Development of novel strategies for enhancing the cycle life of lithium solid polymer electrolyte batteries. Final report

Lithium/solid polymer electrolyte (Li/SPE) secondary batteries are under intense development as power sources for portable electronic devices as well as electric vehicles. These batteries offer high specific energy, high energy density, very low self-discharge rates, and flexibility in packaging; however, problems have inhibited their introduction into the marketplace. This report summarizes findings to examine processes that occur with Li/SPE secondary batteries upon cyclic charging/discharging. The report includes a detailed analysis of the impedance measured on the Li/SPE/IC and IC/SPE/IC systems. The SPE was a derivative of methoxyethoxyethoxyphosphazene (MEEP) with lithium triflate salt as the electrolyte, while the intercalated cathodes (IC) comprised mixtures of manganese dioxide, carbon powder, and MEEP as a binder. Studies on symmetrical Li/SPE/Li laminates show that cycling results in a significant expansion of the structure over the first few tens of cycles; however, no corresponding increase in the impedance was noted. The cycle life of the intercalation cathode was found to be very sensitive to the method of fabrication. Results indicate that the cycle life is due to the failure of the IC, not to the failure of the lithium/SPE interface. A pattern recognition neural network was developed to predict the cycle life of a battery from the charge/discharge characteristics.