High-temperature storage and cycling of C-LiFePO₄/graphite Li-ion cells

The high-temperature storage and cycling characteristics of prismatic Li-ion cells with carbon-coated LiFePO₄ cathodes, MCMB graphite anodes and a LiPF₆/EC-DEC electrolyte were investigated. The cells showed a significant capacity fade when cycled at 37 and 55 °C. Li-Sn reference electrode studies indicated that the interfacial impedance of the graphite electrode increased significantly during high-temperature cycling. Carbon-coated LiFePO₄ electrodes were found to release iron ions into the electrolyte when aged at these temperatures; EDAX analyses confirmed the presence of iron at the surface of the graphite electrodes. The observed impedance rise of the graphite electrodes and the consequent capacity fade of the cells were attributed to the formation of interfacial films that were produced on the graphite electrodes as a result of possible catalytic effects of the metallic iron particles. The cycling stability of the cells was improved significantly when the LiPF₆ electrolyte salt was replaced with the lithium bis-oxalatoborate LiB(C₂O₄)₂ salt.