Optimized Nonflammable Concentrated Electrolytes by Introducing a Low-Dielectric Diluent

Concentrated electrolytes of LiN(SO₂F)₂ (LiFSA) and organic phosphates (e.g., trimethyl phosphate, TMP) are receiving intense attention for safe and long-lasting lithium-ion batteries, because of their nonflammable character and unusual passivation ability toward negative electrodes. However, their high viscosity and low ionic conductivity have hampered their practical application. In this work, a low-dielectric diluent, 1,1,2,2-tetrafluoroethyl 2,2,3,3,-tetrafluoropropyl ether (HFE), is introduced into concentrated LiFSA/TMP electrolytes. Upon dilution, the viscosity drastically decreases to 11.0 mPa s and the ionic conductivity slightly increases to 0.87 mS cm^-1. More importantly, both of the nonflammable character and the unusual passivation ability are retained even after dilution. A spectroscopic analysis shows that the diluted LiFSA/TMP:HFE has a local coordination state similar to that in the concentrated LiFSA/TMP, which leads to the formation of a FSA anion-derived inorganic surface film. This work suggests the importance of the peculiar local coordination state in designing safe battery electrolytes with better passivation ability.