Covalency does not suppress O<sub>2</sub> formation in 4d and 5d Li-rich O-redox cathodes.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 34016979.
- Also identified by DOI 10.1038/s41467-021-23154-4 and PMC identifier 8137948.
- Licence recorded as CC BY.
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Abstract
Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O<sup>2-</sup> ions charge compensated by extraction of Li<sup>+</sup> ions. Recent results have shown that for 3d transition metal oxides the oxidized O<sup>2-</sup> forms molecular O<sub>2</sub> trapped in the bulk particles. Other forms of oxidised O<sup>2-</sup> such as O<sub>2</sub><sup>2-</sup> or (O-O)<sup>n-</sup> 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<sub>2</sub> is formed in the bulk particles on O<sup>2‒</sup> oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li<sub>2</sub>RuO<sub>3</sub>, Li<sub>2</sub>Ru<sub>0.5</sub>Sn<sub>0.5</sub>O<sub>3</sub> and Li<sub>2</sub>Ir<sub>0.5</sub>Sn<sub>0.5</sub>O<sub>3</sub>. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O<sub>2</sub>, i.e. the greater covalency of the 4d and 5d compounds still favours O<sub>2</sub>. RIXS and XAS data for Li<sub>2</sub>IrO<sub>3</sub> 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.