Interfacial Oxygen Locking via Gradient Structural Design: A Route to Air-Stable and High-Performance Sodium Layered Oxides

In the pursuit of sustainable energy storage solutions, sodium-ion batteries have garnered significant attention due to the abundance and low cost of sodium precursors. However, critical issues such as moisture sensitivity, sluggish kinetics, and phase degradation, particularly the irreversible lattice oxygen redox and parasitic side reactions, have significantly constrained their development. Here, we create a coherent gradient lattice reconstruction on an O3-type layered oxide surface via nonaqueous solvent-assisted ion exchange to mitigate O₂ loss and interfacial issues. The experimental analysis combined with calculations verifies that the Ca-gradient and Na-deficient surface architecture remarkably reduces unfavorable anionic redox contribution and oxygen release. Riveting calcium into the alkali metal layer helps improve conductivity and moisture stability, further stabilizing TM ions at high states-of-charge. Through meticulous design of calcium incorporation, surface-Ca-doped Na_1-<i>x</i>Ni_0.33Fe_0.33Mn_0.33O₂ exhibits excellent cycling stability (97.4% capacity retention over 300 cycles in full cells) and rate capability (166.9 mAh g^-1 at 0.1 C with 93.9 mAh g^-1 at 10 C). This study underscores the potential of surface engineering as a viable strategy to advance the performance of sodium-ion batteries, providing mechanistic insights for developing more efficient and durable energy storage systems.