Revealing Electronic Signature of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-ion Battery Material Designs
Yang Yu, Pinar Karayaylali, Stanislaw Nowak, Livia Giordano, Magali Gauthier, Wesley Hong, Ronghui Kuo, John T. Vinson, Sokaras Dimosthenis, Cheng-Jun Sun, Nenian Charles, Filippo Maglia, Roland Jung, Yang Shao-Horn
Anion redox in lithium transition metal oxides such as Li2RuO3 and Li2MnO3, has catalyzed intensive research efforts on the seek of first-row transition metal oxides that may boost up the energy density for lithium-ion batteries. The physical origin for the observed anion redox activities remain debatable and more direct experimental evidence is needed. In this work, we show electronic signature of oxygen-oxygen coupling, a direct evidence central to lattice oxygen redox (O2-/(O2)n-) in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. Experimental Ru L3-edge high-energy-resolution fluorescence detected X-ray absorption spectra (HERFD-XAS), supported by ab-initio simulations, revealed that the increase in the high-energy shoulder intensity upon lithium de-intercalation was resulted from increased O-O coupling, inducing (O-O) !∗ with # overlap with Ru d- manifolds, in agreement with O K-edge XAS spectra. Experimental and DFT-simulated O K-edge X- ray emission spectra (XES) further supported this observation since the broadening of the oxygen non-bonding feature upon charging, also originated from (O-O) !∗ states. This lattice oxygen redox of Li2-xRuO3 was accompanied with a small amount of O2 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 as revealed with Ru L3-edge HERFD- XAS. These observations indicated that Ru redox contributed more to discharge capacities compared to the first cycle. This study has pinpointed the key spectral fingerprints related to lattice oxygen redox from a molecular level, and constructed a transferrable framework to rationally interpret the spectroscopic features by combining advanced experiments and theoretical calculations to design materials for Li-ion batteries and electrocatalysis applications.