Metal-ligand redox in layered oxide cathodes for Li-ion batteries

Abstract

This study refutes the commonly used ionic-bonding model that demarcates transition metal (TM) and oxygen redox using an archetypal Ni-rich layered oxide cathode, LiNi_0.8Mn_0.1Co_0.1O₂. Here, charge compensation during delithiation occurs without formal (ionic) Ni oxidation. Instead, oxygen-dominated states control the redox process, facilitated by strong TM-O hybridization, forming bulk-stable 3d⁸L and 3d⁸L² electronic states, where L is a ligand hole. Bulk O–O dimers are observed with O K-edge resonant inelastic X-ray scattering but, critically, without the long-range TM migration or void formation observed in Li-rich layered oxides. Above 4.34 V vs. Li⁺/Li, the cathode loses O, forming a resistive surface rock-salt layer that causes capacity fade. This highlights the importance of cathode engineering when attempting to achieve higher energy densities with layered oxide cathodes, especially in those where O dominates the charge compensation mechanism.