RESUMO
Na3V2(PO4)2F3 (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, Na3V2(PO4)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 network-modified 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.
RESUMO
Layered Na2FePO4F (NFPF) cathode material has received widespread attention due to its green nontoxicity, abundant raw materials, and low cost. However, its poor inherent electronic conductivity and sluggish sodium ion transportation seriously impede its capacity delivery and cycling stability. In this work, NFPF by Ti doping and conformal carbon layer coating via solid-state reaction is modified. The results of experimental study and density functional theory calculations reveal that Ti doping enhances intrinsic conductivity, accelerates Na-ion transport, and generates more Na-ion storage sites, and pyrolytic carbon from polyvinylpyrrolidone (PVP) uniformly coated on the NFPF surface improves the surface/interface conductivity and suppresses the side reactions. Under the combined effect of Ti doping and carbon coating, the optimized NFPF (marked as 5T-NF@C) exhibits excellent electrochemical performance, with a high capacity of 108.4 mAh g-1 at 0.2C, a considerable capacity of 80.0 mAh g-1 even at high current density of 10C, and a high capacity retention rate of 81.8% after 2000 cycles at 10C. When assembled into a full cell with a hard carbon anode, 5T-NF@C also show good applicability. This work indicates that co-modification of Ti doping and carbon coating makes NFPF achieve high rate and long cycle performance for sodium-ion batteries.
