Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage.

Vanadium-substituted iron oxide aerogels (2 : 1 Fe : V ratio; VFe₂O_x) are synthesized using an epoxide-initiated sol–gel method to form high surface-area, mesoporous materials in which the degree of crystallinity and concentration of defects are tuned via thermal treatments under controlled atmospheres. Thermal processing of the X-ray amorphous, as-synthesized VFe₂O_x aerogels at 300 °C under O₂-rich conditions removes residual organic byproducts while maintaining a highly defective γ-Fe₂O₃-like local structure with minimal long-range order and vanadium in the +5 state. When as-synthesized VFe₂O_x aerogels are heated under low partial pressure of O₂ (e.g., flowing argon), a fraction of vanadium sites are reduced to the +4 state, driving crystallization to a Fe₃O₄-like cubic phase. Subsequent thermal oxidation of this nanocrystalline VFe₂O_x aerogel re-oxidizes vanadium +4 to +5, creating additional cation vacancies and re-introducing disordered oxide domains. We correlate the electrochemical charge-storage properties of this series of VFe₂O_x aerogels with their degree of order and chemical state, as verified by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. We find that the disordered O₂-heated VFe₂O_x aerogel yields the highest Li⁺- and Na⁺-insertion capacities among this series, approaching 130 mA h g⁻¹ and 70 mA h g⁻¹, respectively. Direct heat-treatment of the VFe₂O_x aerogel in flowing argon to yield the partially reduced, nanocrystalline form results in significantly lower Li⁺-insertion capacity (77 mA h g⁻¹), which improves to 105 mA h g⁻¹ by thermal oxidation to create additional vacancies and structural disorder. [ABSTRACT FROM AUTHOR]