Design Of MXene-based Electrode Materials And Study On Electrochemical Energy Storage Behavior

In recent years,renewable energy storage via electrochemical energy storage devices has been extensively used in various fields,in which supercapacitors and rechargeable batteries have been the most effective.Supercapacitors have the advantages of rapid charge/discharge,high power density and long lifespan.While the rechargeable batteries provide the benefits of small size,large capacity and high energy density.In particular,magnesium-ion battery（MIB）is expected to be an alternative battery system to lithium-ion battery（LIB）due to the large reserves,high theoretical capacity and high safety of magnesium metal.Notably,as an essential component in electrochemical energy storage devices,the electrochemical behavior of electrode materials can directly affect the energy storage performance.Therefore,it is imperative for researchers to design and develop high-performance electrode materials to replace conventional energy storage devices and meet the energy demand in different scenarios.As a novel two-dimensional（2D）material,transition metal carbides/nitrides MXenes have been a class of electrode material of great interest since their discovery.Their excellent conductivity and pseudocapacitive active surface enable MXene-based electrode materials exhibit remarkable electrochemical performance in supercapacitors.Similarly,MXene-based electrode materials also show great potential for application in MIB due to the abundant surface-active sites,favorable tunable interlayer spacing and surface functional groups,especially the unique multilayer adsorption structure when storing magnesium.However,MXene materials will inevitably undergo restacking during cycling,which can lead to prolonged ion transport paths,lack of contact between surface active sites and electrolyte ions,and hence substantial and rapid capacity decay.To address these issues,the solution of this thesis is to compound other nanomaterials（such as metal oxides,metal hydroxides and metal chalcogenides）on MXene substrate or to prepare flexible free-standing electrode materials by exfoliation and delamination.Meanwhile,the electrochemical energy storage mechanism of various MXene-based electrode materials is analyzed in detail,and application-oriented energy storage devices are constructed.The main work is as follows:1.A Co₃O₄/Ti₃C₂T_x MXene composite electrode material based on multilayer Ti₃C₂T_xMXene was successfully prepared and assembled into an asymmetric supercapacitor（ASC）device.To address the restacking phenomenon of MXene materials,we introduced Co₃O₄nanoparticles with high pseudocapacitive properties on multilayer Ti₃C₂T_x MXene conducting substrate via a simple and low-cost hydrothermal self-assembly strategy,the synergistic effect between Ti₃C₂T_x MXene and Co₃O₄ nanoparticles was fully utilized to improve the low specific capacitance and restacking phenomenon of Ti₃C₂T_x MXene,as well as the poor cycling performance and conductivity of metal oxide（Co₃O₄）.The Co₃O₄/MXene composite electrode exhibited a high specific capacitance of 1081 F g^-1 at a current density of 0.5 A g^-1 and its electrochemical behavior was analyzed by quantitative kinetic.In addition,the ASC device displayed an energy density of 26.06 Wh kg^-1 at a power density of 700 W kg^-1 along with long-cycle stability over 8000 cycles.2.Although we successfully compounded Co₃O₄ nanoparticles on MXene substrate,which effectively suppressed the restacking phenomenon of MXene material.However,the limited specific capacitance of the electrode material limited the energy density of ASC devices.In order to further improve the energy density of supercapacitor devices,we selected the V₂CT_x MXene nanosheets with high reactivity as the substrate material and assembled the layered double hydroxide（LDH）materials along with Co hetero ion doping via simultaneous doping-electrostatic synergistic assembly strategy,and successfully synthesized a Co-doped Ni Mn-LDH/V₂CT_x MXene（CNMV）composite electrode material based on V₂CT_x MXene nanosheets.Due to the hetero ion doping effect,the nanoflower-like CNMV electrode with 2D/2D hierarchical architecture exhibited a high specific capacitance of 1005 F g^-1 at 1 A g^-1,and the electrochemical behavior was analyzed by quantitative kinetic.The assembled ASC device exhibited a high energy density of 30.16Wh kg^-1 at a power density of 700 W kg^-1.3.The high energy density ASC device constructed in the previous chapter was tested in the practical lighting of LED lights,and the limited energy density of the supercapacitor was not sufficient for stable long-time powering of electronic devices in practical applications.Therefore,in other to further explore high energy density energy storage devices,we developed MXene-based electrode materials for application in MIB.Based on previous studies involving mono-metal MXene（Ti₃C₂T_xand V₂CT_x）,we chose the double-metal Ti VCT_x MXene material that combines the high structural stability of Ti-based MXene and the high reactivity of V-based MXene.The flexible freestanding Ti VCT_xMXene film electrode was successfully fabricated through a facile and controllable liquid-phase etching method,intercalation of organic base and vacuum filtration.Its large interlayer spacing was suitable for storing Mg²⁺,and as a cathode material for MIB and Mg-Li hybrid battery,it exhibited the high discharge specific capacities of 111 and 135m Ah g^-1 at a current density of 0.05 A g^-1.In addition,we have performed quantitative kinetics and ex-situ characterizations to investigate the magnesium storage behavior in detail.4.To further fabricate high-performance Ti VCT_x MXene-based cathode materials for MIB,we successfully prepared a Ni Se₂-Co Se₂/Ti VCT_x（NCSe/Ti VC）composite electrode material based on Ti VCT_x MXene nanosheets via bimetallic MOF self-sacrificial template method and a co-assembly strategy of 2D LDH and 2D MXene.The introduction of MXene promoted the synergistic effect of composites and greatly improved the conductivity and reversible capacity of metal chalcogenides（Ni Se₂-Co Se₂）.The constructed NCSe/Ti VC composite with three-dimensional,porous and spherical structures was applied as a cathode of MIB,exhibiting a high specific capacity of 136 m Ah g^-1 at 0.05 A g^-1 current density.In addition,the magnesium storage mechanism was verified by quantitative kinetic analysis and ex-situ characterizations,the most favorable magnesium adsorption sites on the composite were identified and determined using density functional calculations.Finally,the assembled flexible pouch-cell device displayed good practicality.To address the phenomenon of restacking and capacity decay of MXene-based materials,the high performance MXene-based electrode materials were finally fabricated from various perspectives such as method and structure,and their electrochemical behavior and energy storage mechanisms in electrochemical energy storage devices were analyzed in detail.