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

Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O ions charge compensated by extraction of Li ions. Recent results have shown that for 3d transition metal oxides the oxidized O forms molecular O trapped in the bulk particles. Other forms of oxidised O such as O or (O-O) 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 O is formed in the bulk particles on O oxidation in the archetypal Li-rich ruthenates and iridate compounds, LiRuO, LiRuSnO and LiIrSnO. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O, i.e. the greater covalency of the 4d and 5d compounds still favours O. RIXS and XAS data for LiIrO 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.