Metal–Ligand Redox in Layered Oxide Cathodes for Li-Ion Batteries

Understanding charge compensation in Li-ion battery cathodes is crucial for improving specific capacity and cycle life. This study clarifies some of the ambiguities and inaccuracies of the commonly used ionic-bonding model that requires separate transition metal (TM) and oxygen redox regimes, using an archetypal layered oxide cathode, LiNi0.8Mn0.1Co0.1O2. Contrary to the prevalent TM-centric ionic model, this research reveals that charge compensation upon Li removal occurs without formal Ni oxidation. Instead, oxygen dominates the redox process, facilitated by strong TM–O hybridisation, forming bulk stable 3d8L and 3d8L2 electronic states, where L is a ligand hole. This model supports the observation of O K-edge resonant inelastic X-ray scattering features, often attributed to bulk O–O dimers, irrespective of the state of delithiation. Furthermore, there is no evidence of any crystallographic TM migration or void formation. Above 4.34 V vs. Li+/Li, the cathode loses surface O, forming a resistive surface rock salt layer that eventually causes capacity fade. This highlights the importance of cathode engineering when attempting to achieve higher energy densities with layered oxide cathodes where O dominates the charge compensation process.