Hydrothermal transforming phase structure and chemical composition of V2O5 for elevating electrochemical property of zinc ion batteries

Abstract

V₂O₅ was hydrothermally modified in NaOH or KOH solutions at 180 °C for 24 h. The NaOH-modified powders had a nanorod-like structure with a crystal structure matching Na₂V₆O₁₆·nH₂O. The TG/DTA results of Na₂V₆O₁₆·nH₂O powders show a mass reduction of 4.24 % at 300 °C, corresponding to n of 1.496. KOH-modified powders have large rods and irregular structures with a crystal structure matching KV₃O₈. Its TG/DTA spectrum shows a very small percentage change, just 0.37 % at 600 °C. Cyclic voltammetry (CV) curves of a Na₂V₆O₁₆·nH₂O cathode in a 2 M ZnSO₄ electrolyte exhibit higher oxidation and reduction current densities than those of pure V₂O₅ and KV₃O₈ electrodes.The best capacity of a Na₂V₆O₁₆·nH₂O electrode is 296.10 mAh g^-1 at a current density of 50 mA g⁻¹, which is higher than those of pure V₂O₅ (102.90 mAh g^-1) and KV₃O₈ (91.07 mAh g^-1) electrodes. EDS and XPS results reveal that the charge and discharge states involve de-insertion and insertion of Zn²⁺ ions out of/into the electrodes. Computational analysis of Zn intercalation into V₂O₅, Na₂V₆O₁₆·nH₂O, and KV₃O₈ structures displays increasing electron density on neighboring V atoms, which explains the increasing V⁴⁺/V⁵⁺ ratio in the discharged state as evidenced by XPS spectra.