Oxygen Redox RIXS Signatures in Li- and Na-Ion Battery Cathodes: From Ex Situ to Operando

The growing demand for energy, coupled with an increasing reliance on intermittent renewable sources such as wind and solar, makes reliable energy storage essential and drives research into new battery chemistries. Oxide-based cathodes are common in both Li- and Na-ion batteries, and researchers have increasingly asked whether the oxygen anions in these materials participate in the electrochemical reaction by accepting charge otherwise assigned to the cations. This so-called oxygen redox could, in some systems, provide additional capacity. The soft X-ray techniques X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) are well suited for studying the underlying mechanisms at the atomic level and expand our understanding of battery behavior. Notably, XAS and RIXS have revealed signatures of trapped molecular O2 in certain charged cathodes, a finding often linked to oxygen redox and still actively debated. This thesis explores oxygen redox related mechanisms in Li- and Na-ion cathodes. Oxygen K-edge analysis reveals unexpected intrinsic lattice responses to electrochemical cycling and identifies extrinsic molecular oxygen species by their distinct RIXS and XAS signatures. In the disordered rocksalt Li2VO2F, V and O XAS indicate that oxygen supplies part of the charge compensation typically ascribed to vanadium, without delivering extra capacity. In the commercial cathode LixNi0.90Co0.05Al0.05O2, RIXS reveals signatures of molecular O2 and OH groups. Two well-studied oxygen-redox cathodes, Li1.2Ni0.13Co0.13Mn0.54O2 and Na0.67Mg0.28Mn0.72O2, that appear to form trapped O2 in nanovoids are revisited, showing state-of-charge-dependent phonon excitations and polaron formation that may precede molecular O2. To better understand the nature of the contained O2, oxygen-implanted gold foil is studied as a model system for trapped oxygen in cathodes. RIXS signatures are shown to be sensitive to both intermolecular O2 as well as nanovoid-wall interactions. Finally, the thesis reports progress toward in-situ/operando RIXS measurements that probe cathodes under more realistic conditions. Promising initial results are presented, and remaining challenges and potential solutions are discussed. Altogether, these findings highlight the versatility of soft X-ray XAS and RIXS for investigating complex battery systems and represent a step toward true operando RIXS.