Thermal stability of LiPF₆ salt and Li-ion battery electrolytes containing LiPF₆

The thermal stability of the neat lithium hexafluorophosphate (LiPF₆) salt and of 1 molal (m) solutions of LiPF₆ in prototypical Li-ion battery solvents was studied with thermogravimetric analysis (TGA) and on-line Fourier transform infrared (FTIR). Pure LiPF₆ salt is thermally stable up to 107 °C in a dry inert atmosphere, and its decomposition path is a simple dissociation producing lithium fluoride (LiF) as solid and PF₅ as gaseous products. In the presence of water (300 ppm) in the carrier gas, its decomposition onset temperature is lowered as a result of direct thermal reaction between LiPF₆ and water vapor to form phosphorous oxyfluoride (POF₃) and hydrofluoric acid (HF). No new products were observed in 1 m solutions of LiPF₆ in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) by on-line TGA-FTIR analysis. The storage of the same solutions in sealed containers at 85 °C for 300-420 h did not produce any significant quantity of new products as well. In particular, no alkylflurophosphates were found in the solutions after storage at elevated temperature. In the absence of either an impurity like alcohol or cathode active material that may (or may not) act as a catalyst, there is no evidence of thermally induced reaction between LiPF₆ and the prototypical Li-ion battery solvents EC, PC, DMC or EMC.