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; VFe2Ox) 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 VFe2Ox 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 VFe2Ox 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 VFe2Ox 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 VFe2Ox 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 VFe2Ox 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 VFe2Ox 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.