Synthesis And Electrochemical Performance Of Spinel Lithium Manganese Oxides

The cathode material spinel LiMn₂O₄ was prepared by the sol-gel method using asparagine, glycine acid and aspartic acid as chelating agents respectively at low temperature. FT-IR, NMR, TG-DTA, XRD and element analysis were applied to infer the structure and composition of lithium manganese precursors, to analyze the formation process and thermo-decomposition of lithium manganese precursors and to discuss the formation mechanism of spinel LiMn₂O₄. The morphology, phase, electrochemical properties of spinel LiMn₂O₄ and the kinetics of electrode were investigated by XRD, SEM, element analysis and charge-discharge test. The results show that aspartic acid can reduce decomposing temperature of lihium manganese precursors and phase-pure spinel LiMn₂O₄ can be obtained at 400℃. At the same calcination temperature, the samples prepared using the aspartic acid as the chelating agent have smaller particles and weaker agglomeration and are better than the samples prepared using the glycine acid and asparagine as the chelating agents on crystallization, initial discharge specific capacity and cycling stabilization. The initial discharge capacity of the sample prepared at 400℃is 110.3mAh·g^-1. After 20 cycles, the discharge capacity decreases to 86.4% of the initial capacity. The kinetics of electrode was investigated by EIS. The equivalent circuits medeling was proposed to fit experimental EIS, and the kinetics parameters were calculated. The average diffusion coefficient of lithium of sample prepared at 750℃is 1.003×10^-9cm²Â·s^-1.Spherical spinel LiMn₂O₄ powders were synthesized by sintering the spherical MnO₂ precursor mixed with LiOH·H₂O by controlling the molar ratio of metal ions Li⁺/Mn²⁺. The structures and morphologies of samples were characterized by XRD and SEM. Effect of surface coated LiCoO₂ on the morphologies, structures and electrochemical properties of spherical LiMn₂O₄ was studied. The results indicate that spherical LiMn₂O₄ has high tap-density and excellent size distribution. The initial discharge specific capacities of samples tested at 25℃and 55℃are 128.2mAh·g^-1 and 125.0mAh·g^-1 respectively. After 50 cycles, the discharge capacity decreases to 90% and 68% of the initial capacity respectively. 5%LiCoO₂-coated LiMn₂O₄ has better cycling performance at elevated temperature and more initial specific capacities than pure spherical spinel LiMn₂O₄ at elevated temperature. But the excessive LiCoO₂ can reduce the initial capacity at room temperature.