Effects of the post heat treatment of the ultrasonically spray-pyrolysed LiMn2O4 powders on the performance of the secondary battery

• Main Authors: Tsung-Hsien Teng, 鄧宗憲
• Other Authors: Mu-Rong Yang
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/38995411272384462241

碩士 === 大同大學 === 材料工程研究所 === 91 === The Li-Mn-O spinel is one of the most promising candidates for the cathode materials of Li-ion secondary batteries. Much study has been devoted to extracting the more excellent electrochemical properties from the material. The electrochemical properties of this material are greatly influenced by the particle size, the microstructure and the chemical composition, which depend on the preparation methods and the preparation conditions. In this study, nanocrystalline LiMn2O4 spinel oxides have been synthesized successfully with ultrasonic spray pyrolysis method from an aqueous mixing solution of lithium nitrate and manganese nitrate. The as-sprayed powder is hollow spherical and has no evidence of the phase other than spinel phase. The effects of post heat-treatment on the physicochemical properties of the LiMn2O4 powders have been carefully examined by means of X-ray diffraction spectra, scattering electron microscopy, BET measurement, ICP-AES, particle size analyzer of laser scattering and X-ray absorption spectroscopy. The results indicated that the post heat activation would influence the microstructure and crystallization of the pyrolysed particles. This profile variation in the potential plateau is considered to be caused by an increase in the amount of active Mn3+. The more amount of active Mn3+, the flatter and longer the plateau is. This result is in agreement with the trend of lattice parameter and the lattice parameter increases with increasing the heat treatment time. The capacity loss increases with increasing the heat treatment time. From the fitting results, the (Mn-O) and (Mn-Mn) bond distances are found to become larger as the heat-treatment time is prolonged. This confirms a reduction of Mn oxidation state with the high heat-treatment time. A conclusion is in a good agreement with the XRD results which show an increase in the lattice parameter upon increasing the heat-treatment time. The cell with 4-hour heat-treatment powder as cathode gives best electrochemical performance and exhibited an initial highest capacity of 129mAhg-1. The capacity after 30 cycles can still reaches to 121mAhg-1. Among the parameters of the valency change, lattice constant, crystallinty and Debye-Waller factor discussed in the study, the short range ordering (Debye-Waller factor, σ) is most critical to the electrochemical performance of the cell. In other words, the Debye-Waller factor can be used as an important index to evaluate the quality of the LiMn2O4 powder as cathode materials in lithium ion secondary cell. The post heat treatment is an effective way to improve the ultrasonically spray-pyrolysed powders in terms of the electrochemical characteristic.