Dependence of AlPO4 coating thickness on overcharge behaviour of LiCoO2 cathode material at 1 and 2 C rates

The overcharge behaviour of AlPO4-coated LiCoO2 cathodes as a function of coating thickness is investigated in Li-ion cells at 1 and 2 C overcharging rates. The AlPO4 coating thickness is controlled by the concentration of the coating solution. During 1 C-overcharging to l2 V, the 5 V plateau region, which is indicative of both electrolyte oxidation and an exothermic reaction between the electrolyte and the cathode, decreases with increasing AlPO4 coating thickness (AlPO4 concentration). Therefore, a Li-ion cell containing a P100-coated LiCoO2 cathode (AlPO4 coating thickness estimated as 1000 Å) does not show the 5 V plateau, and displays the lowest cell external temperature of 60 °C, even at 12 V. This suggests that the reaction at the 5 V plateau strongly influences the final external temperature of the cell. When a higher current is applied (2 C), cells with bare or P10-coated LiCoO2 cathodes either burst or explode, and reach external temperatures of over 275 °C. By contrast, cells with P30 and P100-coated cathodes show only a swollen external appearance with temperature profiles similar to those at the 1 C-overcharging rate.