Bulk-Interface Synergy Enables Stable High-Voltage P2-Type Layered Oxide Cathodes for Fast-Charging Sodium-Ion Batteries

P2-type layered oxides hold great promise for high-energy sodium-ion batteries (SIBs) but are hindered by irreversible P2-O2 transitions and interfacial degradation that accelerate capacity fading. Herein, we present a bulk-interface dual-engineering strategy through synergistic Mg²⁺ doping and CeO₂ surface modulation to overcome these challenges. The designed P2-type Na_0.67Mg_0.1Ni_0.23Mn_0.67O₂-CeO₂ (NNMMO-Ce) cathode integrates structural reinforcement and redox synergy: bulk Mg²⁺ stabilizes the lattice, widens Na⁺ diffusion channels, and suppresses destructive high-voltage transitions, while a conformal CeO₂ nanolayer buffers lattice strain (∼0.9%), prevents intragranular cracking, and enables the storage and release of (O₂)^<i>n</i>- species through reversible Ce³⁺/Ce⁴⁺ redox activity. This coupled mechanism coordinates electron-ion transport, minimizes polarization effects, and significantly reduces charge transfer resistance as well as the escape of lattice oxygen. Consequently, NNMMO-Ce exhibits superior performance with 94.0% capacity retention at 0.1 C and 66.2 mAh g^-1 at 20 C, along with highly reversible P2-OP4 transitions and >35-fold enhanced Na⁺ diffusion. When paired with a hard-carbon anode, the full cell delivers a high energy density of 258.97 Wh kg^-1 and excellent cycling stability over 2-4.35 V. This work establishes a cooperative bulk-interface strategy for constructing high-capacity, fast-charging, and long-lived SIB cathodes.