AgNa(VO₂F₂)₂: A trioxovanadium fluoride with unconventional electrochemical properties

We present structural and electrochemical analyses of a new double-wolframite compound: AgNa(VO₂F₂)₂ or SSVOF. SSVOF is fully ordered and displays electrochemical characteristics that give insight into electrode design for energy storage beyond lithium-ion chemistries. The compound contains trioxovanadium fluoride octahedra that combine to form one-dimensional chain-like basic building units, characteristic of wolframite (NaWO₄). The 1D chains are stacked to create 2D layers; the cations Ag⁺ and Na⁺ lie between these layers. The vanadium oxide-fluoride octahedra are ordered by the use of cations (Ag ⁺, Na⁺) that differ in polarizability. In the case of sodium-ion batteries, thermodynamically, the use of a sodium anode introduces a 300 mV loss in overall cell voltage as compared to a lithium anode; however, this can be counter-balanced by introduction of fluoride into the framework to raise the reduction potentials via an inductive effect. This allows sodium-ion batteries to have comparable voltages to lithium systems. With SSVOF as a baseline compound, we have identified new materials design rules for emerging sodium-ion systems that do not apply to lithium-ion systems. These strategies can be applied broadly to provide materials of interest for fundamental structural chemistry and appreciable voltages for sodium-ion electrochemistry.