Achieving High-Performance Na₃V₂(PO₄)₂F₃ Cathode Material through a Bifunctional N-Doped Carbon Network

Abstract

Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) is emerging as a popular cathode for sodium-ion batteries owing to its stable structure, high operating voltage, and large energy density. However, its practical application is hindered by its low conductivity. In addition, due to the loss of fluorine during synthesis, Na 3 V 2 (PO 4 ) 3 (NVP) impurity is often easily generated, resulting in a decrease in actual operating voltage. Herein, a bifunctional carbon network composed of an N-doped carbon layer and carbon bridge is constructed around NVPF particles. Through pyrolysis of polydopamine (PDA), the NVPF particles are covered in situ by an N-doped carbon layer, and the carbon bridge generated by polytetrafluoroethylene (PTFE) is also coated with N-doped carbon. Besides, PTFE also serves as a fluorine supplement to ensure that pure NVPF is obtained. As a result, the bifunctional N-doped carbon networkmodified NVPF delivers a high reversible capacity (125.7 mA h g −1 at 0.2 C) and appreciable cycle stability (92.7% at 1 C over 300 cycles, and 89.8% at 10 C over 1500 cycles). When assembled into a full cell with a commercial hard carbon anode, it displays a discharge median voltage of up to 3.62 V at 0.2 C. Furthermore, it achieves a high energy density of 373.7 W h kg −1 at a power density of 461.2 W kg −1 , with an excellent specific energy retention of 78.2% after 200 cycles. Therefore, this modification method is expected to be extended to other fluorine-containing materials with poor electrical conductivity. KEYWORDS: sodium-ion batteries, cathode, Na 3 V 2 (PO 4 ) 2 F 3 , bifunctional carbon network, N-doping