Abstract:

Sodium-ion batteries have substantial potential in large-scale energy storage and low-speed electric vehicles owing to their raw material abundance, low cost, safety, and relatively low environmental impact. Recently, sodium vanadium fluorophosphate [Na₃V₂(PO₄)₂F₃, NVPF] has become a focus of research into cathode materials for sodium-ion batteries. Key attributes of NVPF are its stable three-dimensional framework structure, high theoretical capacity (128 mA·h/g), and high working voltage (approximately 3.8 V). However, low electronic conductivity and slow ion diffusion rate resulted in both low actual capacity and unsatisfactory rate performance, which had hindered further development. Therefore, researchers have been able to considerably improve electrochemical performance by optimizing the synthesis process, coating, ion doping, and structural design. Together, these improvements have greatly enhanced the potential for application of NVPF in sodium-ion batteries. Based on a review of recent relevant literature, this paper first introduces the cell characteristics of NVPF. Next, it investigates four Na⁺ extraction/insertion mechanisms (i.e., the solid solution reaction, step-by-step Na⁺ extraction/insertion, three-step Na⁺ extraction/insertion, and two-step Na⁺ extraction/insertion mechanisms). It also briefly summarizes three common synthesis methods (i.e., the high temperature solid-state, hydrothermal, and sol-gel methods) and their advantages and disadvantages. Then, recent progress with enhanced NVPF (modified by coating, ion doping, and optimized structural design) is described in detail. Finally, the practical development of the synthesis and modification of NVPF cathode materials and the NVPF full cell are explored in the context of future real-world applications of NVPF in sodium-ion batteries.

Key words: sodium-ion batteries, cathode materials, sodium vanadium fluorophosphate, preparation, modification