Understanding the redox sequence of VFe₂Ox and the role of dopants (Al, Zn, Zr) in shifting capacity and cycling profiles

Vanadium ferrite (VFe₂Ox) is a deliberately defective spinel system that can incorporate substantial Li⁺ and exhibits a high charge-discharge rate, particularly when structured as a nanoscale aerogel. Zr, Zn and Al, can readily be made to enter the structure substitutionally and have strong, but differing effects on the capacity of the material. The earth abundant, cost-effective constituent elements give this class of materials strong potential as future Li ion battery cathodes, but optimizing the stoichiometry for maximum capacity and stability will require understanding the redox sequence and role of intentional dopants.



We use density functional theory calculations in concert with in-situ and operando X-ray absorption near-edge spectroscopy (XANES) spectra obtained from measurements at the Naval Research Laboratory to uncover the quantum mechanical-level effects that underpin relevant energy storage behaviors of doped and undoped VFe₂Ox. Our experimental V K-edge and Fe K-edge spectra indicate reduction of both species during discharge, but cannot distinguish between tetrahedral and octahedral Fe redox sites or fully resolve the valency of each element as a function of state of charge. Using a hybrid form of density functional theory that accounts for the strong correlation present in 3d elements, we show that both V and Fe are indeed reduced, but that only tetrahedral Fe is redox active until fully converted to Fe²⁺. Furthermore, both octahedral and tetrahedral Fe³⁺ are high spin configuration, but the 5m_B moments in the fully filled majority spin channel at each symmetry site are anti-aligned. This allows for easy exchange of electrons and facilitates conduction. We also calculate XANES spectra based on first principles calculations³ to be compared directly to those measured in the lab. This allows us to understand the effect of Al, Zr, and Zn dopants on the redox sequence and relate these results to site preference and capacity. Our combined experimental and calculational investigation sheds light on how these complex materials store Li ions and points toward future alterations that may further improve their properties.