新型多级孔碳微球的宏量可控制备、锂硫电池性能及固硫机制的研究

The lithium-sulfur battery is considered to be one of the most attractive rechargeable batteries due to its high energy density and capacity. The poor rate capability and cycling stability caused by the poor conductive properties of the cathode materials and the lithium polysulfide (especially S4-8 ions) being dissolved and lost in the electrolyte have become the “bottleneck” in the development of lithium-sulfur batteries. Herein, we present a rational design and large-scale synthesis of novel graphene-based heteroatoms doped carbon microspheres with hierarchical porous structure by spray drying method and followed by carbonizing under elevated temperature. Such carbon microspheres encapsulating sulfur are used as cathode materials of lithium-sulfur battery with several aims in mind: 1) heteroatom doping enhancing the chemical adsorption of polysulfides; 2) hierarchical porous structure minimize lithium polysulfides dissolution in the electrolyte and preserving fast transport of lithium ions to the coated sulfur; 3) the micropores from the carbonization of the graphene and oligosaccharide composites restraining the formation of S4-8 ions and facilitating good transport of electrons from the poorly conducting sulfur, which synergistically improve its performance including capacity, rate capability and cycling stability. Exploring the key factors affecting their structure of the carbon microspheres is to realize their controlled large-scale synthesis. Detailed research being devoted to increase the electrochemical performance by modifying the porous structure of carbon microspheres, the heteroatoms doping, the content of graphene and sulfur, etc. is to establish well the relationship between the structure of presented materials and electrochemical performance. Studying on the diffusion and migration behavior of polysulfides being affected by the porous structure and the different doping atoms of carbon microspheres is conducted. Investigating the electrochemical reaction process under different stages of the charge or recharge, and calculating the interaction energy between the oxygen or other atom of the presented carbon materials and sulfur atom by Density Functional Theory (DFT) are to effectively reveal the sulfur retention mechanism of presented materials. The presented research will provide experience for the design and development of cathode materials with more excellent performance for lithium-sulfur batteries.

锂硫电池作为一种新型二次电池，具有高容量、高能量密度等优点。但存在硫的导电性差、多硫化锂（特别是S4-8离子）易溶解、流失等原因导致电池倍率性能和循环稳定性差等缺陷，这大大限制了其广泛应用。为此，本项目拟采用喷雾干燥法联合碳化手段，简便且宏量可控制备新型杂原子掺杂的石墨烯基多级孔碳微球，并组装硫应用于锂硫电池正极。利用掺杂杂原子增强对硫化物的化学吸附、多级孔结构对硫化物溶解的多级抑制、石墨烯和低聚糖复合物碳化形成的微孔限制S4-8离子生成，协同提高电池倍率性能和循环稳定性。探讨影响多级孔碳微球结构的关键因素，实现可控制备；研究杂原子掺杂、孔结构、石墨烯、微球尺寸等因素对电池性能和多硫化物的扩散、迁移行为的影响，揭示电池倍率性能和循环稳定性等性能与微观结构的关系；研究正极材料在充放电过程的电化学行为，结合密度泛函理论计算，揭示杂原子掺杂碳微球的固硫机制，为设计开发高性能锂硫电池提供指导。

锂硫二次电池具有高容量、高能量密度等优点而备受关注。但仍存在硫的导电性差、多硫化锂易溶解、流失等原因导致电池倍率性能和循环稳定性差等缺陷，大大限制了其广泛地应用。因此，本项目设计并可控合成一系列新颖纳米结构碳作为硫载体应用于锂硫电池正极，提升锂硫电池性能。详细探究实验参数对碳材料结构的影响，研究微观结构对电池性能的影响，以及探究充放电过程中硫中间价态的变化，从而揭示材料微观结构与性能之间的关系，筛选优化出性能优异的锂硫电池正极材料。具体研究如下： 1）发展一种喷雾干燥法联合碳化手段，简便地制备多孔杂原子掺杂石墨烯基碳微球作为硫载体。该碳硫复合材料作为电池正极表现出了较好性能；2）设计并合成大比表面积花型结构碳球担载硫作为正极材料，优化后电池表现出很好的性能。制备过程中引入氨气，同时实现对花型碳球的氮掺杂和造孔，可进一步提升以该材料作为正极的电池容量和循环稳定性；3）利用壳聚糖作为氮和碳源直接合成氮掺杂碳纳米花作为硫载体应用于电池正极，结果表明该系列材料同样表现出高的容量 (1633 mAh/g 0.2C), 好的倍率性能 (916 mAh/g 5C)及优异的循环稳定性（1C循环500圈，每圈衰减率为0.07%）,更有意思的是回收的正极碳材料在燃料电池阴极氧还原中表现出与Pt/C接近的性能；4）双模板法构建新颖结构石墨烯基三明治型分级多孔结构杂化碳纳米片，并以此碳片装载较高含量硫而应用于锂硫电池正极。利用的夹心层石墨烯、薄碳纳米片、大孔容以及丰富微介孔等结构特点的协同效应，提升锂硫二次电池性能。相关研究成果以项目负责人为第一或通讯作者发表学术论文6 篇，其中影响因子大于4的5篇，被引用70次；申请发明专利2项，已授权1项；在国际会议和国内会议分别做口头报告1 次，培养硕士研究生3名和本科生多名。相关研究成果曾在浙江省自然科学基金委、浙江日报等网站上报道。