Thermal stability of LiPF6-based electrolyte and effect of contact with various delithiated cathodes of Li-ion batteries

Thermal stability of LiPF6-based electrolyte (1 M LiPF6/EC + DMC) was studied by in-situ FTIR spectroscopy and C80 calorimetry, which indicated that the electrolyte underwent furious polymerization and decomposition reactions and sharp heat flow was generated below 225 °C. The thermal stability of the electrolyte in contact with various delithiated cathodes (LixCoO2, LixNi0.8Co0.15Al0.05O2, LixNi1/3Co1/3Mn1/3O2, LixMn2O4, LixNi0.5Mn0.5O2, LixNi0.5Mn1.5O4 and LixFePO4) was also investigated by C80 calorimetry. The results show that the cathode materials except for LixFePO4 usually have an enhancement effect on the decomposition of the electrolyte, but LixFePO4 exhibits a suppression effect on the reactions of the electrolyte. LixFePO4 is found to be with excellent thermal stability. Among the other cathodes, LixCoO2, LixNi0.8Co0.15Al0.05O2, LixNi0.5Mn0.5O2 and LixNi0.5Mn1.5O4 promote the decomposition of electrolyte by releasing oxygen and thus considered not favorable for safety, but LixNi1/3Co1/3Mn1/3O2 with a lesser reaction heat and LixMn2O4 with even less heat flow in the low temperature range (50–225 °C) are believed as promising cathodes for better safety. By comparing X-ray diffraction (XRD) patterns of these cathode materials at room temperature and those heated to 300 °C in the presence of the electrolyte, we have found that LixFePO4 only has experienced tiny structure change, which is greatly different from the other cathode materials.