Thermal Stability of Electrolytes with Mixtures of LiPFsub 6 and LiBFsub 4 Used in Lithium-Ion Cells

Thermal stability studies of electrolytes with mixtures of and were carried out using differential scanning calorimetry. The solvent was a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in the volume ratio of 3:3:1, respectively. We expected the occurrence of two independent exothermic peaks associated with at the lower temperature and lithium at the higher temperature, due to decomposition reactions resulting in the Lewis acids and Instead, the mixed salt electrolyte exhibited a single exothermic peak. We deduced that the HF produced by the reaction of with solvent was the reason for the existence of one exothermic reaction peak. The HF may react with to give which is very unstable and decomposes easily to HF and at a lower temperature than the decomposition temperature of itself. By comparison, a thermal study of a mixed salt electrolyte including and showed that the exothermic reaction of with solvents is also influenced by HF produced in the reaction of with solvents but that the strength of the influence is small compared with its effect on an electrolyte mixture including © 2004 The Electrochemical Society. All rights reserved.