【作者】 陈伟；

【导师】 熊杰；

【作者基本信息】 电子科技大学 ， 电子科学与技术， 2021， 博士

【摘要】 锂硫（lithium sulfur,Li-S）电池因其超高比能量密度,成为最具潜力超越锂离子电池的下一代储能器件。然而由于存在硫电极以及锂金属电极界面离子动力学的不可控,Li-S电池商业化受到严重制约。例如硫电极反应界面多硫化物阴离子析出以及锂离子缓慢传输引起硫电极较差循环以及倍率性能;锂金属电极侧,界面锂离子的低浓度以及强溶剂化,致使锂金属电极在循环中面临诸多如锂枝晶、界面极化、固态电解质膜（solid electrolyte interphase,SEI）重构等挑战,加剧Li-S电池电化学性能衰减。针对以上问题,本文从离子调控角度出发,通过减缓硫电极中多硫化物阴离子析出以及促进锂离子扩散,提出提升硫电极循环以及倍率性能的研究方向;通过提高锂金属电极界面离子浓度和调控锂离子溶剂化,提出避免锂金属表面枝晶生长的策略,为最终Li-S电池商业化提供解决方案。具体研究内容如下:1、针对硫电极界面多硫化物阴离子析出问题,利用聚乙烯亚胺（polyethyleneimine,PEI）与六亚甲基二乙腈酸酯（hexamethylene diisocyanate,HDI）或聚乙二醇二缩水甘油醚（poly（ethylene glycol）diglycidyl ether,PEGDGE）交联聚合,制备两种氨基功能化极性粘结剂（分别简写为:AFG与PPA粘结剂）,探讨在硫电极反应界面形成S-N、Li-O、Li-N、S-O等极性化学键对多硫化物阴离子的吸附作用。实验表明:界面引入极性化学键,可减缓多硫化物阴离子析出,提升硫电极循环稳定性。例如采用AFG粘结剂,硫电极循环稳定性可提升至600圈且容量保持率为91.3%。此外,PPA粘结剂表明在粘结剂中引入亲水性基团如羟基（hydroxy,-OH）基可提高电极材料与集流体的粘附性,避免活性材料脱离集流体。最后,利用原位技术如原位紫外可见光谱（In-situ UV-vis）和原位拉曼光谱（In-situ Raman）,证实了硫电极反应界面引入AFG或PPA粘结剂可有效降低多硫化物阴离子析出速率,提升硫电极循环稳定性。2、针对硫电极内部锂离子传输问题,提出利用具有层间离子通道的离子导体锂基蒙脱土（lithium montmorillonite,Li-MMT）作为离子通道,研究降低锂离子在电极内部的扩散势垒,促进锂离子反应动力学与硫电极倍率性能的关联性。实验表明,Li-MMT/S复合材料在硫含量80%,单位S负载4 mg cm^-2,Li-S电池在电流密度15 m A cm^-2下可实现345 m Ah g^-1的放电容量。另外,Li-MMT的路易斯酸/碱位点可对多硫化物产生路易斯酸碱作用,使S负载为5.2 mg cm^-2,电流密度8m A cm^-2下稳定循环超过350圈且容量衰减率低至每圈0.049%。3、针对锂金属电极界面低锂离子浓度问题,采用聚氧化乙烯（polyoxyethylene,PEO）作为支撑骨架,在醚类电解液中添加离子吸附型Li-MMT制备一种离子“浓缩”型电解液（简写为:MIP基电解液）,实现对电解液体相中的锂离子“浓缩”,最大限度提升电极界面锂离子浓度,促进锂均匀成核,减少锂枝晶。实验表明,采用MIP基电解液,在电流密度0.5 m A cm^-2、容量1 m Ah cm^-2下,Li||Cu半电池可稳定循环超过300次;Li||Li对称电池能稳定循环超过1200 h,且所制备的Li-S电池前100圈平均库伦效率可达99.62%。此外,借助In-situ Raman技术,可视化了锂离子电镀过程中电极界面离子浓度变化,确认了MIP基电解液对锂金属电极界面离子浓度调控的有效性。4、针对锂金属电极界面锂离子溶剂化问题,通过氧化亚磷酸三（2,4-二叔丁苯基）酯（oxidized tris（2,4-di-tert-butylphenyl）phosphite,O-TBP）耦合四丁基氟化铵（tetrabutylammonium fluoride,TBAF）,在锂金属电极表面制备可削弱锂离子溶剂化以及引入氟阴离子（F^-）参与锂离子配位的功能化层（简写为:RMH层）,实现减少溶剂分子参与SEI构建和提升SEI中氟化锂（lithium fluoride,Li F）含量,促进锂核横向生长,避免锂枝晶形成。实验表明,利用RMH修饰的锂金属（RMH@Li）可在电流密度3 m A cm^-2、容量3 m Ah cm^-2下稳定循环超过800 h。通过理论模拟、Raman探测、冷冻电镜以及X射线光电子能谱（x-ray photoelectron spectroscopy,XPS）深剖等表征技术,确认了RMH@Li可促使锂金属电极表面无枝晶化生长,所制备的Li-S软包电池,经过50次循环后,可贡献～600 m Ah容量。此外,RMH层中O-TBP可避免锂金属被潮湿空气中水腐蚀,使RMH@Li暴露于RH=50～60%潮湿空气1h,仍拥有金属光泽且其电化学性能未受影响。更多还原

