| Transition metal carbides and nitrides(MXene),with their high electronic conductivity,abundant surface terminations,excellent hydrophilicity,and high packing density,have become promising candidates in the applications of energy storage.However,the layer restacking of MXene caused by the strong interlayer van de Waals interaction hindered their further practical applications.Aiming to obtain high performance MXene-based electrodes,in this paper,rational structure design and morphology control were conducted for 2D MXene through various strategies,such as carbon-coated method,electrostatic inducement,ice-template approach,and self-assembly,resulting in high performance 3D porous MXene electrodes and MXene/transition metal oxide heterostructures for Li/Na/K ion batteries,flexible,free-standing 3D MXene-based films for supercapacitors,and flexible silicon-based electrode for lithium ion batteries using MXene as a multi-functional conductive binder.(1)A unique 3D carbon-coated MXene architecture was obtained by coating a polydopamine layer on the surface of MXene nanosheets followed with a carbonization process under high temperature.The surface-coated carbon layer can not only prevent the MXene from being oxidized under high temperature,but also facilitate the transformation of 2D MXene to 3D architectures by easing the electrostatic repulsion on the MXene surface,which can restrain the layer restacking of MXene nanosheets,facilitate the exposure of surface active sites and the permeation of electrolyte,and shorten the ion diffusion pathways,rendering the 3D carbon-coated MXene architecture with high specific capacity and superior rate capability when used as anode materials for lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs).Consequently,the 3D carbon-coated MXene architecture delivers a specific capacity of 499.4 mAh g-1 after 200 cycles at 0.2 C for LIBs,and a capacity of 257.6 mAh g-1 was shown after 200 charge/discharge processes at 50 mA g-1 for SIBs.Moreover,high capacities of 101.5 mAh g-1(100 C)and 77.8 mAh g-1(10 mA g-1)are still maintained for LIBs and SIBs,respectively,indicating the effectiveness of surface-coated carbon layer and construction of 3D architecture in improving the lithium/sodium storage properties of MXene in terms of specific capacity,rate capability,and cycling stability.(2)2D MXene nanosheets were transformed into a unique porous MXene architecture with 3D conductive network and high specific area through the electrostatic interaction,where the positive-charged melamine was absorbed on the surface of negative-charged MXene nanosheets to ease the electrostatic repulsion between MXene nanosheets.The construction of 3D architecture can improve the electrolytic accessibility of MXene surface sites,shorten the ion diffusion pathways,and thus enhance the potassium storage capacity and rate capability of MXene.Moreover,potassium bis(fluorosulfonyl)imide(KFSI)salt was selected as the electrolyte to replace the conventional potassium hexafluorophosphate(KPF6)salt,which can form a robust inorganic solid electrolyte interface(SEI)on the electrode surface during cycling,resulting in an enhanced Coulombic efficiency.Consequently,the as-prepared 3D porous MXene exhibits a capacity of 211.7 mAh g-1 at 20 mA g-1,and can still retain 66.7 mAh g-1 as the current density increased to 2000 mA g-1.Moreover,a capacity of 80.5 mAh g-1 with a capacity retention of 100.0%could be maintained after 2000 cycles at 1000 mA g-1,demonstrating that the potassium storage properties of MXene anode could be effectively improved by constructing the 3D architectures and optimizing the electrolyte system.(3)A freeze-drying route is proposed to replace the conventional vacuum-drying route during the fabrication procedure of MXene film,which can transform the interlayer water molecules into ice template to expanded the dense MXene layers,resulting in a flexible,free-standing MXene/CNTs film with 3D conductive network and large interlayer spacing after the sublimation of ice template.Compared with the vacuum-dried dense MXene film electrode,the freeze-dried 3D MXene/CNTs film electrode has significantly enhanced electrolytic permeation,which is conducive to improve the electrode/electrolyte interfacial interaction,thus achieving the enhancement of the electrochemical performance.When used as the electrode for supercapacitors,the 3D MXene/CNTs film electrode delivers a high specific capacitance of 375 F g-1 at 5 mV s-1,and can still maintain 251.2 F g-1 as the current density increased to 1,000 mV s-1.In addition,the capacitance retention of the 3D MXene/CNTs film electrode after 10000 cycles at 10 A g-1 is up to 95.9%,indicating its good performance in terms of capacitance,rate capability,and cycle durability.(4)A novel strategy to construct various MXene/transition metal oxide heterostructures is proposed for lithium ion batteries by assembling various transition metal oxides(TiO2 nanorods,SnO2 nanowires,and Fe3O4 nanodots)on the MXene nanosheets through van de Waals interaction using MXene as the conductive substrate.The MXene can enhance the conductivity of heterostructures,alleviate the volume variation of transition metal oxides during cycling as well as restrict the aggregation of transition metal oxide nanoparticles caused by their small size.In turn,the transition metal oxide nanoparticles can also prevent the MXene nanosheets from being restacked.The synergistic interaction of these two components endows the constructed heterostructures with significantly enhanced specific capacities,superior rate capabilities,and cycling stabilities when used as the anode materials for lithium ion batteries.This research provides a reference for the construction and applications of MXene-based heterostructures.(5)A novel electrode fabrication procedure to prepare flexible,free standing Si@C electrode is proposed using MXene as a conductive binder.The MXene-Si@C possess excellent flexibility due to the unique 2D structure and superior mechanical property.Besides,the constructed 3D MXene conductive network can not only improve the conductivity of Si@C electrode,but also alleviates the huge volume expansion of Si@C during cycling,resulting in better rate performance and cycling stability.Benefited by this unique structure,the MXene-Si@C exhibits much better lithium storage properties than the electrode fabricated by the conventional slurry-coating process,which delivers a capacity of 1040.7 mAh g-1 after 150 cycles at 420 mA g-1 and still maintain 553 mAh g-1 at a high current density of 8.4 A g-1.This strategy gives a new insight in fabricating flexible silicon-based electrodes. |
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