Unraveling the role of structural water in bilayer V2O5 during Zn2+-intercalation: insights from DFT calculations

Abstract

Bilayer structured V₂O₅·nH₂O has recently been studied as a promising cathode material for aqueous Zn²⁺-batteries (ZIBs) due to its ion-intercalatable layer structure and high theoretical capacity. An interesting observation in this system is the beneficial effect of structural water (nH₂O) on the electrochemical performance, but a fundamental understanding of the underlying reason is still lacking. Herein, we report a systematic density functional theory investigation into why and how structural water in the bilayer V₂O₅·nH₂O affects the structure, voltage, migration barrier and capacity during the Zn²⁺-intercalation process. The results suggest that the structural water acts as extra host sites to accept electrons from Zn, resulting in stronger ionization of Zn²⁺ than that on dry V₂O₅ and thus a higher open-circuit voltage (OCV). It is also found that structural water creates a smoother electrostatic environment between V₂O₅ sheets for easy Zn²⁺ diffusion. Benefitting from the combined “charge shielding” and “O in H₂O interaction with Zn²⁺” effect, V₂O₅·H₂O and V₂O₅·1.75H₂O exhibit lower Zn²⁺-diffusion barriers and higher OCVs than non-hydrated V₂O₅. Overall, this DFT study provides mechanistic insights into the importance of structural water in promoting the Zn²⁺-intercalation process in bilayer V₂O₅.