Synthesis of Spinel LiMn2O4 by the Sol−Gel Method for a Cathode

Ind. Eng. Chem. Res. 1997, 36, 4839-4846

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Synthesis of Spinel LiMn2O4 by the Sol-Gel Method for a Cathode-Active Material in Lithium Secondary Batteries Yang-Kook Sun Central Research Institute of Chemical Technology, Samsung Advanced Institute of Technology, 103-21 Moonji-dong, Yusong-gu, Daejeon 305-380, Korea

In-Hwan Oh* Division of Chemical Engineering, Korea Institute of Science and Technology, 39-1 Hawolkok-dong, Seongbuk-ku, Seoul 136-791, Korea

Kwang Yul Kim Department of Environmental Engineering, Chungbuk National University, San 48 Gaishin-dong, Cheongju 360-763, Korea

Spinel LiMn2O4 powders were synthesized by a sol-gel method using an aqueous solution of metal acetates containing poly(acrylic acid) (PAA) as a chelating agent. The dependence of the physicochemical properties of the spinel LiMn2O4 powders, such as crystallinity, lattice constant, and specific surface area, on the calcination temperature and the PAA quantity was investigated. It was found that a pure crystalline phase of spinel LiMn2O4 without impurities could be formed at 250 °C in air from the gel precursors. Polycrystalline LiMn2O4 powders calcined at 300-800 °C for 10 h were found to be composed of very uniformly-sized particulates with an average particle size of 30-600 nm and a specific surface area of 3.3-65 m2/g, depending on the processing conditions. Therefore, the sol-gel method required much lower calcination temperature and shorter calcination time than the conventional solid-state reaction. Electrochemical studies on the charge/discharge characteristics of the Li/LiMn2O4 cells show that LiMn2O4 powders calcined at 800 °C delivered a high initial capacity of 135 mA h/g and exhibited a good cycling behavior with only 9.5% loss from the initial discharge capacity at the 168 cycles during the charge/ discharge experiments. Introduction Spinel LiMn2O4 has been extensively studied as the most promising cathode material for lithium secondary batteries with high energy density. This material offers several distinct advantages: it is easier to prepare, cheaper, and less toxic than the layered oxides such as LiCoO2 and LiNiO2 (Guohua et al., 1996; Manev et al., 1993; Xia et al., 1995; Yamada et al., 1995). For commercial applications, it is important to produce LiMn2O4 powders with excellent cyclability and capacity retention at relatively high current densities. However, LiMn2O4 has problems related to capacity fading and limited cyclability in the 4 V region when compared to layered oxides. The cause of capacity fading with cycling has not been clearly studied, but some possible factors have been proposed (Gummow et al., 1994; Jang et al., 1996): (1) an instability of the electrolyte at the charged state, (2) a slow dissolution of the LiMn2O4 electrode because of the disproportion reaction (2Mn3+ f Mn4+ + Mn2+), and (3) a phase transition from cubic to tetragonal symmetry due to Jahn-Teller distortion in the deeply discharged state. To improve capacity fading and cyclability of the LiMn2O4 powders in the 4 V region, addition of excess lithium to the stoichiometric LiMn2O4 spinel (Guyomard and Tarascon, 1994), manganese-substituted stoichiometric LiMn2O4 spinel (Gummow et al., 1994; Guohua et al., 1996; Tarascon et al., 1991), synthesis atmosphere (Richard et al., 1994; Xia et al., 1995), and oxygen stoichiometry (Gao and Dahn, * To whom correspondence should be addressed. Fax: +822-958-5199. E-mail: [email protected]. S0888-5885(97)00227-3 CCC: $14.00

1995, 1996; Tarascon et al., 1991; Xia et al., 1995) have been studied. The quality of the LiMn2O4 powders used for lithium secondary batteries strongly depends on its synthetic method. It is desirable that the synthetic temperature of LiMn2O4 be as low as possible without affecting its electrochemical properties. The LiMn2O4 powders have usually been prepared by solid-state reaction which consists of mechanical mixing of lithium hydroxide, carbonate, or nitrate with manganese oxyhydroxide, carbonate, or oxide, followed by high-temperature calcination and extended grinding. This method, however, has several disadvantages: inhomogeneity, irregular morphology, larger particle size with broader particle size distribution, and poor control of stoichiometry. To facilitate rapid diffusion of lithium ions and thus to achieve excellent capacity, it is necessary to obtain powders with good homogeneity, uniform morphology with narrow size distribution, and high surface area. There have been a few reports on the synthesis of the cathode-active materials for lithium secondary batteries by chemical synthesis processes (Barboux et al., 1991; Jang et al., 1996; Liu et al., 1996; Ogihara et al., 1990; Prabaharan et al., 1995; Tsumura et al., 1993). Recently, authors have synthesized ultrafine LiCoO2 powders with an average particle size of 30-50 nm and a surface area of 2.3-17 m2/g at 550 °C for 1 h through the sol-gel method using poly(acrylic acid) (PAA) as a chelating agent (Sun et al., 1996) and highly crystalline LiNiO2 powders at 750 °C for 5 h using poly(vinylbutyral) (PVB) (Sun and Oh, 1997). In this study, LiMn2O4 powders were synthesized by a sol-gel method using PAA as a chelating agent at © 1997 American Chemical Society

