Impact of sodium vanadium oxide (NaV₃O₈, NVO) material synthesis conditions on charge storage mechanism in Zn-ion aqueous batteries

The electrochemical charge storage of sodium vanadate (NaV₃O₈or NVO) cathodes in aqueous Zn-ion batteries has been hypothesized to be influenced by the inclusion of structural water for facilitating ion transfer in the material. Materials properties considered important (morphology, crystallite and particle size, surface area) are systematically studied herein through investigation of two NVO materials, NaV₃O₈·0.34H₂O [NVO(300)] and NaV₃O₈·0.05H₂O [NVO(500)], with different water content, acicular morphologies with different size and surface area achievedviapost-synthesis heat treatment. The electrochemistry of the two materials was evaluated in aqueous Zn-ion cells with 2 M ZnSO₄electrolyte using cyclic voltammetry, galvanostatic cycling, and rate capability testing. The thinner NVO(300) nanobelts (0.13 µm) demonstrate greater specific capacities and higher effective diffusion coefficients relative to the thicker NVO(500) nanorods. Notably however, while cells containing NVO(500) deliver lower specific capacity, they demonstrate enhanced capacity retention with cycling. The structural changes accompanying oxidation and reduction are elucidatedvia ex situX-ray diffraction, transmission electron microscopy, andoperandoV K-edge X-ray absorption spectroscopy (XAS), where NVO material properties are shown to influence the ion insertion.OperandoXAS verified that electron transfer corresponds directly to change in vanadium oxidation state, affirming vanadium redox as the governing electrochemical process.