| Summary: | 碩士 === 大同大學 === 材料工程研究所 === 90 === Portable electronic devices such as cellular telephones, camcorders and notebook computers, known as 3C, are becoming smaller and lighter through the development of LSI and VLSI. Therefore, the demand for lighter and higher specific energy secondary batteries to power this device has increased. The lithium-ion battery is considered to be the most suitable system among these state-of-the-art techniques. The spinel lithium manganese oxide is a promising candidate for the cathode materials of Li-ion secondary batteries due to its low cost, high safety in usage and non-toxicity.
Many methods to synthesize the lithium manganese oxide have been achieved or attempted. In this study, the ultrasonic spray pyrolysis, which has been successfully employed to manufacture commercially various ceramic powders, will be attempted to synthesize the LiMn2O4.
The precursors used in this study are nitrates to prepare aqueous starting solution for ultrasonic atomization in ambient temperature (15-20℃). The pure spinel LiMn2O4 powder, which was verified from the absence of the other phases in the XRD spectra, has been successfully synthesized by using ultrasonic spray pyrolysis process at synthesis temperature as low as 600℃. The optimal pyrolysis temperature is 800℃ in terms of the electrochemical properties. The preparation is remarkably fast (only less than 1 sec) in contrast to the solid state reaction or solution methods (many hours). The composition of LiMn2O4 powder, determined by the AA spectra and ICP-AES, can be easily controlled by the composition of starting composition. The morphologies of powder depend on the flow rate of the carries gas (air) as predicted. The higher flow (8SLM) will lead to the spherical hollow balls with narrow size distribution around 1m of secondary particle size. The primary particle size is about 70-120nm, determined by SEM and particle size analyzer of laser scattering. The lower flow rate will also result in the spherical hollow particles but with wider size distribution. The electrochemical performance of the LiMn2O4 powders can be evaluated through the coin cell assembly by the charging-discharging cycle test and through T-type cell by using cyclic voltammetry (CV). The results indicate the stoichiometric lithium manganese oxide exhibits best performance, i.e., high initial discharging capacity and low capacity fading with cycling. The mono-dispersed stoichiometric lithium manganese oxide powder prepared by higher flow rate and subsequent grinding will give the optimal experimental parameter for further study.
The post heat treatment will substantial increase the short range ordering of the synthesized powders. The increase in crystallinity will be beneficial to the electrochemical performance of LiMn2O4. At 800oC, 4 hours sintering will get best result in the properties of the cell performance.
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