Thermal Stability of LiPF6 Salt and Li-ion Battery ElectrolytesContaining LiPF6

The thermal stability of the neat LiPF6 salt and of 1 molal solutions of LiPF6 in prototypical Li-ion battery solvents was studied with thermogravimetric analysis (TGA) and on-line FTIR. Pure LiPF6 salt is thermally stable up to 380 K in a dry inert atmosphere, and its decomposition path is a simple dissociation producing 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 POF3 and HF. No new products were observed in 1 molal solutions of LiPF6 in EC, DMC and EMC by on-line TGA-FTIR analysis. The storage of the same solutions in sealed containers at 358 K for 300 420 hrs. did not produce any significant quantity of new products as well. In particular, noalkylflurophosphates 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