A facile strategy for developing uniform hierarchical Na3V2(PO4)2F3@carbonized polyacrylonitrile multi-clustered hollow microspheres for high-energy-density sodium-ion batteries

Abstract Na superionic conductor structured Na3V2(PO4)2F3 (NVPF) has received considerable attention as a cathode material for sodium-ion batteries because of its higher energy density and three-dimensional open structure for Na+ diffusion channels. However, its insulating structure [PO4] results in significantly inferior conductivity, which severely limits the electrochemical performance of NVPF. In this study, uniform carbonized polyacrylonitrile-coated hierarchical Na3V2(PO4)2F3 multi-clustered hollow microspheres (NVPF-H@cPAN) were synthesized via a facile ethylene glycol-assisted hydrothermal approach, followed by a wet chemical method and heat treatment. Further, the possible formation mechanism of hierarchical Na3V2(PO4)2F3 multi-clustered hollow microspheres was investigated. The concentration of ethylene glycol and hydrothermal reaction time were found to play essential roles in the formation mechanism of the microspheres. The as-prepared NVPF-H@cPAN delivered a high discharge capacity of 116.2 mAh g−1 at 0.2C and impressive cycling stability of 85% at 5C after 2000 cycles. When assembled as NVPF-H@cPAN||commercial hard carbon (NVPF-H@cPAN||CHC) for sodium-ion full cells, which demonstrates a specific capacity of 107 mAh g−1 at 0.2C, thus achieving an energy density of 376.4 Wh kg−1 at a power density of 141 W kg−1. These results can be attributed to the structural regulation of multi-level clusters, which improved the Na+ diffusion rate and carbon coating strategy. Such a combination of structural regulation and carbon-coating strategy can provide a strategy to synthesize materials with multi-level clusters of hollow microspheres to increase the electronic conductivity and enhance the electrochemical performance of energy-storage devices.