Preparation And Electrochemical Performance Study Of Vanadium-based Cathode Materials For Aqueous Zinc-ion Batteries

The shortage of lithium resources and the flammability and explosion of organic electrolytes have made increased cost and safety problems of lithium-ion batteries.The aqueous zinc ion batteries（ZIBs）have received extensive attention from researchers due to the advantages of high safety,low cost,environmental friendliness,and ease of manufacturing.Thanks to the high theoretical capacity and good compatibility of zinc anodes,the main research goal of ZIBs is to develop high-performance cathode materials.Benefit from the high theoretical capacity and layered crystal structure,vanadium-based materials are considered as potential candidates for ZIBs cathodes.However,due to the intrinsic property of metal oxides,they usually suffer from electronic conductivity,and their structural stability is poor during fast and deep discharge conditions,which limits their practical applications.The current strategies for the above-mentioned problems are mainly pre-intercalation and surface/interface modification.Unfortunately,these strategies cannot take into account capacity,rate and cycle performance.This dissertation starts with the composition,structure and morphology of the material,and carries out multi-angle adjustment and optimization simultaneously,so as to comprehensively improve the comprehensive performance.In addition,the phase transition mechanism and reaction mechanism involved in the conversion and electrochemical reaction of the material are also explored,which provides theoretical and technical support for the application of vanadium-based cathode materials,and provides research for the development of new materials method.Type B vanadium dioxide suffers from low specific capacity and poor rate performance.Herein,an oxygen-deficient vanadium dioxide/graphene composite（O_d-VO₂-rG）was prepared using a two-step method of solvothermal and chemical reduction.Theoretical calculations and electrochemical tests confirmed that Zn²⁺can undergo a highly reversible adsorption and desorption process at oxygen defect sites,thereby contributing additional capacity.Moreover,the rich VO₂/graphene interface and the three-dimensional porous conductive framework also facilitated ion transport and electronic conduction of electrodes.Therefore,without sacrificing cycle stability,the specific capacity and rate performance of O_d-VO₂-rG have been improved,and the overall performance optimization has been achieved.O_d-VO₂-rG has a specific capacity of 372 m Ah g^-1 at 0.1 A g^-1,which exceeds the theoretical specific capacity of 325 m Ah g^-1.At 20 A g^-1,the specific capacity still maintains 190 m Ah g^-1 and the capacity retention is close to 90%after 5000 cycles at 10 A g^-1.Low-valence vanadium oxides usually exhibit poor performance for Zn²⁺storage.Herein,the vanadium-based metal organic framework（V-MIL-47）was used as the precursor to prepare carbon-coated vanadium trioxide hollow microcubes（V₂O₃@C）by the high-temperature pyrolysis.The average size of V₂O₃ nanoparticles is below20 nm,and they are evenly embedded into the conductive carbon arrays for fast electronic conduction.Besides,the hierarchical porous hollow structure ensures facilitated ion diffusion.After that,the V₂O₃ is transformed into double-layer hydrated vanadium pentoxide（V₂O₅·n H₂O,VOH）using the in-situ anodic oxidation strategy by simultaneously adjusting the morphology of the material and the concentration of the electrolyte.Such design has achieved the application of V₂O₃ as a high-performance cathode for ZIBs.The VOH@C shows a high capacity of 452m Ah g^-1 at 0.1 A g^-1,excellent rate performance(250 m Ah g^-1 at 20.0 A g^-1)and long-time life（nearly 100%capacity retention after 10000 cycles）.Furthermore,we used a variety of ex-situ characterization methods to systematically study the reversible deintercalation of Zn²⁺between the VOH layers during the charge and discharge process.Due to the low conversion efficiency in the above-mentioned in-situ anodic oxidation process,herein,a three-dimensional porous vanadium dioxide nanoribbon/graphene composite（VO₂-rG）was prepared using a two-step method.This composite has the following advantages:1)Ultra-thin nanoribbons increases active sites;2)The rich VO₂/graphene interface significantly improves the electronic conduction of the interface;3)Three-dimensional porous graphene conductive framework facilitates ion/electron transfer.Therefore,the VO₂-rG can achieve an ultra-efficient in-situ self-transformation process.During the first charging,VO₂-rG is completely converted into VOH-rG without any water decomposition.Such composite achieves high specific capacity(470 m Ah g^-1 at 0.1 A g^-1),satisfactory rate(280 m Ah g^-1 at 20 A g^-1)and long cycle stability(100%capacity retention after 5000cycles at 10 A g^-1).In addition,the assembled quasi-solid flexible battery also shows great electrochemical performance.This ultra-efficient in-situ self-transformation strategy provides a new method for exploring advanced energy storage materials.