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was determined by the BET method (Quantachrome Autosorb-1) with nitrogen adsorption. A three-electrode electrochemical cell was used for the galvanostatic charge-discharge experiments. The reference and counter electrodes were constructed from the lithium foil (Cyprus Foote Mineral), and the electrolyte used was a 1:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1 M LiAsF6 (Mitsubishi). The cathode consisted of 72 wt % active material, 20 wt % ketzen black EC, and 8 wt % Teflon binder. These components were dispersed in isopropyl alcohol and spread on Exmet, followed by pressing and drying at 120 °C for 3 h. After the cells were assembled in an argon-filled drybox, the charge-discharge cycling was galvanostatically carried out at a current density of 1 mA/cm2 and a rate of 1/1.92 C with a cut-off voltage of 3.6-4.3 V (vs Li/Li+). Results and Discussion

Figure 1. Flowsheet of the procedure to prepare polycrystalline LiMn2O4 powders by the sol-gel method.

considerably lower temperature and shorter time. The physicochemical and electrochemical properties of the LiMn2O4 powders were investigated. Experimental Section LiMn2O4 powders were prepared according to the procedure as shown in Figure 1. A stoichiometric amount of lithium and manganese acetate salts (Junsei, EP grade) with a cationic ratio of Li:Mn ) 1:2 was dissolved in distilled water and completely mixed with an aqueous solution of PAA (Wako Pure Chem., EP grade). PAA was used as a chelating agent in making gels. Nitric acid was slowly added to this solution with constant stirring until a pH of 1-3 was achieved. The resulting solution was evaporated at 70-80 °C for 10 h until a transparent sol was obtained. To remove water further, the transparent sol was heated at 70-80 °C while being mechanically stirred with a magnetic stirrer. As the evaporation of water proceeded, the sol turned into a viscous transparent gel. For the preparation of the gel precursors with different molar ratios of PAA to the total metal ions, the same procedure was repeated, with the molar ratio of PAA to the total metal ions being changed to 0.5, 1.0, 1.67, and 2.0. The gel precursors obtained were decomposed at 300 °C for 1 h in air to eliminate organic contents and then calcined at 200-800 °C for 10 h in air to obtain polycrystalline spinel LiMn2O4 powders. The thermal decomposition behavior of the gel precursors was examined by means of thermogravimetry (TG) and differential thermal analysis (DTA; Netzsch STA409). Powder X-ray diffraction (XRD; Rigaku Rint2000) using Cu KR radiation was used to identify the crystalline phase of the materials calcined at various temperatures. Rietvelt refinement was then performed on the X-ray diffraction data to obtain lattice constants. The morphological change of the materials after the calcination of gel precursors was examined using a field emission scanning electron microscope (SEM; Hitachi S-4100), and the specific surface area of the material

