Thermal stability of electrolytes with Li xCoO 2 cathode or lithiated carbon anode

Abstract

The thermal stability of electrolytes with Li x CoO 2 cathode or lithiated carbon anode was reviewed including our recent results. From our experiments, it was found that Li x CoO 2, delithiated by a chemical method using H 2SO 4 showed two exothermic peaks, one beginning at 190 °C and the other beginning at 290 °C. From high-temperature XRD, it was found that the first peak, from 190 °C, was the phase transition from a monoclinic ( R-3 m) to a spinel structure ( Fd3 m). The spinel structure Li x CoO 2 showed a very small cycling capacity. Probably, cation mixing was induced by the heat treatment. The DSC measurements of Li 0.49CoO 2 with 1 M LiPF 6/EC+DMC showed two exothermic peaks. The peak starting at 190 °C probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230 °C was electrolyte oxidation caused by released oxygen from Li 0.49CoO 2. From DSC profiles of chemically delithiated Li 0.49CoO 2 and 1 M PC electrolytes with various Li salts, it was found that the inhibition effect of the surface reaction starting at 190 °C was large when LiBF 4, LiPF 6, and LiClO 4 were used. The thermal stability of electrochemically lithiated graphite with 1 M LiPF 6/EC+DMC and PVdF-binder has been investigated. DSC revealed a mild heat generation starting from 130 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 °C. The peak at 140 °C 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.