Cross-site lattice coordination modulates interfacial stability and ion transport behavior in sodium-ion cathode materials

P2‑type Na_0.67Fe_0.5Mn_0.5O₂ cathodes often suffer from slow Na⁺ diffusion and interfacial instability, limiting their electrochemical performance. Here, a Cu/Ca co-doping strategy was developed to simultaneously enhance structural stability and Na⁺ transport. In the designed Na_0.65Fe_0.3Mn_0.5Cu_0.2Ca_0.02O₂ (NFMCC) cathode, Cu²⁺ occupies the transition-metal layer, modulating the local electronic structure and suppressing Jahn-Teller distortion. DFT calculations show that Ca²⁺ preferentially occupies the Na layer, acting as a pillar to stabilize the framework and widen Na⁺ diffusion channels. Electrochemical tests indicate accelerated Na⁺ diffusion and reduced interfacial polarization. The optimized NFMCC delivers a discharge capacity of 158.4 mAh·g⁻¹ at 20 mA·g⁻¹ and retains 80.4% capacity after 500 cycles at 1000 mA·g⁻¹. The full cell NFMCC//HC achieves an energy density of 279.8 Wh·kg⁻¹ at 84.3 W·kg⁻¹. These results demonstrate that Cu/Ca co-doping effectively improves both the structural and electrochemical performance of P2 cathodes, providing a promising strategy for designing high-stability, high-performance sodium-ion battery cathodes.