Preparation Of Ti₃C₂T_x MXene Composites And Their Applications In Lithium/Sodium Ion Hybrid Capacitors

Lithium/sodium-ion hybrid capacitor（LIC/SIC）has attracted widespread attention due to its advantages of high energy density and high power density.LIC/SIC is composed of a battery-type anode and a capacitive-type cathode.The battery-type anode follows the sluggish redox reaction mechanism,the capacitor-type cathode obeys the rapid adsorption/desorption capacitive mechanism,which inevitably gives rise to an unbalanced kinetic between two electrodes and fails to ensure the simultaneous output of high energy density and high power density.Therefore,it is extremely necessary to develop battery-type anode materials with fast reaction kinetics.Recent studies have found that Ti₃C₂T_x MXene material with high conductivity,large layer spacing and low ionic barrier can greatly alleviate the kinetics imbalance in LIC/SIC and realize the simultaneous output of high energy density and high power density in LIC/SIC devices when used as a battery-type anode.Nevertheless,Ti₃C₂T_x still suffers from the problems ofπ-πstacking and low theoretical specific capacity,which limit its application in LIC/SIC.This thesis takes Ti₃C₂T_xas candidate material for battery-anode,combined with other materials with high specific capacity and fast reaction kinetics to improve the problems of inherentπ-πstacking and low specific capacity of Ti₃C₂T_x through structural regulation and morphology innovation.As a result,a series of Ti₃C₂T_x based materials with high specific capacity and fast reaction kinetics have been obtained,and the electrochemical properties and mechanisms of these Ti₃C₂T_x based materials in lithium/sodium storage,LIC and SIC were systematically studied.The research contents of this thesis are as follows:The d-Ti₃C₂T_x/VGSs composite was prepared by mixing the delaminated Ti₃C₂T_x（d-Ti₃C₂T_x）with vertical graphene sheets（VGSs）.Among,d-Ti₃C₂T_x with large interlayer spacing can effectively inhibitπ-πself-stacking and accelerate ion/electron transfer kinetics,VGSs have good electrical conductivity and shorten Li⁺migration paths.Benefiting from the synergistic effect of d-Ti₃C₂T_x and VGSs,d-Ti₃C₂T_x/VGSs-2 composite shows excellent lithium storage performance,with capacity of 155 m Ah g^-1after 3000 cycles at 5.0 A g^-1.The assembled d-Ti₃C₂T_x/VGSs-2//AC LIC shows high energy density and long cycle lifespan,with maximum energy density of 95.1 Wh kg^-1and the cycle lifespan can reach 8300cycles.The VS₄/d-Ti₃C₂T_x material was prepared by the composite of d-Ti₃C₂T_x and VS₄nanoarrays.The VS₄ nanoarrays can effectively facilitate permeation of electrolyte,shorten the diffusion paths of ions and accommodate the volume expansion.The d-Ti₃C₂T_x with larger interlayer spacing can effectively facilitate electron/ion transfer,ensuring faster reaction kinetics.Benefiting from the synergistic effect of d-Ti₃C₂T_xand VS₄ nanoarrays,VS₄/d-Ti₃C₂T_x composite shows excellent lithium storage performance,with capacities of 1328 m Ah g^-1after 170 cycles at 0.1 A g^-1 and 721m Ah g^-1 after 500 cycles at 1.0 A g^-1.The assembled VS₄/d-Ti₃C₂T_x//AC LIC and SIC exhibit high energy densities and ultralong cycle lifespans.The maximum energy density of LIC can reach to 122.5 Wh kg^-1 and the cycle lifespan reaches up to 20000cycles.The maximum energy density of SIC can reach to 117.0 Wh kg^-1and the cycle lifespan is 5000 cycles.The SnCu₂Se₄/d-Ti₃C₂T_x/NPC material with hierarchical structure was prepared through the composite of SnCu₂Se₄ nanoparticles,d-Ti₃C₂T_x and N-doped porous carbon.Among,the d-Ti₃C₂T_x can restrainπ-πself-stacking and facilitate the insertion/extraction of ions,SnCu₂Se₄ nanoparticles accelerate the charge transfer rate via shortened paths,NPC can promote the penetration of electrolyte,shorten the transport path of ions and further enhance the electrical conductivity.Benefiting from the synergistic effect of d-Ti₃C₂T_x,SnCu₂Se₄ and NPC,SnCu₂Se₄/d-Ti₃C₂T_x/NPC composite shows excellent lithium storage performance,with capacity of 114 m Ah g^-1after 6000 cycles at 10 A g^-1.The SnCu₂Se₄/d-Ti₃C₂T_x/NPC composite also shows excellent sodium storage performances,with capacities of 468 m Ah g^-1 after 200cycles at 0.1 A g^-1and 296 m Ah g^-1 after 900 cycles at 1.0 A g^-1.The assembled SnCu₂Se₄/d-Ti₃C₂T_x/NPC//AC LIC and SIC exhibit high energy densities and ultralong cycle lifespans,with maximum energy densities of 147.9 Wh kg^-1 and 158.6Wh kg^-1,respectively.The cycle lifespans of LIC and SIC can reach 10000 cycles with high capacity retentions.