Preparation And Electrochemical Energy Storage Study Of Ti₃C₂T_x MXene Based Flexible Electrode Materials

The wearable electronic products has promoted the rapid development of flexible energy storage devices.Among them,flexible supercapacitors and flexible aqueous zinc ion batteries have a widly application prospect in the energy storage of wearable electronic products.However,for high energy density flexible energy storage devices,the preparation of flexible electrode materials with high capacity and long service life is the key to the manufacture of flexible energy storage and conversion devices.At present,the preparation of flexible electrode materials with high capacity and excellent electrochemical performance still faces some challenges,such as low load of active materials,slow ion diffusion caused by material accumulation,complex preparation methods and harsh conditions,which seriously limit the practical development of flexible electrode.Transition metal carbides and nitrides（MXene）,a family of two-dimensional（2D）inorganic compounds,have aroused extraordinary interest as electrode materials for energy storage and conversion when it is compounded with itself or its derivatives and other materials.Herein,in this paper,the Ti3C2Tx MXene was applied as the research object,a series of high loading flexible electrode materials（MXene@Fe2O3/CC,3D-PHMF,and VCM@CC）are prepared by hierarchical assembly,ice template and sacrificial template method,which applied to flexible supercapacitor and flexible aqueous zinc ion energy storage.Besides,the electrochemical performance characterization and energy storage mechanism of the above materials are studied.The corresponding research contents and results are as follows:The hierarchical MXene@Fe₂O₃/CC flexible negative materials and C-MnO2@CC flexible positive material was prepared by dipping method and carbon template sacrifice method,respectively.For the MXene@Fe2O3/CC electrode material,MXene layer can not only provide capacity as an active material,but also act as an electronic channel to improve the electron transportation and as a protective layer to prevent Fe2O3 from falling off,improving the surface capacity and cycle stability of the whole electrode.When MXene@Fe2O3/CC flexible materials was applied to the Li⁺energy storage of flexible supercapacitor,the areal capacity could reach 725 m F cm^-2 at a current density of 1.0 m A cm^-2.The results showed that the layer-wrapping of MXene was beneficial to improve the capacitance and electrochemical stability of the flexible electrode.In addition,the flower-like C-MnO2@CC flexible cathode materials were also prepared by hydrothermal methods that a mussel-derived polydopamine layer was used as an inductive catalyst.The MXene@Fe2O3/CC as the negative electrode and C-MnO2@CC as the positive electrode were designed and assembled into MXene@Fe2O3//C-MnO2 flexible asymmetric supercapacitor（ASC）,which has an areal capacity of 147 m F cm^-2 at a current density of 1.0 m A cm^-2 and are retention rate of 78.8%after 5000 cycles at a current density of 20 m A cm^-2.Three-dimensional porous MXene flexible electrodes（3D-PHMF）were prepared by the ice template method and applied to aqueous zinc ion capacitor.The Ti3C2Tx MXene nanosheet was cross-linked using proton H⁺to form H-MXene flocculation by weaken the electrostatic repulsion,which can effectively shorten the time to form a gel film and to achieve the preparation of high loading(8 mg cm^-2)porous MXene free-standing flexible electrodes.The porous structure of 3D-PHMF can effectively avoid the layer accumulation and improve electrolyte diffusion.The aqueous Zn//3D-PHMF capacitor delivers a specific capacitance of105.6 m Ah g^-1 at 0.2 A g^-1 and good rate performance of 61.0 m Ah g^-1 at 5 A g^-1with remarkable cycling stability,which remains 90%of specific capacitance after20,000 cycles.Besides,the capacitor also shows outstanding anti-self-discharge ability in which the self-discharge rate is 1.87 m V h^-1.Remarkably,the as-obtained flexible cathode can be designed into a flexible quasi-solid-state device,which shows excellent flexibility and stability.In order to further prepare the high areal capacity flexible electrode materials,the vanadium-based oxide material（V2O5·n H2O）with high specific capacity have been grown in flexible carbon cloth substrate by hydrothermal method that the MXene acted as the sacrificial agent and template agent,commercial V2O5 as the vanadium source.Beside,the CNTs are in-situ introduced when MXene coated with carbon cloth which can improve the conductivity,diffusion of electrolyte and poor rate performance of active materials,due to high active materials loading.When the VCM@CC flexible electrode material was applied to aqueous zinc ion energy storage,its capacity was 2.64 m Ah cm^-2 at a current density 0.5 m A cm^-2and remained above 0.8 m Ah cm^-2 at a current density of 10.0 m A cm^-2 for 2300cycles.In addition,the energy storage mechanism of Zn²⁺and H⁺co-intercalation layers was revealed by means of ex-situ XRD,SEM and XPS characterization.Finally,the assembled Zn@CC//VCM@CC flexible devices exhibit excellent flexibility and high capacity of 1.7 m Ah cm^-2 even at a current density of 1.0 m A cm^-2,realizing the possibility for applications in flexible devices.Finally,the above prepared C-MnO₂@CC flexible electrode material is used to zinc ion flexible energy storage.The zinc storage mechanism of layered V2O5·n H2O is compared and analyzed with the layered manganese based material.Finally,the comparison of zinc ion energy storage performance and mechanism are discussed with three-dimensional porous MXene and V₂O₅·n H₂O.