Operation of MXene-Derived Zinc-Preintercalated Bilayered Vanadium Oxide Cathode in Aqueous Zn-Ion Batteries

Layered hydrated vanadium oxides, particularly those with bilayered structures, show remarkable electrochemical performance as cathodes for aqueous Zn-ion batteries (AZIBs). However, their wide-scale adoption is hindered by limited understanding of their charge storage mechanisms in different Zn-containing electrolytes. Here, we demonstrate the first synthesis of a MXene-derived Zn-preintercalated bilayered vanadium oxide (MD-ZVO) with a nanoflower-like morphology comprised of two-dimensional (2D) nanosheets, achieved via a two-step dissolution-recrystallization process. The strategic Zn²⁺ preintercalation establishes well-defined ion diffusion pathways, while the nanoflower-like assembly of 2D nanosheets enhances structural integrity, together contributing to improved electrochemical performance over other layered vanadium oxides. A systematic evaluation of four electrolytes (2 M ZnSO₄, 2.6 M Zn-(OTf)₂, 2 M ZnCl₂, and 30 m ZnCl₂) showed that MD-ZVO electrodes delivered high reversible capacities (450 and 315 mAh g^-1 at 0.1 A g^-1), excellent rate capability (223 mAh g^-1 for both electrolytes at 1.0 A g^-1), and good electrochemical stability (84% and 48% over 1000 cycles at 1.0 A g^-1) in saturated 2.6 M Zn-(OTf)₂ and highly concentrated 30 m ZnCl₂, respectively. The material's superior electrochemical stability in concentrated electrolytes is attributed to suppressed vanadium oxide dissolution during cycling. In situ and ex situ XRD analyses of MD-ZVO electrodes reveal larger contribution of Zn²⁺-associated species for charge storage in cells containing 2.6 M Zn-(OTf)₂ and proton dominant charge transfer in cells containing 30 m ZnCl₂. Additionally, the combination of in situ and ex situ characterization demonstrates the reversible formation of Zn _x OTf _y (OH)_2x-y ·nH₂O in cells using 2.6 M Zn-(OTf)₂ and Zn₅(OH)₈Cl₂·H₂O in cells using 30 m ZnCl₂ on the MD-ZVO electrode surface over extended cycling. This work highlights the superior performance of nanoflower MD-ZVO for cathodes in aqueous Zn-ion batteries, which benefits from the proper selection of highly concentrated electrolytes that enable better utilization of the cathode material.