Low-Entropy Multimetal Doping-Modified Na₃V₂(PO₄)₃ Cathodes: Synergetic Enhancement for High-Performance Sodium-Ion...

Sodium-ion batteries (SIBs) have emerged as promising alternatives to lithium-ion batteries for large-scale energy storage systems, but their development is hindered by the lack of high-performance cathode materials. Na₃V₂(PO₄)₃ (NVP), with a NASICON structure, is a potential cathode candidate; however, its insufficient structural stability and sluggish Na⁺ diffusion kinetics limit its practical applications. Herein, a low-entropy doping strategy is proposed to modify NVP by incorporating multimetal ions (Ti, Cr, Fe, Mn, and Ca) to obtain low-entropy NVP-based materials (NV_2-xM_xP) via a sol-gel method followed by annealing. The optimized NV_1.8M_0.2P delivers a discharge capacity of 97.09 mAh g^-1 at 0.5 C, retains 59.19 mAh g^-1 at 20 C (60.97% capacity retention), maintains 91.76% capacity after 200 cycles at 1 C, and still retains 85.01% of its initial capacity after 4000 cycles at 10 C. X-ray diffraction (XRD) Rietveld refinement results reveal that low-entropy doping induces unit cell contraction of NV_2-xM_xP, thereby enhancing its structural stability. Partial density of states (PDOS) calculations indicate that this doping strategy reduces the bandgap of NVP from 1.32 to 0.173 eV, significantly enhancing electronic conductivity. Electrochemical impedance spectroscopy and galvanostatic intermittent titration technique reveal that NV_1.8M_0.2P exhibits a lower charge transfer resistance (449.2 Ω) and a significantly higher Na⁺ diffusion coefficient (3.8 × 10^-6 cm² s^-1) compared to pristine NVP (8.3 × 10^-8 cm² s^-1). Furthermore, ex situ XRD and X-ray photoelectron spectroscopy verify the reversible structural transformation of NV_1.8M_0.2P and the V³⁺ ↔ V⁴⁺ redox reaction during cycling. This low-entropy doping strategy not only provides an effective approach for optimizing NVP-based cathodes but also offers a valuable guideline for designing advanced electrode materials for high-performance SIBs.