Nature of the Oxygen-Loss-Induced Rocksalt Layer and Its Impact on Capacity Fade in Ni-Rich Layered Oxide Cathodes

In Ni-rich layered oxide cathodes, cycling above the oxygen-loss threshold voltage (∼4.3 V vs Li⁺/Li) promotes structural transformations at the cathode surface. These transformations can result in various thermodynamically favorable rocksalt-like (RSL) structures (NiO, NiO_x, and/or Li_yNi_zO) that have different Li⁺ transport properties. Elucidating the precise phase type in the RSL can help determine design strategies to improve Li⁺ kinetics and identify design rules to suppress capacity fade in Ni-rich cathodes. This study utilizes surface-sensitive X-ray absorption spectroscopy in combination with first-principles simulations and distinguishes the layered oxide spectroscopic features from those of surface-reduced layers of pure NiO and Li_xNi_1-xO. The transport of lithium ions through this oxygen-loss-induced surface-reconstructed layer is studied with operando X-ray diffraction in a pouch cell as a function of cycling aging and constant voltage protocols.