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.

The commercialization of sodium-ion batteries is based on developing low-cost, highly stable, and safe cathode and anode electrodes. However, the promising hard carbon anode and layered oxide cathode suffer from low sodium-embedded potential near 0.1 V and severe phase transitions, which cause safe problem and short lifespan, respectively. Herein, we design a low-strain bipolar P2-Na_0.7Ni_0.25Fe_0.2Ti_0.55O₂ to solve the mentioned obstacles, whereas (Ni, Fe) and Ti provide charge compensation when it is used as cathode and anode, respectively. It is revealed that the bipolar layered oxide undergoes solid–solution reaction when used as cathode or anode, and exhibits volume-complementary feature in a sodium-ion full-cell, as identified by in-situ X-ray diffraction. Remarkably, the safe symmetric sodium-ion full-cell exhibits excellent cyclic stability with 91.7% capacity retention after 200 cycles. This work will provide a new horizon for designing safe and stable sodium-ion batteries.