Rapid Supercritical Fluid-Based Hydrothermal Synthesis of Na3(VOPO4)2F as a Cathode Material for Na-Ion Batteries

Sodium vanadium oxyfluorophosphates are highly promising cathode materials for large-scale sodium-ion batteries due to their robust NASICON-type structure, high theoretical capacity (up to ≈130 mAh g–1), and elevated operating potential (≈3.8 V). Yet, the widespread commercial adoption is stalled by the materials’ poor intrinsic electronic conductivity and the complex, high-cost nature of conventional preparation methods, such as high-temperature solid-state reactions and prolonged hydrothermal synthesis, which limit production efficiency and industrial scalability. To address these challenges, this study investigates the synthesis and properties of sodium vanadium oxyfluorophosphate utilizing a green and scalable supercritical fluid processing route. The material was synthesized using a batch-type supercritical hydrothermal method at 400 °C with an exceptionally short residence time of 40 min, forming the nanoparticles in the range of 50–80 nm. Electrochemical analysis demonstrated that the synthesized material exhibited highly competitive kinetics. Cyclic voltammetry revealed two distinct V4+ to V5+ redox peaks between 3.5 and 3.6 V and 4 and 4.1 V as a two-step sodiation/desodiation process. The material displayed admirably low polarization hysteresis, with the initial discharge capacity reaching 108 mAh g–1 at a C/10 rate. The material demonstrated good rate capability, retaining 52 mAh g–1 at 10C and recovering 106 mAh g–1 at the C/10 rate, and the diffusion coefficient was quantified in the order of 10–10 to 10–11cm2 s–1. This work successfully establishes the supercritical fluid processing as a powerful, rapid, and high-efficiency platform for producing kinetically enhanced cathode materials, providing a critical reference for the future industrial development of high-power and long-lifespan Na-ion batteries.