Zwitterionic polymer-derived nitrogen and sulfur co-doped carbon-coated Na3V2(PO4)2F3 as a cathode material for sodium ion battery energy storage

Abstract

Polyanion-type Na₃V₂(PO₄)₂F₃ (NVPF) represents a new potential cathode material due to its sodium super ionic conductor (NASICON) structure. However, unmodified Na₃V₂(PO₄)₂F₃ is restricted by its intrinsic properties, and its limitation of poor conductivity make it an unsatisfactory cathode material. Herein, a zwitterionic polymer is used as a new nitrogen and sulfur source to successfully synthesize nitrogen and sulfur co-doped carbon-coated Na₃V₂(PO₄)₂F₃ (NVPF-NSC) through a sol–gel method combined with freeze-drying technology. The obtained three-dimensional framework both alleviates agglomeration between particles and improves the electrical conductivity and electrochemical performance of the material. Based on these advantages, NVPF-NSC-2 (the zwitterionic polymer was added at 25% of NVPF mass) exhibits a high specific discharge capacity (109 mA h g⁻¹ at 0.5 C) and excellent cycle performance (with a capacity retention ratio of 92.1% after 500 cycles at 5 C and a capacity retention ratio of 87.6% after 500 cycles at 10 C with a retained capacity of 83 mA h g⁻¹). Ex situ X-ray diffraction (ex-XRD) tests were performed to explore the charge/discharge deintercalation mechanism of Na⁺, with the results demonstrating the excellent cycle stability of the material. The results reported herein demonstrate that using zwitterionic polymers as a new heteroatom source is feasible and effective, providing a new idea for the future large-scale application of sodium ion batteries.