Coupling Anionic Oxygen Redox with Selenium for Stable High-voltage Sodium Layered Oxide Cathodes

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, we incorporated selenium as a synergistic redox active center of oxygen to improve the stability of Na-ion cathodes. Our 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 Na0.6Li0.2Mn0.8O2 cathode remains as high as 84% after 50 cycles, while the pristine Na0.6Li0.2Mn0.8O2 cathode experiences a reduction to 39% of its initial capacity. Our 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 Na0.6Li0.2Mn0.8O2 cathodes. Our findings highlight the potential of redox coupling design to address the issue of voltage fade caused by irreversible anionic redox.