High-concentrated Electrolyte Design Enables Lithium-ion Pouch Batteries to Stably Operate at Extremely High Temperatures (Supporting Information)

Due to the poor thermal stability of the lithium hexafluorophosphate (LiPF₆) electrolyte system, commercial lithium-ion batteries (LIBs) are difficult for normal operation at high temperatures above 55 °C. The limitation of the LiPF₆ electrolyte severely limits the practical application of LIBs under extremely high temperatures conditions. Here, a high-concentration electrolyte based on lithium bis(fluorosufonyl)imide (LiFSI) as electrolyte salt and ethyl methyl carbonate (EMC) as solvent is proposed, which possesses superior electrochemical stability and thermal stability. The LiCoO₂/graphite (Gr) pouch battery with the LiFSI high-concentration electrolyte (5.0 mol L⁻¹ (M)) has been shown excellent cycling performance even at 100 °C, an impressive capacity retention of 87.7 % can be still maintained after 100 cycles at 1.0 C-rate. The superior high temperatures performance is mainly attributed to the unique solvated structure, along with the robust solid electrolyte interphase (SEI) rich in anions. This work presents an effective strategy for promoting the development of high-temperature lithium-ion batteries.