Controllable Preparation And Electrochemical Energy Storage Performance On MXene-Based Composites

Environmental pollution and energy crisis are expected to be alleviated through green,clean and renewable new energy devices.As promising candidate energy storage methods,high-performance alkali metal-ion batteries and supercapacitors must be supported by new electrode materials with high energy density,long cycle life and low cost.High specific energy electrode materials,such as metal hydroxides,sulfides,phosphorus(P)and antimony(Sb),generally have problems such as low conductivity,serious volume expansion,and slow kinetics,making it difficult to obtain excellent electrochemical performance.Two dimensional(2D)layered materials have received extensive attention on account of their unique advantages.Among them,transition metal carbide/nitride(MXenes)are expected to further improve the performance of high specific materials due to its excellent electrical conductivity,abundant surface functional groups,low ion diffusion barrier,adjustable interlayer spacing and interface chemistry.However,there are still challenges in the application of MXenes:the theoretical specific capacity of MXene is low,and self-stacking between nanosheets is prone to cause limited active sites and low material utilization.In response to this problem,this thesis constructed the composite of high specific capacity material and titanium carbide(Ti3C2Tx)MXene by implementing the strategy of composite construction.Subsequently,based on the surface and interface behaviors,we explored the controllable preparation and structural regulation of Ti3C2Tx MXene composite,revealing the structure-activity relationship between the microstructure of composite and electrochemical performance,ion storage kinetics and energy storage mechanisms.The main contents are summarized as follows:(1)The poly(diallyldimethylammonium chloride)(PDDA)-NPCN/Ti3C2Tx heterostructure was elaborately designed through liquid phase flocculation and electrostatic attraction self-assembly approaches,and applied to potassium ion batteries.The PDDA-NPCN/Ti3C2Tx anode exhibits a high reversible capacity of 358.4 mAh g-1 at a current density of 0.1 A g-1 after 300 cycles,which is much higher than that of the single component ex-Ti3C2Tx(106.2 mAh g-1)and N-rich porous carbon nanosheets(NPCNs)(280.9 mAh g-1).The layered structure and large specific surface area of the PDDA-NPCN/Ti3C2Tx heterostructure could ensure the intimate contact between Ti3C2Tx and NPCNs,which can efficiently take advantage of both components and abundant active sites to improve material utilization.The PDDA-NPCN/Ti3C2Tx heterostructure with an enlarged interlayer spacing and unique three dimensional(3D)porous interconnected conductive networks can buffer volume changes,accelerate ions diffusion and electrons transfer to further improve reaction kinetics.Theoretical calculations show that the PDDA-NPCN/Ti3C2Tx heterostructure enhances the interface adsorption of K+and accelerates electrons transfer,synergistically improving electrochemical performance(2)The Sb/Na-Ti3C2Tx composite was successfully prepared through cation induction,electrostatic attraction and carbothermic reduction processes and applied to potassium ion batteries,which successfully realized the controllable preparation and uniform loading of ultrafine Sb nanoparticles.The ultrafine Sb nanoparticles can effectively shorten K+diffusion path and expose more active sites to significantly improve capacity utilization.3D porous Na-Ti3C2Tx structure significantly enhance the charge transfer kinetics and provide unblocked K+diffusion channels to accelerate ions diffusion and electrons transport,but also can effectively relieve the volume expansion and prevent the aggregation and pulverization of Sb nanoparticles during the cycling process to improve the structural stability.The Sb/Na-Ti3CTx anode with a significant synergistic effect exhibits a reversible capacity of 392.2 mAh g-1 at a current density of 0.1 A g-1 after 450 cycles,which is much higher than that of Na-Ti3C2Tx(147.9 mAh g-1)and Sb(8.6 mAh g-1).Theoretical calculations show that the Sb/Na-Ti3C2Tx composite has an obvious interaction,which improves the interfacial adsorption capacity of K+and promotes the potassiation process(3)The MoS2/in-Ti3C2Tx composite with 3D interconnected networks was successfully synthesized through ion exchange and electrostatic interaction strategies,and applied to potassium ion batteries.The MoS2/in-Ti3C2Tx anode delivers a reversible capacity of 346.2 mAh g-1 at a current density of 0.1 A g-1 after 300 cycles and a reversible capacity of 157.3 mAh g-1 even at 2.0 A g-1.The excellent potassium storage performance of the MoS2/in-Ti3C2Tx anode benefits from the unique structural design and the efficient battery-capacitive dual-model energy storage(DMES)mechanism.On one hand,the unique layered structure not only prevents the self-stacking of Ti3C2Tx nanosheets,accelerating ions diffusion and electrons transmission,but also effectively inhibits the volume expansion and aggregation of MoS2 nanosheets.On the other hand,MoS2 and Ti3C2tX are typical battery-type and capacitive-type anode materials,respectively.The synergistic effect between both components not only can effectively strengthen interfacial charges transfer efficiency and high interfacial structural durability,but also can further polarizes atomic charges within the composite,thereby improving the electrochemical performance.(4)The PDDA-BP/TI3C2Tx heterostructure was prepared at the molecular scale through liquid phase flocculation and electrostatic attraction self-assembly methods and applied to sodium ion batteries.Due to the intimate contact of both components,the heterostructure with molecular-level thickness exposes more active sites and provides effective charges transfer and ions diffusion channels,accelerating electrochemical kinetics.The results show that the PDDA-BP/Ti3C2Tx anode exhibits an ultra-high reversible capacity of 1112 mAh g-1 at a current density of 0.1 A g-1 after 500 cycles,which is much higher than that of the single component Ti3C2Tx 85.0 mAh g-1)and phosphorene(BP)(43.0 mAh g-1).Theoretical calculations show that Ti3C2Tx can not only buffer the volume expansion of phosphorene and prevent it aggregation,but also promote Na+ diffusion,meanwhile act as synergistic adsorption sites to accelerate sodiation process,thereby improving sodium storage performance.(5)The alternately stacked Ni-Co-Al-LDH/Ti3x2Tx heterostructure was successfully prepared through liquid phase flocculation and electrostatic attraction orderly self-assembly processes,and applied to surpercapacitors.The Ni-Co-Al-LDH/Ti3C2Tx electrode can obtain a specific capacity as high as 748.2 F g-1 at a current density of 1 A g-1,and maintain the specific capacity of 569.8 F g-1 even at 20 A g-1.The excellent electrochemical performance of the Ni-Co-Al-LDH/Ti3C2Tx electrode benefits from the unique structure and synergistic effects between Ti3C2Tx and Ni-Co-Al layered double hydroxide(Ni-Co-Al-LDH).On one hand,the Ni-Co-Al-LDH/Ti3C2Tx heterostructure exhibits the shorter ions diffusion path and the higher electrons transport efficiency.On the other hand,the intimate contact between Ti3C2Tx and Ni-Co-Al-LDH improves the electrode conductivity,exposes more active sites,and enhances the utilization of material.In addition,the Ni-Co-Al-LDH/Ti3C2Tx heterostructure was used as the positive electrode of the all-solid-state flexible asymmetric supercapacitor,while activated carbon(AC)and polyvinyl alcohol(PVA)/KOH were used as the negative electrode and the solid electrolyte,respectively,which shows good mechanical flexibility and excellent electrochemical performance.