Degradation Analysis of LiNi_0.8Co_0.15Al_0.05O₂ for Cathode Material of Lithium-Ion Battery Using Single-Particle Measurement

The effects of structural changes on electrochemical performances in cathode-active materials have to be understood to improve the durability of lithium-ion batteries. Here, cycle testing was conducted on a commercial lithium-ion cell using a LiNi0.8Co0.15Al0.05O2 cathode. Uncycled cells and those cells that were cycled 400 and 800 times were disassembled to obtain their cathodes, which were analyzed using scanning transmission electron microscopy and single particle measurement. After completing 400 cycles, a NiO-like phase is formed on the outermost surface of the particle. Furthermore, after 800 cycles, a NiO-like structure was also formed inside the particle. The rate performance of each single cathode particle that was obtained from the composite cathode was investigated to evaluate its exchange current density (i0) and Li+ apparent diffusion coefficient (D). The i0 decreased from 1.5 × 10–1 mA cm–2 (uncycled) to 0.3 × 10–1 mA cm–2 (cycled 400 times) and 0.01 × 10–1–0.05 × 10–1 mA cm–2 (cycled 800 times). D decreased from 2.0 × 10–10 cm2 s–1 (uncycled) to 1.3 × 10–10 cm2 s–1 (cycled 400 times) and 0.2 × 10–10 cm2 s–1 or less (cycled 800 times). It was clarified both electrochemically and quantitatively that the decomposition phase at the outermost surface, which was formed during the initial 400 cycles, causes a decrease in the exchange current density and that the decomposition phase inside the particle, which was formed in the range of 400 to 800 cycles, causes a decrease in the apparent diffusion coefficient of the particle.