Fe/V Ratio-Engineered NASICON Cathodes with Electrospun Nanofiber Templating for Long-Life Na-Ion Batteries

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Abstract Polyanionic sodium (Na) super ionic conductor (NASICON) cathodes are attractive for sodium-ion batteries owing to their robust 3-dimensional frameworks and fast Na+ transport. Na3V2(PO4)3, the benchmark NASICON material, delivers near-theoretical two-electron capacity but is constrained by vanadium cost/toxicity and limited electronic conductivity. Motivated by employing an earth-abundant, eco-friendly redox center (Fe3+/Fe2+) while retaining V-centered activity, we systematically introduced and tuned Fe in Na3+xFexV2–x(PO4)3 (x = 0.5, 0.75, 1.25, and 1.5) to increase the Na content and access the synergistic redox of Fe and V. We implemented an ex situ electrospinning-templated architecture in which electrospun fibers enabled the formation of an interconnected NFVP network, providing a continuous electron pathway and mechanical integrity. Given the well-documented difficulty of obtaining phase-pure NFVP by direct routes, often yielding secondary phases, this templated synthesis combined with composition control provides an effective path to a phase-pure, V-lean NASICON structure. Among the series, Na3+xFexV2–x(PO4)3 with x = 0.75 shows the optimum electrochemical activity and durability, delivering 78 mAh g–1 at 0.5C and retaining 90.5% after 1000 cycles and 88% after 1500 cycles at 2C. In situ X-ray absorption near-edge structure results show that the Fe and V K-edge energies shift reversibly during Na+ insertion/extraction at different charging/discharging voltage plateaus, respectively, confirming highly reversible Fe2+/Fe3+ and V3+/V4+ redox within the applied potential window. These results demonstrate a V-lean, Fe-rich NASICON cathode optimized by composition tuning, delivering promising electrochemical performance and cycling stability, and offering a stable and cost-efficient SIB cathode.

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