关键词: Na₃V₂(PO₄)₂O₂F, NVPF, 共沉淀法, 电化学性能, 钠离子电池

Abstract:

High-temperature solid-state method, hydro-thermal method and solvo-thermal method have been mainly employed to synthesize Na₃V₂(PO₄)₂O₂F (NVPF) cathode materials. However, these methods are energy consuming and complicated, which is not applicable for a large scale industrial production. In this study, a rather low-temperature (70℃) co-precipitation strategy was proposed to synthesize NVPF cathode materials. The as-prepared NVPF cathode materials showed a spherical shape with a diameter of 400 ~ 500 nm, and exhibited a sodium storage of 105.6 mAh·g^-1 and an efficiency of 90.06%. After a simple thermal process, the specific capacity of the material increased to 124.3 mAh·g^-1, and the first cycle efficiency increased to 96.06%. More specifically, a series of experiments with different heat temperatures were done and the results revealed that the best electrochemical performance of NVPF cathode material was achieved with the heat treatment of 600℃ for 2 h under argon atmosphere. Techniques including XRD, SEM, FT-IR, TG-MS, and carbon content analysis and Rietveld analysis were used in order to figure out the effect of the thermal process. The results revealed that the heat treatment could remove the crystal water that led to many side reactions and lowered the cycle efficiency, remove the adsorbed hydroxyl resulted from liquid-phase synthesis, as well as increase the crystallinity of NVPF cathode materials and coated a tinny amount of carbon on the surface of the materials through the decomposition of the remained C₂O₄^2-, thus, improving the electrochemical performance of the NVPF cathode materials. Additionally, a full cell with a capacity of 24 mAh composed of a NVPF cathode and a commercial hard carbon anode was fabricated and tested. The cell exhibited an excellent cycle and rate performance. It remained 94.6% of its initial capacity after 1200 cycles at 1 C and 86% of the reference rate (0.33 C) capacity even at 4 C. Furthermore, this method is attractive to the large-scale industrial production of NVPF cathode materials.

Key words: Na₃V₂(PO₄)₂O₂F, NVPF, co-precipitation, electrochemical performance, sodium ion battery