【作者】 刘洋；

【导师】 贺高红； 李祥村；

【作者基本信息】 大连理工大学 ， 化学工程， 2021， 博士

【摘要】 锂硫电池因理论容量高、低价环保等优势成为极具发展前景的新一代电池。但锂硫电池充放电过程中硫的体积膨胀易导致其从电极脱落、且正极产生的多硫化锂由于穿梭效应向负极扩散,造成电池容量的迅速衰减;另一方面,硫/硫化锂较差的导电性以及硫化锂沉积物难以转化成锂和硫,导致电池缓慢的电化学反应速率和较差的循环稳定性。针对上述问题,本文构建兼具吸附性能和催化活性的金属基多功能电极/隔层材料,通过调控材料的孔结构和催化活性,抑制多硫化锂穿梭效应、缓解硫体积膨胀效应,同时提高电池材料导电性和多硫化锂催化转化反应动力学速率,提升锂硫电池综合性能,并揭示了锂硫电池的催化转化反应机制。为解决多硫化锂穿梭效应及硫体积膨胀效应,首先制备了核壳结构氧化铁纳米微球支撑石墨烯的三明治结构电极材料。核壳氧化铁较大的比表面积(121.8 m2 g-1)为多硫化锂提供吸附/催化活性位点,抑制多硫化锂的穿梭效应;同时氧化铁纳米微球支撑片层石墨烯为硫的体积膨胀和多硫化锂的存储提供了充足空间。电池在1 C电流密度下循环100圈后容量仍保持607.7 mAh g-1(每圈衰减率为0.32%),高于纯石墨烯电极的容量(476.7 mAh g-1)。为提高材料的催化活性,使硫化锂更易生成锂和硫,论文构建具有催化性能的碳膜隔层材料,进一步扩大储硫空间。以载铂空心微球(Pt@C)为催化剂,将其均匀分散于聚丙烯腈(PAN)铸膜液通过相转化法构建了高连通网络载铂碳膜(Pt/CM)隔层材料。膜内分散的Pt纳米颗粒催化沉积硫化锂向锂和硫转化,提升电化学反应速率。Pt@C减缓了相转化过程PAN的聚集,使碳膜孔隙率从44.7%提升至88.5%,丰富的孔结构缓冲体积膨胀并提供多硫化锂存储空间,且碳膜多孔网络结构有利于电子传递。PAN中的氮元素转化为碳膜中的吡啶氮、吡咯氮、石墨烯氮,对多硫化锂的吸附能分别为-1.541、-1.605和-1.459 eV,能高效吸附固定多硫化锂、抑制其穿梭效应,提高电池容量。锂硫电池在1 C时循环100圈后放电容量达691.7 mAh g-1,每圈衰减率为0.22%。为进一步提高铂的催化活性,论文提出在多孔碳球内设计制备了铂纳米颗粒外部不同程度镍包覆的核壳结构双金属催化剂,并结合密度泛函理论(DFT)计算进一步揭示了 Pt@Ni合金协同催化的反应机理:部分包覆结构的Pt@Ni合金催化剂界面处紧密结合,促进了 Ni的电子向Pt转移并为电化学反应提供电子。失电子状态下的镍更易活化硫化锂分子,促进硫化锂分解(硫化锂在Pt和PtNi上分解能分别为0.486 eV和0.101 eV),降低了反应能垒(Pt:5.19 eV,PtNi:4.34 eV),从而提高了锂硫电池的电化学反应动力学速率和循环稳定性。同时,Pt@Ni合金催化剂展示较强的多硫化锂亲和吸附性能,抑制穿梭效应。1 C电流密度循环100圈后电池容量654.1 mAh g-1,循环300圈仍保持600.0 mAh g-1,每圈衰减率为0.18%。为了避免传统电极制备过程使用非活性物质集流体,进一步提高活性物质含量,并增强电极内部电子转移,论文提出相转化法构建超细碳化铁(粒径30 nm)掺杂的非对称碳膜一体化电极,非对称碳膜致密层可替代传统铝箔集流体,不仅提高了活性物质硫的含量,还增强材料与集流体间的电子传递和电极的整体导电性。碳膜的孔结构(孔径3.9 nm,孔隙率81.2%)缓解了硫体积膨胀效应并提高硫的担载(4.6 mg cm-2的高载硫量)。碳化铁的强吸附能力有利于引导多硫化锂向膜孔内传递扩散、抑制其穿梭效应,其高导电性还促进多硫化锂/硫化锂的催化转化,增强电化学反应动力学,使锂硫电池具有每圈0.11%的低衰减率和2.6 mAh cm-2的高面容量,1 C电流密度循环100圈电池容量为879.8 mAhg-1。膜电极易规模化制备,为高能量密度和长程稳定性锂硫电池的实际应用开辟了新思路。更多还原

