Constructing MXene-Based Hierarchical Structures For Suppressing Shuttle Effect In Lithium–Sulfur Batteries

Lithium-sulfur(Li-S)batteries have much higher theoretical energy density than traditional Li-ion batteries and show the greatest potential to become one of the next-generation energy storage devices.However,the large-scale application of Li-S batteries is still restricted by the following aspects.1)Low conductivity of the active materials leads to the sluggish kinetics of electrochemical reaction;2)Dissolution and diffusion of the intermediate polysulfides result in the loss of active materials;3)Volume effect of the electrode causes the poor stability or even failure of the electrodes.In this dissertation,to suppress the shuttle effect of Li-S batteries,we employed the polar MXene(d-Ti₃C₂)as the conductive skeleton and starting material to construct the hierarchical-structural S hosts,in order to optimize the structure and electrochemical performance of the electrodes.As the key materials,the S hosts with multi-scale and multi-functional coupling can effectively inhibit the shuttling of polysulfides,enhance the electrochemical reaction kinetics,and improve the structural stability,realizing high S utilization and long cycling life.The main research contents are as follows.Firstly,two hierarchical nanostructures were constructed.Hollow porous carbon sphere(HPCS)@d-Ti₃C₂ was prepared with electrostatical self-assembly method,while the hollow nitrogen-doped carbon(NC)@d-Ti₃C₂ was fabricated using a process of co-precipitation followed by in situ derivatization.The re-stacking of d-Ti₃C₂ and the aggregation of hollow carbon(HPCS and NC)were successfully avoided,which is conducive to increasing specific surface area and pore volume as well as improving the S loading capacity and the polysulfide confined capability of d-Ti₃C₂.In addition,the d-Ti₃C₂ framework provides electron/ion transmission path,which promotes the conversion kinetics of polysulfides.Cooperating the physical confinement and chemisorption functions,the S hosts with multi-scale coupling alleviate the polysulfide shuttling,resulting in the improvement of S utilization and cycle performance.After 500 cycles at 1 C,the HPCS@d-Ti₃C₂/S delivers a reversible capacity of500 mA h g^-1 and an average capacity decay rate of 0.069%/cycle,and the NC@d-Ti₃C₂/S remains a reversible capacity of 613 mA h g^-1 and the decay rate of 0.055%/cycle.These results revealed the importance of the synergistic polarity and conductivity of S hosts for improving the polysulfide adsorption and conversion efficiency.Secondly,in view of the limitations of physical confinement on suppressing the shuttle effect,three-dimensional(3D)Mn O₂ nanosheets@d-Ti₃C₂ aerogel(MNSs@d-Ti₃C₂),serving as the nano-S host,was constructed by self-assembly combined sol-gel method.MNSs are adsorbed on 3D d-Ti₃C₂ to form heterogeneous structure,so that MNSs@d-Ti₃C₂ possesses a large specific surface area,which is beneficial to the exposure of active sites for polysulfide adsorption and catalytic conversion.Meanwhile,3D d-Ti₃C₂ framework provides charge transfer path;the heterogeneous interface constructed by MNSs and d-Ti₃C₂ shortens ion transport channel,reduces ion diffusion resistance,and promotes the electrochemical reaction kinetics.As a result,MNSs@d-Ti₃C₂/S maintains a reversible capacity of 521 mA h g^-1 after500 cycles at 1 C,corresponding to a capacity decay rate of 0.059%/cycle.Besides,it delivers a reversible capacity of 475 mA h g^-1 over 500 cycles with S loading of 3.7 mg cm^-2.Such excellent electrochemical performance originates from the multi-functional coupling of chemisorption and catalytic conversion of the polar S hosts,which can improve the conversion efficiency of polysulfides and suppress the shuttle effect,ensuring high S utilization and cycling stability.Thirdly,in order to improve the S loading and the structure stability of the electrode,we designed a hierarchical composite with multiple functions,including physical confinement,chemisorption and catalytic conversion through a modular-assembly method.Hollow core-shell structural Mn O₂ nanorods@hollow porous carbon(MNR@HPC),fabricated by template method,was self-assembled with d-Ti₃C₂ to construct a hierarchical-structural MNR@HPC@d-Ti₃C₂(labelled as MCT).As the S host of cathode,HPC provides physical spaces for confining active materials;MNR gives catalytic interface for promoting the conversion of polysulfides;the d-Ti₃C₂ conductive framework not only serves as an electronic conduction path but provides active sites for polysulfide adsorption.Owing to the synergistic effect of structure and function,MCT exhibits high-efficiency polysulfide adsorption and catalytic conversion capabilities.As a result,MCT/S with S content of 75%delivers a reversible capacity of 591 mA h g^-1 over 600 cycles at 2 C,corresponding to an average capacity decay rate of 0.044%/cycle.In addition,with S loading of 4.15 mg cm^-2,it shows a reversible capacity of 816 mA h g^-1 and 80%capacity retention over 200 cycles at0.2 C.Therefore,constructing S hosts with hierarchical structure and multi-function coordination can not only suppress the shuttle effect and improve the S utilization rate but also profit to the structure stability of electrode and the reversible conversion of polysulfides,which ensure the excellent electrochemical performance.Last,to tackle the problems of low polysulfide conversion efficiency and inferior cycling stability,oxygen-vacancy-rich Ti_nO_2n-1 quantum dots-embedded porous carbon nanosheet(T_nQDs@PCN)was in situ derived from d-Ti₃C₂,which serves as the S host,and constructed into a three-phase interface of‘Catalytic centers-Active materials-Conductive substrates'for the cathode.Structurally,T_nQDs are uniformly dispersed on PCN and maintain a good contact interface,which facilitates the rapid migration of electrons/ions and promotes the conversion kinetics of active materials and uniform deposition of conversion products.Functionally,T_nQDs,as the polar catalytic centers,promote polysulfide adsorption and conversion,effectively suppress the shuttle effect,and improve the S utilization;PCN not only provides charge transfer path and ion diffusion channel,but also physically confines active materials.Thus,T_nQDs@PCN buffers the volume expansion and ensures the structure stability of the electrode,resulting in the improvement of cycle performance.With S content of 79%,T_nQDs@PCN/S delivers a reversible capacity of 660 mA h g^-1 over 1000 cycles at 2C,corresponding to an average attenuation rate of only 0.012%/cycle.Even with S loading of4.8 mg cm^-2 and electrolyte dosage(E/S)of 4.5?L mg^-1,the cathode shows a reversible capacity of 736 mA h g^-1 and capacity retention rate of 82%over 500 cycles at 0.5 C.