Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte

Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes in lithium-ion battery (LIB). Here we theoretically investigated low (LC) and high (HC) concentration systems of LiN(SO2CF3)(2) (Li-TFSA) salt in acetonitrile (AN) solution, to elucidate the mechanism of improving the low reduction stability of AN at the HC condition, by density functional theory based molecular dynamics (DFT-MD) sampling of the solvation character with extra electron(s). We demonstrated that TFSA anions sacrificially accept the reductive electron at the HC condition, which is ascribed to formation of specific network structure and the resulting shift of electron affinity of the anions. We also found that, even in the LC condition, TFSA eventually decomposes with one electron reduction. This sacrificial anion reduction hinders two electron reductive decomposition of AN, leading to improved electrochemical stability. The mechanism may give a guiding principle for the design of better LIB electrolytes.