Stabilizing the Bulk Lattice and Passivating the Interface/Surface via an In Situ Constructed Dual‐Layer Modified for Ultra‐Stable Sodium‐Ion Cathodes

O3-type layered oxides are promising cathodes for sodium-ion batteries(SIBs) due to their high capacity, yet they suffer from irreversible O3-P3 phase transitions, lattice strain, transition metal dissolution, and moisture sensitivity, leading to rapid capacity decay. Conventional modified often fail to address bulk phase instability and exhibit poor adhesion. Here, an in situ AlPO4/Na3xAl1-xPO4 dual-layer modified was constructed on NNFM via a liquid-phase mixing and high-temperature sintering process. The inner Na3xAl1-xPO4 stabilizes the bulk structure through strong Al─O bonds, delaying phase transitions and enabling fast Na+ transport dynamic, while the outer AlPO4 acts as a sacrificial layer, consuming harmful species, blocking H2O/CO2, suppressing sodium hydrolysis and transition metal dissolution. The modified AC-NNFM cathode achieves 85.8% capacity retention after 300 cycles at 1 C, and delivers a reversible capacity of 96.8 mAh·g-1 at 5 C while maintaining excellent stability over 500 cycles at high temperature. Furthermore, the assembled AC-NNFM//HC pouch full battery demonstrates an 8.4% improvement in capacity retention after 300 cycles at 1 C along with markedly reduced charge-discharge temperature rise. This work demonstrates a synergistic bulk-anchoring and interface-passivation strategy for advanced SIB cathode design.