锂硫电池正极多孔材料的设计及其固硫机理的研究

Lithium-sulfur (Li-S) batteries are secondary batteries, which are based on metal lithium as negative electrode and elemental sulfur as a positive electrode. Li-S batteries are considered to be one of the most promising energy storage devices for next generation high energy power system, due to their high specific capacity and energy density. In addition, sulfur is abundant, low cost, and environmentally friendly. Therefore, Li-S batteries are “green batteries”. So far, the development of Li-S batteries is greatly hindered by the high solubility of polysulfide in organic liquid electrolyte during the discharge process leads to the active mass loss and the shuttle effect. The polysulfide departs from the anode skeleton because of relocation diffusion, resulting in the problem that attenuation of battery capacity is faster and cyclic performance is lower. In order to solve the major problem of sulfur electrode, in this project, we will explore the design of functional covalent organic frameworks (COFs) as cathode framework materials for Li-S batteries using for limiting the loss of sulfur. COFs represent a new type of porous organic material, which is ingeniously constructed with organic building units via strong covalent bonds, with good thermal stability, regular structure and adjustable pore sizes. Due to the structure of COFs materials can be designed, we propose combing with theoretical calculation and experiment to elucidating the mechanisms of porous materials fixed sulfur, and then design and synthesis high performance porous materials as cathode materials for Li-S batteries. Design a series of COFs materials with different topology structure, pore sizes and functional groups as the research object. Based on density functional theory (DFT) and molecular dynamics simulation, research on the effect of topology structure, pore sizes and functional groups of COFs materials to the solubility and diffusion of polysulfide in the different organic liquid electrolyte solutions, respectively. And then, illustrate the mechanism of COFs materials fixed sulfur in micro-level. On the basis of theoretical research, the structure of COFs materials will be optimized, and the functional COFs materials based on the suppression of the shuttle effect will be designed and synthesized. With the further research on the structure and properties of COFs materials, it can provide guidance for the structure design of porous materials using in the cathode framework of Li-S batteries.

在新的储能体系中，锂硫电池是一种极具发展潜力的高能量密度锂二次电池,然而硫正极在充放电循环中形成易溶于有机电解质的多硫化物并产生“穿梭效应”，导致电池的循环性能差、倍率性能不理想，阻碍了锂硫电池的实际应用。围绕解决硫电极存在的关键问题，本项目提出以具有结构可设计性、易功能化的新型纳米功能材料---共价有机框架（COFs）多孔材料作为电池正极骨架，采用“理论设计-实验验证-理论优化”的螺旋上升式研究策略，通过量子力学计算和分子动力学模拟，研究COFs多孔材料中拓扑结构、孔径和功能基团对多硫化物在不同电解质中的溶解性和扩散性能的影响，阐明多孔材料的固硫机理。在理论研究的基础上，设计开发出有效固硫的COFs多孔正极材料，并通过电化学手段对其电池性能进行测试，验证理论研究提出的固硫机理。随着对COFs多孔材料的结构与性能的深入研究，可为锂硫电池正极多孔材料的结构设计提供理论指导。

近年来，锂硫电池具有高理论比容量（1672 mAh·g-1）和高比能量（2600 Wh·kg-1）被认为是最有应用前景的新型能源存储设备之一。锂硫电池是以硫单质为正极、锂金属为负极的二次电池。其中，硫单质具有环境友好、价格低廉和储量丰富等优点。然而硫正极在充放电循环中形成易溶于有机电解质的多硫化物并产生“穿梭效应”，导致电池的循环性能差、倍率性能不理想，阻碍了锂硫电池的实际应用。围绕解决硫电极存在的关键问题，本项目提出以具有结构可设计性、易功能化的新型纳米功能材料---共价有机框架（COFs）多孔材料作为电池正极骨架，采用“理论设计-实验验证-理论优化”的螺旋上升式研究策略，通过量子力学计算和分子动力学模拟，研究COFs多孔材料中拓扑结构、孔径和功能基团对多硫化物在不同电解质中的溶解性和扩散性能的影响，阐明多孔材料的固硫机理。我们构筑一系列COFs材料结构，从孔道、功能基团和单原子等角度研究了在不同溶剂中，COFs材料与多硫化锂的相互作用及其电催化性能。研究发现，孔道中层间的活性位点相比于表面的活性位点更有助于吸附多硫化锂；在DOL和DME溶剂中，O原子和N原子或两个O原子的协同作用可以更好的锚定多硫化锂；金属Mn和Zn可以很好催化解离Li2S，通过酞菁锚定单金属原子的COF作为正极不仅表现出较好的M-N-C双位点锚定Li2Sx分子能力，而且还表现出单原子催化剂的优势。我们将这一研究策略和结果应用于设计新材料及研究催化性能的本质上。研究成果共发表SCI论文12篇，其中JCR一区8篇，另有2篇论文在审稿中。2019年获辽宁省自然科学学术成果三等奖。随着对COFs多孔材料的结构与性能的深入研究，可为锂硫电池正极多孔材料的结构设计提供理论指导。