Mesoporous Spinel Vanadium-Based Materials As Cathode Materials For Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries（AZIBs）are considered to be the next energy storage battery systems with great potential due to their low cost,simple preparation process,aqueous electrolyte and harmlessness to the environment.However,due to the sluggish diffusion kinetics of zinc ions and the easy dissolution of cathode materials,it is still a very challenging task to find suitable cathode materials for AZIBs.Vanadium-based materials have low production costs,abundant valence,and simple synthesis methods.However,there are still few researches on vanadium-based cathode materials with excellent capacity,high-rate performance and long-cycle life.In this paper,two mesoporous spinel-type vanadium-based materials were synthesized by a simple solvothermal method,and their mesoporous morphology and spinel structure were characterized in detail through a series of ex-situ testing methods.They are further applied as cathode materials for AZIBs,and their electrochemical performance and energy storage mechanism characteristics have been studied and summarized.The main research contents and results of this work are as follow:（1）Urchin-like Mg V₂O₄microspheres（MVO）were synthesized by solvothermal method.Its unique urchin-like structure has a high specific surface area(102.5 m²g^-1),which can provide abundant active sites,shorten ion diffusion channels,and strengthen the structural stability.The rate performance test shows that the Zn//MVO battery has good stability under various current densities.The battery can provide a high capacity of 272 m Ah g^-1at 0.2 A g^-1;it has excellent long-cycle stability even after 500 cycles at4.0 A g^-1,with a reversible capacity of 128.9 m Ah g^-1.In addition,the energy density calculated based on the MVO cathode is 171.5 Wh kg^-1at a power density of 140.6 W kg^-1.Through ex-situ XRD,XPS and TEM,it was observed that the MVO gradually changed from order to disorder and produced phase transition during the electrochemical cycle.These results demonstrated an unusual mechanism characteristic when compared with many other vanadium-based cathodes.（2）According to the mesoporous structure of MVO,we adjusted the solvothermal reaction time and calcination method to synthesize three different morphologies of zinc vanadate,namely irregular nanoparticles Zn V₂O₄（ZVO-1）,Zn V₂O₄Microspheres（ZVO-2）and Zn O-coated Zn V2O4 hollow microsphere structure（ZVO-3）,further study the effect of mesoporous morphology on electrochemical performance.The results show that the ZVO-3 cathode with a larger specific surface area exhibits better electrochemical performance,with an initial Coulombic efficiency of 97%,a better rate capacity(93.3 m Ah g^-1at 8.0 A g^-1),and more excellent cycle performance(338 m Ah g^-1at 0.1 A g^-1,70%capacity retention after 50 cycles)and outstanding long-cycle performance(after 2000 cycles at 4.0 A g^-1current density,the specific capacity is 75.6m Ah g^-1).In addition,the electrochemical kinetic analysis further showed that the ZVO-3 cathode experienced a pseudo-capacitance-led electrode reaction,which greatly increased the diffusion rate.Mechanism studies further show that spinel-type vanadium-based materials have a universal phase transition process from order to disorder.