Sodium Vanadium Fluorophosphates Morphology Effects on Rechargeable Sodium Ion Batteries

With growing demands of using sustainable energy, a reliable energy storage system is truly essential. Sodium ion battery is one of the candidates which features in low cost and reasonable battery performances for future energy storage. In our previous works, promising cathode material, carbon-coated sodium vanadium fluorophosphates (Na3V2(PO4)2F3/C, NVPF/C) nanocomposites with sodium superionic conductor (NASICON) structures, have been investigated and displayed high capacities at high operation voltage. Some thorny issues arising, however, during the small-scale production. First, the discharging capacities decrease as the loading of active materials increase. Second, the original synthesis method is time consuming and low yield. Considering the economic and technical feasibility, we introduce some measures to improve both the manufacture process and the battery performance. The adoption of spray-drying not only effectively reduces the production time of NVPF/C but transforms the shape of NVPF/C from blocks to spheres. In addition, different recipes of additives results in different morphology of NVPF/C particles. Here we present three types of NVPF/C, including blocks, hollow spheres and solid spheres, which are confirmed by SEM. X-ray diffraction indicated that the purity of NVPF crystalline increased by the new process. Coin cells assembled with Na foils reversible discharge capacities of 105 mAh g−1 at 1C, accompanied with high active material loading over 10 mg/cm2.