X-ray Photoemission Spectroscopy Study of Cationic and Anionic Redox Processes in High-Capacity Li-Ion Battery Layered-Oxide Electrodes

Electrode materials based on Li-rich layered oxides are of growing interest for high-energy Li-ion battery applications because of their staggering capacities associated with the emergence of a novel, reversible anionic process. However, the fundamental science at work behind this new process needs to be well understood for further optimization. Here we report on the redox mechanisms in high-capacity Li-rich materials Li₂Ru_1-xM_xO₃ and Li₂Ir_1-xM_xO₃, by combining X-ray photoemission spectroscopy (XPS) core peaks and valence intensity analyses. We fully confirm that these materials electrochemically react with Li via cumulative reversible cationic/anionic redox processes, but more importantly we reveal that, depending on the nature of the metal (Ru or Ir), there is a delicate balance between metal and oxygen contributions. For instance, we show a greater implication of oxide ions for Ir-based electrodes, consistent with the higher covalent character of Ir-O bonds compared to Ru-O bonds. We equally provide evidence that the oxygen redox process is responsible for the high capacity displayed by the Li-rich NMC Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes that are serious contenders for the next generation of Li-ion batteries. These combined results highlight the benefit of collecting both XPS core and valence spectra for a better understanding of anionic redox mechanisms in Li-rich layered oxides.