The transparent gel could be formed for the various ratios of PAA to the total metal ions tested in this study. The transparency of the gel indicates that its composition was very homogeneous. In a sol-gel process where PAA is used as a chelating agent, the carboxylic acid group of PAA, -COOH, forms a chelate with mixed cations, resulting in a sol (Lessing, 1989). If M represents the divalent metal, then the chelating reaction becomes 2(-COOH) + M2+ f -COO-M-OOC- + 2H+. The utility of PAA in the sol-gel process comes from such a chemical bonding of cations onto the polymer chains. The most important thing during the sol formation process is to distribute the cations atomistically throughout the polymeric structure and not to cause a cation segregation and thereby a precipitation. The severity of the segregation largely depends on the solubility of the cations in solution as a function of pH. Heating the sol to moderate temperature causes a condensation reaction between -COOH groups through dehydration with the concurrent formation of a water molecule. As most of the excess water is removed, the sol turns into a gel and extremely high viscosity polymeric resins are developed. The gel can be either cross-linked or un-cross-linked, depending on the stoichiometry of the ratios of reactants. Sine PAA has more functional groups than citric acid which is the conventional chelating agent, it should greatly aid in the formation of a cross-linked gel and the cross-linked gel may provide more homogeneous mixing of the cations and less tendency for segregation during calcination. Figure 2 shows the TG and DTA results of the gel precursor pretreated in vacuum drying at 80 °C prior to thermal analysis, when the molar ratio of PAA to the total metal ions was 1.67. It is seen that a weight loss of the gel precursor occurred at two discrete regions of 60-230 and 230-340 °C and terminated at 340 °C. The weight loss in the temperature range of 60-230 °C corresponds to the decomposition of acetates, which is accompanied by an exothermic peak at 220 °C in the DTA curve. A similar behavior was previously reported where the acetate decomposition in coprecipitates was completed at 250 °C (Barboux et al., 1991). Also, it is presumed that the exothermic peak at 220 °C in the DTA curve is associated with the crystallization of the spinel LiMn2O4 phase, which will be discussed in Figure 3. The weight loss in the temperature range of 230340 °C in the TG curve and a large exothermic peak above 300 °C in the DTA curve can be considered as a result of the combustion of PAA in the gel precursor,

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Figure 2. Thermogravimetric and differential thermal analysis of the gel precursors pretreated in vacuum drying at 80 °C prior to thermal analysis in an air flowrate of 40 cm3/min and a heating rate of 5 °C/min.

Figure 3. X-ray differential patterns of gel-derived materials calcined at various temperatures when the molar ratio of PAA to the total metal ions was 1.67.

since the gel precursor is self-burning if once ignited. This pyrolysis behavior is supported by the observation that the actual temperature monitored abruptly increased to 450 °C despite the program temperature of 250 °C during the decomposition process. The gel precursor also turned into fluffy dark brown powders after being calcined at 300 °C, because many void volumes would be generated in the powders by CO and CO2 evolved from the thermal decomposition of PAA. Figure 3 shows the XRD pattens for the materials calcined at various temperatures for 10 h in air when the molar ratio of PAA to the total metal ions was fixed at 1.67. For the material calcined at 200 °C, a small number of impurity peaks such as Mn2O3 in addition to the LiMn2O4 phase were detected. When a material was calcined at 250 °C, most of the intermediate products disappeared and a pure crystalline phase of spinel LiMn2O4 was observed. This is quite consistent with the DTA result which shows the crystallization peak at 220 °C. Generally, the higher the calcination temperature is, the sharper and higher the diffraction peaks are and thus the better is the crystallinity of the LiMn2O4 phase. When the calcination was carried out above 650 °C, it was seen that the peaks were abruptly sharpened due to the high bulk crystallization of the grains. These results strongly suggested that our preparative method requires much lower calcination temperature and shorter calcination time than the conventional solid-state reaction where calcination temperature is usually 650-750 °C and calcination time is

Figure 4. Effect of the calcination temperature on the specific surface area of the LiMn2O4 powders when the molar ratio of PAA to the total metal ions was 1.67.

48-200 h (Gao and Dahn, 1995, 1996; Guohua et al., 1996; Guyomard and Tarascon, 1994; Manev et al., 1993; Richard et al., 1994; Tarascon et al., 1991). The use of PAA greatly suppresses the formation of precipitates from which the heterogeneity stems, because the cross-linked gel may provide more homogeneous mixing of the cations and less tendency for segregation during calcination. Therefore, the fine mixture state of calcined materials in the homogeneous composition makes it possible to form a single-phase spinel LiMn2O4 under the mild conditions. This may be ascribed to the fact that the materials derived from the gel precursors are of atomic scale and homogeneously mixed with each other and thus have high sinterability. Similar results have already been reported where LiCoO2 and LiNiO2 powders were synthesized through the sol-gel method using PAA and PVB as chelating agents, respectively (Sun et al., 1996; Sun and Oh, 1997). Figure 4 shows the effect of the calcination temperature on the specific surface area of the same materials as shown in Figure 3. For measurement of the specific surface area, multiple samples should have been prepared and appropriate statistics should have also been incorporated in the analysis of the data. In this study, however, only limited samples at a specific condition were tested for such a purpose; namely, two different samples prepared at the same calcination temperature of 650 °C were analyzed to check whether there was any big deviation in the measurements. Since the deviation of the specific surface areas for the above two samples turned out to be only (1.0 m2/g, it has been assumed that the deviation might not greatly differ for other calcination temperatures and a sample for each of the experimental conditions would be sufficient to see the trends. On the other hand, the deviation of the specific surface areas for the multiple measurements of the same sample was within (0.5 m2/g. It is seen from Figure 4 that the specific surface area of the powders decreases linearly with increasing calcination temperature. The specific surface area of 65 m2/g for the material synthesized at 300 °C is larger than that of 48 m2/g prepared at 250 °C by the sol-gel method using carbon black or gelatin as a stabilizing agent reported by Amine et al. (1994). To our best knowledge, the specific surface area of the powders prepared in this study is the largest among the ones that have been reported so far. The specific surface area of 3.3 m2/g for the material synthesized at 800 °C is also compa-

