High-entropy lattice pinning effect enables long lifespan and air stability in O3-type sodium-ion battery cathodes

O3-type NaNi_0.4Fe_0.2Mn_0.4O₂ cathodes suffer from harmful phase transitions and large volume strains, leading to sluggish Na⁺ kinetics and poor cyclability. To address these issues, we propose dual-site doping via a high-entropy strategy, O3-Na_0.96Ca_0.02Ni_0.27Fe_0.2Mn_0.35Al_0.02Cu_0.1Ti_0.05O₂ (NCNFMACTO) cathode. Based on the Rietveld refinement of X-ray diffraction (XRD) data and DFT calculations, it is inferred that Ca²⁺ doping occupies the sodium sites, generating an “anchoring effect” that stabilizes the transition metal layer during the desodiation process. Concurrently, doping the transition-metal layer with multi-principal elements (Al, Cu, Ti) induces a lattice-regulating effect. This reinforces TM-O covalency and impedes slab gliding, thereby suppressing detrimental phase evolution and minimizing volume change. As a result, the material undergoes a reversible O3-P3-O3 phase transition with only 1.1% volumetric strain, significantly enhancing Na⁺ diffusion kinetics. The doped cathode exhibits remarkable cycling stability (91% capacity retention after 200 cycles at 1C and 80.1% capacity retention after 1000 cycles at 5C) and excellent air stability. Moreover, the NCNFMACTO//HC full cell maintains 85.2% capacity retention over 400 cycles at 1C. The full cell demonstrates an excellent energy density of 264.87 Wh·kg^–1 at 44.42 W·kg^–1, fully demonstrating its feasibility. This work provides key insights for designing high-stability sodium-layered oxide cathodes.