锂硫电池用可跟踪修复式锂负极界面设计及其电化学行为研究

Owing to its high energy density and low cost, Li-S battery has become a research and development hot spot for the next generation secondary battery device with high specific energy. However, lithium polysulfides produced during the discharge process can be dissloved in the electrolyte, enabling the transport of sulfur species to the Li anode, where results in corrosion reactions and forming the insoluble Li2S. This gives rise to a shuttle phenomenon, reducing the coulombic efficiency and the reversible capacity. In view of this problem, this project intends to combine lithium metal modification with electrolyte additives application, aiming to design and achieve a traceable repaired dense flexible composite interface film on the lithium anode, inhibiting the shuttle effect and improving the battery cycle stability greatly. Additionally, the formation mechanism of solid electrochemical interface (SEI) film and the mechanism of Li+ migration in SEI film will be revealed based on the theory of density functional theory and interfacial chemistry analysis. The structure-activity relationship of.lithium thin film interface componets will be established. The selection regular for electrolyte additives of Li-S battery will also be established, and a new type of additives based on fluoroether and unsaturated ether for Li-S battery electrolyte will be proposed to target filling the defects and holes of the interface flexible film. Finally, a traceable repair interface film will be obtained on the surface of lithium anode. This project will enrich the research contents of interface chemistry of lithium anode heterogeneous materials. This can also be beneficial to the development of devices using lithium metal as the negative electrode and lay the foundation for the application of the other metal batteries.

锂硫电池具有高的能量密度和低的原材料成本，使其成为新一代高比能二次电池的研究开发热点。但是，锂硫电池放电中间产物多硫化锂会溶解在电解液中，迁移至锂负极并与其发生腐蚀反应，引起穿梭效应，导致电池循环性能差和库伦效率低等问题。针对该问题，本项目拟结合聚合物基金属锂表面修饰和电解液添加剂优选两策略，在锂负极表面设计并实现可跟踪修复式的致密柔性复合界面膜，抑制穿梭效应，大幅度提高电池的循环稳定性。通过密度泛函理论计算和界面化学分析，揭示锂负极界面SEI膜的形成机理和锂离子迁移机制；探明锂负极界面柔性修饰膜的组分构效关系；建立锂硫电池电解液添加剂筛选机制；提出基于氟代醚和不饱和醚的锂硫电池新型电解液添加剂，靶向填补界面柔性修饰膜的缺陷和孔洞；最终获得可跟踪修复式复合界面膜。本项目的开展丰富了锂负极异质材料界面化学研究，研究结果将有益于以金属锂为负极的器件开发，并为其它金属电池的研发奠定基础。

金属锂具有超高的比容量和最低的电压，被认为是锂离子电池发展的终极负极材料，但却存在副反应多、SEI膜不稳定和锂枝晶等问题，限制了金属锂负极的商业应用。本项目以构建电导率高和稳定性好的可跟踪修复式的界面膜为研究目标，进而解决金属锂的上述各项问题，推动金属锂的规模化商业应用。首先，基于溶液涂覆法，在金属锂表面构建了稳定性好和导电率高的界面层，但是，物理方法形成的界面修饰层均匀性不足，对金属锂电池的电化学性能提升有限。第二，合成了纯度达到99%以上的二氟双草酸磷酸锂，并将这种新型锂盐用作锂硫电池的电解液添加剂，这种新型添加剂一方面可以在金属锂表面还原形成界面更加稳定的无机型SEI膜，另外，这种盐具有一定的酸性，会催化DOL发生聚合反应，因而实现SEI膜中有机成分的复合。最终形成的有机-无机杂化的SEI膜不仅具有导电性好和性能稳定的优点，而且，添加剂还会及时修复SEI膜在循环过程中可能造成的破损，具有可跟踪修复的特点。第三，优选出适用于伪高浓度电解液的非溶剂液体HFME，基于电解液溶剂化结构的调控，实现了循环寿命超过150周的钴酸锂/金属锂软包装电池（~1Ah）。第四，基于溶剂化效应，提出了适用于锂硫电池的碳酸酯基电解液，发现了一种完全不同于当前的锂硫电池充放电反应新机制，即基于固相反应的锂硫电池。该体系消除了穿梭效应，有效降低了电解液的使用量和抑制了电池的自放电。总之，本项目从物理涂覆、添加剂使用和电解液溶剂化结构调整等几个方面着手，为金属锂负极构造了稳定性好、电导率高且具有跟踪修复功能的界面层，为金属锂负极的规模化应用奠定了基础，为金属锂电池的商业应用提供了指导。