锂硫电池用PEO基纳米复合固态电解质的双侧改性研究

Due to their high specific energy, low cost and environmental-friendly character, Li-S batteries are promising candidates for electric vehicles. However, the Li-S batteries suffer from low capacity, poor cycling performance and safety issues due to the high solubility of the lithium polysulfides in the commonly used liquid ether-type electrolytes. The use of solid polymer electrolytes instead of liquid electrolytes can completely solve the above issues for Li-S batteries. Nanocomposite-PEO polymer has been considered as appealing solid state electrolyte for Li-S batteries with its high ionic conductivity and excellent mechanical stability. However, its development is impeded by the interface compatibility and stability between electrodes and solid electrolytes. Starting from the viewpoint of molecular surface structure design, the binding energy between different surface functional groups and sulfur and Li will be calculated by density-functional theory (DFT) calculation and suitable surface functional groups will be selected. According to the theoretical results, double-sided surface modification of nanocomposite-PEO polymer will get conducted to improve the interface compatibility, reduce the contact impedance, form a stable solid-solid reaction interface, as well as inhibit the diffusion of lithium polysulfides to the negative electrode. Through various chemical methods, the modified layers were synthesized and regulated by its thickness, porosity, homogeneity and surface configurations to gain a more optimized structure. With this surface modified nano-PEO electrolyte, all-solid-state Li-S batteries will be constructed. The influence of surface modification to nanocomposite-PEO electrolyte on the electrochemical properties of solid-state Li-S batteries was investigated by using the the state-of-the-art chemical and electrochemical characterization methods. This project intends to establish a reliable pathway to study the nanocomposite-PEO polymer electrolyte for all solid Li-S, and further to provide new ideas and theoretical basis to solve the interfacial problems for solid-state Li-S batteries.

锂硫电池因其高比能、低成本、对环境友好等优点在电动汽车等领域极具应用潜力。然而，传统的锂硫电池因使用有机液态电解液而导致穿梭效应及安全隐患。使用纳米掺杂-PEO复合电解质替代液态电解质可从根本上解决以上问题，但其应用又受到正负极界面稳定性的制约。传统固态锂离子电池使用的表面改性手段往往只关注到正极-电解质界面，而且不能系统的针对某一特定电极材料的表面特性来改性。本项目从分子表面结构设计出发，借助理论计算分别筛选出与S、Li有较好亲和力的基团/化合物，进一步通过化学方法合成并调控其厚度、孔隙率、均一度、表面构型等，来对电解质两侧分别进行不对称改性，以达到同时改善正、负电极与电解质的界面相容性，降低界面阻抗的作用。在此基础上，构筑全固态锂-硫电池，揭示电解质不同的表面结构对锂-硫电池电化学性能的影响规律，为解决全固态锂硫电池的界面问题提供新的思路。

本项目的实施针对聚醚类复合电解质作为固态锂-硫电池电解质研究体系，系统深入地探索如何通过界面改性来提高电池的综合性能。借助理论计算筛选出与硫相容性良好的表面基团/化合物，对聚醚类电解质进行表面改性优化设计。进一步通过实验方法，制备优化所得目标电解质膜，通过表征手段测试其多方面性质，优选出综合性能优异的表面改性电解质膜。最后，开发出了一种新型的凝胶聚合物电解质原位界面固化技术，解决了锂-硫电池中电化学活性和稳定性之间的平衡问题，构筑出混合固液锂-硫电池，使锂-硫电池同时具备了高能量密度与长循环寿命的优势。