High-rate stability of Na3V2(PO4)2F3 sodium-ion cathode materials enabled by an entropy-increasing strategy

Abstract

Na₃V₂(PO₄)₂F₃ (NVPF) is a widely studied cathode material for sodium-ion batteries, owing to its remarkable Na⁺ migration capability and robust structural stability. However, its application as a high-performance cathode material is hindered by its inherently low electronic conductivity. In this study, an entropy-increase strategy was employed to significantly enhance the electronic conductivity of the NVPF cathode while simultaneously optimizing its ion diffusion kinetics. These improvements collectively contribute to superior rate performance and enhanced cycling stability. Specifically, the configurational entropy (S_conf) of the material system was increased by multi-element doping, which on the one hand changed the internal electron density of the material through the cocktail effect, and on the other hand alleviated the structural collapse and performance degradation during high-rate charge and discharge cycles, thereby significantly improving the electronic conductivity and structural stability. Nonetheless, the volatilization of fluorine during synthesis introduces impurity phases, which counteract the benefits of the entropy-enhancement strategy. To address this issue, while preserving the original doping scheme, 6.5 at% NaF is introduced as an additive to compensate for fluorine loss. The configurational entropy of the resulting entropy-enhanced Na₃V_1.9(Na,Ca,Cr,Ti,Nb,Mo)_0.1/6(PO₄)₂F₃-6.5NF (E NVPF-6.5NF) reaches 0.288R. Hall effect measurements demonstrated significant improvements in electronic conductivity. The electronic conductivity increased by three orders of magnitude, from 3.9 × 10⁻⁶ S cm⁻¹ to 1.2 × 10⁻³ S cm⁻¹. The entropy-enhanced E NVPF-6.5NF also exhibited excellent rate performance, delivering a discharge capacity of 115.2 mA h g⁻¹ at a high rate of 10C. Furthermore, after 500 cycles at rates of 2C, 3C, and 4C, the material retained over 90% of its initial capacity, demonstrating exceptional cycling stability.