LiFSI vs. LiPF6 electrolytes in contact with lithiated graphite: Comparing thermal stabilities and identification of specific SEI-reinforcing additives

Lithium bis(fluorosulfonyl) imide (LiFSI) is regarded as an alternative to the classical LiPF6 salt in today's LiFePO4/graphite-based Li-ion batteries electrolyte owing to its slightly higher conductivity and lower fluorine content. In an attempt to better evaluate the safety issues, here we report the comparative study of the LiFSI and LiPF6 based electrolyte/lithiated graphite interface thermal behavior. DSC measurements with LiFSI-based electrolyte reveal a sharp exotherm with large heat release though at higher onset and peak temperatures compared to LiPF6-based electrolyte. With the help of GC/MS, 19F NMR and ESI-HRMS analyses, we assume that this highly energetic peak around 200 °C, which is dependant upon the lithium content, is mainly related to electrochemical reduction of FSI− anion. In a strategy to limit the probability and damage of thermal runaway event, electrolyte additives such as vinylene carbonate (VC), fluoro ethylene carbonate (FEC), di-isocyanato hexane (DIH) and toluene di-isocyanate (TDI) have been investigated and shown to significantly lower the energy associated with the exothermic phenomenon.