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 Li2Ru1-xMxO3 and Li2Ir1-xMxO3, 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 Li1.2Ni0.13Co0.13Mn0.54O2 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.
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© 2015 American Chemical Society.
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