Structural origin of overcharge-induced thermal instability of Ni-containing layered-cathodes for high-energy-density lithium batteries

Using a combination of time-resolved X-ray diffraction (XRD), in situ transmission electron microscopy (TEM), and first principles calculations, we explore the structural origin of the overcharge induced thermal instability of two cathode materials, LiNi_0.8Co_0.15Al _0.05O₂ and LiNi_1/3Co_1/3Mn _1/3O₂, which exhibit significant difference in thermal stabilities. Detailed TEM analysis reveals, for the first time, a complex core-shell-surface structure of the particles in both materials that was not previously detected by XRD. Structural comparison indicates that the overcharged Li_xNi_0.8Co_0.15Al_0.05O₂ (x < 0.15) particles consist of a rhombohedral core, a spinel shell, and a rock-salt structure at the surface, while the overcharged Li_xNi _1/3Co_1/3Mn_1/3O₂ consists of a similar core-shell-surface structure but a very different CdI₂-type surface structure. The thermal instability of Li_xNi _0.8Co_0.15Al_0.05O₂ can be attributed to the release of oxygen because of the rapid growth of the rock-salt-type structure on the surface during heating. In contrast, the CdI₂-type surface structure of the overcharged Li_xNi_1/3Co _1/3Mn_1/3O₂ particles delays the oxygen-release reaction to a much higher temperature resulting in better stability. These results gave deep insight into the relationship between the local structural changes and the thermal stability of cathode materials, which is vital to the development of new cathode materials for the next generation of lithium-ion batteries.