【作者】 姜波；

【导师】 张乃庆；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2022， 博士

【摘要】 锂硫电池因具有理论能量密度高、价格低廉和环境友好等优势得到广泛关注,被认为是新一代高比能储能系统中最具有应用前景的电池体系之一。然而,锂硫电池面临着活性物质硫利用率低、多硫化锂穿梭严重以及反应动力学迟缓等诸多问题,其实际能量密度和循环稳定性仍难以满足商业化应用的要求。将具有良好吸附能力和高催化活性的材料用于抑制多硫化锂的穿梭和提升硫物种相互转化的反应动力学是提高电池的实际能量密度和循环稳定性的有效策略,成为锂硫电池领域的研究热点。本论文借助晶面工程、界面工程等策略调控金属氧化物基催化材料的表/界面结构,并通过实验和理论模拟研究了各种调控策略对锂硫电池性能的改善作用和相关机理。采用水热合成法制备了凹面Fe₂O₃纳米立方体负载在还原氧化石墨烯上的复合材料（C-Fe₂O₃-G）。实验和理论模拟表明,凹面Fe₂O₃纳米立方体表面暴露的高指数Fe₂O₃{13-44}和{12-38}晶面上分布着丰富的配位不饱和Fe活性位点,而使得C-Fe₂O₃-G具有强吸附能力和高催化活性。C-Fe₂O₃-G作催化剂不仅能够高效地化学锚定多硫化锂抑制其穿梭,而且显著地加速了多硫化锂转化的反应动力学并降低了硫化锂分解的能垒,进而使得锂硫电池放电容量和循环稳定性得到明显提升。电池在2 C电流密度下充放电循环1600次后仍保持了491 mAh g^-1的比容量,相应的单次循环的容量衰减率仅为0.025%。为了证实借助晶面效应调节锂硫电池的电化学性能具有广泛的适用性,以吸附性能比d区金属氧化物Fe₂O₃更出色的p区金属氧化物SnO₂为模型进一步研究了晶面效应在锂硫电化学中的作用机制。利用结构导向剂合成两种SnO₂纳米晶/还原氧化石墨烯复合材料（SnO₂{332}-G和SnO₂{111}-G）,两种SnO₂纳米晶分别为暴露了{332}晶面和{111}晶面的SnO₂纳米八面体。与{111}晶面相比,表面含有更多配位不饱和Sn位点的{332}晶面不仅与多硫化锂分子具有更强的结合能力而且更有效地降低了硫化锂的分解能垒,因而使得SnO₂{332}-G不仅更有效地抑制了多硫化锂的穿梭且更高效地提高了硫物种相互转化的反应动力学,进而更显著地提升了锂硫电池的电化学性能。电池在2 C电流密度下实现了2000次的充放电循环,平均每次循环的容量衰减率低至0.021%。在前两章研究工作的基础上,进一步探索了载体暴露晶面的差异对异质材料负载生长形成的界面结构的影响,并研究了暴露不同异质界面的催化材料对锂硫电池电化学性能的影响。通过两步生长法在暴露高指数{113}和{104}晶面的Fe₂O₃纳米十八面体表面生长CeO₂纳米晶,制备了暴露高能界面的Fe₂O₃-CeO₂复合纳米颗粒。研究表明,高能Fe₂O₃-CeO₂界面处存在强界面电子转移。得益于大量具有强界面相互作用的高能异质界面的形成,Fe₂O₃-CeO₂复合纳米颗粒展现出对多硫化锂的强吸附能力和对硫物种转化的高催化性能。电池在2 C电流密度下经过2000次充放电循环后仍具有546 mAh g^-1的可逆容量,单次循环的平均容量衰减率仅为0.016%。在10.17 mg cm^-2的高硫负载条件下,电池经过200次循环后仍具有8.65mAh cm^-2的面容量。与晶面工程和界面工程相比,应变工程对材料表面特性的调控更具有连续性且调控能力更强,能够通过精细地调节材料表面原子的间距实现对材料表面特性高效且精准的调控。采用水热合成反应制备了FeOOH纳米棒,再将其在不同温度下进行热处理得到了不同压缩应变的FeOOH催化材料。实验和理论计算证实,晶格压缩应变能够减弱催化材料对多硫化锂的吸附能力,适量的压缩应变使得FeOOH催化材料对多硫化锂展现出适中的吸附能力进而能更好地发挥催化作用。含有4.2%晶格压缩应变的FeOOH催化材料在有效地锚定多硫化锂的同时高效地催化了硫物种的相互转化。锂硫电池在2 C电流密度下完成了3000次的充放电循环,平均每次循环容量衰减率低至0.013%。在E/S比为4.6μL mg^-1的贫电解液条件下,硫载量为9.17 mg cm^-2的电池循环70次后保留了7.17 mAh cm^-2的面容量。更多还原

