Development of solid polymer lithium secondary batteries

To develop a long-life secondary lithium battery using solid-state polymer electrolyte system, several factors of the polymer electrolyte were examined by using metal lithium and LiCoO2 electrodes. The lithium salts used were LiBF4, LiN(SO2CF3)2 (abbreviated as LiTFSI) and LiN(SO2CF3)(SO2C4F9) (abbreviated as LiTNSI), and the matrix polymer was cross-linked random copolymer of ethylene oxide and propylene oxide. Passivation films formed on the metal lithium surface after storage at 60°C were analyzed by X-ray photoelectron spectroscopy (XPS). The battery performance was found to depend significantly both on species and concentration of lithium salts. At present, our best achievement of the cycle ability for the 4 V class secondary battery was over 300 cycles at a current rate of C/8 at 60°C by using LiTFSI.