CAS OpenIR
金属有机框架衍生的碳基复合材料在锂电池中的应用
方大量
Subtype博士
Thesis Advisor陈仕谋
2019-07-01
Degree Grantor中国科学院大学
Degree Discipline材料学
Keyword锂离子电池,锂硫电池,金属有机框架,碳基复合材料
Abstract

近年来,随着电动汽车行业的快速发展,工业界对锂电池的能量密度提出了更高要求。以石墨为负极、以钴酸锂为正极的传统锂离子电池,由于其有限的能量密度,已经难以满足要求。因此,开发具有更高能量密度的锂离子电池或新电池体系(如锂硫电池)是锂电池未来的主要发展方向。作为锂电池的关键组成部分,电极和隔膜材料对锂电池的整体性能都起到了决定性的影响,所以高能量密度锂电池的研发重心在于电极和隔膜材料的设计和制备。金属有机骨架(MOFs)衍生的碳基复合材料作为锂电池电极或隔膜修饰材料,具有较轻的质量、良好的电子和离子传导性、丰富的孔道和均匀分布的活性位点等优势。此外,这类碳基复合材料还具备形貌和组分可控、来源丰富和孔径可调等特性便于机理研究,因此近年来得到了广泛关注。本论文主要针对具有高能量密度锂离子和锂硫电池的关键问题,以MOFs为前驱体设计和制备了一系列MOFs衍生的新型碳基复合材料,并作为锂离子和锂硫电池的电极或隔膜修饰层去研究它们对电池电化学性能的影响。同时通过DFT计算、原位透射电镜(TEM)等手段深入揭示结构、组分与性能之间的关系。具体的工作如下:1、针对高能量密度负极材料的体积膨胀问题,受自然界中的“板块运动”现象启发提出了“滑移”策略来实现负极材料在高振实密度和低空隙的条件下还能具有优异的电化学性能:通过高温硫化ZIF-7纳米片前驱体,制备出了具有高振实密度(0.86 g cm-3)和低总孔体积(0.092 cm-3 g-1)的ZnS-QDs@mNC复合材料。通过原位TEM表征证实了,ZnS-QDs@mNC在嵌锂过程中,构成它的碳纳片之间会发生相对位置的滑动。这种特殊的“滑移”行为能充分地利用纳米片之间有限空隙来容纳ZnS-QDs@mNC的体积膨胀,从而显著地减小其体积变化(完全嵌锂后体积仅增大了6.5%)。因此,ZnS-QDs@mNC复合材料作为锂离子电池负极材料具有优异的长循环稳定性和倍率性能,即使在2.76 mg cm-2的高负载下依旧能稳定循环。该工作为如何在高振实密度、低空隙率的条件下还能实现高能量密度负极材料的长循环稳定性,提供了一种新思路。2、针对高能量密度负极材料体积膨胀和导电性差的问题,提出了纳米空心结构结合双碳层包覆的设计方案:以ZIF-67为前驱体,制备出了双碳层包覆空心Co3O4纳米颗粒的H-Co3O4/NC@C负极材料。纳米尺度的Co3O4颗粒有利于缩短锂离子的传输距离,而其空心结构能有效地缓冲嵌脱锂过程中产生的体积变化。双碳层包覆不仅能增强材料的导电性,而且还有助于维持稳定的结构:内层的多孔碳能很好地容纳Co3O4的体积变化而外层致密的碳壳有利于在其表面形成稳定的SEI膜。因此,H-Co3O4/NC@C作为锂离子电池的负极材料具有优异的倍率性能和长循环稳定性。3、针对锂硫电池中多硫化物的穿梭效应,受自然界中蜘蛛网捕捉昆虫原理的启发提出了物理阻隔和化学作用相结合的策略:以ZIF-67为前驱体制备了一种类蜘蛛网结构的纳米复合材料(Co/mSiO2-NCNTs),该材料由空心的介孔二氧化硅(mSiO2)纳米球、Co纳米颗粒和贯穿二者的相互交织的氮掺杂碳纳米管(NCNTs)网络所构成。将Co/mSiO2-NCNTs修饰到商业隔膜上,然后运用到锂硫电池中发现它能像蜘蛛网捕捉昆虫一样结合物理和化学作用来有效捕捉和利用多硫化物,从而很好地抑制了其穿梭效应。此外,结合实验和分子动力学模拟证实了多硫化物在mSiO2球表面上是可逆吸脱附的。这种可逆吸脱附现象不仅能有效地抑制穿梭效应,还有助于活性物质的充分利用。并且Co/mSiO2-NCNTs中的Co和N杂原子对多硫化物的转化起到了协同催化作用,有助于加快多硫化物的反应速率。因此,相比于普通隔膜,Co/mSiO2-NCNTs修饰的隔膜能显著提升锂硫电池的循环和倍率性能。该工作证明了合理的结构设计和组分调控是解决多硫化物穿梭效应的一种有效途径。4、针对锂硫电池高负载贫电解液的条件下硫利用率低的问题,提出协同调节多硫化物转化和沉积的策略:以长在碳布纤维上的Co-MOFs纳米片阵列为前驱体,经过一步高温煅烧转化成Co,N掺杂的具有多级有序结构的碳基复合材料(Co, N-CNTs-CNS/CFC)。丰富的活性位点和快速的电子/离子传输通道使得Co, N-CNTs-CNS/CFC可以高效地锚定和转化多硫化物。通过理论计算和实验证实了Co和N的掺杂能共同增强该材料对多硫化物的吸附作用从而加快多硫化物的转化。并且特殊的多级结构还能实现硫化锂的三维(3D)沉积和生长,显著提高硫的利用率。因此,负载上一定量多硫化物后的Co, N-CNTs-CNS/CFC作为锂硫正极材料具有优异的长循环稳定性和倍率性能。甚至当硫的负载量高达10.20 mg cm-2且电解液与硫的比值(E/S)低至6.94 μL mg-1时,还能达到7.42 mA h cm-2的可逆容量且循环稳定。该工作为如何在高负载且贫电解液的条件下还能实现较高活性物质利用率提供了一种新的解决方案。;Recently, the rapid development of electric automobile industry proposes higher requirements on the energy density, rate performance and cycling life of lithium batteries. Due to its limited energy density, the conventional lithium-ion batteries with graphite as anodes and LiCoO2 as cathodes can not meet these requirements. Therefore, it is necessary to exploit new high-energy-density anode materials or even new battery systems, such as lithium sulfur batteries. As key parts of lithium batteries, both electrodes and separators have great influence on the whole properties of lithium batteries, thus, to develop high-energy-density lithium batteries should mainly focus on the design and fabrication of electrode