Preparation Of MXene-based Composites And Research On The Performance Of Sodium-ion Batteries

Nowadays,the ever-growing material demand is in conflict with the environmental problems and energy shortage.Renewable clean energy such as wind power and solar energy demand efficient energy storage devices.Lithium-ion batteries（LIBs）are large-scale commercially used rechargeable battery devices at present and will be widely applied in various electronic mobile devices and electric vehicles in the next few decades.However,researches focus on the development of sodium ion batteries（SIBs）due to the scarcity of lithium resources.In addition,traditional anode and cathode materials are not suitable for SIBs because of the sharply different physical properties and electrochemistry of sodium ions.Recently,two-dimensional（2D）layered MXene material has shown unique physical and chemical properties.Its excellent electrical conductivity,large layer spacing can improve the mass transfer kinetics and of sodium ionscycling stability.Nevertheless,graphene-like MXene with high specific surface makes it easy to restack and aggregate during charge/discharge and prepararion,resulting in the loss of active sites and rapid decrease of cycling capacity.The design of MXene based composites can exert the synergistic effect to achieve both high capacity and cyclic stability.In this paper,by controlling the synthesis method and surface modification to improve the structural stability,the as-prepared anode materials achieve excellent rate performance and cycling life with reversible capacity.The hierarchical nanostructured MXene@ReS₂@C composites were successfully synthesized by one-pot hydrothermal method.The 2D/2D composite has 2D sheets stacked structure with large lateral size and few layers of MXene,which could provide abundant active sites for ReS₂ nanosheets.The as-prepared materials present excellent electrochemical performance,obtaining 138 mAh g^-1 at 5.0 A g^-1 and maintaining 202mAh g^-1 at 2.0 A g^-1 for 200 cycles.The experimental results show that MXene substrate can disperse Res₂ nanosheets,both achieving a porous structure and increasing the internal charge transfer.ReS₂ nanoshhets also prevents re-stacking of few-layered MXene,while carbon coating further enhances charge transfer on the surface of ReS₂and inhibits its volume expansion.The interface interaction of MXene-ReS₂-Carbon can simultaneously improve rate performance,cycling performance and reversible capacity.In the second part,ZIF-67 was grown on the surface of MXene by wet chemical method as precursor,and the MXene@CoP@NC composite was prepared by annealing and in situ phosphorization.It could produce CoP nanoparticles and in-situ nanocarbon coating under long time and low temperature annealing.MXene as a substrate precents CoP particles from agglomeration and the carbon nanolayer also effectively limits the volume expansion.Furthermore,porous carbon provides direct channels for the transport of Na⁺ions and shorten the transport distance.It achieved a reversible specific capacity of 153 mAh g^-1 at 1.0 A g^-1 and maintained 1000 cycles of cycling,with only0.02% capacity loss per cycle.In the last part,MXene/ZIF-67 dodecahedron was synthesized by combining the2D/2D structure and the carbon coating.The MXene@Co₉S₈/CoM₂S₄ composite were prepared by ion exchange and in situ sulfurization methods.Sulphur powder can create porous network in the process of evaporation and increase the reactive sites,elevating the reversible specific capacity.The 2-methylimidazole from precursor act as carbon source to stabilize the crystal structure of Co₉S₈/CoM₂S₄.The discharge capacities of MXene@Co₉S₈/CoM₂S₄ at the current densities of 0.1,0.2,0.5,1.0,2.0 and 5.0 A g^-1are 325,309,284,256,226 and 172 mAh g^-1.It also retained a reversible capacity of 196 mAh g^-1 after 350 cycles at 1.0 A g^-1.