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Figure 6. X-ray differential patterns of the LiMn2O4 powders calcined at 650 °C at the various molar ratios of PAA to the total metal ions: (a) 0.5, (b) 1.0, (c) 1.67, and (d) 2.0. Figure 5. Effect of the calcination temperature on the lattice constant of the LiMn2O4 powders when the molar ratio of PAA to the total metal ions was 1.67.

rable to that of about 3 m2/g for the one obtained at 750 °C by melt-impregnation (Xia et al., 1995). Figure 5 shows the effect of the calcination temperature on the lattice constant a, obtained from the Rietvelt refinement on the XRD data, in the cubic unit cell of the LiMn2O4 powders when the molar ratio of PAA to the total metal ions was 1.67 and the gel precursors were calcined in air for 10 h. As in the case of the specific surface area, the multiple Rietvelt refinements have been applied to the same sample prepared at a specific condition to find out that the deviation of the lattice constants was only (0.001 Å for the sample prepared at 750 °C and (0.002 Å for that at 500 °C. It is seen from the figure that the lattice constant increases almost linearly up to 8.2332 Å with increasing calcination temperature from 250 to 800 °C. It is reported that the lattice constant of Li1+xMn2-xO4 powders should always be minimized at a fixed composition to have less oxygen vacancies (Gao and Dahn, 1996). It is speculated that the lattice constant of the cubic unit cell is closely related to the value of the manganese average oxidation state in the spinel phase. A lower calcination temperature results in a more oxidized manganese cation because manganese ions are stable preferentially as Mn4+ at lower temperatures. The ionic radius of Mn4+ is smaller than that of Mn3+, and this would lead to slight contraction of the unit cell (Masquelier et al., 1996). The lattice constants of the LiMn2O4 powders calcined at 750 and 800 °C are seen to be 8.2313 and 8.2332 Å, respectively. The effect of the PAA quantity on crystallinity, specific surface area, and lattice constant is represented in Figures 6-8, respectively. Figure 6 shows the XRD patterns for the materials calcined at 650 °C in air for 10 h at the various molar ratios of PAA to the total metal ions: 0.5, 1.0, 1.67, and 2.0. It is seen from the XRD patterns that the spinel LiMn2O4 phase could be formed regardless of the molar ratios of PAA to the total metal ions tested in this study. However, the dependence of crystallinity on the PAA quantity is not clearly shown here, because the sharpness of the peaks is comparable to each other. Crystallinity will be discussed below in terms of the lattice constants. The dependence of the specific surface area for the same materials as shown in Figure 6 on the molar ratio of PAA to the total metal ions is shown in Figure 7. With increasing the molar ratio of PAA to the total metal ions, the specific surface area is expected to decrease because

Figure 7. Effect of the molar ratio of PAA to the total metal ions on the specific surface area of the LiMn2O4 powders calcined at 650 °C.

Figure 8. Effect of the molar ratio of PAA to the total metal ions on the lattice constant of the LiMn2O4 powders calcined at 650 °C.

of the increased combustion heat generated, but the rsults show an opposite trend. For example, the specific surface areas are 4.8 and 25.4 m2/g, respectively, when the molar ratios of PAA to the total metal ions are 0.5 and 2.0. This phenomenon will be explained in the following paragraph. In order to investigate the structural differences in the spinel LiMn2O4 at the various molar ratios of PAA to the total metal ions, the Rietvelt refinement was performed on the XRD data to obtain lattice constants. Figure 8 shows the effect of the molar

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Figure 9. Scanning electron micrographs of the LiMn2O4 powders calcined at (a) 300, (b) 500, (c) 750, and (d) 800 °C.

