These observations indicated that Ru redox contributed more to discharge capacities after the first cycle.
This lattice oxygen redox of Li 2-xRuO 3 was accompanied by a small amount of O 2 evolution in the first charge from differential electrochemistry mass spectrometry (DEMS) but diminished in the subsequent cycles, in agreement with the more reduced states of Ru in later cycles from Ru L 3-edge HERFD-XAS. Experimental and simulated O K-edge X-ray emission spectra (XES) further supported this observation with the broadening of the oxygen non-bonding feature upon charging, also originated from (O-O) σ* states. Experimental Ru L 3-edge high-energy-resolution fluorescence detected X-ray absorption spectra (HERFD-XAS), supported by ab-initio simulations, revealed that the increased intensity in the high-energy shoulder upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ*-like states with π overlap with Ru d-manifolds, in agreement with O K-edge XAS spectra.
In this work, we have shown electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O 2−/(O 2) n−), in charged Li 2-xRuO 3 after Ru oxidation (Ru 4+/Ru 5+) upon first-electron removal with lithium de-intercalation. The physical origin of observed anion redox remains debated, and more direct experimental evidence is needed. Anion redox in lithium transition metal oxides such as Li 2RuO 3 and Li 2MnO 3, has catalyzed intensive research efforts to find transition metal oxides with anion redox that may boost the energy density of lithium-ion batteries.
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