锂硫电池用碳材料功能化设计及电化学性能研究

发布时间：2018-07-23 19:36

【摘要】：锂硫电池具有理论能量密度高、环境友好、成本低廉等优点,但仍存在活性物质利用率低、循环稳定性和安全性差等问题,成为制约其实用化的瓶颈。本论文以碳质材料为基础,从碳材料导电网络的构建和限域效应出发,结合孔结构调控、异质原子掺杂与表面化学修饰,并从锂硫电池的整体结构设计出发,制备出具有高能量密度、长循环寿命的新型锂硫电池。本论文采用L-抗坏血酸为还原剂,在温和条件下,一步还原-诱导自组装制备出具有三维相互连通结构的石墨烯-碳纳米管-硫复合材料。在液相自组装过程中,石墨烯-碳纳米管导电网络的形成伴随着纳米硫颗粒的均匀负载。这种相互连通的导电网络为电子和锂离子提供了快速的传输通道,同时也能容忍充放电过程中硫电极的体积膨胀。此外,复合材料中残留的含氧官能团能与硫形成强的化学键合。因此,在较高放电倍率1C下,经过450圈循环后容量仍能维持在657 mAh g~(-1),表现出良好的循环稳定性。在碳材料中采用氮、硫等异质原子掺杂,可以起到调控碳材料性质,提高碳与硫之间的相互作用。本论文采用L-半胱氨酸作为掺杂剂和结构调控剂,葡萄糖为碳源,通过水热处理和KOH活化,制备出具有高比表面积和层次孔结构的N,S双掺杂多孔碳微球,结合层次孔的物理限域和表面化学键合作用,增强了硫与碳基体之间的化学亲和力。为了进一步提高电子和离子的传输,本文采用氧化石墨烯作为模板,葡萄糖为碳源,吡咯为氮源,制备了一种具有三明治结构的石墨烯基氮掺杂多孔碳纳米片。石墨烯可以确保快速的电子和锂离子的传输,而两侧薄的氮掺杂多孔碳可以容纳硫并限制多硫化物的溶解。在2C倍率下,该碳-硫正极的首次放电容量可达625 mA h g~(-1),经过200圈循环后容量仍有461 mAh g~(-1),每圈的容量损失仅为0.13%。从锂硫电池整体结构设计出发,采用静电喷涂的方法在碳-硫电极表面均匀涂覆了一层厚度薄、质量轻的多功能涂层。该复合涂层可以通过多孔碳的物理限域与导电聚合物与多硫化物间形成的化学键合的协同作用,有效地限制了多硫化物的溶解,并能够重新利用捕捉到的多硫化物,提高了活性物质的利用率,从而提高了锂硫电池的倍率性能和循环寿命。
[Abstract]:Lithium-sulfur batteries have the advantages of high theoretical energy density, environmental friendliness and low cost, but there are still some problems such as low utilization of active substances, poor cycle stability and safety, etc. In this paper, based on carbon materials, the structure of carbon conductive network and its limiting effect are discussed, and the structure of lithium-sulfur battery is designed, which is based on the pore structure control, heterogeneity atom doping and surface chemical modification, as well as the overall structure design of lithium-sulfur battery. A new type of lithium sulfur battery with high energy density and long cycle life was prepared. In this paper, graphene-carbon nanotube-sulfur composites with three-dimensional interconnected structure were prepared by one step reduction-induced self-assembly under mild conditions using L- ascorbic acid as reducing agent. During liquid phase self-assembly, the formation of graphene-carbon nanotube conductive network is accompanied by uniform loading of nano-sulfur particles. This interconnected conductive network provides a fast transport channel for electrons and lithium ions and can also tolerate the volume expansion of sulfur electrodes during charge and discharge. In addition, the residual oxygen functional groups in the composites can form strong chemical bonding with sulfur. Therefore, at a high discharge rate of 1C, the capacity can still be maintained at 657 mAh g ~ (-1) after 450 cycles, showing good cyclic stability. The addition of heterogeneous atoms such as nitrogen and sulfur in carbon materials can regulate the properties of carbon materials and improve the interaction between carbon and sulfur. L- cysteine (L- cysteine) was used as dopant and structural regulator, glucose was used as carbon source, and high specific surface area and layered pore structure were obtained by hydrothermal treatment and KOH activation. The chemical affinity between sulfur and carbon matrix is enhanced by combining the physical limit of the layered pore and the surface chemical bond cooperation. In order to further improve the transport of electrons and ions, a graphene oxide nitrogen-doped porous carbon nanochip with sandwich structure was prepared by using graphene oxide as template, glucose as carbon source and pyrrole as nitrogen source. Graphene can ensure rapid electron and lithium transport while thin nitrogen doped porous carbon on both sides can contain sulfur and limit the dissolution of polysulfide. At 2C rate, the initial discharge capacity of the carbon-sulfur positive electrode can reach 625 Ma h g ~ (-1). After 200 cycles, the capacity is still 461 mAh g ~ (-1), and the capacity loss per cycle is only 0.13%. Based on the overall structure design of lithium-sulfur battery, a thin and light multifunctional coating was coated on the surface of carbon-sulfur electrode by electrostatic spraying. The composite coating can effectively limit the dissolution of polysulphides and reuse trapped polysulphides through the synergism of the physical limits of porous carbon with the chemical bonding between conducting polymers and polysulphides. The utilization ratio of active substances is improved, and the ratio performance and cycle life of lithium-sulfur battery are improved.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TM912;TQ127.11

【相似文献】