Stabilizing Interlayer Repulsion in Layered Sodium-Ion Oxide Cathodes via Hierarchical Layer Modification.

Layered sodium-ion oxides hold considerable promise in achieving high-performance sodium-ion batteries. However, the notorious phase transformation during charging, attributed to increased O─O repulsion, results in substantial performance decay. Here, a hierarchical layer modification strategy is proposed to stabilize interlayer repulsion. During desodiation, migrated Li from the transition metal layer and anchored Ca in sodium sites maintain the cationic content within the sodium layer. Meanwhile, partial oxygen substitution by fluorine and the involvement of oxygen in redox reactions increase the average valence of the oxygen layer. This sustained cation presence and elevated anion valence collectively mitigate increasing O─O repulsion during sodium extraction, enabling the NaCa[LiNiMn]OF (NCLNMOF) cathode to retain a pure P2-type structure across a wide voltage range. Unexpected insights reveal the interplay between different doping elements: the robust Li─F bonds and Ca steric effects suppressing Li loss. The NCLNMOF electrode exhibits 82.5% capacity retention after 1000 cycles and a high-rate capability of 94 mAh g at 1600 mA g, demonstrating the efficacy of hierarchical layer modification for high-performance layered oxide cathodes.