Thermal stability of alkyl carbonate mixed-solvent electrolytes for lithium ion cells

The thermal stability of some mixed-solvent electrolytes used in lithium cells was measured by differential scanning calorimetry (DSC) using airtight containers. The electrolytes used were ethylene carbonate (EC) + diethylcarbonate(DEC), EC + dimethylcarbonate(DMC), propylene carbonate (PC) + DEC, and PC + DMC in which was dissolved 1 M LiPF₆ or 1 M LiClO₄. The influence of water addition and lithium metal addition on the thermal behavior of these electrolytes was also investigated. The exothermic peak of LiPF₆ electrolytes containing DEC was found at 255 °C, and the peak temperature of the electrolytes containing DEC was 15-20 °C lower than that of LiPF₆ electrolytes containing DMC. This effect was also observed in the electrolytes including LiClO₄. DMC was found to be more reactive than DEC. The thermal behavior of various kinds of LiPF₆ electrolytes with lithium metal was measured by DSC. The exothermic reaction of 1 M LiPF₆/EC + DEC, 1 M LiPF₆/EC + DMC, and 1 M LiPF₆/PC + DMC with lithium metal began at the melting point of lithium metal because of the break down of the solid electrolyte interface (SEI). The temperature was approximately 180 °C, whereas the self-heating of 1 M LiPF₆/PC + DEC occurred before the melting point of lithium metal. The temperature at which the self-exothermal reaction began was 140 °C. Therefore, the lithium metal in this electrolyte was found to be thermally unstable. When water was added to the above electrolytes with lithium metal, the exothermic reaction began at less than 130 °C, probably due to a collapse of SEI in response to the HF that was a product of the reaction between LiPF₆ and the added water.