Study On Aqueous Zinc-ion Full Cell Based On VS₄/CNTs Cathode And MIL-125（Ti）/Zn Anode

Aqueous zinc-ion batteries have the advantages of high theoretical specific capacity,low cost,environmental friendliness,safety and non toxicity.Therefore,it has broad market prospects in large-scale electric energy storage applications.However,the industrialization of water-based zinc ion batteries still faces many problems.In terms of cathode materials,the cycle life,rate capacity and specific capacity need to be further improved,which requires cathode materials to have good zinc embedding ability,rapid diffusion ability of zinc ions and stable structure.In addition,zinc anode electrode in aqueous zinc-ion battery system also faces several problems such as zinc dendrite,anode electrode corrosion and hydrogen evolution reaction,resulting in poor battery reversibility and low cycle life.Given that the problems existing in the anode and cathode electrodes of aqueous zinc ion battery,this paper has carried out the relevant material characterization and electrochemical performance test of VS₄/CNTs cathode and MIL-125（Ti）/Zn anode,and deeply explored their charge/discharge mechanism.Finally assembled them into the full battery for electrochemical performance test.（1）VS₄/CNTs composite cathode was prepared by a simple hydrothermal reaction method.VS₄/CNTs have unique nanoflowers morphology and rich mesoporous inside,it increasing the contact area between the cathode and electrolyte,which is conducive to the absorption of electrolyte and more sufficient electrochemical reaction.In addition,CNTs also increase the electron transmission capacity of the cathode electrode.For the special electrochemical phenomena of the first cycle cyclic voltammetry curve（CV）and charge-discharge profiles,the ex situ XRD,XPS,SEM and HRTEM were characterized,and the phase transition reaction mechanism from VS₄ to ZVO in initial cycles is proposed.Subsequently,the Zn²⁺is reversible insertion/extraction in the open framework.The VS₄/CNTs cathode provides a high discharge specific capacity of 265 m Ah g^-1 at a current density of 0.25 A g^-1.Furthermore,the cathode also has excellent long cycle performance,and the capacity retention rate can reach 93%after 1200 cycles at 5 A g^-1 current density.The results show that VS₄/CNTs with flake nanoflowers structure have broad prospects as cathode materials for aqueous zinc-ion batteries.（2）To solve the problems of uneven growth of zinc dendrite and hydrogen evolution of negative electrode,a titanium-based metal organic framework material（MOF）material MIL-125（Ti）was proposed as the protective coating of zinc anode.When MIL-125（Ti）is in contact with the electrolyte,Zn²⁺in the electrolyte can react with titanium in MIL-125（Ti）,and uniform MOF-5 nanosheets were formed on the surface of MIL-125（Ti）.This increases the contact area between the anode and electrolyte,thus reducing the interfacial resistance of Zn²⁺in the deposition process.During the process of cycles,it accelerates the speed and degree of replacement.Through the characterization of the recycled electrode,we found that MIL-125（Ti）coating can effectively inhibit the uneven growth of zinc dendrite and hydrogen evolution reactions.When the symmetrical cell is tested in 3 M Zn SO₄ electrolyte,it can cycle for 2500 h at a current density of 1m A cm^-2 with a capacity limitation of 1 m Ah cm^-2,and the polarization voltage is only 120 m V.Besides,the average CE of 99%after600 cycles can be achieved at the half-cell test which shows ultra-high cycle life and stability of MIL-125（Ti）.After stable cycling over 2800 cycles,average CE up to 99.97%at a current density of 1 A g^-1.Even if the current density increased to 5 A g^-1,the cycle numbers is much longer than that of bare zinc batteries.The results show that MIL-125（Ti）coating can effectively improve the cycle life and stability of aqueous Zinc-ion battery,proving that the market of MIL-125（Ti）coating in dendrite free aqueous zinc ion battery is quite prospective.