Vanadium-based polyoxometalate as new material for sodium-ion battery anodes

Affordable energy storage is crucial for a variety of technologies. One option is sodium-ion batteries (NIBs) for which, however, suitable anode materials are still a problem. We report on the application of a promising new class of materials, polyoxometalates (POMs), as an anode in NIBs. Specifically, Na₆[V₁₀O₂₈]·16H₂O is being synthesized and characterized. Galvanostatic tests reveal a reversible capacity of approximately 276 mA h g^-1 with an average discharge potential of 0.4 V vs. Na/Na⁺, as well as a high cycling stability. The underlying mechanism is rationalized to be an insertion of Na⁺ in between the [V₁₀O₂₈]^6- anions rather than an intercalation into a crystal structure; the accompanying reduction of V^+V to V^+IV is confirmed by X-ray Photoelectron Spectroscopy. Finally, a working full-cell set-up is presented with the POM as the anode, substantiating the claim that Na₆[V₁₀O₂₈]·16H₂O is a promising option for future high-performing sodium-ion batteries.