Thermal risk evaluation of concentrated electrolytes for Li-ion batteries

Abstract

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.

Keywords

• Concentrated electrolyte
• Salt concentration
• Thermal stability
• Thermal runaway
• Li-ion battery