Empowering Reversible Anionic Redox in Sodium Layered Oxide Cathodes via Ionic Impedance Matching Interphase

Anionic redox reaction (O^2-/O^-, ARR) in low-cost sodium manganese-based layered oxide cathodes enable high energy density, but often suffer from rapid degradation under high voltages due to interfacial instability. Here, we design an ionic impedance matching interphase to mitigate this challenge by harmonizing mechanical compatibility and ionic transport. The interphase forms as NaTi₂(PO₄)₃ undergoes thermally driven conversion accompanied by ion migration, inducing gradient surface reconstruction on P2-Na_5/6Li_1/4Mn_3/4O₂ and yielding a composite architecture comprising an outer Na₃PO₄ and an inner Ti-rich spinel-like layer. The spinel-like layer shows strong lattice compatibility with layered bulk and is anchored to conductive Na₃PO₄ through robust Ti-O-P linkages, which alleviates internal stress and stabilizes interfacial chemistry, while its intermediate conductivity lowers interfacial resistance. Moreover, this architecture shields electrolyte components from reactive oxygen species, suppresses gas evolution, and introduces lattice-permeated Ti doping, which reinforces Ti-O covalency to stabilize lattice oxygen and mitigate Jahn-Teller distortion. These synergistic effects yield a stabilized interfacial environment that promotes reversible redox electrochemistry, achieving high discharge capacity (∼ 230 mAh g^-1) and low voltage decay (<0.05 V) over extended cycling. This work deepens mechanistic insights into interfacial regulation and provides an effective route for stabilizing anionic redox chemistry for high-energy sodium-ion batteries.