Structural stability of Li1-xNi0.85Co0.15O2 and Li1-xNi0.85Co0.12Al0.03O2 cathodes at elevated temperatures

Layered LiNi0.85Co0.15O2 and LiNi0.85Co0.12Al0.03O2 oxides have been synthesized by a gel method and a hydroxide precursor method. The LiNi0.85Co0.12Al0.03O2 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 Li1-xNi0.85Co0.15O2 and Li1-xNi0.85Co0.12Al0.03O2 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.