Degradation of surface film on LiCoO2 electrode by hydrogen fluoride attack at moderately elevated temperature

The failure mechanism of LiCoO2 electrode at moderately elevated temperature (70 °C) is examined. To this end, a Li/LiCoO2 cell is cycled 10 times within 3.0-4.3 V (vs. Li/Li+) to deposit a surface film on the LiCoO2 electrode, followed by storage at 70 °C for 10 h. The charge/discharge cycling data obtained at 25 °C after the storage reveal that the cell is damaged by the storage. The Coulombic efficiency decreases because the charging capacity is larger than that observed before the storage. The increased charging capacity is due to electrolyte oxidation. The postmortem analyses of the stored electrodes show that the surface film on the LiCoO2 electrode becomes thinner after storage and with a notable compositional change compared to before storage. The increased charging capacity after the storage can be attributed to the extra electrolyte oxidation on the damaged (thinned) LiCoO2 electrode surface. The loss of passivating ability due to surface film damage is responsible for the extra electrolyte oxidation and poorer Coulombic efficiency. Meanwhile, the chemical composition of the damaged film is very similar to that of the surface film stored with added hydrogen fluoride (HF, 400 ppm) in the electrolyte. It is thus suggested that the surface film is damaged by attack of HF generated from LiPF6 during the high-temperature storage. Evidently, a decrease of LiPF6 concentration in the electrolyte mitigates the film damage and the associated electrode degradation.