Covalency does not suppress O2 formation in 4d and 5d Li-rich O-redox cathodes

Abstract

Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O^2- ions charge compensated by extraction of Li⁺ ions. Recent results have shown that for 3d transition metal oxides the oxidized O^2- forms molecular O₂ trapped in the bulk particles. Other forms of oxidised O^2- such as O₂^2- or (O-O)^n- with long bonds have been proposed, based especially on work on 4 and 5d transition metal oxides, where TM-O bonding is more covalent. Here, we show, using high resolution RIXS that molecular O₂ is formed in the bulk particles on O^2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li₂RuO₃, Li₂Ru_0.5Sn_0.5O₃ and Li₂Ir_0.5Sn_0.5O₃. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O₂, i.e. the greater covalency of the 4d and 5d compounds still favours O₂. RIXS and XAS data for Li₂IrO₃ are consistent with a charge compensation mechanism associated primarily with Ir redox up to and beyond the 5+ oxidation state, with no evidence of O-O dimerization.