Surface Film Degradation on LiCoO2 Electrode by Hydrogen Fluoride Attack at Moderately Elevated Temperature and CuO Addition to Mitigate the Degradation

The degradation of the surface film on a LiCoO2 (LCO) electrode by hydrogen fluoride (HF) attack at moderately elevated temperature (70°C) and the effect of CuO as a HF scavenger were studied. Toward this end, a Li/LCO half-cell was "pre-cycled" to deposit a surface film on the LCO electrode and then stored at 70°C to simulate exposure to moderately elevated temperature. Then, the Li/LCO half-cell was "re-cycled" at 25°C to examine any film damage that occurred during high-temperature storage. It was found that HF attack during storage damages the surface film. Electrolyte oxidation is followed on the film-damaged LCO surface owing to a loss of passivating ability. Concomitantly, a surface film is deposited, which "repairs" the damaged surface. In parallel, the LCO electrode is lithiated (reduced) by the Li+ and electrons produced during electrolyte oxidation. The net result is self-discharge (lithiation) of the LCO electrode. This process (film damage and repair) continues until the Li+ and electron storage sites in LCO are completely filled. The HF scavenging action of CuO added into the LCO electrode effectively mitigates the self-discharge of the LCO electrode by decreasing the HF concentration, which alleviates the damage to the surface film.