Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O2−
ions charge compensated by extraction of Li+ ions. Recent results have shown that for 3d
transition metal oxides the oxidized O2− forms molecular O2 trapped in the bulk particles.
Other forms of oxidised O2− 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 O2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds,
Li2RuO3, Li2Ru0.5Sn0.5O3 and Li2Ir0.5Sn0.5O3. 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.