ratio of PAA to the total metal ions on the lattice constant of the same materials as shown in Figure 6. With increasing the molar ratio, the lattice constant and thus the crystallinity of the spinel LiMn2O4 powders are seen to increase, although the extent of increase is not as much as in the case of increasing calcination temperature in Figure 5. The reason why the crystallinity and the specific surface area of the LiMn2O4 powders simultaneously increase with the quantity of PAA used can be explained as follows. The less the PAA quantity in preparing gel precursors is, the shorter the Li-Mn cation distance is, and thus the higher the probability of the crystallization between the cations is. Therefore, bigger particles with a low specific surface area will be produced at the lower PAA quantity. On the contrary, when the quantity of PAA used increases, the cross-linked gel precursors suppress cation mobility and effectively prevents the cations from contacting each other. Therefore, the particles do not grow in size and the specific surface area becomes high. During the formation of small particles with a large specific surface area by such a wrapping effect of PAA on the cations, the combustion heat from

PAA contributes to the increase of the crystallinity of the particles. From the DTA curve of the gel precursors in Figure 2, it is seen that the exothermic peak above 300 °C corresponds to the combustion of PAA. It is presumed that PAA not only works as a chelating agent but also provides combustion heat required for synthesis of the LiMn2O4 powders (Lessing, 1989; Sun et al., 1996). The greater the amount of PAA used in preparing the gel precursors, the greater the combustion heat generated from PAA. The combustion heat may not be enough to facilitate sintering between particles to reduce the specific surface area of the powders but should be sufficient to increase the crystallinity of the LiMn2O4 powders in this study, resulting in an increase in the lattice constant at the higher PAA quantity. During the calcination of the gel precursors, CO and CO2 gases evolved from the thermal decomposition of PAA yield many void volumes in the LiMn2O4 powders. It has been observed that the materials were more puffed up after calcination of the gel precursors at the increased amount of PAA even though the calcination temperature remained unchanged. Therefore, a plentiful amount of combustion heat from the increased PAA quantity might

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Figure 10. Scanning electron micrographs for the LiMn2O4 powders calcined at 650 °C when the molar ratios of PAA to the total metal ions were (a) 1.67 and (b) 0.5.

be though to increase the crystallinity, while the wrapping effect of PAA resulted in the smaller LiMn2O4 powders with an increased specific surface area. When excess PAA is used, a negative effect can be induced by raising the temperature too high in a short period of time and by decreasing the partial pressure of oxygen from the increased amount of CO or CO2 during the decomposition of PAA. On the other hand, if the PAA quantity is too small, the combustion heat becomes insufficient to increase crystallinity and even to yield the spinel LiMn2O4 phase (Taguchi et al., 1993). The PAA quantity used in this study was out of this range, and such a negative effect was not observed. Figure 9 shows scanning electron microscopy (SEM) images for the powders calcined at various temperatures in air for 10 h, with the molar ratio of PAA to the total metal ions being fixed at 1.67. The presence of looselyagglomerated spherical particles with an average grain size of about 30 nm was observed from the powders calcined at 300 °C. For the materials calcined at 500 °C, no morphological changes were observed compared with the ones calcined at 300 °C. As the calcination temperature increased, growth kinetics was favored and thus agglomerated spherical particles were changed to a larger particulate. When the gel precursors were heated at 750 °C, the particle size of the particulates increased to about 100 nm with a fairly narrow particlesize distribution. For the materials calcined at 800 °C, it was observed that the particle size of the particulates abruptly increased to about 600 nm with a narrow particle-size distribution. Furthermore, it is notable that the surface of the particulates has many wrinkles which might consist of the porous structures. In order to investigate the morphological features of the LiMn2O4 powders at the different molar ratios of PAA to the total metal ions, SEM images were shown in Figure 10 for the molar ratios of 1.67 and 0.5 for the powders calcined at 650 °C in air for 10 h. The surface of the powders with the molar ratio of 1.67 contains loosely-agglomerated spherical particles with an average grain size of about 50 nm, though larger agglomerated particles are observed. For the materials prepared with the molar ratio of 0.5, it is seen that the particle size of the particulates becomes about 200 nm, which

Figure 11. Cyclic voltammograms for the LiMn2O4 powders calcined at 750 °C when the scan rate was 0.1 mV/s.