【Abstract】 Lithium-sulfur battery(Li-S batteries)is attractive for its advantageous such as high theoretical capacity,low cost and environmentally friendly,which is considered as one of the most promising next-generation rechargeable battery systems.However,the volume expansion of sulfur result in the detachment of active materials,polysulfides generated at the cathodes shuttle towards the anodes,leading to the rapid capacity degradation.The other hand,low conductivity of sulfur and Li2S is difficult to transform Li and S,result in the sluggish reaction kinetics and poor cycle life.To address the above problems,this paper designs multifunction electrodes/interlayer materials with adsorption and electrocatalytic activity by controlling the pore structures and catalytic activity,restraining the shuttle effect as well as buffering the volume expansion effect,and further improves the conductivity and enhances the reaction kinetics of polysulfides catalytic conversion to obtain a high performance and practical Li-S batteries,and fundamentally explores the mechanism of the polysulfide conversion.To resolve the shuttle effect of polysulfides and buffer the volume expansion effect of sulfur,sandwich-like hierarchical reduced graphene oxide doping core-shell Fe2O3 microspheres(RGO/Fe2O3)was prepared as electrodes.Core-shell Fe2O3 with stable structure and large surface area(121.8 m2 g-1)suppresses the shuttling of LiPSs by chemical bonding instead of merely physical adsorption,the meantime exhibits the electrocatalytic effect on the transformation of intermediate polysulfides to lower order polysulfides.promoting the conversion in both discharging and charging period;highly conductive the RGO with multi-lamellar structure promotes the rapid electron/ion transfer;RGO and Fe2O3 support each other,reducing the volume expansion,enhancing the cycling stability with the enhanced discharge capacity from 476.7 mAh g-1 to 607.7 mA h g-1 at 1 C after 100 cycles in Li-S batteries,with the 0.32%decay per cycle.To further improve the catalytic activity of materails,accelerate the conversion of Li2S to form Li and S,the paper introduces the carbon membranes as interlayer with catalytic activity,and expands the sulfur storage space.Pt@C is used as catalyst,which is dispersed in PAN casting solution amd obtain the Pt/CM interlayer by phase inversion.Pt nanoparticles dispersed in membranes catalyst Li2S to form Li and S,increasing the electrochemical reaction kinetics.Pt@C slowes down the accumulation of PAN during phase inversion and increase the porosity of carbon membranes from 44.7%to 88.5%.The abundant pore structures buffer the volume expansion effect and provide the space for polysulfides reservation;interconnected networks enhance the electron transport.The N in PAN form pyridine N,pyrrole N and graphene N in carbon membranes,the binding energy of pyridine N,pyrrole N and graphene N is-1.541 eV,-1.605 eV and-1.459 eV respectively,providing the efficient adsorption for polysulfides and suppressing the shuttle effect.The electrochemical performance was improved with the capacity of 691.7 mAh g-1 at 1 C after 100 cycles significantly,decay per cycle is 0.22%.To improve catalytic activity of Pt material,a Pt@Ni core-shell bimetallic catalyst is prepared in porous carbon spheres,and the mechanism of synergic catalysis is first proposed based on DFT:The interface of Pt and Ni core-shell catalysts can facilitates the charge transfer between the two metals,the bimetallic catalysts offer a synergic effect on catalyzing reactions for catalytic oxidation of insoluble polysulfides to Li and S.The electronic migration from Ni to Pt becomes a driving force for Ni activating Li2S2/Li2S molecules(decomposition energy barrier of Li2S on Pt and PtNi is 0.486 eV and 0.101 eV)by largely reduce the reaction energy barrier(Pt:5.19 eV,PtNi:4.34 eV),enhance the reaction kinetics and cycle stability of Li-S batteries.At the same time,bimetallic catalysts show the strong adsorption of LiPSs,suppress the shuttle effect.The capacity of batteries is 654.1 mAh g-1 at 1 C after 100 cycles,and maintains 600.0 mAh g-1 after 300 cycles with 0.18%decay per cycle。To avoid the use of current collector inactive materials,further improve the active material mass and enhance the electronic transfer in the electrode,a flexible asymmetric porous carbon membrane doped with iron carbide(FexC)nanoparticles(partical size is 30 nm)is designed by a facile phase-inversion method,realizing the integrated electrode.The dense conductive lower layer can replace A1 foil as a current collector,which improves active material mass of S,and enhances electronic transfer in the electrode and overall conductivity of the electrode.The unique triple-layer-structured pores of multifunctional membrane(pore size is 3.9 nm,porosity is 81.2%)relieve the volume expansion and improve the S loading(4.6 mg cm-2 high sulfur loading).The high adsorption of FexC promote their gradual diffusion into the porous carbon membrane on the opposite side of the electrolyte,suppressing the shuttle effect,the high electrical conductivity effectively promote entrapped polysulfide redox kinetics and fast electron transfer.The areal capacity of batteries is 2.6 mAh cm-2 and the capacity is 879.8 mAh g-1 at 1 C after 100 cycles with 0.11%decay per cycle.The flexible preparation method offers a new strategy to achieve smooth trapping-diffusion-conversion of polysulfides for large-scale processes toward high-performance Li-S batteries.更多还原