Distinguishing bulk redox from near-surface degradation in lithium nickel oxide cathodes

Ni-rich layered oxide cathodes can deliver higher energy density batteries, but uncertainties remain over their charge compensation mechanisms and the degradation processes that limit cycle life. Trapped molecular O₂ has been identified within LiNiO₂ at high states of charge, as seen for Li-rich cathodes where excess capacity is associated with reversible oxygen redox. Here we show that bulk redox in LiNiO₂ occurs by Ni-O rehybridization, lowering the electron density on O sites, but importantly without the involvement of molecular O₂. Instead, trapped O₂ is related to degradation at surfaces in contact with the electrolyte, and is accompanied by Ni reduction. O₂ is removed on discharge, but excess Ni²⁺ persists forming a reduced surface layer, associated with impeded Li transport. This implicates the instability of delithiated LiNiO₂ in contact with the electrolyte in surface degradation through O₂ formation and Ni reduction, highlighting the importance of surface stabilisation strategies in suppressing LiNiO₂ degradation.