Probing and Tuning Lattice Oxygen Redox in Li2RuxM(1-x)O3 for Design of High-Energy Li-Ion Batteries

Abstract

The participation of anion redox in lithium transition metal oxides such as Li2RuO3 and Li2MnO3 for positive electrodes have opened a new avenue to boost up the energy density of current lithium-ion batteries. There are many schools of thoughts for the physical origin proposed to be responsible for anion redox and more direct and clear experimental evidence of anion redox is needed to further understand and utilize this phenomena. In the first part of this work, we demonstrated the electronic signature of oxygen-oxygen coupling, 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 and simulated Ru L3-edge X-ray absorption spectra (XAS) revealed that the increase in the high-energy shoulder intensity upon lithium de-intercalation resulted from increased O-O coupling, inducing (O-O) σ* with π overlap with Ru d-manifolds, in agreement with the similar changes in the O K-edge XAS spectra. Further support came from experimental and simulated O K-edge X-ray emission spectra (XES), where the broadening of the oxygen nonbonding feature upon charging also originated from creation of (O-O) σ* states. By combining differential electrochemistry mass spectrometry (DEMS) with core-level spectroscopies, we are able to quantify the percentage of this lattice oxygen redox across different Li2RuxM(1-x)O3 (M=Ti, Cr, Mn, Fe, Ir, Sn) chemistries, through which we proposed a preliminary electronic descriptor for reversible bulk oxygen redox activity. This study constructed a transferrable framework to rationally interpret the spectroscopic features by combining experiments and theory, and pinpointed the key spectra footprints related to lattice oxygen redox from a molecular level. We also proposed a simple descriptor to rationalize and facilitate novel positive electrode materials design for reversible anion redox activities in high-energy Li-ion batteries.