Evolutionary Zn ion storage mechanism of hierarchical nanotubular spinel FeV₂O₄ for Zinc–Metal full cells

A fundamental understanding about the evolution of energy storage mechanism by the structure reconstruction during initial cycling is crucial in establishing their correlation with the unprecedented electrochemical performance of electrodes. Herein, we demonstrate evolutionary zinc (Zn) ion storage mechanism and kinetics of the hierarchical nanotubular FeV₂O₄ (FeV₂O₄@NT) coated by turbostratic carbon layers through the anodic hydration-induced crystal reconstruction, where the smaller nanocrystalline FeV₂O₄ spinel phase was embedded into the newly-formed amorphous V-O-Fe phase at the heterointerface. These FeV₂O₄@NT cathodes achieved the synergistically combined energy storage mechanism of intercalation and pseudocapacitance for the greatly improved electrochemical performance of pouch full cells, which was not achieved by existing spinel oxides. With the polyoxometalate modified Zn metal anode at the optimized condition of 3.0 M Zn(OTf)₂/(ACN/H₂O-0.15) in the voltage range of [0.3–1.6 V (vs. Zn/Zn²⁺)], the POM-Zn||FeV₂O₄@NT full cells achieved a high capacity of 456.8 mAh g^–1 at 200 mA g^–1, high rate capacity of 222.0 mAh g⁻¹ even at 10,000 mA g^–1, and a large energy density of 337.5 Wh kg^–1. Furthermore, the chemical pillar and Fe vacancies in the amorphous V-O-Fe phase allowed FeV₂O₄@NT electrode to achieve a low capacity decay rate of 0.0152 % per each cycle over 1,000 cycles.