Studies On Structure Design And Storage Mechanism Of Vanad-based Cathode Materials For Aqueous Zinc Ion Batteries

Aqueous zinc-ion batteries（AZIBs）,as a new promsing energy storage solution,have become a research hotspot and an important development direction in the field of energy storage,due to the comprehensive advantages of low cost,high safety,environmentally friendly and high power density.Because of the capability of multiple electron transfer and high theoretical specific capacity,vanadium（V）-based compounds have been considered as promising Zn²⁺storage hosts for zinc-ion batteries.However,their practical application are usually hindered by the poor cycle performance and rate capability.Thus,the design of ultra-high rate and ultra-long lifespan vanadium oxide cathode material is still be the great challenge for developing high-performance AZIBs.In this thesis,several kinds of V-based cathode materials were successfully prepared by the hydrothermal method,in-situ electrochemical etching induction,eutectic method combined with stripping and pre-intercalation,etc.Then,the structure and electrochemical performance of those vanad-based cathode materials,and the evolution of morphology and microstructure as well as the reaction kinetics process upon cycling were systematically investigated by combining the orectical calculation and various in-situ and ex-situ experimental technologies.The main contents of this thesis can be mainly concluded as follows:（1）3D-VO₂/MXene composite with 3D sandwich structure was prepared by a one-step hydrothermal method using few-layer Ti₃C₂T_x（MXene）as carrier and vanadium（IV）oxy acetylacetonate as vanadium source.As expected,such structural design can not only avoid the aggregation of low-dimensional VO₂ and MXene components,but also creat abundant pore structure,shorten the Zn²⁺diffusion path and boost the ion diffusion rate.The large pore structure and high specific surface area are also beneficial for electrolyte penetration,accelerating the the electrode activation,and achieving fast charge/discharge capability.In addition,the introduction of MXene nanosheets can significantly enhance the electrical conductivity of the composites and improve the electron transfer.The Zn²⁺diffusion kinetics were investigated by intermittent current titration（GITT）,cyclic voltammetry（CV）,alternating current impedance（EIS）and self-discharge tests upon cycling.In-situ X-ray diffraction（XRD）and ex-situ X-ray photoelectron spectroscopy（XPS）,combining with the multidimensional visual structure model were confirmed that the zinc ions embedded in the crystal structure from（003）and（510）crystal plane without the collapse of the structure,reveals the storage mechanism.As a result,both rate capability and cycle stability of the 3D-VO₂/MXene composite can be significantly improved.The discharge capacity can reach to 415 m Ah/g at the current density of 0.1 A/g after 110 cycles,with the capacity retention of 138.6%.Moreover,such 3D-VO₂/MXene composite can retain the specific capacity of 141.9 m Ah/g at 5 A/g after 5000 cycles,demonstrating it a superior high-rate performance.（2）Commercial V₄Al C₃ MAX can be directly used as cathode material for AZIBs,since it would transform into V₂O₅/C composite induced by in-situ electrochemical etching.Compared to the previous works,this strategy can completely avoid the Lengthy preparation and the use of volatile strong acid,which is a really green and efficient preparation method.In order to reveal the effect of cut-off voltage on the electrochemical etching,in-situ electrochemical Raman spectroscopy（Raman）,XRD,field emission scanning electron microscopy（FE-SEM）and XPS characterizations were used to explore the morphology and structure evolution of the electrode upon cycling.Besides,the relationship between transport kinetics and phase transition of electrode was also uncovered via the CV,EIS and GITT studies.As demonstrated,such V₄Al C₃ MAX derived V₂O₅/C composite electrode can provide a reversible specific capacity of 158m Ah/g at the current density of 10 A/g after 3000 cycles.In addition,a high specific capacity of 102 m Ah/g can be achieved even at the larger current density of 20 A/g.Such electrochemical performance is also much be than that of other MXene-based electrode materials in previous reports.（3）A 3D porous VSe₂/V₂O₅·n H₂O/RGO composite was prepared by hydrothermal method using the ammonium met-vanadate and selenium powder as vanadium and selenium sources,hydrazine hydrate as reducing agent and graphene oxide as carrier.In theory,such composite integrates the functionality of three components:1)VSe₂ with high electrical conductivity can promote the Zn²⁺migration;2)the binding water in V₂O₅·n H₂O can act as shielding layer to weaken the electrostatic interaction between interlayers,and 3)conductive RGO network can boost the electron transfer.The Zn²⁺storage mechanism and kinetic evolution of electrode were studied by XRD,FESEM,XPS and online EIS.The results indicated that such 3D porous VSe₂/V₂O₅·n H₂O/RGO composite can significantly shorten the Zn²⁺diffusion distance,alleviate the structural collapse,promote the kinetics of Zn²⁺insertion/extraction,and perform excellent cyclic stability.Benefited from the synergy mechanism,such rationally designed electrode can display excellent electrochemical performance with a specific capacity of 220 m Ah/g at the current density of 5 A/g.（4）VSe_2-x·n H₂O was designed at the molecular-atomic level from macrocosm to microcosm,and prepared by in-situ dissection and intercalation in hydrothermal process.Microscopically,VSe₂crystals with dense bulk structure were dissected into accordion structure with large interlayer spacing,and the layers had a large number of microporous,the changes in morphology can provide more active sites.At the molecular level,the pre-embedding of water molecules enables increasing the interlayer spacing from 6.1 to 13（?）,and is conducive to the rapid embedding and unembedding of zinc ions.At the atomic level,the introduction of Se defects into the units can significantly improve the Zn²⁺adsorption capacity.As a result,when at the current density of 1 A/g,the specific capacity and energy density of VSe_2-x·n H₂O electrode reached to 440 m A h/g and 452 Wh/kg,respectively.Moreover,when the current density was improved to 10 A/g,the specific capacity and power density of electrode were still as high as 227 m A h/g and 29442 W/kg,respectively.The capacity retention of 79%after 5000 cycles further confirms its excellent rate performance and high power density,which is far better than most of the reported layered metal chalcogenide electrode materials.Meanwhile,quasi-solid AZIBs were also constructed by using the VSe_2-x·n H₂O as the positive electrode,3M Zn（CF₃SO₃）₂-polyacrylamide（PAM）as the solid electrolyte and Zn metal as the negative electrode.This device can achieve nearly 100%coulombic efficiency after 1600 cycles at the cryogenic condition of-20 ^oC.Even at the lower temperature of-40 ^oC it can still deliver the specific capacities of 220,196,160,143,129,116 and 108 m Ah/g,at 1,2,5,8,10,15 and 20 A g^-1,respectively.Besides,when the current density was returned to 1A g^-1,the discharge capacity can be restored to the initial 220 m Ah/g,demonstrating a superior cryogenic performance.Beyond that,a single soft package battery of Zn/3M Zn（CF₃SO₃）₂-PAM/VSe_2-x·n H₂O was assembled,which can steadily support the operation of timer at 0°,90°and 180°folding.After frozened at-20^oC for 24 h,it was also demonstrated that a soft package battery assembled by 5 layers of positive-negative electrodes can output a open circuit voltage of 7.83 V and light up the LED lamp.The specific capacity can still reach to 172 m Ah/g after 50 cycles at the current density of 1A/g.In general,the Zn²⁺adsorption mechanism,transport kinetics and ion diffusion kinetics of Zn²⁺were systematically studied by first-principle calculation combined with in-situ XRD,in-situ Raman and in-situ scanning electrochemical testing.The mechanism uncovered in this work is expected to provide a theoretical basis for the practical application of AZIBs.