Innovative 2D dioxonium vanadium oxide: enhancing stability in aqueous zinc-ion battery cathodes

Vanadium oxide-based compounds have attracted significant interest as battery materials, especially in aqueous Zn-ion batteries, due to favorable properties and compatibility in Zn-ion systems. In a simple hydrothermal method with moderate conditions, a novel vanadium oxide compound has been synthesized using ammonium metavanadate with oxalic acid as a reducing agent. Various characterization techniques confirmed the formation of layered V₃O₈(H₃O)₂ nanoplatelets with a tetragonal crystal structure. The as-prepared cathode material was tested in coin cells against a Zn metal anode in two aqueous electrolytes of the same concentration: ZnSO₄·7H₂O and Zn(CF₃SO₃)₂. Electrochemical results showed high reversibility of Zn insertion/de-insertion and impressive cycling stability with aqueous Zn(CF₃SO₃)₂ electrolyte. Notably, the cathode material delivered a specific capacity of 150 mA h g^-1 at 100 mA g^-1 and a relatively constant coulombic efficiency near 100%, indicating impressive stability during cycling and reversibility of charge/discharge electrochemical reactions. Post-mortem characterization exposed a significant structural change in the as-prepared cathode material from nanoplatelets to nanoflakes after full discharge, which reverted to nanoplatelets after charging, reflecting the high level of reversibility of the material. DFT calculations revealed a structural change in the material after cycling, providing mechanistic insights in Zn²⁺-ion storage.