Enhanced Electrochemical Performance of Ca-Doped Na₃V₂(PO₄)₂F₃/C Cathode Materials for Sodium-Ion Batteries

Na₃V₂(PO₄)₂F₃ (NVPF) with a NASICON structure has garnered attention as a cathode material owing to its stable 3D structure, rapid ion diffusion channels, high operating voltage, and impressive cycling stability. Nevertheless, the low intrinsic electronic conductivity of the material leading to a poor rate capability presents a significant challenge for practical application. Herein, we develop a series of Ca-doped NVPF/C cathode materials with various Ca²⁺ doping levels using a simple sol-gel and carbon thermal reduction approach. X-ray diffraction analysis confirmed that the inclusion of Ca²⁺ does not alter the crystal structure of the parent material but instead expands the lattice spacing. Density functional theory calculations depict that substituting Ca²⁺ ions at the V³⁺ site reduces the band gap, leading to increased electronic conductivity. This substitution also enhanced the structural stability, preventing lattice distortion during the charge/discharge cycles. Furthermore, the presence of the Ca²⁺ ion introduces two localized states within the band gap, resulting in enhanced electrochemical performance compared to that of Mg-doped NVPF/C. The optimal NVPF-Ca-0.05/C cathode exhibits superior specific capacities of 124 and 86 mAh g^-1 at 0.1 and 10 C, respectively. Additionally, the NVPF-Ca-0.05/C demonstrates satisfactory capacity retention of 70% after 1000 charge/discharge cycles at 10 C. These remarkable results can be attributed to the optimized particle size, excellent structural stability, and enhanced ionic and electronic conductivity induced by the Ca doping. Our findings provide valuable insight into the development of cathode material with desirable electrochemical properties.