Structural Design And Energy Storage Mechanism Of Vanadium-based Aqueous Zinc-ion Batteries Cathode Materials

In recent years,owing to the advantages of low cost,high safety,and environmental friendliness,aqueous rechargeable batteries have shown great potential in the field of large-scale industrial energy storage.In addition to the above advantages,zinc metal s as the anode of aqueous zinc-ion batteries（ZIBs）have the characteristics of high theoretical specific capacity(820 m Ah g^-1)and low redox potential（-0.76 V vs.standard hydrogen electrode）,which makes ZIBs become a competitive candidate energy storage device.Among the cathode materials currently developed,the vanadium-based compounds not only have a layered or tunnel-like crystal structure,which enables the rapid insertion and extraction of zinc ions,but also have multiple redox valence states,which result in a high specific capacity and energy density.However,due to the inherent characteristics of zinc itself,such as large ionic radius and high charge density,a large electrostatic force will be generated with the host material during the de-intercalation process,which will accelerate the bending vibration of the crystal structure of the host material and cause the deformation of the crystal structure.With the number of charge and discharge increases,the crystal structure will finally collapse,resulting in unfavorable electrochemical performance.In this thesis,in order to improve the structural stability,cycling stability and reversible specific capacity of vanadium-based compound cathode materials,the ion diffusion kinetics rate and zinc storage mechanism of aqueous ZIBs were studied.Combined with strategies such as structural design and composite modification of vanadium-based materials,the main results obtained are as follows:（1）In-situ construction of yolk-shell structured porous V₂O₅ wrapped by intertwined carbon fibers（V₂O₅@void@V₂O₅@CFs）is implemented through a simple combined hydrothermal and calcination route.Using 3M Zn（CF₃SO₃）₂ aqueous solution was as the electrolyte to assemble the Zn//V₂O₅@void@V₂O₅@CFs aqueous ZIBs.Benefiting from the synergistic effect of the porous yolk-shell structure with void space and the wrapping of interwoven carbon fibers,the crystal structure strain caused by the intercalation and deintercalation of zinc ions become slower,the ion diffusion and electron conduction rate is promoted,and the close contact area between V₂O₅@void@V₂O₅and the conductive network can be ensured.Therefore,Zn//V₂O₅@void@V₂O₅@CFs cells can show the high specific capacity,cycling stability,excellent power density and energy density.The in-situ EIS and theoretical calculations were performed to study the reasons for its superior performance.The results show that the introduction of CFs not only improves the electrical conductivity of V₂O₅@void@V₂O₅@CFs electrode material,but also enlarges the the adsorption and surface capacitance behaviors of Zinc ions,which improved the rate capability of V₂O₅@void@V₂O₅@CFs electrode.The ex-situ XPS/XRD test revealed the zinc storage mechanism of the electrode during the charge and discharge process.In addition,a large-size,high-load pouch battery was fabricated,which verifies the possibility of practical application of V₂O₅@void@V₂O₅@CFs cathode material.（2）In view of the low conductivity of V₂O₅ material and the structural instability caused by the narrow interlayer spacing,a one-step hydrothermal method was used to synthesize manganese ions intercalated into hydrated vanadium pentoxide（Mn VOH）cathode material.Mn²⁺as“interlayer pillars”to expand the interlayer distance of V₂O₅ crystals,the“charge shielding effect”of interlayer water molecules were combined to alleviate the structural collapse phenomenon caused by the repeated insertion and extraction of zinc ions.In order to improve the ion diffusion and electron conduction rate between electrodes,carbon nanotube-interconnected Mn VOH（Mn VOH-CNTs）were synthesized by a composite modification strategy.The assembled Zn//Mn VOH-CNTs cells display excellent electrochemical performance,which mainly attribute to the unique stable structure and strong electrostatic attraction between manganese ions and vanadium oxide layers.On the one hand,manganese ions weaken the interaction force between zinc ions and vanadium oxide layers during the de-intercalation process by generating electrostatic networks with VO₆ polyhedrons.On the other hand,the network of CNTs interwoven with Mn VOH helps to obtain fast electron transport channels,endowed the batteries with high capacity,fast ion transport rate,and high power and energy density.Therefore,this work not only proves that layered Mn VOH-CNTs cathode materials with manganese ions as“interlayer pillars”have great application potential in large-scale energy storage devices,but also provides new structural engineering methods for other oxides directions.