Oxygen Redox Chemistry in Rechargeable Li-Ion and Na-Ion Batteries

Cationic redox-based oxide cathodes have formed the basis of alkali-ion batteries. The limited energy density provided by cationic redox can be overcome by triggering oxygen redox. Hence, oxygen redox has received extensive interest from the perspective of materials development and fundamental understanding. Although oxygen redox is widely studied, there is still much to understand. Consolidating various concepts of oxygen redox (static/dynamic) can inform a unified understanding. Moreover, materials with oxygen redox create significant challenges such as oxygen evolution, voltage hysteresis/fading, and chemical/structural transformations. A consensus is yet to be achieved on the interrelationship of these phenomena due to the chemical/structural complexities at different length and time scales. Given the great potential of these materials for next-generation batteries, a review of the recent understanding of oxygen redox is timely. In this review, the mechanistic understandings, and challenges and their mitigation with oxygen redox are summarized by integrating various schools of thought.