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; VFe 2 O 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 VFe 2 O x aerogels at 300 °C under O 2 -rich conditions removes residual organic byproducts while maintaining a highly defective γ-Fe 2 O 3 -like local structure with minimal long-range order and vanadium in the +5 state. When as-synthesized VFe 2 O x aerogels are heated under low partial pressure of O 2 ( e.g. , flowing argon), a fraction of vanadium sites are reduced to the +4 state, driving crystallization to a Fe 3 O 4 -like cubic phase. Subsequent thermal oxidation of this nanocrystalline VFe 2 O 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 VFe 2 O 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 O 2 -heated VFe 2 O 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 VFe 2 O 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.