Different thermal degradation mechanisms: Role of aluminum in Ni-rich layered cathode materials

Despite increasing demands for higher energy density cathode materials, they can be bigger threats unless thermal stability is guaranteed. Herein, the thermal stability of Li_xNi_0.835Co_0.15Al_0.015O₂ (NCA83) and Li_xNi_0.8Co_0.15Al_0.05O₂ (NCA80) is compared by using in-situ transmission electron microscopy. Analysis demonstrates that NCA83 and NCA80 degrade thermally by distinct mechanisms. Al prevents the transition to CoO₂-type O1 phase by suppressing O-slab gliding by residual Li. At 67% SOC, in the sub-surface area, thermal degradation of NCA80 is mainly due to reduction of Ni, whereas thermal degradation of NCA83 is a result of concurrent reduction of Ni and Co. The difference indicates that NCA83 has both earlier transition to the rock-salt structure and poorer thermal stability than NCA80. This study presents a protocol to properly evaluate new high energy density cathode materials, and provides important insights into the thermal degradation mechanism of Ni-based layered oxides.