Modification And Electrochemical Performance Of MnO₂-Based Electrode Materials For Aqueous Zinc Ion Batteries

Based on the energy structure of the world today,energy storage technologies that utilize renewable energy sources such as solar and tidal energy are emerging.Currently,this type of technology has met nearly 26%of the world’s electricity consumption.However,this type of renewable energy is severely affected by geographical and environmental factors,and sustainable solutions are urgently needed to deal with this dilemma.Batteries are at the forefront of this technological revolution as the most widely used portable energy storage device on the market today.Since the 1990s,batteries represented by lithium-ion batteries（LIBs）have dominated the energy storage market due to their high energy conversion efficiency,simple mechanical structure and long cycle life.However,these batteries also have some disadvantages,such as LIBs require the use of expensive lithium metal and more demanding assembly environment for production,the high cost of seriously restrict its further development.For several years,researchers have tried to use monovalent or multivalent metal ions,such as Na⁺,K⁺,Mg²⁺and Zn²⁺,which are abundant in nature,to replace lithium metal in the hope of achieving low cost and also excellent energy storage properties.Among many ion batteries,aqueous zinc ion batteries（AZIBs）,which are based on Zn²⁺with a similar radius to Li⁺for energy storage,have been favored by many researchers.AZIBs are composed of three main components:cathode,electrolyte and Zn anode.In contrast to the high ionic conductivity of the aqueous electrolyte and the high specific capacity provided by the Zn anode,the development of cathode materials has greatly hindered the industrialization process.For example,the widely used vanadium-based and organic cathode materials are unable to meet the demand for high working potential and high capacity.In response to this situation,this thesis selects manganese-based materials,which are low-cost,abundant and meet the requirements of high working potential and high capacity,as the target of the research,and carries out the next series of research works.As a typical manganese-based cathode material,MnO₂ has many advantages such as crystalline variety and safety,but there are many problems such as severe component dissolution and structure collapse when it is used in AZIBs as cathode.Based on this,this thesis focuses on the poor capacity and cycling stability of MnO₂cathode materials in AZIBs,and proposes to improve the charge storage capacity and cycling stability of MnO₂-based cathode materials from both electrode material structure optimization and electrode interface kinetics optimization,respectively.The following studies have been carried out:（1）Study on enhancement of electrochemical properties of MnO₂-based electrode materials by transition group metal ion pre-intercalation strategyTaking the typical transition group metal ion Cu²⁺as an example,Cu²⁺pre-intercalated MnO₂-based electrode materials were successfully prepared by variable potential electro-deposition,which exhibited excellent capacity performance up to 425.9 m Ah g^-1(at a current density of 0.1 A g^-1).In-depth electrochemical mechanisms（two-electron transfer capacity）suggest that the pre-intercalated Cu²⁺impedes the proton stripping process inside the MnO₂-based electrode material,resulting in the continuous dissolution of MnO₂ during the initial cycling stage,allowing it to exhibit a completely different third discharge platform from the other two comparison materials and provide a high capacity at nearly 1/3 of the total specific capacity(about 150 m Ah g^-1).However,as the cycling process proceeds and MnO₂ continues to dissolve without supplementation,the performance will continue to degrade,eventually relying on the surface control process to provide most of the capacity.（2）Ferroelectric material-assisted electrode interface kinetic optimization strategy enhancing the electrochemical performance of MnO₂-based electrode materialsTo address the above-mentioned problems（poor cycling stability）of ion pre-intercalated MnO₂-based electrode materials,we optimize the electrode interface dynamics by introducing a ferroelectric material（β-PVDF）to achieve surface electric field equilibrium on MnO₂-based electrode materials.Theβ-PVDF coated MnO₂-based electrode material（ZMO@β-PVDF）was successfully prepared by electro-deposition method combined with infiltration and drying.The simulation results show that the presence of ferroelectric coating layerβ-PVDF significantly changes the electric field distribution on the cathode side,which makes the cathode electrochemical kinetics accelerate while the reactivity of ZMO increases significantly.Further investigation of the energy storage mechanism and kinetic analysis indicates that the accelerated cathode kinetics of AZIBs is entirely due to the unique ferroelectricity ofβ-PVDF,making the ZMO@β-PVDF//Zn cell still exhibits optimal capacity retention of 68.6%after 120 cycles at a current density of 0.3 A g^-1.In contrast,the non-ferroelectric coating layer CMC cannot exhibit a similar effect.