Achieving Zero Phase Transition in P2-Type Layered Oxides via Targeted Chemical Design for Zero-Strain Sodium Storage.

Abstract

P2-type layered oxide cathodes dominate sodium-ion batteries (SIBs) due to exceptional sodium ion kinetics. However, longstanding phase transitions (e.g., P2-to-O2) not only compromise this inherent kinetic advantage but also cause severe stress strain undermining structural stability. Here, we propose a stage-specific chemical design that targetly addresses de-sodiated interlayer O^2- repulsion, the structural origin of phase transitions in P2 cathodes. The designed Na_0.67Ni_0.05Fe_0.05Ti_0.05Cu_0.2Mn_0.65O₂ (NFTCM) cathode shows a record Na-layer spacing (3.67 Å) with reduced negative charge on oxygen ions, maximally lowering O^2--O^2- repulsion during the entire desodiation process. As evidenced by in situ X-ray diffraction, the NFTCM cathode shows a true zero-phase-transition behavior with a record-low volume variation of 0.062% upon cycling. This stable, zero-strain Na ions storage behavior contributes to exceptional rate capability (121 mA h/g at 10C) and remarkably stable cycling, retaining 93.7% capacity after 600 cycles. Furthermore, operando neutron diffraction data indicate that the eliminated phase transition also enables a robust oxygen framework, a crucial factor in stabilizing the ion storage process of layered oxides.