Thermal stability of the solid electrolyte interface on carbon electrodes of lithium batteries

The type of lithium salts (LiPF6 and LiClO4) was found to have a strong impact on the thermal stability of the solid electrolyte interface (SEI) layer formed on graphite electrodes. According to Fourier transform infrared spectra, the dominant species of the SEI layer is the ethylene carbonate reduction product. Hence, the change of the impedance of the SEI layer is determined by its structure. The oscillating phenomena of the thickness of the SEI layer in the LiPF6 system can be attributed to an alternating deterioration and reformation of SEI, illustrating its inherent thermal instability. As for the LiClO4 system, because it is less reactive, the buildup of the SEI layer is stable and gradual. In addition, PF5, a decomposition product of LiPF6, was identified to be a chief source of the thermal instability of SEI in the LiPF6 system. (C) 2004 The Electrochemical Society.