Defect Structure Design And Performance Optimization Of Vanadium Dioxide As Cathode Materials For Aqueous Zinc-ions Batteries

At present,aqueous batteries,especially aqueous zinc ion batteries（AZIBs）,has attracted extensive attention due to its high safety and low cost.Vanadium-based compounds,especially vanadium-based oxides,have been widely studied as the cathode of AZIBs.In this paper,polypyrrole in-situ coated oxygen-deficient vanadium dioxide hydrate nanosheets and fluorine doped vanadium dioxide hydrate were respectively synthesized as cathode materials of AZIBs.The main contents are as follows:（1）the oxygen deficient hydrate vanadium dioxide with PPy coating（Od-HVO@PPy）was synthesized by a one-step hydrothermal method.In more detail,the in-situ polymerization makes PPy coat on the surface of hydrate vanadium dioxide then form oxygen vacancies in hydrate vanadium dioxide.Oxygen vacancies facilitate more reversible Zn2+adsorption on the surface of Od-HVO@PPy,and conductive polymer coating enhance the electronic transfer and suppress the cathode dissolution.Hence,the Od-HVO@PPy electrode presents a desirable reversible capacity of 337.1 mAh g^-1 at 0.2 A g^-1 and delivers a superior cycling stability(159.1 mAh g^-1 retained after 1000 cycles at 10 A g^-1)within the good structural stability.（2）The structure of vanadium dioxide hydrate was modified by one step hydrothermal reaction,and the fluorine-doped hierarchical structure was successfully constructed.Because the radii of F atom and O atom are similar（0.133 nm vs 0.132 nm）,part of O in vanadium dioxide can be effectively doped by F.In this work,the specific capacity of F-HVO is 320 mA h g^-1 at 0.4 A g^-1,and retains 233 mAh g^-1 reversible capacity after more than 700 cycles at the current density 5 A g^-1.This excellent electrochemical performance can be attributed to the following two reasons:（1）valence state engineering expanded HVO lattice space,provides highly reversible electrochemical active sites.（2）the hierarchical structure of nanorods with porous structure could enhance electron transfer kinetics and ion diffusion,so as to improve the storage ability of the zinc ions.