【Abstract】 Li-S battery delivers the possibility beyond the state-of-the-art lithium-ion battery due to its high specific energy density.However,the uncontrollable ionic dynamicses at S and Li metal electrode interface hinder the actual application.For instance,the shuttling of polysulfide anions and the slow migration of Li ions inside the S electrode result Li-S battery with poor cycling stability and rate performance,respectively.While low concentration and strong solvation of Li ions nearby Li metal electrode interface cause Li metal electrode with many challenges such as Li dendrite growth,interface polarization and SEI reconstruction during cycles,aggravating the degradation of Li-S electrochemical performance,thus hindering the actual applications.Herein,in this dissertation,investegations via slowing down the shullte of polysulfide anions in elelctrolyte,promoting the diffusion of lithium ions within sulfur electrolde,increasing the ion concentration at the lithium metal surface and regulating the solvation of lithium ions nearby lithium metal electrode are preformed,aiming to improve the interface stability of S and Li electrolde for promoting commercialized Li-S battery with higher electrochmical performance.The researches are as follows:1.For the shullte of polysulfide anions in elelctrolyte,the polysulfide anions are suppressed via two amino functionalized polar binders which are prepared by crosslinking polyethylenimine（PEI）with hexamethylene diisocyanate（HDI）or poly（ethylene glycol）diglycidyl ether（PEGDGE）,denoted as AFG and PPA binders,respectively.The regulation effects of two binders on polysulfide anions at the reaction interface of S electrode by adopting polar chemical bonds such as S-N,Li-O,Li-N and S-O are investigated.Results show that introducing polar chemical bonds at the reaction interface of S electrode can slow down the shuttle of polysulfide anions in electrolyte and improve the cycling stability of S electrode.For instance,S electrode cycles stability with capacity retention rate of 91.3%over 600 cycles as using AFG binder.Moreover,PPA binder allows the adhesive strength between S electrode material and current collector reachs up to 0.4 Mpa,indicating that hydrophilic groups such as-OH group embed into binder framework can improve the adhesion of binder,ensuring the structural integrity of S electrode.In addition,in-situ techniques such as in situ UV-Vis and Raman are provided to confirm the advantages of proposed binders.2.For the diffusion issues of Li ions,atomic interlamellar ion path lithium-montmorillonite（Li-MMT）via forming interlayer ion channel within S electrode to decrase the migration barrier,facilitating Li ion diffusion within the entire S electrode,is employed.It is found that Li-MMT enhances the Li ion transport and redox kinetic processes at reaction interface,decreasing the concentration polarization under high S loading and high current density.When conbined with 80%sulfur content,the prepared Li-MMT/S composites enable Li-S battery with a discharge capacity of 345 m Ah g^-1 with4 mg cm^-2 S loading even at a current density of 15 m A cm^-2 after constructing Li-MMT as fast ion transfer paths.In addition,the Lewis acid/base site of Li-MMT provides lewis acid-base effect on polysulfides,ensuring stable cycles of S electrode more than 350cycles with a capacity decay rate of 0.049%per cycle when sulfur loading is 5.2 mg cm^-2and current density is 8 m A cm^-2.3.For the barren region of Li ions at the Li metal surface,lithiophilic MMT coupled with polyethylene oxide（PEO）as additive in the ether-based electrolyte,denoted as MIP-based electrolyte,is reasonably designed to concentrate the Li ions near the surface of MMT and improve the Li ion concentration on the anode surface（e.g.,Li metal anode）,thus promoting the homogeneous nucleation of Li ions as well as suppressing Li dendrite growth.The experimental results show that MIP based electrolyte allows Li||Cu half cell with stable Li plating/stripping more than 300 cycles at a current density of 0.5 m A cm^-2with a capacity of 1 m Ah cm^-2.Moreover,the Li||Li symmetric battery enables stable cycles more than 1200 h as repalcing the ether based electrolyte with MIP based electrolyte,allowing Li-S battery with 99.62%average coulomb efficiency among 100cycles.In addition,the variation of ionic concentration at the electrode interface during plating process is visualized via in-situ Raman technology,confirmed the effectiveness of MIP-based electrolyte on regulating ionic concentration nearby the Li electrode interface.4.For the solvation of Li ions at the Li metal electrode interface,oxidized tri（2,4-di-tert-butylphenyl）phosphite（O-TBP）coupled with tetrabutylammonium fluoride（TBAF）,denoted as RMH layer,customized on the interface of Li metal electrode is investigated on the effect of constructing SEI components.Using theoretical simulation,Raman detection,cryo-electron microscopy,X-ray photoelectron spectroscopy deep profiling and other characterization techniques,it is clear that weakening the solvation intensity of Li ions with solvent molecules and introducing fluoride anion（F^-）nearby the electrode interface can reduce the participation of solvent molecules and increase lithium fluoride（Li F）component in SEI,forcing the lateral growth of Li nuclei,and avoiding the formation of lithium dendrites.The experimental results show that RMH modified Li metal（RMH@Li）stably cycles more than 800 h at a current density of 3 m A cm^-2 with a capacity of 3 m Ah cm^-2.Moreover,RMH@Li enables Li-S pouch cells contributing600 m Ah/pouch over 50 cycles with a capacity decay rate of 0.26%per cycle.In addition,the O-TBP component in RMH layer reduces the molecular hydrogen bonding cooperation between Li and water molecules,thus effectively prevents the moisture-sensitive Li from water corruption.In the experiment,RMH@Li still miantians nature features and negligible influences on its electrochemical properties even RMH@Li is exposed to RH=50～60%humid air for 1h.更多还原