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, LiNi0.8Co0.15Al0.05O2 and LiNi1/3Co1/3Mn1/3O2, 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 LixNi0.8Co0.15Al0.05O2 (x < 0.15) particles consist of a rhombohedral core, a spinel shell, and a rock-salt structure at the surface, while the overcharged LixNi1/3Co1/3Mn1/3O2 consists of a similar core–shell-surface structure but a very different CdI2-type surface structure. The thermal instability of LixNi0.8Co0.15Al0.05O2 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 LixNi1/3Co1/3Mn1/3O2 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.