is almost 4 times larger than that of the materials with the molar ratio of 1.67 at the same calcination temperature. Figure 11 shows the cyclic voltammogram for the LiMn2O4 electrode prepared at 750 °C with a scan rate of 0.1 mV/s. Two pairs of clearly-separated oxidation peaks, located at 3.99 and 4.13 V, and reduction peaks, located at 4.06 and 4.20 V, obviously reflect the reversible two-stage process for Li ion intercalation/deintercalation in the LiMn2O4 electrode. The results observed in this study are consistent with those reported previously (Rossow et al., 1990; Xia et al., 1995). The ratio of the cathodic peak height to the anodic peak height (Ipc/Ipa) and the separation peak potential between the cathodic peak potential and the anodic peak potential (Epc - Epa) are nearly 1 and 60 mV, respectively. Also, the areas of the two redox peaks are almost equal. These results indicate that the intercalation/deintercalation of Li ions each occurs in the two-stage process and is reversible in this material. It is well-known that the cycle life and capacity of the LiMn2O4 powders depend on the oxygen stoichiometry and the electro-

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Figure 13. Variation of the specific discharge capacity with the cycle number for the cells of Figure 12 using the LiMn2O4 powders calcined at (a) 800 and (b) 750 °C.

Figure 12. Cycling charge/discharge curves over the potential range of 3.4-4.3 V at a current density of 1 mA/cm2 for the Li/1 M LiASF6-EC/DEC solution/porous LiMn2O4 cells using the LiMn2O4 powders calcined at (a) 800 and (b) 750 °C.

chemical characteristics. Tarascon et al. (1994) reported that the 4.5 V peak in the cyclic voltammogram was larger in their quenched sample than in slowly-cooled samples which exhibited a significant reduction of cell capacity fading during cycling. Gao and Dahn (1996) also reported that the samples with a larger number of oxygen vacancies show larger 3.3 and 4.5 V peaks in the differential capacity of the voltage curves and thus have poorer capacity retention upon cycling. From Figure 11, those peaks at 3.3 and 4.5 V are not observed at all. It is inferred from the result that the materials prepared in this study have good homogeneity and oxygen stoichiometry, but further study on this point is likely to be needed. Figure 12 shows the discharge characteristics and cycling performance of the Li/LiMn2O4 cells at a constant charge/discharge current density of 1 mA/cm2 and a rate of 1/1.92 C for the different LiMn2O4 calcination temperatures of 750 and 800 °C for 10 h. All the samples show that the discharge curves have two plateaus which are the characteristics of the manganese oxide spinel structure (Barbox et al., 1991; Thackeray et al., 1983). The Li/LiMn2O4 cell in Figure 12a, where the LiMn2O4 powders were calcined at 800 °C, initially delivers 135 mA h/g, which is identical to the result of the Pechini process (Liu et al., 1996). However, the capacity of the Li/LiMn2O4 cell in this study remains

134 mAh/g at the first 10 cycles, which is superior to the result of the Pechini process, 127 mAh/g (Liu et al., 1996). The capacity slowly decreases with cycling and remains 122 mA h/g at the first 168 cycles, declining by only 7.5 mA h/g to 127 mA h/g after 50 cycles, by 10.7 mA h/g to 124 mA h/g after 100 cycles, and by 12.8 mA h/g to 122 mA h/g after 150 cycles. By contrast, the LiMn2O4 powders calcined at 750 °C in Figure 12b initially deliver 126 mA h/g and decline by 5 mA h/g to 121 mA h/g after 50 cycles, by 13 mA h/g to 113 mA h/g after 100 cycles, and by 16 mA h/g to 110 mA h/g after 140 cycles. Variation of the specific discharge capacity with the cycle number is separately represented in Figure 13. The capacity losses over the first 168 and 142 cycles for the LiMn2O4 powders calcined at 800 and 750 °C are 9.5% and 12.7% of the initial discharge capacity, respectively. Considering the relatively high charge/ discharge current density, the above decay rates imply very good capacity retention with cycling, especially for the LiMn2O4 powders calcined at 800 °C. It is wellknown that the powders prepared at low temperatures have lower capacity but better cycling behavior than the ones prepared at high temperatures, because the former have low crystallinity and high specific surface area, and thus a good retention spinel structure caused by lithium ion intercalation/deintercalation and higher lithium ion diffusion coefficient (Liu et al., 1996; Pistoia et al., 1992). It is seen from the figure, however, that the LiMn2O4 powders calcined at 800 °C show not only superior capacity but also good cyclability compared to the ones calcined at 750 °C. Therefore, the superior capacity and cyclability of the LiMn2O4 powders calcined at 800 °C are closely related with the higher crystallinity and retention ability of the spinel structure with cycling. It is well established that higher synthetic temperature promotes better crystal growth of the LiMn2O4 powders and this is quite consistent with the result of Rietvelt refinement that the lattice constant of 8.2332 Å for the materials calcined at 800 °C is bigger than that of 8.2313 Å for the materials calcined at 750 °C. It has also been reported that the structure of the crystal surface plays an important role since the surface directly affects the lithium transport properties and the importance of unimpeded diffusion in the tunnels increases with decreasing specific surface area of the LiMn2O4 powders (Gao and Dahn, 1996). The surface structure of the LiMn2O4 powders calcined at 800 °C, which has

