Enhancing Li-Ion Charge Storage in Disordered Vanadium Ferrite Aerogels via Multivalent Substitution

Abstract

We have developed a family of vanadium ferrite (VFe2Ox) aerogels that exhibit promising performance for Li-ion storage [1]. These materials are synthesized by a simple epoxide-driven sol–gel route that yields aerogels with high surface area (200–300 m2 g–1) and no discernible crystallinity by powder X-ray diffraction. Subsequent calcination at 300°C is required to decompose residual organics from the synthesis, but even after such thermal treatment, the VFe2Ox aerogels retain their disordered nature. Because of their high surface area and lack of defined crystal structure, VFe2O x aerogels exhibit the electrochemical characteristics of pseudocapacitive materials―broad voltammetric envelopes and sloping discharge curves; fast charge–discharge response; and low charge–discharge voltage hysteresis―but with capacity levels commensurate with many battery-type materials. We find that substituting ~10 at% of the V sites with nominally electroinactive Al3+ enhances specific capacity by >25% from 100 mAh g–1 for VFe2Ox to 128 mAh g–1 for Al-VFe2Ox , while also improving capacity retention at high current density. X-ray photoelectron spectroscopy (XPS) shows that incorporation of Al3+ induces the formation of V4+ sites that likely increase local electronic conductivity in Al-VFe2Ox. Ex-situ XPS analysis of materials electrochemically conditioned to various charge–discharge states reveals distinct differences in the utilization of V and Fe redox sites for VFe2Ox versus Al-VFe2Ox. First principles modeling shows the effect of Al-doping on V and Fe oxidation as a function of dopant concentration and site location within the amorphous material. Lessons learned from this paper will inform future studies on rational substituents and targeted defective compositions for high-rate lithium-ion charge storage. Chervin, Christopher N., et al. "Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage." Journal of Materials Chemistry A22 (2015): 12059-12068.Chervin, Christopher N., et al. "Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution.” RSC Adv., under review, March 2021.