Toward high-performance sodium storage cathode: Construction and purification of carbon-coated Na3V2(PO4)2F3 materials

Na3V2(PO4)2F3 (NVPF) is an appealing cathode for sodium ion batteries (SIBs) owing to its moderate specific capacity, high working voltage and excellent stability. However, uncontrollable preparation and low electronic conductivity limit its production in large-scale. Herein, carbothermal reduction (CTR) is employed to prepare NVPF materials and the effect of carbon sources on the physiochemical and electrochemical properties of NVPF has been systematically investigated. Inorganic carbon sources (Super P, graphite) with low removability mix with vanadium source heterogeneously, resulting in the uncompleted reduction of vanadium, severely aggregated NVPF particles, and loose contact of carbon coated layer. In contrast, pyrolysis amorphous carbon derived from organic carbon sources (citric acid, glucose) is tightly coated onto the NVPF surface, leading to enhanced sodium ion transmission kinetics and excellent electrochemical performance. After purifying via a simple washing step, the glucose prepared NVPF material displays the best electrochemical performance, which delivers a reversible specific capacity of 122 mAh g−1 at 0.1C and remarkable long-term cycle stability with 83.2% capacity retention at 10C after 1000 cycles. This work proposes a controllable and scalable preparation method for pure phase NVPF with distinguished electrochemical performance, and we believe such a study will advance the development of SIBs.