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many wrinkles as can be seen in the SEM images, might facilitate lithium ion diffusion in the electrode so that cycling behavior is improved. In the future, however, more detailed morphological and electrochemical studies shall be needed to understand these phenomena and to improve the cyclability of the cells further. Conclusions Spinel LiMn2O4 powders were synthesized by a solgel method using an aqueous solution of metal acetates containing PAA as a chelating agent. The transparent gels and thus spinel LiMn2O4 phase could be formed for the various ratios of PAA to the total metal ions tested in this study, say 0.5, 1.0, 1.67, and 2.0, and the transparency of the gel indicates that its composition was very homogeneous. TG, DTA, and XRD results reveal that a pure crystalline phase of spinel LiMn2O4 without impurities could be obtained at a temperature as low as 250 °C. The crystallinity and the lattice constant of the LiMn2O4 powders were seen to increase with increasing the molar ratio fo PAA to the total metal ions as well as the calcination temperature. The specific surface area decreased due to the sinterability of the powders as the calcination temperature increased, but it increased almost linearly with an increase in the PAA quantity because of the wrapping effect of PAA on cations, which prevented cations from contacting each other and lowered the possibility of the crystallization between the cations resulting in smaller particles. Polycrystalline LiMn2O4 powders calcined at 300-800 °C for 10 h were found to be composed of very uniformlysized paticulates with an average particle size of 30600 nm and a specific surface area of 3.3-65 m2/g, depending on the processing conditions. Therefore, the sol-gel method in this study required much lower calcination temperature and shorter calcination time than the conventional solid-state reaction where calcination temperature is usually 650-750 °C and calcination time is 48-200 h. It is concluded in the viewpoint of the synthetic method that the LiMn2O4 powders with a wide variety of physicochemical properties such as particle size, specific surface area, and microcrystallite morphology can be controlled by simply varying the pyrolysis conditions and chelating agent quantity. Electrochemical studies on the charge/discharge characteristics of the Li/LiMn2O4 cells show that the LiMn2O4 powders calcined at 800 °C delivered a high initial capacity of 135 mA h/g and exhibited a good capacity retention with cycling, with only 9.5% loss from the initial discharge capacity at the 168 cycles of the charge/ discharge experiments. The high initial capacity and good cycling behavior of the LiMn2O4 powders calcined at higher temperatures are closely related with the higher crystallinity and retention of the spinel structure with cycling. Literature Cited Amine, K.; Yasuda, H.; Fujita, Y. Synthesis Characterization and Electrochemical Studies of Spinel Li1+xMn2O4. The 35th Battery Symposium in Japan, 1994; 2C15, p 157. Barboux, P.; Tarascon, J. M.; Shokoohi, F. K. The Use of Acetates as Precursors for the Low-Temperature Synthesis of LiMn2O4 and LiCoO2 Intercalation Compounds. J. Solid State Chem. 1991, 94, 185. Gao, Y.; Dahn, J. R. Thermogravimetric Analysis to Determine the Lithium to Manganese Atomic Ratio in Li1+xMn2-xO4. Appl. Phys. Lett. 1995, 55, 2487. Gao, Y.; Dahn, J. R. Synthesis and Characterization of Li1+xMn2-xO4 for Li-Ion Battery Applications. J. Electrochem. Soc. 1996, 143, 100.

