We applied photon-only X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study the bulk properties of Li-rich disordered rock salt oxyfluoride cathodes Li₂VO₂F, Li₂V_0.5Ti_0.5O₂F and Li₂V_0.5Fe_0.5O₂F. We have systematically investigated V 3d- and O 2p-states, both of which are crucial for charge compensation during battery cycling. A combined analysis of both spectroscopies reveals that the vanadium ions deviate from the expected V³⁺/V⁵⁺ redox couple for all systems. Moreover, our results suggest that the oxygen ions partake in charge compensation to a considerable degree. Large O 2p-band width changes as a function of doping material are observed. While the diverging trend makes it less plausible that these states have a significant influence on degradation, we find evidence that the presence of O 2p–V 3d hybridized states may constitute a common factor for the previously found improvement in long term cycling of the Ti/Fe-substituted materials. This study demonstrates the power of using a combined spectroscopy approach to understand the tuning properties of Li₂VO₂F to find ways of designing better cathode materials based on this parent compound.

The rising demand for high-performance lithium-ion batteries, pivotal to electric transportation, hinges on key materials like the Ni-rich layered oxide LiNixCoyAlzO2 (NCA) used in cathodes. The present study investigates the redox mechanisms, with particular focus on the role of oxygen in commercial NCA electrodes, both fresh and aged under various conditions (aged cells have performed >900 cycles until a cathode capacity retention of ∼80%). Our findings reveal that oxygen participates in charge compensation during NCA delithiation, both through changes in transition metal (TM)–O bond hybridization and formation of partially reversible O2, the latter occurs already below 3.8 V vs. Li/Li+. Aged NCA material undergoes more significant changes in TM–O bond hybridization when cycling above 50% SoC, while reversible O2 formation is maintained. Nickel is found to be redox active throughout the entire delithiation and shows a more classical oxidation state change during cycling with smaller changes in the Ni–O hybridization. By contrast, Co redox activity relies on a stronger change in Co–O hybridization, with only smaller Co oxidation state changes. The Ni–O bond displays an almost twice as large change in its bond length on cycling as the Co–O bond. The Ni–O6 octahedra are similar in size to the Co–O6 octahedra in the delithiated state, but are larger in the lithiated state, a size difference that increases with battery ageing. These contrasting redox activities are reflected directly in structural changes. The NCA material exhibits the formation of nanopores upon ageing, and a possible connection to oxygen redox activity is discussed. The difference in interaction of Ni and Co with oxygen provides a key understanding of the mechanism and the electrochemical instability of Ni-rich layered transition metal oxide electrodes. Our research specifically highlights the significance of the role of oxygen in the electrochemical performance of electric-vehicle-grade NCA electrodes, offering important insights for the creation of next-generation long-lived lithium-ion batteries.

Vibrationally-resolved resonant inelastic X-ray scattering (VR-RIXS) at the O K-edge is emerging as a powerful tool for identifying embedded molecules in lithium-ion battery cathodes. Here, we investigate two known oxygen redox-active cathode materials: the commercial Li_xNi_0.90Co_0.05Al_0.05O₂ (NCA) used in electric vehicles and the high-capacity cathode material Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LRNMC) for next-generation Li-ion batteries. We report the detection of a novel vibrational RIXS signature for Li-ion battery cathodes appearing in the O K pre-peak above 533 eV that we attribute to OH-groups. We discuss likely locations and pathways for OH-group formation and accumulation throughout the active cathode material. Initial-cycle behaviour for LRNMC shows that OH-signal strength correlates with the cathodes state of charge, though reversibility is incomplete. The OH-group RIXS signal strength in long-term cycled NCA is retained. Thus, VR-RIXS offers a path for gaining new insights to oxygen reactions in battery materials.