Thermal stability of electrolytes with mixtures of LiPF₆ and LiBF₄ used in lithium-ion cells

Thermal stability studies of electrolytes with mixtures of LiPF₆ and LiBF₄ 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 LiPF₆ at the lower temperature and lithium LiBF₄ at the higher temperature, due to decomposition reactions resulting in the Lewis acids PF₅ and BF₃. Instead, the mixed salt electrolyte exhibited a single exothermic peak. We deduced that the HF produced by the reaction of LiPF ₆ with solvent was the reason for the existence of one exothermic reaction peak. The HF may react with LiBF₄ to give HBF₄, which is very unstable and decomposes easily to HF and BF₃ at a lower temperature than the decomposition temperature of LiBF₄ itself. By comparison, a thermal study of a mixed salt electrolyte including LiPF ₆ and LiN(SO₂CF₃)₂ showed that the exothermic reaction of LiN(SO₂CF₃)₂ with solvents is also influenced by HF produced in the reaction of LiPF₆ with solvents but that the strength of the influence is small compared with its effect on an electrolyte mixture including LiBF₄.