Thermal stability of propylene carbonate and ethylene carbonate–propylene carbonate-based electrolytes for use in Li cells

Abstract

The thermal stability of mixed-solvent electrolytes used in lithium cells was investigated by differential scanning calorimetry (DSC) through the use of airtight containers. The electrolytes used were propylene carbonate (PC) and ethylene carbonate (EC)+PC, in which was dissolved 1 M LiPF 6, 1 M LiBF 4, 1 M LiClO 4, 1 M LiSO 3CF 3, 1 M LiN(SO 2CF 3) 2, or 1.23 M LiN(SO 2CF 3)(SO 2C 4F 9). The influence of lithium metal or the Li 0.5CoO 2 addition on the thermal behavior of these electrolytes was also investigated. The peak temperature of PC-based electrolytes increased following the order of LiPF 6<LiClO 4<LiBF 4<LiN(SO 2CF 3) 2<LiSO 3CF 3<LiN(SO 2CF 3)(SO 2C 4F 9). The order of peak temperature of EC–PC-based electrolytes shows a similar tendency to that of EC–PC-based electrolytes, with the exception of the LiN(SO 2CF 3) 2 electrolyte. The EC–PC-based electrolytes with Li metal show a more stable profile compared with the DSC curves of the PC-based electrolytes with the Li metal. The solid electrolyte interphase (SEI) covers the surface of the Li metal and prevents further reduction of the electrolytes. EC may form a better SEI compared with PC. The PC-based electrolytes of LiSO 3CF 3, LiN(SO 2CF 3) 2 and LiN(SO 2CF 3)(SO 2C 4F 9) with the coexistence of Li 0.49CoO 2 show a broad peak at around 200 °C, which may be caused by the reaction of the Li 0.49CoO 2 surface and electrolytes. The PC-based electrolytes of LiPF 6, LiClO 4 and LiBF 4 with Li 0.49CoO 2 show exothermic peaks at higher temperatures than 230 °C. The peak temperatures of the EC–PC-based electrolytes with the coexistence of Li 0.49CoO 2 are nearly the same temperature as the EC–PC-based electrolytes.