基于固-固反应机制的硫电极及其在锂/硫电池中的应用

Developing advanced rechargeable batteries with substantially enhanced energy density and greatly reduced cost are in great demand for widespread electric storage applications, such as zero-emission electric vehicles and renewable power stations. In this technology development, lithium-sulfur (Li-S) batteries have attracted particular attention as a promising candidate for next generation energy storage devices because of their high theoretical energy density, wide availability of sulfur resources, all of which offer competitive advantages for large scale applications. However, realizing a practical Li–S battery is still difficult, primarily due to the poor cycleability of sulfur cathode, which is caused by the dissolving loss of lithium polysulfides generated as intermediates during cycling in the electrolyte. To solve this problem, a new strategy to develop high performance sulfur cathode is proposed in this project by embedding sulfur into the interior micropores of a Li+-insertable carbon matrix with the simultaneous use of a carbonate electrolyte, which can occur nucleophilic reaction with polysulfide intermediates to form an insoluble film on the surface of S/C composite, so as to prevent the direct contact of embedded sulfur with bulk electrolyte. In such a case, Li-ions and electrons transport through the carbon matrix into the interior of the cathode and then react with the embedded sulfur in the S/C solid–solid interfaces, thus avoiding the dissolution loss of polysulfide intermediates into the electrolyte. Based on our previous work, this project aims at designing and synthesizing porous carbon matrix, which has required Li-ion insertion potential and abundant interior micropores structure, optimizing the preparation condition for S/C composite, investigating the relationship between structure and performance, and characterizing the application performance of S/C cathode in Li-S batteries. The structural design and working mechanism of such a sulfur cathode could be extended to a variety of poorly conductive and easily soluble redox-active materials for battery applications.

开发低成本、长寿命的高比能二次电池是电动汽车、储能电站等新能源产业发展的关键。锂/硫二次电池具有理论比能量高及硫电极资源丰富的特点，是新体系二次电池的研究热点。然而，因反应中间产物多硫化锂溶解流失而导致的循环稳定性问题严重制约了锂/硫二次电池的实用化进程。为此，本项目提出构建固-固反应机制硫电极，发展不涉及中间产物溶解流失的锂/硫电池的新思路。其原理为：采用具有合适嵌锂电位及丰富体相微孔结构的多孔碳分散负载硫，同时配合使用有机碳酸酯类电解液，利用有机碳酸酯与多硫离子之间因存在亲核反应易在硫电极表面成膜的特征，隔离电解液与体相硫，使得锂离子仅能通过导电碳以固相传输形式到达硫/碳界面，参与硫的电化学反应，以此避免中间产物的溶解流失。项目拟在前期工作基础上，设计合成出能够高度满足原理要求的高孔容微孔碳，优化制备固-固反应硫电极，研究其结构与性能的关联性，以及其在锂/硫电池中的应用性能。

目前，开发低成本、长寿命的高比能二次电池是电动汽车、储能电站等新能源产业发展的关键。锂/硫二次电池具有理论比能量高及硫电极资源丰富的特点，是新体系二次电池的研究热点。然而，因反应中间产物多硫化锂溶解流失而导致的循环稳定性问题严重制约了锂/硫二次电池的实用化进程。为此，本项目提出构建固-固反应机制硫电极，发展不涉及中间产物溶解流失的锂/硫电池的新思路。实现该思路的关键是寻找满足固相反应原理要求的导电碳材料，不仅需具有丰富的体相微纳孔隙结构，而且应具备合适的嵌锂电势及容量，从而保证能为包埋在其中的纳米硫颗粒反应提供足量的锂离子。项目在前期工作基础上，设计并合成出2~3种兼具以上两种特征的碳材料，通过气相转移方式制备了相应的固-固反应硫电极，表征了碳材料和硫碳复合物的结构和形貌，测试了碳材料的嵌锂特性以及硫电极的电化学性能，分析了电极性能与结构的关联性以及其在锂/硫电池中的应用性能。其中，所制备的聚丙烯腈热解碳/BP2000复合碳材料是一种在1.0~3.0V（Vs. Li+/Li）电压范围内具有嵌锂特性的多孔碳，以其为载体的聚丙烯腈/硫复合电极，在100mAg-1的电流密度下，首周放电比容量达948mAhg-1（按复合材料的总质量计算），循环至300周，电极放电比容量降低为661mAhg-1，电极容量保持率为72.4%，与采用不具备嵌锂性能的多孔碳为载体的聚丙烯腈/硫复合电极进行对比，其比容量和稳定性均有明显提高。由此验证了固-固反应机制硫电极的构建原理，明确了利用具有高电位嵌锂性能的体相微孔碳材料分散负载硫，提高硫电极性能的实际可行性，为发展高循环稳定性硫电极提供新原理和新方向。