Thermal stability of Li_xCoO₂ cathode for lithium ion battery

It is well known that charged Li_x)CoO₂ (x < 1) is metastable, and that oxygen evolution has been observed at temperatures above 200 °C. Li_xCoO₂, delithiated by a chemical method using H₂SO₄, was investigated by means of differential scanning calorimetry (DSC) with/without an electrolyte (1 M LiPF₆/ethylene carbonate (EC) + dimethyl carbonate (DMC)). The lithium content x in the delithiated Li_xCoO₂ was determined by atomic absorption spectroscopy. The DSC profile of Li_0.49CoO₂ showed two exothermic peaks, one beginning at 190 °C and the other beginning at 290 °C. From high-temperature X-ray diffraction (XRD), it was found that the first peak, from 190 °C, was the phase transition from a monoclinic (R3̄m) to a spinel structure (Fd3m). The DSC measurements of Li_0.49CoO₂ with the electrolyte at various mixing ratios showed two exothermic peaks, one beginning at 190 °C and the other at 230 °C. The exothermic heat of each peak was proportional to the amount of Li_0.49CoO₂. 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_0.49CoO₂. The exothermic heat from 190 to 230 °C based on cathode weight was 420 ± 120 J/g, and that from 230 to 300 °C was 1000 ± 250 J/g.