Gummow, R. J.; de Kock, A.; Thackeray, M. M. Improved Capacity Retention in Rechargeable 4 V Lithium/Lithium-Manganese Oxide (Spinel) Cells. Solid State Ionics 1994, 69, 59. Guohua, L.; Ikuta, H.; Uchida, T.; Wakihara, M. The Spinel Phases LiMyMn2-yO4 (M ) Co, Cr, Ni) as the Cathode for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1996, 143, 178. Guyomard, D.; Tarascon, J. M. The Carbon/Li1+xMn2O4 System. Solid State Ionics 1994, 69, 222. Jang, D. H.; Shin, Y. J.; Oh, S. M. Dissolution of Spinel Oxides and Capacity Losses in 4 V Li/LixMn2O4 Cells. J. Electrochem. Soc. 1996, 143, 2204. Lessing, P. A. Mixed-Cation Powders via Polymeric Precursors. Ceram. Bull. 1989, 68, 1002. Liu, W.; Farrington, G. C.; Chaput, F.; Dunn, B. Synthesis and Electrochemical Studies of Spinel Phase LiMn4O4 Cathode Materials Prepared by the Pechini Process. J. Electrochem. Soc. 1996, 143, 879. Manev, V.; Momchilov, A.; Nassalevska, A.; Kozawa, A. Rechargeable Lithium Battery with Spinel-related MnO2, II. Optimization of the LiMn2O4 Synthesis Conditions. J. Power Sources 1993, 41, 305. Masquelier, C.; Tabuchi, M.; Ado, K.; Kanno, R.; Kobayashi, Y.; Maki, Y.; Nakamura, O.; Goodenough, B. Chemical and Magnetic Characterization of Spinel Materials in the LiMn2O4-Li2Mn4O9-Li4Mn5O12 System. J. Solid State Chem. 1996, 123, 255. Ogihara, T.; Yanagawa, T.; Ogata, N.; Yoshida, K.; Mizuno, Y.; Yonezawa, S.; Takashima, M.; Nagata, N.; Ogawa, K. Preparation of LiNiO2 by the Alcohate Method for a Cathode Active Material of Lithium Secondary Battery. Denki Kagaku 1990, 6, 1343. Pistoia, G.; Wang, G.; Wang, C. Li+ Insertion into Mn Spinel Phases. Solid States Ionics 1992, 58, 285. Prabaharan, S.; Michael, M. S.; Kumar, T. P.; Mani, A.; Athinarayanaswamy, K.; Gangadharan, R. Bulk Synthesis of Submicrometer Powders of LiMn2O4 for Secondary Lithium Batteries. J. Mater. Chem. 1995, 5, 1035. Richard, M. N.; Fuller, E. W.; Dahn, J. R. The Effect of Ammonia Reduction on the Spinel Electrode Materials, LiMn2O4 and Li(Li1/3Mn5/3)O4. Solid State Ionics 1994, 73, 81. Rossow, M. H.; de Kock, A.; de Picciotto, L. A.; Thackeray, M. M. Structural Aspects of Lithium-Manganese-Oxide Electrodes for Rechargeable Lithium Batteries. Mater. Res. Bull. 1990, 25, 173. Sun, Y.-K.; Oh, I.-H. Synthesis of LiNiO2 Powders by a Sol-Gel Method. J. Mater. Sci. Lett. 1997, 16 (1), 30. Sun, Y.-K.; Oh, I.-H.; Hong, S.-A. Synthesis of Ultrafine LiCoO2 Powders by the Sol-Gel Method. J. Mater. Sci. 1996, 31, 3617. Taguchi, H.; Yoshioka, H.; Matsuda, D.; Nagao, M. Crystal Structure of LaMnO3+δ Synthesized Using Poly(Acrylic Acid). J. Solid State Chem. 1993, 104, 460. Tarascon, J. M.; Wang, E.; Shokoohi, F. K.; McKinnon, W. R.; Colson, S. The Spinel Phase of LiMn2O4 as a Cathode in Secondary Lithium Cells. J. Electrochem. Soc. 1991, 138, 2859. Tarascon, J. M.; McKinnon, W. R.; Coowar, F.; Bowmer, T. N.; Amatucci, G.; Guyomard, D. Synthesis Conditions and Oxygen Stoichiometry Effects on Li Insertion into the Spinel LiMn2O4. J. Electrochem. Soc. 1994, 141, 1421. Thackeray, M. M.; David W. I. F.; Bruce, P. G.; Goodenough, J. B. Lithium Insertion into Manganese Spinel. Mater. Res. Bull. 1983, 18, 461. Tsumura, T.; Shimizu, A.; Inagaki, M. Synthesis of LiMn2O4 Spinel via Tartrates. J. Mater. Chem. 1993, 3, 995. Xia, Y.; Takeshige, H.; Noguchi, H.; Yoshio, M. Studies on a LiMn-O Spinal System (Obtained by melt-impregnation) as a Cathode for 4 V Lithium Batteries, Part 1. Synthesis and Electrochemical Behavior of LiMn2O4. J. Power Sources 1995, 56, 61. Yamada, A.; Miura, K.; Hinokuma, K.; Tanaka, M. Synthesis and Structural Aspects of LiMn2O4(δ as a Cathode for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1995, 142, 2149.

Received for review March 18, 1997 Revised manuscript received July 9, 1997 Accepted July 22, 1997X IE970227B

Abstract published in Advance ACS Abstracts, September 1, 1997. X