新型"豆荚式"碳纳米花@碳纤维自支撑膜的设计制备及在高硫载量锂硫电池中的应用研究

To solve the defects of low specific capacity and poor stability etc. for high sulfur loading density lithium sulfur batteries, the novel "pods" carbon nanoflower @ carbon fiber free-standing membrane is designed and fabricated by coaxial electrospinning and used as a sulfur host with a large pore volume and high conductivity of cathode, strong adsorption of polysulfide ions. The influence of changing parameters of the precursor types (such as N-doped polymer, molybdenum compound / polymer composites etc.) and the corresponding concentration, spinning parameters, carbonization process etc. on the structure and chemical composition of carbon nanoflower core and carbon shell, and cavity size of “pod” fiber is studied to establish a regulation mechanism for “pod” structure. The relationship between fiber structure and electrical properties, mechanical properties and sulfur loading of free-standing membrane is discussed. The influence of sulfur loading density, mechanical strength and conductivity of free-standing membrane on capacity, rate capability and cycle stability is investigated. The influence of the structure, chemical composition and size of the “pod” carbon fiber on the dispersing of Li2S, the sulfur loading and the migration of polysulfide ions is also investigated in detail. The valence state of sulfur ion in charge and discharge process is studied in situ XRD technology. Combined with quantitative calculation, the sulfur retention mechanism and synergistic effect of cavity limiting sulfur, the "physical chemical" multiple adsorption for carbon core and shell are revealed. The presented research will provide useful inspiration for the design and development of cathode materials for high energy density lithium sulfur batteries with long cycling stability and excellent rate capability.

为解决高载硫面密度锂硫电池比容量低、循环稳定性差等缺陷，本项目拟基于同轴静电纺丝技术可控制备“豆荚式”碳纳米花@碳纤维自支撑膜，并作为大孔容、高导电、强吸附多硫离子的正极固硫载体。研究前驱体类型（如含氮高分子、钼基化合物/高分子等）及浓度、纺丝参数、碳化工艺等对豆荚内核碳纳米花和外壳碳层的结构、化学组成及空腔尺寸的影响规律，建立豆荚结构的调控机制。探讨纤维结构与自支撑膜电学、力学性质及载硫量的关系，考察硫担载面密度、自支撑膜机械强度和导电性对电池容量、倍率及循环稳定性的影响规律。深入研究豆荚式碳纤维结构、化学组成及尺寸对硫化锂分散、载硫量和束缚多硫离子溶解迁移的影响本质，借助原位XRD技术研究充放电过程中硫离子价态变化，并结合量化计算，阐明空腔载硫、碳内核与壳层“物理-化学”多重吸附的固硫机制和协同效应。上述研究成果将为长寿命、高倍率、高能量密度锂硫电池正极材料的结构设计提供有益的启示。

锂硫二次电池具有高容量、高能量密度等优势，但仍存在硫导电性差、多硫离子易流失、锂硫反应动力学缓慢原因导致电池倍率性能与循环稳定差等缺陷，且在高载硫面密度时问题尤其突出。为此，本项目设计并可控合成一系列新型纳米结构碳材料及功能化碳，应用于锂硫二次电池正极或者插层膜中，大幅提升锂硫电池性能。详细探究试验参数对碳材料结构的影响，研究微观结构与电池性能的关联，深入探究充放电过程中锂硫反应动力学问题，结合理论计算、仿真模拟等手段，揭示构效关系、固硫机制以及多硫离子催化转化机制等，获得系列性能优异的锂硫电池正极材料，为高性能锂硫电池正极的设计提供有益的启示。具体研究内容如下：1）采用同轴静电纺丝技术可控构筑出“豆荚式”结构碳纳米花@碳纤维自支撑膜，并作为大孔容、高导电、强吸附多硫离子的正极固硫载体，并通过有限元模拟仿真揭示结构与性能关系，以实现纯碳基材料的高能量密度。当面积载硫量增加到13.4 mg·cm-2时，电极电池的表面容量可达15.5 mAh·cm-2并保持较好的容量保持率；2）通过对中空碳球内部及多孔碳材料的理性设计、制备显著改善硫的导电性差及利用率低的问题，从而提高锂硫电池稳定性；3）将单原子钼以四个氮原子配位形式锚定在花型碳纳米球上，实现对多硫化物的有效吸附、增强锂离子扩散、促进多硫化物催化转化，从而有效抑制多硫化物穿梭效应；4）开发多种改性碳基纳米材料作为先进锂硫电池的功能层。能够快速将液相多硫化锂催化转化固相硫化锂，从而有效抑制多硫化物穿梭效应，同时能够诱导锂负极表面形成稳定的固体电解液中间相层，进而减少锂负极枝晶的形成；5）受多硫离子催化转化的概念，设计构建多种元素及其化合物等掺杂进碳基复合纳米材料作为电解水催化剂，揭示其电催化的反应机理。