Thermal risk evaluation of concentrated electrolytes for Li-ion batteries

Concentrated electrolytes have been attracting increasing attention due to their unique properties. However, despite the concern about their thermal stability, few research has been done on their exothermic behaviors, especially with the coexistence of electrodes. Herein, we report the results of detailed investigation into the thermal properties of LiBF₄, LiPF₆, LiTFSI, and LiFSI/carbonate concentrated solutions and their thermal behaviors with the coexistence of fully lithiated graphite. Concentrated LiBF₄ solutions showed no practical application possibilities because they were unstable on C₆Li. Increasing the salt concentration decreased the thermal stability of LiPF₆/PC solutions with the coexistence of C₆Li. The organic salt dominated the thermal behavior of the solution when mixed with C₆Li. A drastic exothermic reaction happened at 210–220 °C when C₆Li was mixed with LiFSI solutions, indicating a very high thermal risk of LiFSI carbonate solutions as LIB electrolytes. In contrast, LiTFSI solutions showed much milder reactions with C₆Li. On the other hand, because of the different LiF content in SEI, the exothermic onset temperature of the C₆Li mixture with the concentrated solution increased in the order of LiFSI > LiTFSI > LiPF₆. Comprehensively, concentrated LiTFSI electrolytes should be a good choice for LIB from the standpoint of battery safety.