【作者】 张恒；

【导师】 孙正明；

【作者基本信息】 东南大学 ， 材料学， 2021， 博士

【摘要】 锂硫电池比传统锂离子电池具有更高的理论能量密度,是最具前景的下一代储能器件之一。然而,锂硫电池的大规模应用依然受到以下几方面的制约:1)电极活性物质的低电导率导致电化学反应动力学迟缓;2)中间产物多硫化物的溶解和扩散导致活性物质的流失;3)体积效应导致电极的稳定性差甚至失效等。本论文从抑制锂硫电池的穿梭效应出发,以极性的MXene（d-Ti₃C₂）作为导电骨架和前驱体,构筑多级结构的硫载体,以期优化电极结构和提高电化学性能。作为电极的关键材料,多尺度和多功能耦合的硫载体,可有效抑制多硫化物穿梭,提升电化学反应动力学,提高电极结构稳定性,保证高硫利用率和长循环寿命。主要研究内容如下:首先,采用静电自组装和共沉淀-原位衍生的策略,分别构筑了中空多孔碳（HPCS）@d-Ti₃C₂和中空氮掺杂碳（NC）@d-Ti₃C₂纳米多级结构。中空碳（HPCS和NC）与d-Ti₃C₂复合,避免了d-Ti₃C₂的堆垛和中空碳的团聚,增大比表面积和孔体积,提升固硫和限域多硫化物的能力;同时d-Ti₃C₂骨架为电子/离子的快速传输提供路径,促进多硫化物转化动力学。多尺度耦合的硫载体通过物理限域和化学吸附的双功能协同,缓解了多硫化物穿梭,提高了硫利用率和循环性能。在1 C的倍率下循环500圈后,HPCS@d-Ti₃C₂/S电极表现出500 mA h g^-1的可逆容量和0.069%/圈的平均容量衰减率,而NC@d-Ti₃C₂/S的可逆容量为613 mA h g^-1对应0.055%/圈的衰减率。从而揭示了硫载体的极性与导电性协同,对提升多硫化物吸附和转化效率的重要性。其次,针对物理限域在穿梭效应抑制中的局限性,利用自组装结合溶胶-凝胶法构筑了Mn O₂纳米片（MNSs）@d-Ti₃C₂三维结构的硫载体。MNSs吸附在d-Ti₃C₂表面构建异质结构,使MNSs@d-Ti₃C₂具有较大的比表面积,促进多硫化物吸附和催化活性位点的暴露;同时,d-Ti₃C₂三维网络提供电荷转移路径,MNSs与d-Ti₃C₂构建的异质界面缩短离子传输通道,减小扩散阻力,提升电化学反应动力学。MNSs@d-Ti₃C₂/S在1 C下循环500圈后保持了521 mA h g^-1的可逆容量,对应0.059%/圈的衰减率;而且在3.7mg cm^-2的硫载量下循环500圈仍有475 mA h g^-1的可逆容量。优异的电化学性能表现,得益于极性硫载体通过化学吸附和催化转化的多功能耦合,提升多硫化物转化效率,抑制穿梭效应,保证了正极的高硫利用率和循环稳定性。再次,为提高硫载量和电极结构稳定性,通过模块组装的方法设计了具有物理限域、化学吸附和催化转化等多功能的复合硫载体。采用模板法制备了中空多孔碳（HPC）包覆Mn O₂纳米棒（MNR）的核壳结构（MNR@HPC）,并与d-Ti₃C₂自组装构筑了多级结构的MNR@HPC@d-Ti₃C₂（记为MCT）。作为硫载体,HPC为活性物质的限域提供物理空间,MNR为促进多硫化物的转化提供催化界面,d-Ti₃C₂导电骨架不仅作为电子传导路径而且为多硫化物吸附提供活性位点。在结构和功能协同作用下,MCT展现出高效的多硫化物吸附和催化转化能力。在75%的硫含量下,MCT/S在2 C下循环600圈后的可逆容量为591 mA h g^-1,对应平均0.044%/圈的容量衰减率;另外,硫载量为4.15 mg cm^-2的电极在0.2 C下循环200圈表现出816 mA h g^-1的可逆容量和80%的容量保持率。因此,构筑多尺度和多功能耦合的硫载体,既有利于穿梭效应的抑制和硫利用率的提高,又有利于电极结构的稳定性和多硫化物的可逆转化,保障了硫正极优异的电化学性能。最后,针对多硫化物转化效率低和循环稳定性差等问题,通过d-Ti₃C₂的原位衍生制备了含有氧空位的Ti_nO_2n-1量子点修饰多孔碳纳米片（T_nQDs@PCN）,为硫正极构筑了具有“催化中心-活性物质-导电基质”三相界面的硫载体。在结构上,T_nQDs均匀负载在PCN上且保持良好的界面接触,有助于电子/离子的快速传输,促进活性物质转化动力学的提升和转化产物的均匀沉积。在功能上,T_nQDs作为极性催化中心,促进多硫化物的吸附和转化,有效抑制穿梭效应,提高硫利用率;PCN不仅提供电荷转移路径和离子扩散通道,而且物理性限域活性物质,缓冲电极的体积变化,保障了电极结构的稳定性和循环性能的提升。硫含量为79%的T_nQDs@PCN/S电极在2 C下循环1000圈保持了660 mA h g^-1的可逆容量,对应仅0.012%/圈的平均衰减率;当硫载量为4.8 mg cm^-2且电解液用量（E/S）仅为4.5μL mg^-1时,电极在0.5 C下循环500圈仍具有736 mA h g^-1的可逆容量和82%的容量保持率。更多还原

【Abstract】 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 4.5μL 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.更多还原