Study On Vanadium Based Materials For Energy Storage In Aqueous Zinc-ion Battery

In recent years,the characteristics of high safety,low cost,high capacity and environmental friendliness of aqueous zinc ion batteries（AZIBs）based on their neutral or slightly acidic electrolyte have once again aroused great interest of scholars.Among a wide range of water rechargeable batteries,azibs have attracted much attention because of their unique advantages,such as high theoretical capacity(820 m Ah·g^-1),low redox potential（-0.76 V,relative to standard hydrogen electrode）,high stability,rich reserves,low cost and easy availability.However,bulk vanadate based cathode materials suffer from a rapid capacity decaying as a result of low electronic conductivity,inferior ion diffusion kinetics and unstable structure during the zinc ion de/intercalation in host material.Meanwhile,the inescapable zinc dendrite formation and reactive irreversibility during long cyclic testing also lead to growing polarization and poor cyclic performance.To address the above problems,this study carried out the following works:（1）By adjusting the type of reducing agent,a new hydrothermal system was used to prepare ammonium vanadium bronze(NH₄V₄O₁₀)for comparative study.After phase characterization and electrochemical performance tests,it is found that the layer spacing and the stability of the framework of vanadium based cathode materials play a decisive role in the electrochemical performance.With the help of same Zn@carbon paper（Zn@CP）anode,its superiority of organic pillar N（CH₃）₄⁺stands out from the inferior initial capacity 76 m Ah·g^-1and capacity retention 26%after 5000 cycles at 50 A·g^-1,when the NH₄⁺acts as inorganic pillar(NH₄V₄O₁₀).As for N（CH₃）₄V₈O₂₀,the inferior initial capacity 105 m Ah·g^-1 and capacity retention 97%（2）In order to further expand the layer spacing of vanadium oxide layer,a strategy of structural design by synergistically introducing quaternary ammonium cation（tetramethyl-ammonium ion）into VO layer structure is put forward and investigated elaborately.The results show that the desired N（CH₃）₄V₈O₂₀ material synthesized by facile one-step hydrothermal method owns lattice spacing of 11.5?for faster kinetics,higher capacity and in AZIBs.Through kinetic calculation,it is inferred that there is an obvious in-phase diffusion behavior of zinc ions in N（CH₃）₄V₈O₂₀,and an obvious charge-discharge platform can also be observed,which is different from the most vanadium based cathode materials.（3）For further exploring the effect of zinc negative electrode on the electrochemical properties of the above positive materials,Zn@CP negative electrode material was prepared by constant current electrodeposition,and the promotion effect of the anode materials in AZIBs were comprehensively discussed with reference to bare Zn sheet.The results show that the carbon based structure with good conductivity and high porosity combined with the array zinc anode can not only inhibit the growth of zinc dendrite,but also improve the electrochemical performance.Consequently,the N（CH₃）₄V₈O₂₀//Zn@CP full cell exhibits a higher capacity of 360 m Ah·g^-1at 0.5 A·g^-1.While the current density raises to 100 folds from0.5 A·g^-1 to 50 A·g^-1,appreciable capacity of 105 m Ah·g^-1 and outstanding stability of ultra-long lifespan with capacity retention 97%after 5000 cycles are achieved.Combining the sound investigations of experimental practices and computational research,mechanism of rapid and stable Zn²⁺storage in N（CH₃）₄V₈O₂₀ cathode host material was systemically demonstrated.This work leads to a new strategy of the design for cathodes in AZIBs and unfolds a new and feasible path for the actualization of high-up vanadate based materials.