Thermal stability of LiPF 6-based electrolyte and effect of contact with various delithiated cathodes of Li-ion batteries

Abstract

Thermal stability of LiPF 6-based electrolyte (1 M LiPF 6/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 (Li x CoO 2, Li x Ni 0.8Co 0.15Al 0.05O 2, Li x Ni 1/3Co 1/3Mn 1/3O 2, Li x Mn 2O 4, Li x Ni 0.5Mn 0.5O 2, Li x Ni 0.5Mn 1.5O 4 and Li x FePO 4) was also investigated by C80 calorimetry. The results show that the cathode materials except for Li x FePO 4 usually have an enhancement effect on the decomposition of the electrolyte, but Li x FePO 4 exhibits a suppression effect on the reactions of the electrolyte. Li x FePO 4 is found to be with excellent thermal stability. Among the other cathodes, Li x CoO 2, Li x Ni 0.8Co 0.15Al 0.05O 2, Li x Ni 0.5Mn 0.5O 2 and Li x Ni 0.5Mn 1.5O 4 promote the decomposition of electrolyte by releasing oxygen and thus considered not favorable for safety, but Li x Ni 1/3Co 1/3Mn 1/3O 2 with a lesser reaction heat and Li x Mn 2O 4 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 Li x FePO 4 only has experienced tiny structure change, which is greatly different from the other cathode materials.