具有表面修饰层的硫碳复合正极及其储能机理研究

Lithium sulfur battery is a kind of very promising energy storage device for its high energy density. However, many problems including poor conductivity of sulfur and its reduction product, formation of polysulfide and subsequent shuttling effect during charge-discharge process will lead to the irreversible loss of active material, and then finally result in poor cycle performance and low coulombic efficiency on battery. To solve these problems, in this project, based on the fact that polar molecule is carried away by an electric field, we present a kind of sulfur carbon composite cathode with surface modification layer. In the cathode, different materials and structure of the modification layer are deposited on the surface of cathode based on sulfur carbon composite preparation via magnetron sputtering, forming a barrier over polysulfide shuttling and suppressing its dissolving and diffusion in the electrolyte, as well as providing accommodation space for discharge final product Li2S/Li2S2 from decomposition of polysulfide in the electrolyte. The formation mechanism and reaction kinetics of the modification layer will be studied, and the influence of preparation condition on its structure and properties will also be investigated, and then the optimized modification layer and preparation technique for sulfur cathode can been found; the transition behavior and characteristics of lithium and polysulfide ions through the layer in the cathode under effect of electrical field will be researched, and the function mechanism of the modification layer as well as the energy storage mechanism of novel composite cathode will be studied, in order of present a theoretical model describing the dynamic behavior of the electrode. The research work will provide theoretical basis of designing lithium-sulfur battery with high capacity and long cycle life.

锂硫电池是一种极具应用潜力的高能量密度储能器件，但是单质硫和其还原产物的导电性差、充放电过程中易生成溶于电解液的多硫化物并在正负极间穿梭，导致活性物质不可逆损失，使得电池循环性能差、库伦效率低。为解决该问题，项目基于电场作用下极性分子定向移动这一原理，设计了一种具有表面修饰层的硫碳复合正极。该正极以硫碳复合材料制备正极基底，通过磁控溅射手段在其表面生长不同材质和结构的修饰膜层，阻挡多硫化物的溶解扩散和穿梭，并为多硫化物还原产生的Li2S/Li2S2提供宿留空间。项目重点研究修饰层的形成机理和反应动力学，揭示合成条件对于膜层结构和理化性能的影响规律，找到具有优异性能的复合正极表面修饰层及制备工艺；研究电场作用下，锂离子和多硫离子通过表面修饰层在电极中的传输行为与特征，阐明膜层作用机制和新型复合正极的储能机理，建立其工作模型，为设计高容量、长寿命的锂硫电池奠定理论基础。

锂硫电池作为下一代高能量密度二次电池之一，近年来显示出巨大的应用潜力，成为了当前储能材料和器件领域的研究热点。为了解决锂硫电池中由于单质硫及还原产物导电性差、充放电过程中易生成溶于电解液的多硫化物而产生“穿梭效应”，导致活性物质利用率低、电池循环性能差及库伦效率低的问题，项目运用磁控溅射、表面涂覆技术设计并成功在锂硫电池结构中构筑了不同的表面修饰层，获得了具有表面修饰层的硫碳复合正极，达到有效抑制多硫化物溶解穿梭，改善电极循环性能的目的。项目重点研究了硫碳复合材料及电极基体的制备、表面修饰涂层的构筑和性能调控、以及修饰层的阻硫机理和作用机制。通过研究合成条件对表面修饰层的物化性能和阻硫作用的影响，获得了优化的工艺条件；通过一系列电化学手段和方法研究了具有表面修饰层复合硫正极的性能，解析了表面修饰层的作用机理和多重功效，阐释了新型复合电极的反应机制，建立了具有表面修饰层的电极反应动力学模型。研究表明，多孔碳材料制备的硫碳复合基体对活性物质硫具有良好的限域和固定作用，实验通过水热条件下实现了三维多孔石墨烯的自组装和水热碳在石墨烯孔壁上的定向生长，获得了具有680 cm3 g-1的比表面积和0.94 cm3 g-1的孔容的层次孔碳/石墨烯气凝胶，以其制备硫碳复合材料获得到了高导电性和高稳定性的正极基体；研究通过表面涂覆的方法在硫碳复合正极基体上构筑了导电碳和粘接剂的修饰层，有效改善了硫碳复合电极的循环性能，0.5 C电流密度下，经表面涂层修饰后的电极硫含量为60%，首次放电比容量为962 mAh g-1，100次循环后的放电比容量695 mAh g-1；研究采用磁控溅射在碳纳米管纸表面构筑了一层MoS2修饰层，用作锂硫电池夹层时显著改善了硫电极的循环性能，溅射了10分钟MoS2修饰层的电池在0.5 C循环100次后仍有850 mA h g-1的比容量研究验证解析了表面修饰层的对多硫化物具有限域、宿留、吸附、催化转化的作用和协同机制，获得的关键数据、重要结果和科学结论，对于深入理解锂硫电池反应机理，设计高容量、长寿命的锂硫电池提供了理论和实践指导。