Thermal stability of electrolytes with LixCoO2 cathode or lithiated carbon anode

The thermal stability of electrolytes with LixCoO2 cathode or lithiated carbon anode was reviewed including our recent results. From our experiments, it was found that LixCoO2, delithiated by a chemical method using H2SO4 showed two exothermic peaks, one beginning at 190 degreesC and the other beginning at 290 degreesC. From high-temperature XRD, it was found that the first peak, from 190 degreesC, was the phase transition from a monoclinic (R-3m) to a spinel structure (Fd3m). The spinel structure LixCoO2 showed a very small cycling capacity. Probably, cation mixing was induced by the heat treatment. The DSC measurements of Li0.49CoO2 with 1 M LiPF6/EC + DMC showed two exothermic peaks. The peak starting at 190 degreesC probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230 degreesC was electrolyte oxidation caused by released oxygen from Li0.49CoO2.. From DSC profiles of chemically delithiated Li0.49CoO2 and 1 M PC electrolytes with various Li salts, it was found that the inhibition effect of the surface reaction starting at 190 degreesC was large when LiBF4, LiPF6, and LiClO4 were used. The thermal stability of electrochemically lithiated graphite with 1 M LiPF6/EC + DMC and PVdF-binder has been investigated. DSC revealed a mild heat generation starting from 130 degreesC with a small peak at 140 degreesC. The mild heat generation continued until a sharp exothermic peak appeared at 280 degreesC. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 degreesC. The peak at 140 degreesC seems to be caused by the reaction (the solid electrolyte interphase (SEI) formation) of the electrolyte and lithiated graphite, which surface is covered by poly(vinylidene fluoride) (PVdF)-binder without formation of SEI at a lower temperature. (C) 2003 Elsevier Science B.V. All rights reserved.