Elucidating Anionic Redox Chemistry in P3 Layered Cathode for Na-Ion Batteries

The emergence of anionic redox has recently garnered intense interest for lithium/sodium-ion batteries because of the increasing specific capacities of cathodes, which is considered as a transformative approach for designing cathode materials. Nevertheless, the widespread use of such oxygen-related anionic redox is still precluded because of the oxygen release and the correlated irreversible structural transformations and voltage fade. To fundamentally unravel the related mechanism, we have investigated the corresponding anionic redox process based on a new P3-type layered material Na_0.5Mg_0.15Al_0.2Mn_0.65O₂. Here, we prove an excellent structural stability via the operando/ex situ structural evolution within this cathode and further elucidate the complete anionic/cationic redox activity via both surface-sensitive (X-ray photoelectron spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) spectroscopies. Moreover, based on the characterization of the ex situ state to the operando evolution for the whole anionic redox process by Raman and differential electrochemical mass spectrometry, the nature of the reversible oxygen redox chemistry is clarified. Meanwhile, the origin of a small portion irreversible oxygen release generated upon the first charging and its resulting impact on subsequent processes are also fully illuminated. These insights provide guidelines for future designing of anionic redox-based high-energy-density cathodes in lithium/sodium-ion batteries.