Coupling Anionic Oxygen Redox with Selenium for Stable High‐Voltage Sodium Layered Oxide Cathodes

Abstract Utilizing anion redox reaction is crucial for developing the next generation of high‐energy density, low‐cost sodium‐ion batteries. However, the irreversible oxygen redox reaction in Na‐ion layered cathodes, which leads to voltage fading and reduced overall lifespan, has hindered their practical application. In this study, selenium is incorporated as a synergistic redox active center of oxygen to improve the stability of Na‐ion cathodes. The redesigned cathode maintains stable voltage by demonstrating reversible oxygen redox while significantly suppressing the redox activity of manganese. The anionic redox contribution capacity of the selenium‐doped Na 0.6 Li 0.2 Mn 0.8 O 2 cathode remains as high as 84% after 50 cycles, while the pristine Na 0.6 Li 0.2 Mn 0.8 O 2 cathode experiences a reduction to 39% of its initial capacity. The X‐ray photoelectron spectroscopy data and computational analysis further revealed that selenium doping participates in redox as Se +4/5 which stabilizes the charged state and increases the energy step for O─O dimerization, thus improving the stability and lifespan of Na 0.6 Li 0.2 Mn 0.8 O 2 cathodes. The findings highlight the potential of redox coupling design to address the issue of voltage fade caused by irreversible anionic redox.