Structural stability of Li_1-xNi_0.85Co_0.15O₂ and Li_1-xNi_0.85Co_0.12Al_0.03O₂ cathodes at elevated temperatures

Layered LiNi_0.85Co_0.15O₂ and LiNi_0.85Co_0.12Al_0.03O₂ oxides have been synthesized by a gel method and a hydroxide precursor method. The LiNi_0.85Co_0.12Al_0.03O₂ oxide prepared by the hydroxide precursor route exhibits a reversible capacity of around 170 mAh/g with excellent cyclability. With an objective to evaluate the structural stability of nickel-based layered oxides, the variations in the lattice parameters of charged Li_1-xNi_0.85Co_0.15O₂ and Li_1-xNi_0.85Co_0.12Al_0.03O₂ cathodes with temperature have been investigated. The change in lattice parameters have been monitored by (i) heating the charged cathodes at various temperatures in air after the first charge and after 20 cycles, (ii) heating at various temperatures the cycled coin cells in the charged state without opening, and (iii) cycling the cells at various temperatures. The experiments reveal a decrease in the c/a ratio with increasing temperature (T > 50°C) due to a migration of the nickel ions from the nickel planes to the lithium planes. The change in the c/a ratio is more pronounced during the initial number of cycles and it becomes less pronounced as the cathodes are cycled.