고상반응법을 이용한 LiMnPO4/C 합성방법 최적화 및 전기화학적 특성 평가

Abstract

The cathode materials of Olivine structure type (LiMPO4, M=Fe, Mn, Co) have many advantages for lithium rechargeable batteries such as excellent structure stability, high discharge capacity (167∼171 mAh/g about M3+/M2+ redox couple) and nontoxic nature. Among olivine families, LiMnPO4 has higher energy density than LiFePO4 because redox potential of LiMnPO4 is 4.1V vs Li/Li which is higher than that of LiFePO4 (3.5V vs Li/Li+). However, LiMnPO4 shows poor electrochemical performance because of the very low electronic/ionic conductivity, large volume change during charging and discharging, and low diffusivity of Li ions through the solid electrolyte interface. Therefore, many studies have focused on improving them. Recently, several researchers reported that polyol and sol-gel process was useful to control the particle size and shape and get a good electrochemical performances. Especially, thin platelike carbon-coated LiMnPO4 with small particle size showed an excellent performances. However, those powders were made by liquid phase process which was less economical than solid phase process. LiMnPO4 powders synthesized by solid phase process were also reported, but showed the large variation of electrochemical properties among powders, eg. 80mAh/g at C/20 by Kang et al., 95mAh/h at C/10 by Deyu et al., and 140mAh/g at 0.28mA/cm2 by Guohua et al.. Therefore, the main purpose in this study is to investigate the reason why the electrochemical properties of LiMnPO4 powders made by solid phase process are not consistent among the researchers. The another is to establish a reliable solid state process for LiMnPO4 powder with excellent electrochemical properties. Synthesis processes such as powder mixing and annealing process were carefully controlled and the intermediate reaction products were analyzed using TG/DSC, XRD, FT-IR and SEM.