Reversible Molecular and Ionic Storage Mechanisms in High-Performance Zn_0.1V₂O₅·<i>n</i>H₂O Xerogel Cathode for Aqueous Zn-Ion Batteries.

Abstract

The cathode is a critical component for aqueous Zn-ion batteries (ZIBs) to achieve high capacity and long stability. In this work, we demonstrate a dissolution-free, low-Zn-preinserted bilayer-structured V₂O₅ xerogel cathode, Zn_0.1V₂O₅·<i>n</i>H₂O (ZnVO), with excellent capacity and stability using a low-cost ZnSO₄ electrolyte. Its discharge capacity reaches 463 mAh g^-1 at 0.2 A g^-1 and 240 mAh g^-1 at 10 A g^-1, while 93% and 88% of its capacity are retained at 0.2 A g^-1 for 200 cycles and at 10 A g^-1 for 20 000 cycles, respectively. We then show that the outstanding performance of ZnVO is derived from the enlarged gallery spacing by the solvent water intercalation and the water stable V₂O₅ bilayer structure. We further unveil <i>via</i> <i>ab initio</i> molecular dynamics that H⁺ is largely originated from the dissociation of the gallery water, while OH^- moves out of the gallery to form Zn₄(SO₄)(OH)₆·5H₂O with ZnSO₄ electrolyte on the surface of ZnVO; the intercalated Zn²⁺ forms aquo complex [Zn(H₂O)₆]²⁺ with the gallery water. Our theoretical analysis also suggests that the gallery water and solvent water in the electrolyte are statistically the same and functionally equivalent. Overall, this study shows the promise of ZnVO as a practical cathode for ZIBs and offers fundamental insights into the roles of gallery water, solvent water, bilayer V₂O₅ structure, and dual Zn²⁺/H⁺ intercalation mechanisms in achieving high capacity and long stability.