Thermal runaway mechanism of fast-charging lithium-ion batteries using LiFSI-based electrolyte

Abstract

As one of the most promising alternatives to LiPF6 salt in the fast-charging electrolyte, the application of high-content lithium bis(fluorosulfonyl)imide (LiFSI) in the LiFePO4 batteries has been seriously hindered by the higher thermal runaway risk. However, the mechanism of heat release is the most critical factor in finding the solution strategy has not been well understood. Herein, we first discovered that stable N2 formation during the reaction process of LiFSI results in siginicant heat release via thermogravimetric mass spectrometry and gas chromatography characterization. Combined quantum chemical calculations, molecular dynamic simulations and electron paramagnetic resonance studies reveal that the NSO2F free radicals generated from the decomposition of LiFSI initiate a reaction sequence that culminates in the formation of N2. This process is aggravated by high temperature and highly reactive lithiated anode, leading to rapid heat release. Based on these findings, a proof of concept strategy is adopted to quench N-containing free radicals via introducing tris(2,2,2-trifluoroethyl) borate (TTFEB) additive toward higher battery safety and excellent electrochemical performance. This work sheds light on the fundamental understanding for thermal runaway of LiFSI-based electrolyte, which potentially inspires viable improving strategies for LiFSI-based batteries in future.