Deconvolving lithium-ion redox in vanadium–iron oxide aerogels using X-ray absorption spectroscopy and density functional theory

Abstract

Substitution of vanadium into earth-abundant maghemite iron oxide introduces cation vacancies that increase Li⁺ storage capacity concomitant with a positive shift in its electrochemical potential. Expressing vanadium ferrite (VFe₂O_x) as an aerogel offers an opportunity to probe Li⁺ storage in this inherently defective spinel from highly disordered (X-ray amorphous) to nanocrystalline. To understand the redox sequence of the host cations, we use in situ X-ray absorption near-edge spectroscopy (XANES) obtained using an in-lab X-ray absorption spectrometer in concert with density functional theory calculations to uncover the quantum mechanical-level effects that underpin relevant energy-storage behaviors. The Fe K-edge spectra indicate that upon Li⁺ insertion, the change in Fe oxidation state occurs primarily at high voltage (average voltage ∼2.9 V), which is ∼0.7 V higher than the average voltage for γ-Fe₂O₃. Parallel computations using density functional theory show that tetrahedral V and octahedral Fe sites are reduced during lithiation and that the hybridization of Fe and V orbitals imposes a positive shift in voltage for Fe redox. Our combined experimental and computational investigation sheds light on how these complex materials store Li⁺ and increase cell voltage. These findings point toward future compositional alterations that may further improve their properties.