Ionic Liquid–Acrylic Acid Copolymer Derived Nitrogen–Boron Codoped Carbon-Covered Na₃V₂(PO₄)₂F₃ as Cathode Material of High-Performance Sodium-Ion Batteries

In this study, a nitrogen-boron codoped carbon layer, Na3V2(PO4)2F3 sample, obtained by using an ionic liquid-acrylic acid copolymer as the nitrogen-boron source was used as the cathode material for sodium-ion batteries. The optimized and modified nitrogen and boron codoped carbon layer, Na3V2(PO4)2F3 (denoted as NVPF-PCNB-20), illustrated better rate capability and cycling performance. The discharge capacities of NVPF-PCNB-20 at 0.5C and 10C were 109 and 90 mAh g-1, respectively, and the capacity retention rate was 93.2% after 100 cycles at 0.5C and 92.8% after 750 cycles at 10C. Through in situ X-ray diffraction analysis of NVPF-PCNB-20, the results show that the modified Na3V2(PO4)2F3 has excellent cycle reversibility. The scanning electron microscopy and transmission electron microscopy images reveal that NVPF-PCNB-20 particles were finer and covered by a uniform coating. The results show that the ionic liquid-acrylic acid copolymer not only make the material dispersion more uniform but also enhance the electronic conductivity and sodium storage performance of Na3V2(PO4)3F3 effectively. This study may provide an effective way to synthesize nitrogen and boron codoped carbon-coated Na3V2(PO4)2F3 with excellent electrochemical performance.