High-Entropy Strategy Enabling Solid-Solution Energy Storage Mechanism of Na ₃ V ₂ (PO ₄ ) ₃ Cathodes for Long-Term Aqueous Sodium-Ion Batteries

Na₃V₂(PO₄)₃(NVP) is a promising cathode candidate for aqueous sodium-ion batteries (ASIBs), while the practical application of NVP is severely hindered by vanadium dissolution in aqueous electrolytes and electrochemical performance degradation. Herein, a high-entropy strategy was innovatively employed to synthesize Na₃V_1.0(Ti,Cr,Mn,Fe,Nb)_1.0(PO₄)₃ (HE-NVP-1.0) cathode material via a facile sol-gel method. <i>In situ</i> X-ray diffraction confirms that high-entropy doping markedly alters the Na⁺ (de)intercalation mechanism, transforming the typical two-phase reaction between Na₃V₂(PO₄)₃ and Na₁V₂(PO₄)₃ into a continuous solid-solution reaction involving a series of stable intermediate phases, which effectively mitigates lattice strain and structural deterioration and results in a minimal unit cell volume change of merely 0.34% during cycling. <i>Ex situ</i> X-ray photoelectron spectroscopy elucidates the reversible valence transitions of V³⁺/V⁴⁺/V⁵⁺ and Mn²⁺/Mn³⁺ during charge/discharge, while Cr³⁺, Ti⁴⁺, and Nb⁵⁺ remain electrochemically inactive, constituting a stable lattice skeleton. Consequently, HE-NVP-1.0 delivers a reversible specific capacity of 56.2 mA h g^-1 at 0.1 A g^-1, exhibits an excellent rate capability of 80.7% at 5.0 A g^-1, and retains 91.4% of its capacity after 5000 cycles. This work not only provides a novel high-entropy modification strategy to address vanadium dissolution in NVP but also opens new avenues for performance optimization of polyanion-type energy storage materials.