and separator materials. MOF-derived carbon-based composites possess the following advantages such as light weight, excellent electron/ion conductivity, aboundant pores and even-dispersed active sites. In addition, due to their controllable morphologies and components, abundant sources and adjustable pore sizes, these carbon-based composites are also suitable for mechanism researches, thus causing extensive attentions recent years. Aiming at the key problems of lithium-ion and lithium sulfur batteries, this dissertation design and fabricate a series of new MOFs-derived carbon-based composites by using MOFs as precursors and use them as electrodes or separator coating layers in lithium batteries to investigate their effects on the electrochemical properties. Furthermore, the relationship between structure/component and properties is also uncovered by using advanced methods, such as theoretical calculation and in situ TEM. The detail works are as follows:1、Aiming at the large-volume-change problem of high-energy-density anode materials, inspired by the geological plate-movement in nature, a “slippage” strategy was proposed to achieve excellent electrochemical properties of anode materials with low porosities and high tap densities: ZnS-QDs@mNC with a high tap denstiy of 0.86 g cm-3 and low total pore volume of 0.092 cm-3 g-1 was fabricated by sulfurization of ZIF-7 as precursors under high temperature. It is demonstrated that the carbon nanosheets of ZnS-QDs@mNC can slide against each other during lithiation process via in situ TEM. The slippage behaviour can make full use of the limited gaps between the carbon nanosheets to reduce the volume expansion of ZnS-QDs@mNC significantly (the volume expansion is only 6.5% after full lithiation). Therefore, as anodes of lithium-ion batteries, ZnS-QDs@mNC possesses excellent cycling stability and rate performance. Even at a high loading of 2.76 mg cm-2, cycling stability can still be achieved. This work provide a novel approach to achieve long-term cycling stability of high-energy-density anode materials with high tap densities and low porosities.2、Aiming at the large volume expansion and poor electrical conductivity of high-energy-density anode materials, the design of hollow nano-structure with double carbon coating was provided: H-Co3O4/NC@C composed of hollow nano-Co3O4 coated by double carbon layers was developed by using ZIF-67 as precusors. The hollow nano-Co3O4 particles are beneficial for reducing lithium-ion pathway distances and relasing volume-change stress. The double carbon coating can not only improve electron conductivity, but also strengthen the structure stablity: the interior porous carbon layer effectively accommodates the volume change, and the outside compact carbon layer benefits the formation of stable SEI. Therefore, as anodes of lithium ion batteries, H-Co3O4/NC@C has excellent rate performance and cycling stability.3、Aiming at the shuttle