Unraveling the anionic oxygen loss and related structural evolution within O3-type Na layered oxide cathodes

Nowadays, oxygen-related anionic redox reactions (ARRs) attract considerable research attention because of the potential of expanding stored energy in cathode materials for Li/Na-ion batteries. In general, irreversible lattice oxygen loss and associated structural distortion are considered to be a threat to the practical application of the ARRs process. Herein, the anionic redox behavior in an O3-type layered NaMg0.67Ru0.33O2 (Ru5+) cathode has been systematically investigated by various in/ex situ spectroscopic characterization techniques. Unexpectedly, compared to the complex phase transitions (O3/O ' 3/P3) and sluggish dynamic processes observed within ARR-free O3-type NaMg0.5Ru0.5O2 (Ru4+), the distorted structure induced by initial oxygen loss within the NaMg0.67Ru0.33O2 (Ru5+) cathode reveals a simple phase transition (O3/O ' 3) and better structural stability during cycling. This anomaly triggers us to make a fair assessment of the distorted structure induced by loss of lattice oxygen. Furthermore, based on the abnormal phenomenon referred to above, to restrain redundant oxygen loss, appropriate lattice oxygen loss can be rationally considered as a structural modification method for moderate phase transition processes during long-term cycling.