Distinguishing Bulk Redox from Near-Surface Degradation in Lithium Nickel Oxide Cathodes

Ni-rich layered cathodes deliver high energy densities, but uncertainties remain about their charge compensation mechanisms and the degradation processes that limit their cycle life. Recent studies have identified trapped molecular O2 within LiNiO2 at high states of charge, as seen for Li-rich cathodes where excess capacity is associated with reversible O2−/O2 redox. Here we show that bulk redox in LiNiO2 occurs by Ni-O rehybridization, lowering the electron density on O sites, but without significant involvement of molecular O2. Trapped O2 is predominantly associated with surface degradation through contact with the electrolyte, and is accompanied by Ni reduction. O2 is removed on discharge, but an excess of Ni2+ remains forming a reduced surface layer that impedes Li transport. Our findings highlight the importance of separating bulk redox from near-surface degradation, and implicate the instability of delithiated LiNiO2 in contact with the electrolyte in surface degradation through O2 formation.