effect of polysulfides in lithium sulfur batteries, inspired by the mechnism behind that natural spider webs can effectively capture insects, we proposed the combination of physical confinemnt and chemical interaction: a spider-web-like composite Co/mSiO2-NCNTs, composed of the hollow mSiO2 nanospheres/Co nanoparticles threaded by interwoven NCNTs, was fabricated by calcination of ZIF-67 as precusors. When coated on commercial separators and then used in Li-S batteries, Co/mSiO2-NCNTs can effectively capture and reutilize polysulfides through both physical and chemical interaction as natural spider webs catch insects. In additon, the combination of experiments and molecular dynamics simulation demonstrates the reversible adsorption/desorption of polysulfides on the mSiO2. Such behavior can not only inhibit the shuttle effect, but also contribute to the full utilization of active materials. Furthermore, Co and N doping of Co/mSiO2-NCNTs perform a synergistic effect on the conversion of polysulfides, which contritbutes to accelerate the reaction rate of polysulfides. Therefore, compared with the commercial separator, Co/mSiO2-NCNTs coated separator can significantly improve long-term cycling stability and rate performance of lithium sulfur batteries. This work comfirms that the combination of rational structure design and component arrangement is an effective approach to solve the shuttle effect problem.4、Aiming at the low sulfur utilization at high sulfur loading with lean eletrolyte, the synergistic regulation of polysulfides conversion and depositon was proposed: by using Co-MOFs nanosheets (CNSs) arrays grown on the carbon fiber cloth (CFC) as precursors, a Co, N doped carbon-based composite (Co, N-CNTs-CNS/CFC) with a hierarchical structure was fabricated via one-step calcination. Possessing abundant active sites and fast electron/ion pathways makes Co, N-CNTs-CNS/CFC can effectively archor and convert polysulfides. The combination of experiment and DFT calculation confirms that both Co and N doping can enhance the interaction between Co, N-CNTs-CNS/CFC and polysulfides, thus accelerating the reaction kinetics of polysulfides. Besides, the special hierarchical structure can induce 3D Li2S depostion, obviously improving the sulfur utilization. Thus, after loaded with certain polysulfides, Co, N-CNTs-CNS/CFC as sulfur cathodes, exhibits outstanding cycling stability and rate performance. Even when the sulfur loading is increased to 10.20 mg cm-2 with the low electrolty/sulfur (E/S) of 6.94 μL mg-1,cycling stability can still be maintained with a high areal capacity of 7.42 mA h cm-2. This work provides a new approach to achive high sulfur utilization under high sulfur loading with lean electrolyte. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40647
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
方大量. 金属有机框架衍生的碳基复合材料在锂电池中的应用[D]. 中国科学院大学,2019.
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