Synthesis And Properties Of Spinel Limn₂ O₄ As Cathode Material

Lithium ion batteries have attracted much attention in recent years because of the growing demand for energy. Intensive studies have been focused on the cathode materials, especially LiMn2O₄. LiMn2O₄ is a promising candidate as cathode material for lithium ion batteries, owing to its high voltage, high energy density, safety, high abundance, low-cost, non-toxicity and environmental friendlkiness. Unfortunately, its severe capacity loss during cycling, especially at elevated temperature, is still a tough task. Some methods such as optimization of synthesis methods, surface modification or doping have been adopted to reduce the capacity loss of LiMn2O₄ in previous studies.In this thesis, hydrothermal method was used to synthesize MnO₂ nanomaterials under different temperature for 12 h. Then, the MnO₂ nanomaterials were mixed with Li₂CO₃ to synthesize the spinel LiMn2O₄ cathode materials using solid state method. The influence of MnO₂ precursors, calcination temperature and calcination time on the electrochemical performance of LiMn2O₄ were investigated systematically. Under the optimum condition, the as-prepared LiMn2O₄ was modified by FePO₄ surface coating. The thermal analysis was conducted by TG/DTA. The structure, morphology and element of materials were characterized by XRD, SEM, HRTEM and EDAX. The electrochemical performance was tested by charge-discharge, cycling volt-ampere (CV) and Electrochemical Impedance Spectroscopy (EIS).The results show that MnO₂ nanomaterials with different crystallographic types and morphologies can be synthesized via tunable hydrothermal method.β-MnO₂ sphere which was assembled by nanorods are obtained at 120℃for 12 h. Using it as the precursor, spherical LiMn2O₄ with good crystallinity and small particle size can be obtained and presents good electrochemical performance. During the synthesis process of LiMn2O₄ by solid state method, calcination temperature and calcination time are important factors influcing the the electrochemical performance of LiMn2O₄. When reacted at 750℃for 20 h, LiMn2O₄ has the best electrochemical performance.FePO₄ surface coating can effectively enhance the electrochemical performance of LiMn2O₄. LiMn2O₄ coated with 1wt.% FePO₄ has the best electrochemical performance. The initial discharge capacity is 117.2 mAh/g at 1 C, and still remains 98.4 mAh/g after 100 cycles with the rention of 84.0%.