Degradation mechanism of alkyl carbonate solvents used in lithium-ion cells during initial charging

The degradation mechanism of electrolytes in the lithium-ion cell with LiCoO2 and graphite electrodes was investigated by analyzing: (i) the composition of generated gases; (ii) thin films formed on the electrode, and (iii) the compositional change of the electrolyte during the initial charging. The solvents in this work were ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC). LiPF6 was used as a salt. In the one- to three-component systems containing EC, carbon monoxide and ethane were detected, whereas Li2CO3, RCOOLi and (CH2OLi)(2) were the main components of the surface film on the negative electrode. From these results, it can be assumed that the decomposition of the systems was mainly due to the reductive reaction of EC at the initial charging. Through the additional analysis of the electrolyte composition, it was confirmed that the dialkyl-2,5-dioxahexane carboxylate was produced in the electrolyte after initial charging. This suggests the occurrence of trans-esterification. (C) 1997 Elsevier Science S.A.