Preparation And Their Zinc Ion Storage Performance Of Vanadium-based Oxide Electrode Materials

As the next generation energy storage technology,rechargeable aqueous zinc-based batteries are highly promising alternatives to lithium-ion batteries due to the characteristics of safety and eco-friendliness,high theoretical capacity and low cost.Nevertheless,there are few cathode materials can match zinc anodes because of the high charge of zinc ions and the severe solvation effect.Vanadium-based compounds are considered as suitable cathode materials for aqueous zinc ion batteries due to the multiple crystal structures and large layer spacing.However,the poor electrical conductivity and complex charge storage mechanism affect their electrochemical performance.In this thesis,several vanadium-based oxides are prepared as the cathode materials through some structural modification strategies.Their zinc storage capability,charge storage behavior,electrochemical kinetics process and energy storage mechanism are discussed in depth by a series of structural,morphological and electrochemical characterization techniques.The main contents of this work are as follows:（1）Nanosheets assembled VO₂-5@PPy hollow nanospheres are synthesized using a one-step hydrothermal route.The voids between the nanospheres provide many active sites and increase the zinc ion diffusion channels.It ensures that the electrodes can withstand large volume change.The Kirkendall effect is used to form a cavity structure in the sample,which greatly facilitates the charge transfer.The PPy coating effectively improves the conductivity of the host materials.The batteries are assembled using VO₂-5 or VO₂-5@PPy as cathodes,3M Zn（CF₃SO₃）₂as electrolyte and Zn foil as anodes.Compared to the Zn/VO₂-5 batteries,Zn/VO₂-5@PPy batteries show a high specific capacity at 0.1 A g^-1.They can be charged and discharged up to 860 times and maintain a reversible capacity of 143 m Ah g^-1 at 1 A g^-1.This strategy effectively increases the specific capacity and cycle life of vanadium oxides electrodes by accelerating their electrochemical kinetic processes.（2）A combined organic-inorganic strategy is used to prepare amorphous carbon encapsulated VO_x microspheres（VO₂ and V₂O₃）cathodes.The large tunneling structure of VO2 material facilitates the embedding and de-embedding of zinc ions.The small pore size promotes charge transport.Therefore,the Zn/VO₂@C-0.5 battery can still maintain a high output capacity(260 m Ah g^-1,5 A g^-1)after 1000 times of charging and discharging.The pseudocapacitance-controlled electrochemical reaction process enables the electrode to demonstrate reversible rate capabilities.A soft pack device was assembled by the cathode material,zinc anode and hydrogel electrolyte,which can be stabilized for 1000 cycles.It indicates that the material possesses a favorable mechanical stability.The ex-situ characterization techniques are further used to study the processes of elemental valence and phase changes of the cathode materials.The formation of new phases reveals the reaction mechanism of H⁺and Zn²⁺co-embedding in the host structure.（3）Na ions intercalated VO₂·n H₂O nanobelts（NVO）are designed via an ion pre-embedding strategy.The co-embedded H₂O and Na⁺enlarge the tunneling space of VO₂materials and reduce the electrostatic interactions of zinc ions.The assembled Zn/NVO cells demonstrate superior specific capacity and rate capabilit.The cell achieves an extended cycle life of 3000 cycles and a capacity retention of 76%(12 A g^-1).The electrochemical kinetic processes confirm the high pseudocapacitance contribution and fast Zn ion diffusion coefficient of the electrode.This is attributed to Na⁺stabilizing host structure and enhancing their charge transfer ability.The ex-situ test results further reveal the reaction mechanism of H⁺and Zn²⁺co-embedding and deembedding.（4）The two-phase coexisting CaV₂O₆/Na V₆O₁₅ nanoribbon materials are designed by a simple one-step hydrothermal method.The addition of organic precursors provides many coordination and pore sizes for the main materials.Compared with single-phase materials,the synthesized samples possess more reaction sites.The abundant phase interface can shorten the Zn²⁺migration path and facilitate the ions transfer.Active sites and zinc storage capacity of the electrode materials can be improved by regulating the amount of Ca ions.Based on the two-phase synergistic effect,the obtained 5Ca-NVO composite electrodes possess an excellent specific capacity.The small charge transfer resistance and the pseudocapacitance-controlled charge storage behavior accelerate their electrochemical kinetic processes.The results suggest that the cells can still show an ultra-long cycle life（6500 cycles）when the current density is increased to 10 A g^-1.