Covalency does not suppress O2 formation in 4d and 5d Li-rich O-redox cathodes
Robert A. House, John‐Joseph Marie, Joohyuk Park, Gregory J. Rees, Stefano Agrestini, Abhishek Nag, Mirian García‐Fernández, Ke‐Jin Zhou, Peter G. Bruce
Abstract
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 O 2 trapped in the bulk particles. Other forms of oxidised O 2− such as O 2 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 O 2 is formed in the bulk particles on O 2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li 2 RuO 3 , Li 2 Ru 0.5 Sn 0.5 O 3 and Li 2 Ir 0.5 Sn 0.5 O 3 . The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O 2 , i.e. the greater covalency of the 4d and 5d compounds still favours O 2 . RIXS and XAS data for Li 2 IrO 3 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.