Architecture And Electrochemical Properties Of Novel MAX/MXene-based Composite Electrode Materials

Increasing environmental problems and energy challenges have created an urgent demand to develop green,efficient energy storage and conversion systems.The search for new high-performance energy storage and conversion materials is one of the most challenging tasks today.Recently,a new two-dimensional layered carbon/nitride MXenes prepared by etching A layer element from the MAX phase has been extensively studied in energy storage and conversion,water purificaion,electromagnetic interference shielding and other fields due to their advantages of abundant surface terminations,high conductivity and excellent flexibility.With the rise of MXenes materials,the layered ternary compounds——MAX phases with high conductivity,high stability and rich variety have begun to enter the renewable energy industry.In this dissertation,Ti-based MAX/MXene was served as the research object to fabricate electrode materials with high rate performances and long cycle lifetime.Three kinds of composite electrode materials combining with MAX/MXene substrate and Ti-containing materials which were in-situ prepared by utilizing active Ti element in Ti-based MAX/MXene are synthesized?MAX@Titanate,MXene/Metal and MXene/Birnessite?.The morphology,structure and electrochemical properties of composite electrodes were systematically investgated.Moreover,the mechanisms of their high electrochemical activity were elucidated by the theory and experiment.In chapter 3,a synthetic method for a new class of Ti₃SiC₂@H-K₂Ti₈O₁₇?H-KTO?core-shell structure composite was developed by combining the alkaline?KOH?hydrothermal reaction with highly conductive Ti₃SiC₂ and hydrogenation treament.The relationship between the parameters of the annealing process and the sodium-storage properties of the shell-core structure composites were investigated.By optimizing the preparation process,an anode material for sodium ion batteries?NIBs?with high specific capacity,excellent rate performance and long-term cycling life was achieved.Our research results confirmed that the increase in Ti³⁺on the surface layers of H-KTO by hydrogenation increases its conductivity in addition to enhancing the sodium-ion intercalation pseudocapacitive process.Furthermore,the distorted dodecahedrons between Ti and O layers not only provided abundant sites for sodium-ion accommodation but also acted as wide tunnels for sodium-ion transport.In chapter 4,a tailored microwave-assisted synthetic strategy to design Ti₃SiC₂@C-Na₂Ti₇O₁₅?C-NTO?shell-core structure composites was developed.The effects of microwave reaction parameters and active material loading on the sodium storage performance of C-NTO electrode materials in the temperature range of 25-80 ^oC were explored,and an anode material for NIBs with a high specific capacity,high rate performance and long cycle stability at 25-80 ^oC has been obtained.The superior sodium storage performances of the C-NTO composites were attributed to the core-shell architecture,which not only provided fast kinetics by high pseudocapacitance but also prolonged cycling life by preventing nanoparticle agglomeration and facilitates the transportation of sodium ions by large mesopore structure.In chapter 5,a facile method of in situ fabricating MXene/Metal?such as Ag?composites by utilising self-reduction properties of MXene was developed.It was elucidated that the self-reduction property of Ti₃C₂T_x MXene was derived from the existence of its low-valence Ti(such as Ti²⁺and Ti³⁺).The MXene/Ag composite electrode materials exhibited excellent lithium storage properties,especially with an extraordinary long cycle lifetime at high current densities.The main reasons for the long cycle life with high capacity were relevant to the reduced interface resistance and the occurrence of Ti²⁺to Ti³⁺during the cycle process.In addition,the influence of the refinement and conductivity of nanoparticles on the the activation phenomenon during the cycling was also confirmed.In chapter 6,a novel MXene/Birnessite hybrid materials(such as Na_0.55Mn_1.4Ti_0.6O₄)was synthesized by using self-reduction property of MXene.A symmetrical lithium ion battery based on MXene/Na_0.55Mn_1.4Ti_0.6O₄?MXene/NMTO?hybrid materials has been developed.The internal mechanism of MXene/NMTO electrode as a bifunctional electrode mechanism was revealed.The assembled MXene/NMTO symmetrical lithium ion battery exhibited an average voltage of 2.81 V and an energy density of³93.4 Wh kg^-1.The high capacity,superior rate performance and the excellent cycle stability of the dipolar MXene/NMTO electrodes in a wide current range are mainly related to the high conductivity,multivalent Mn and large bimodal mesoporous structure.In chapter 7,a new alk-MXene/NW-Ag_0.9Ti_0.1 nanowire composite was synthesized by directly reduction AgNO₃ aqueous solution with MXene treated by alkaline intercalation?alk-MXene?after the addition of PVP,which exhibited unexpected electrocatalytic activity for oxygen reduction reaction.The addition of PVP induced the formation of 5-fold nanotwin Ag seeds,which then grow into Ag/Ti(Ag_0.9Ti_0.1)bimetallic nanowires.The unique bimetallic nanowires favored a four-electron transfer process,and exhibited high current density and good stability by offering numerous oxygen adsorption sites and shortening the diffusion path of adsorbed oxygen.