The thermal runaway mechanism of fast-charging lithium-ion batteries using LiFSI-based electrolytes

Abstract

As one of the most promising alternatives to LiPF₆ salt in a fast-charging electrolyte, the application of high-content lithium bis(fluorosulfonyl)imide (LiFSI) in LiFePO₄ batteries has been seriously hindered by a higher thermal runaway risk. However, the mechanism of heat release, which 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 significant heat release via thermogravimetric mass spectrometry and gas chromatography characterization. Combined quantum chemical calculations, molecular dynamics 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 a 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 the tris(2,2,2-trifluoroethyl) borate (TTFEB) additive toward higher battery safety and excellent electrochemical performance. This work sheds light on the fundamental understanding of thermal runaway of LiFSI-based electrolytes, which potentially inspires viable strategies for improving LiFSI-based batteries in future.