(362e) Synergistic Coupling of Anionic Oxygen Redox with Selenium for Stable High-Voltage Sodium Layered Oxide Cathodes

The increasing need for accessible and sustainable energy storage solutions has brought sodium-ion (Na-ion) cathode materials^1,2 to light as possible contenders for the upcoming generation of battery cathodes. Unfortunately, Na-ion layered cathodes' irreversible oxygen redox reaction has made it difficult for them to be widely used, resulting in voltage deterioration and a shorter lifespan. In this work, we conduct a thorough investigation of the Na_0.6Li_0.2Mn_0.8O₂ classic model Na-ion cathode material, demonstrating a notable spatial heterogeneity in its redox couple progression from the bulk to the surface. When oxygen is released, cationic valence states stratify from the surface to the bulk, causing surface cations to become "activated" and engage in electrochemical redox while inhibiting anionic redox at the same time. This process progressively spreads to the particle's bulk phase.

We design Se-oxidative doped surfaces to counteract this undesirable transfer of redox coupling. Significantly reducing manganese redox activity, this revised cathode exhibits steady voltage behavior and maintains reversible oxygen redox even after 100 cycles. While the pristine Na_0.6Li_0.2Mn_0.8O₂ cathode decreases from 58% to 14%, the anionic redox capacity contribution of the Se-doped Na_0.6Li_0.2Mn_0.8O₂ cathode stays high at 45% from an initial 62%. The comprehensive computational electronic analyses have offered us valuable insights into the enhanced stability observed in this context. These results highlight the possibility of using sophisticated synergistic coupling design to lessen the problem of irreversible anionic redox-induced voltage deterioration.