Effective Formation of Superior Surface Films on Si Negative Electrodes in a Highly Concentrated Fluorinated Carbonate Ester Solvent/Diluent Electrolyte Solution System

Abstract

Highly concentrated electrolyte solutions that used methyl 2,2,2-trifluoroethyl carbonate (MFEC) as a single solvent were studied for Si nano-flake powder negative electrodes. The nearly saturated 3.68 mol/L LiN(SO2F)2 (LiFSI)/MFEC electrolyte solution improved the charge/discharge cycle performance of the Si electrode. This electrolyte solution was moderately diluted with bis(2,2,2-trifluoroethyl) carbonate (TFEC), which did not dissolve LiFSI, to such an extent that the solvation structure was not disturbed. The solvates containing FSI– anions and MFEC molecules in the diluted electrolyte solution effectively decomposed to form a LiF-rich thin surface film on a Si electrode. As a result, the swelling of a Si electrode upon charge/discharge cycling was further suppressed, and the charge/discharge cycle performance greatly improved.