【Abstract】 Lithium-sulfur（Li-S）batteries have attracted widespread attention due to their high theoretical energy density,low price,and environmental friendliness.Li-S batteries are recognized as one of the most promising battery systems in the new generation of high-energy energy storage systems.However,Li-S batteries are facing many problems,such as low utilization of active sulfur,severe lithium polysulfide shuttle,and sluggish reaction kinetics.As a result,the actual energy density and cycle stability of Li-S batteries still do not meet the requirements of commercial applications.Applying catalytic materials with good adsorption capacity and high catalytic activity to inhibit the shuttle of lithium polysulfides（Li PSs）and enhance the reaction kinetics of the interconversion of sulfur species is an effective strategy to improve the practical energy density and cycle stability of the batteries,which has become a research hotspot in the field of Li-S batteries.In this thesis,the surface/interface structures of metal oxide-based catalytic materials are regulated by strategies such as crystal facet engineering and interface engineering,and the improvements and related mechanisms of various regulation strategies on the performance of Li-S batteries are studied through experiments and theoretical simulations.The composite materials（C-Fe₂O₃-G）of concave Fe₂O₃ nanocubes supported on reduced graphene oxide were prepared by hydrothermal synthesis.Experiments and theoretical simulations show that C-Fe₂O₃-G have strong adsorption capacity and high catalytic activity due to the abundant unsaturated coordinated Fe active sites on the high index Fe₂O₃{13-44}and{12-38}crystal facets exposed on the surface of the concave Fe₂O₃ nanocubes.Therefore,C-Fe₂O₃-G catalysts can not only efficiently chemically anchor Li PSs to inhibit the shuttle effect,but also significantly accelerate the reaction kinetics of Li PSs conversion and reduce the energy barrier of lithium sulfide（Li₂S）decomposition,thus significantly improving the discharge capacity and cycle stability of Li-S batteries.The batteries maintain a specific capacity of 491 mAh g^-1 after 1600charge-discharge cycles at 2 C,and the corresponding single-cycle capacity decay rate is only 0.025%.In order to test the wide applicability of adjusting the electrochemical performance of Li-S batteries by means of the crystal facet effect,the p-block metal oxide SnO₂,which has better adsorption performance than the d-block metal oxide Fe₂O₃,is used as a model to study the mechanism of crystal facet effect in Li-S electrochemistry.Two kinds of SnO₂ nanocrystals/reduced graphene oxide composites（SnO₂{332}-G and SnO₂{111}-G）were synthesized with the assistance of the structure-directing agents.The two kinds of SnO₂ nanocrystals are SnO₂ nano-octahedrons with{332}and{111}facets,respectively.Compared with the{111}facets,the SnO₂{332}facets with more coordination-unsaturated Sn sites not only have a stronger binding ability with Li PSs,but also reduce the decomposition energy barrier of Li₂S more effectively.Therefore,SnO₂{332}-G not only inhibit the shuttle of Li PSs more effectively,but also catalyze more efficiently the reaction kinetics of sulfur species conversion,thereby significantly improving the electrochemical performance of Li-S batteries.The batteries achieve2000 charge-discharge cycles at 2 C,and the average capacity decay rate is as low as0.021%per cycle.On the basis of the research works in the previous two chapters,the influence of the exposed crystal facets of the carriers on the interface structures formed by heterogeneous material loading growth is further explored,and the impact of catalytic materials with different heterointerfaces on the electrochemical performance of Li-S batteries was investigated.The Fe₂O₃-CeO₂ composite nanoparticles with high-energy interfaces were prepared by growing CeO₂ nanocrystals on the surface of Fe₂O₃octadecahedrons with high-index{113}and{104}crystal facets via a two-step growth method.The results show strong interfacial electron transfer at the high-energy Fe₂O₃-CeO₂ interfaces.Benefiting from the formation of abundant high-energy heterointerfaces with strong interfacial interactions,the Fe₂O₃-CeO₂ composite nanoparticles exhibit strong adsorbability for Li PSs and high catalytic performance for the conversion of sulfur species.The batteries still have a reversible capacity of 546mAh g^-1 after 2000 charge-discharge cycles at 2 C,and the average capacity attenuation rate of a single cycle is only 0.016%.Even with a high sulfur loading of 10.17 mg cm^-2,the batteries still exhibit an areal capacity of 8.65 mAh cm^-2 after 200 cycles.In compared with crystal facet engineering and interface engineering,strain engineering can achieve efficient and accurate control on material surface characteristics by finely adjusting the spacings of atoms on the surface of materials.FeOOH nanorods are fabricated via hydrothermal synthesis reaction,and then thermally treated at different temperatures to obtain FeOOH catalysts with different compressive strains.Experiments and theoretical calculations validate that the lattice compression strain can weaken the adsorption capability of the catalytic material for Li PSs,and the appropriate compressive strain makes FeOOH catalysts exhibit the moderate adsorption capability for Li PSs,which is conducive to optimize their catalytic effects.The FeOOH catalytic material with 4.2%lattice compressive strain effectively anchor Li PSs and catalyze the interconversion of sulfur species.The assembled Li-S batteries accomplish3000 charge-discharge cycles at 2 C,and the average capacity decay rate of each cycle is as low as 0.013%.Under the condition of lean electrolyte with E/S ratio of 4.6μL mg^-1,the batteries with sulfur loading of 9.17 mg cm^-2 retain an areal capacity of 7.17mAh cm^-2 after 70 cycles.更多还原