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 rock-salt like (RSL) structures (NiO, NiOx, and/or LiyNizO) 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 surface reduced layers of pure NiO and LixNi1-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.