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

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 I M LiPF6, I M LiBF4, I M LiClO4, I M LiSO3CF3, 1 M LiN(SO2CF3)(2), or 1.23 M LiN(SO2CF3)(SO2C4F9). The influence of lithium metal or the Li0.5CoO2 addition on the thermal behavior of these electrolytes was also investigated. The peak temperature of PC-based electrolytes increased following the order of LiPF6 < LiClO4 < LiBF4 < LiN(SO2CF3)(2) < LiSO3CF3 < LiN(SO2CF3)(SO2C4F9). 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(SO2CF3)(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 LiSO3CF3, LiN(SO2CF3)(2) and LiN(SO2CF3)(SO2C4F9) with the coexistence of Li0.49CoO2 show a broad peak at around 200 degreesC, which may be caused by the reaction of the Li0.49CoO2 surface and electrolytes. The PC-based electrolytes of LiPF6, LiClO4 and LiBF4 with Li0.49CoO2 show exothermic peaks at higher temperatures than 230 degreesC. The peak temperatures of the EC-PC-based electrolytes with the coexistence of Li0.49CoO2 are nearly the same temperature as the EC-PC-based electrolytes. (C) 2002 Elsevier Science B.V. All rights reserved.