Origin of reversible oxygen redox reactions in high energy density layered oxides

Oxygen redox reactions (ORRs) are considered a new strategy in reaching a high-energy density for rechargeable batteries. Here, we propose the concept “band coherency” for identifying the origin of reversible ORRs in alkali-excess compounds, i.e., Na2RuO3 and Li2RuO3. Band coherency redox chemistry exhibits non-discrete transition metal (TM) nd–O 2p electron activity. This can be explained by the charge variations of O and Ru, including thermodynamic-phase stability. After the cation-based redox reaction (Ru4+/Ru5+), a dominant ORR, accompanied by the partial Ru-redox reaction, takes place in the band-coherency region. Subsequently, pure anion redox through oxygen occurs. This three-step redox mechanism is consistent with the electrochemical behavior of the oxygen-redox-tuned cathodes, until the band-coherency region shows great ORR reversibility. Triggering band coherency is a rational-design principle in using ORRs, excluding pure anionic activity and maintaining their high-energy-density properties upon cycling for the next generation of alkali-ion batteries.