Composition analysis of the passive film on the carbon electrode of a lithium-ion battery with an EC-based electrolyte

This work examines the formation of a passive film on the carbon electrode of lithium-ion batteries. With a single solvent of EC (ethylene carbonate), the structure of the passive film is found to be (CH2OCOOLi)2. In a DEC (diethyl carbonate) or DMC (dimethyl carbonate) system, C2H5OCOOLi and Li2CO3 are formed on the surface of the carbon electrode. According to results from mass spectra, CO2 gas is the main product when EC is decomposed. By contrast, DEC is decomposed into CO and C2H6, and DMC into CO and CH4. These findings suggest that the composition of the passive film depends on the chosen solvent. In a binary solvent system which contains EC, the passive film contains chiefly (CH2OCOOLi)2, which is identical to a single EC solvent system.