锂硫电池用单离子聚合物基功能化改性隔膜的构筑及多硫阴离子穿梭效应抑制研究

Due to its high energy density, lithium sulfur battery has become a hot research topic in chemical energy storage. However, several issues including low sulfur utilization, rapid capacity decay and low coulombic efficiency have severely impeded its practical application. In this regard, a functional composite membrane based on the one side decoration of the commercial Celgard membrane was proposed using sp3 hybridized B (or Al) based single-ion polymer electrolytes and conductive carbon materials. The functional composite membrane play two roles: one is to evenly distribute the single-ion polymer electrolytes (i.e. negative charge) using the carbon materials as host, avoiding the polysulfide shuttle effect through “charge repulsion”. The other is to reduce the internal impedance of the cathode side through the addition of highly conductive carbon materials and function as upper current collector to reuse sulfur species. Besides, sp3 B based electrolytes are beneficial to the formation of stable SEI layer. Consequently, the interphase between cathode and separator could be improved. This project intends to synthesize a series of sp3 B (or Al) based single-ion polymer electrolytes, to systemically study the effect of molecular structure of single-ion polymer electrolytes (including the main chain structure, the side chain structure or different pore size), the morphology of carbon materials and the nature of binders on the compositions, microstructure and the suppression of polysulfide “shuttle effect” of the interlayer. Then the composite membrane will be applied in real Li-S cells, to study the relationship between the derivation of the composite separator during cycling and Coulombic efficiency as well as discharge capacity, elucidating the interaction mechanism between single-ion polymer electrolytes and polysulfide. This project will provide new materials and methodology to address the shuttle issues of polysulfide and optimize the interphase between cathode and separator of Li-S battery. Therefore, it is of importance to both fundamental research and practical development of Li-S battery technology.

锂硫电池能量密度高，是储能领域研究的热点。但其存在硫利用率低、容量衰减快、库伦效率低等问题。申请人拟构建新型sp3硼基(铝基)单离子聚合物与导电碳对商业化隔膜进行改性，以碳材料为载体实现单离子聚合物（即电荷）的均匀分布，利用静电排斥效应抑制多硫阴离子的穿梭，同时利用碳材料的导电性降低正极侧界面电阻、实现含硫组分的回收利用，而且sp3硼基电解质促进固体电解质界面的形成，改善正极隔膜界面。本项目通过设计、合成一系列sp3硼基(铝基)单离子聚合物，系统地研究聚合物分子结构（主链、支链、孔径）、碳材料形貌、粘结剂性质对隔膜结构、透气度、微观形貌及多硫化物穿梭效应抑制的作用，进而研究循环中复合隔膜结构演变与锂硫电池库伦效率、放电容量的关系，阐明单离子聚合物与多硫化物的作用机制。该项目对解决多硫阴离子的穿梭和正极隔膜界面的改性提供新材料和新思路，期望对锂硫电池产业化应用具有指导价值。

基于单质硫磺S8作为活性正极材料的锂硫电池在放电过程中会产生多硫化锂，当与醚类电解质相匹配组装电池时，长链多硫化锂会发生溶解，继而在浓度梯度的作用下发生迁移，从正极侧透过商业化隔膜到达负极侧，继而被金属锂还原成短链多硫化锂。而部分短链多硫化锂在充电过程中受到电场作用会再次穿过隔膜回到正极侧，被氧化成长链多硫化锂，这样的过程称为穿梭效应，不仅导致锂硫电池库伦效率减小，而且导致实际的放电容量持续减少。为了解决上述问题，项目利用单离子聚合物电解质和不同形貌的碳材料共同对商业化隔膜进行修饰，通过“静电效应”和物理吸附共同作用抑制穿梭效应，提升锂硫电池性能。本项目研究内容分为以下几部分：第一，单离子聚合物电解质的制备与表征。先制备具有稳定结构、含有芳香环的高分子骨架，然后在高分子骨架上引入磺酸根，进而锂化形成锂离子传导基团。第二，碳材料的制备与修饰。通过调节金属与配体关系形成配合物，进而碳化制备多孔碳或者通过硬模板法再脱模板制备多孔碳。使用合成的单离子聚合物电解质和碳材料通过粘结剂对商业化隔膜正极侧进行修饰。第三，研究修饰后的隔膜对多硫化锂的阻隔作用机制。组装H型电解池，直接通过颜色变化直观的判断修饰隔膜对于多硫化锂的阻隔作用，进而对单离子聚合物以及碳材料的结构进行筛选。第四，组装扣式电池进行测试。利用电化学工作站测试电池循环性能、倍率性能以及电化学阻抗。研究表明单离子聚合物电解质和碳材料共同修饰隔膜可以有效抑制多硫化物的穿梭，碳材料的引入不仅提升正极侧的电子传导，降低了电池的阻抗，而且可以作为集流体提升硫的利用率，利于倍率性能和循环性能的提升。本项目的研究成果为锂硫电池的进一步产业化提供了理论和实验基础。