Investigation On Design,fabrication And Electrochemical Properties Of Vanadium-based Cathode Materials

Among aqueous ion batteries,zinc-ion batteries（ZIBs）are considered as a promising alternative for grid-scale energy storage due to their low cost,high safety,abundant resources and high theoretical capacity.In order to facilitate the advancement of aqueous ZIBs,the design of suitable cathode materials is of paramount importance.In recent years,vanadium-based cathode materials have attracted considerable attention due to their high theoretical capacity and the abundance of valence and structure.Nevertheless,their sluggish kinetics,volume expansion effects,and material dissolution have resulted in suboptimal rate performance and accelerated capacity decay.Consequently,the design of cathode materials with distinctive structures is a promising avenue for addressing these challenges,paving the way for the practical application of ZIBs.This dissertation presents an investigation of vanadium-based cathode materials,with a focus on the optimization of morphology and microstructure.The core concept of regulating the electrolyte/electrode interface to achieve rapid intercalation/deintercalation of zinc ions is also discussed.A variety of vanadium-based cathode materials with distinct morphologies and structures are developed to enhance the reversible capacity,rate performance and cycling stability of vanadium-based materials.In parallel,the mechanism of formation and electrochemical performance of vanadium-based cathode materials were subjected to a comprehensive analysis using modern characterization/testing techniques to investigate the constitutive relationship between the electrode materials and the electrochemical performance.Furthermore,the morphology and crystalline phase structure evolution of the electrode materials during charging and discharging were deeply analyzed to reveal the zinc storage mechanism of the materials when used as cathode materials for aqueous ZIBs.The principal research contents of this dissertation are as follows:（1）Hollandite-type VO_1.52（OH）_0.77 nanorod arrays（HVOOH@ACC）were constructed in-situ on a pretreated conductive carbon cloth substrate by hydrothermal synthesis and used as cathode materials for ZIBs.The Hollandite-phase 2×2 tunneling stabilization structure and the crystalline water in the H-VOOH material provide a directional channel for the reversible de-embedding of the zinc ions.Furthermore,the nanorods facilitate the migration distance of ions within the electrolyte,while the in-situ construction of the electrode material on a conductive substrate ensures the smooth electron transfer,thus enabling the fast and stable kinetic process of the electrochemical reaction and significantly enhancing the electrochemical performance of the electrode material.The structural advantages of the prepared H-VOOH@ACC electrodes result in a reversible specific capacity of 305.6 mAh g^-1 at 0.1 A g^-1 and excellent cycling stability（96.5%capacity retention after 1000 cycles）.（2）Straw-like AgVO₃（AVO-15）was synthesized via a hydrothermal method,and the mechanism of the evolution of the structure and morphology of AVO was investigated in depth through the modulation of hydrothermal reaction parameters.When used as cathode materials for ZIBs,the straw-like AVO-15 electrode exhibited a specific capacity of 292.3 mAh g^-1 at a current density of 0.1 A g^-1,with a capacity retention of 90.9%after 1200 cycles at a current density of 3 A g^-1.The reasons for the excellent performance of the straw-like AVO-15 electrodes were analyzed by kinetic mechanisms.Firstly,the straw-like AVO-15 assembled by uniform nanowires significantly increased the effective contact area,which was conducive to achieving fast ion transport.Secondly,the tight junctions of the nanowires at the core position enable them to resist extreme volume changes caused by repetitive electrochemical reactions,thereby ensuring the remarkable electrochemical stability of the straw-like AVO-15 structure.Finally,the single-crystal structure with aβ-phase exhibits excellent conductivity and reaction kinetics.The zinc storage mechanism of the straw-like AVO-15 was subjected to a comprehensive ex-situ characterization,suggesting that the stability of AVO can be improved by a rational morphology and structural design.（3）Zn V₂O₄ hollow nanospheres（ZVO-HN）assembled from nanoparticles with spinel structure were successfully synthesized by a solvothermal method,and the formation mechanism of ZVO-HN was investigated in depth through the regulation of hydrothermal reaction parameters.ZVO-HN with three-dimensional nanostructures exhibit excellent electrochemical properties as cathode materials for ZIBs.The internal cavity of the hollow structure can effectively alleviate the stress caused by volume changes,thus improving the cycling stability of the structure.Meanwhile,ZVO-HN possesses a large specific surface area,which provides sufficient space for the storage and penetration of the electrolyte,enabling more intercalation sites for zinc ions and shortening the diffusion path of ions,thereby improving the kinetics of electrochemical reactions and enhancing the surface capacitance contribution.More importantly,a more stable structure is formed as the host material in subsequent cycles after the phase transition induced in the ZVO-HN electrode during the initial charging process,with long cycling stability(84.3%capacity retention after 1500 cycles at a current density of 5 A g^-1)and excellent rate performance(retention rate from 5 A g^-1 back to 0.1 A g^-1 reaches95.2%).（4）Layered（NH₄）₂V₁₀O₂₅·8H₂O（NVO）cathode materials with non-metallic cations replacing metal ions were designed to attenuate the electrostatic effect of the host material on zinc ions.CTAB pre-embedded sea urchin-like（NH₄）₂V₁₀O₂₅·8H₂O（U-NVO）nanorods and oxygen vacancy-rich tremella-like（NH₄）₂V₁₀O₂₅·8H₂O nanosheets（T-NVO）were constructed.Firstly,the pre-embedding of CTAB enlarges the layer spacing to 1.32 nm,providing sufficient channels for the de-embedding of zinc ions.At the same time,the sea urchin-like structure increases the contact area between the electrode material and the electrolyte,shortening the diffusion path of electrolyte ions.These distinctive structural characteristics permit the U-NVO cathode to exhibit a specific capacity of 401.7 mAh g^-1 at 0.1 A g^-1 and to demonstrate excellent rate characteristics,with a retention rate of 99.6%when cycling from 0.1 to 5 A g^-1 and back to 0.1 A g^-1.The cathode also demonstrated excellent cycling stability,retaining 87.5%of its capacity after 2600 cycles).Secondly,the introduction of L-cysteine enhanced the electrical conductivity of the electrode material,inhibited the dissolution its the material,provided more active sites to facilitate the rapid transport of zinc ions,and significantly improved its rate performance.