s-1. The to the reactions at the interface of the polymer electrolyte/ chemical diVusivity within both electrodes decreases with electrode (Li and oxide electrode) and the inclined line in the increasing number of cycles.However, it should be noted that lower frequency range is due to Warburg impedance which Fig. 12 Variation of chemical diVusivity of the porous (a) LiMn2O4 Fig. 10 Variation of discharge capacity with the number of cycles of and (b) LiMn2.95Ni0.05O4 electrodes as a function of the number of cycles.(a) Li/polymer electrolyte/LiMn2O4 and (b) LiMn1.95Ni0.05O4 cells. J. Mater. Chem., 1998, 8(11), 2399–2404 2403the chemical diVusivity of lithium ions markedly decreases References with cycling for the LiMn2O4 electrode. It is speculated that 1 T. Ohuzuka, M. Kitagawa and T. Hirai, J. Electrochem. Soc., the decrease in chemical diVusivity could be attributed to the 1990, 137, 760.decreased number of vacant sites ailable for the diVusion of 2 D. Guyomard and J. M. Tarascon, Solid State Ionics, 1994, 69, lithium ions which resulted from volume change and 222. Jahn–Teller distortion of the spinel host structure. Liu et al. 3 R. J. Gummow, A. de Kock and M. M. Thackeray, Solid State Ionics, 1994, 69, 59.reported that the LiMn2O4 powders after 80 cycles had a 4 D. H. Jang, Y. J. Shin and S. M. Oh, J. Electrochem. Soc., 1996, tetragonal phase, which suggests the onset of the Jahn–Teller 143, 2204. eVect causes a severe structural distortion, leading to a decrease 5 Y. Xia, Y. Zhou and M. Yoshio, J. Electrochem. Soc., 1997, 144, in vacant sites.12 The capacity fading of the LiMn2O4 and 2593.LiMn1.95Ni0.05O4 electrodes appears to be related to the 6 D. Guyomard and J. M. Tarascon, Solid State Ionics, 1994, 69, 222. decrease in chemical diVusivity as well as the increase in 7 X. Qiu, X. Sun, W. Shen and N. Chen, Solid State Ionics, 1997, interfacial resistance between polymer electrolyte and electrode 93, 335. (Li and oxide electrode). The good capacity retention for the 8 Y.-K.Sun, Solid State Ionics, 1997, 100, 115. LiMn1.95Ni0.05O4 electrode compared to the stoichiometric 9 Li Guohua, H. Ikuta, T. Uchida and M. Wakihara, LiMn2O4 is attributed to the suppression of the Jahn–Teller J. Electrochem. Soc., 1996, 143, 178. distortion, smaller volume change of the unit-cell (smaller 10 R. Bittihn, R. Herr and D.Hoge, J. Power Sources, 1993, 43–44, 223. lattice constant) and higher chemical diVusivity in the 11 G. Pistoia and G. Wang, Solid State Ionics, 1993, 66, 135. LiMn1.95Ni0.05O4 electrode as mentioned above. 12 W. Liu, K. Kowal and G. C. Farrington, J. Electrochem. Soc., 1997, 143, 3590. 13 A. D. Robertson, S. H. Lu, W. F. Averill and W. F. Howard, Jr., J. Electrochem. Soc., 1997, 144, 3500. 14 T.Tsumura, A. Shimizu and M. Inagaki, J. Mater. Chem., 1993, Conclusions 3, 995. The spinel LiMn2O4 powders with submicron, monodispersed, 15 W. Liu, G. C. Farrington, F. Chaput and B. Dunn, J. Electrochem. Soc., 1996, 143, 87. and highly homogeneous particles were synthesized by a 16 Y.-K. Sun, I.-H. Oh and K. W. Kim, Ind. Eng. Chem. Res., 1997, sol–gel method using an aqueous solution of metal acetate 36, 4839.containing glycine as a chelating agent. While the crystallinity 17 A. Hooper and B. C. Tofield, J. Power Sources, 1984, 11, 33. and lattice constant of the LiMn2O4 powders were increased, 18 B. C. H. Steele, G. E. Lagos, P. C. Spurdens, C. Forsyth and A. D. the particle size of the LiMn2O4 powders was decreased with Foord, Solid State Ionics, 1983, 9 and 10, 391. 19 P. G. Bruce and F. Krok, Solid State Ionics, 1989, 36, 171. an increase in glycine quantity. Polycrystalline LiMn2O4 pow- 20 B. V. Ratnakumar, S. DiStefano and C. P. Bankston, J. Appl. ders calcined at 250–800 °C were found to be composed of Electrochem., 1989, 19, 813. very uniformly sized particulates with an average particle size 21 R. Koksbang, I. I. Olsen, P. E. Tonder, N. Knudsen and of 70–600 nm depending on the processing conditions. The D. Fauteux, J. Appl. Electrochem., 1991, 21, 301. initial capacity of the cell with the LiMn1.95Ni0.05O4 was lower 22 C. Masquelier, M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y. Maki, O. Nakamura and J. B. Goodenough, J. Solid State than the cell with the stoichiometric LiMn2O4, but the cycle Chem., 1996, 123, 255. performance was improved at the expense of capacity. The 23 M. M. Thackery, W. I. F. David, P. G. Bruce and good capacity retention of the LiMn1.95Ni0.05O4 electrode J. B. Goodenough, Mater. Res. Bull., 1983, 18, 461. compared to the stoichiometric LiMn2O4 is attributed to the 24 Y.-M. Choi and S.-I. Pyun, Solid State Ionics, 1997, 99, 173. suppression of the Jahn–Teller distortion, smaller volume 25 B. E. Conway, J. Electrochem. Soc., 1991, 138, 1569. change of the unit-cell and higher chemical diVusivity of lithium ions. Paper 8/04483J 2404 J. Mater. Chem., 1998, 8(11), 2399–2404