Unraveling the mechanism of optimal concentration for Fe substitution in Na₃V₂(PO₄)₂F₃/C for Sodium-Ion batteries

It is generally accepted that there is always an optimal concentration of transition metal substitution for the best possible electrochemical performance in cathode materials. This mechanism is crucial, yet normally overlooked. Herein we present a model based on Fe substitution in Na₃V₂(PO₄)₂F₃ (Na₃V_2-2xFe_2x(PO₄)₂F₃), with the optimal stoichiometry at x=0.03, and discover this optimal concentration is mainly determined by the volcanic electronic conductivity and electron activation energy. Experimental measurements discover the electron charge transfer process from V to Fe ions. Initially, Fe ions hog the electrons upon increasing substitution, but gradually the overwhelming of the charge transfer leads to the drop of electron transition ability of the major V ions. First-principle calculations confirm the volcanic trend and charge transfer process in electronic properties, resulting in the Fermi level moves close to conduction band and then moves away in n-type Na₃V₂(PO₄)₂F₃ crystal upon increasing substitution. Meanwhile, in situ electrochemical impedance spectroscopy detects the Na⁺ diffusivity increases monotonously upon increasing substitution. These findings unravel the mechanism for optimum transition metal substitution, providing a new understanding for similar phenomena in other cathode materials.