Leveraging high-entropy substitution to achieve V⁴⁺/V⁵⁺ redox couple and superior Na⁺ storage in Na₃V₂(PO₄)₃-based cathodes for sodium-ion battery

Sodium super ionic conductor-structured Na₃V₂(PO₄)₃ (NVP) has garnered considerable attention owing to its excellent operational voltage and 3D framework, but the limited ionic conductivity and substantial volume fluctuation impede its practical application. In this work, high-performance Na₃V_1.45(Fe,Al,Cr,Mn,Ni)_0.5Mo_0.02Zr_0.03(PO₄)₃ (HE-NVP) is designed by the partial substitution of V³⁺ by seven metal ions in the NVP using high-entropy substitution. Due to the profound effect of high-entropy substitution, the M-O bond length in the HE-NVP is finely tuned, effectively reducing the distortion of MO6 octahedron. Simultaneously, high entropy substitution suppresses adverse phase transitions in the high voltage range above 4.0 V (vs Na⁺/Na) and enhances structural stability. The activated V^4+/5+ elevates energy density to an impressive 394.2 Wh kg^-1 at 0.5 C within the voltage range of 2.2–4.3 V, while the specific capacity remains 80.2 mAh g^-1 after 800 cycles at 5 C, exhibiting capacity attenuation per cycle at only 0.025 %. Ex-situ XRD reveals that the sodium storage process of HE-NVP undergoes a single solid solution phase reaction with a small volume change rate of 0.91 %. This study offers a promising avenue for the development of advanced polyanionic phosphate cathodes.