富锂层状正极材料薄膜电极/电解液本征界面特性的研究

Due to high capacity and new charge-discharge mechanism, Li-rich layered cathode material has attracted much attention recently. However, its high charge-discharge voltage plateau is beyond the stable electrochemical window of electrolyte, which causes electrolyte decomposition and fast cyclic performance deterioration. Research on the interfacial characteristics between Li-rich layered cathode and electrolyte and constructing a stable solid electrolyte interface (SEI) on Li-rich layered cathode are the key to satisfy long life and high energy density for Li-ion battery. However, binder is usually used to prepare positive electrode, which will adhere on the surface of cathode materials and hardly to be cleaned up. Thus it will interfere with the analysis of SEI film. So, in this project, we will prepare pristine Li-rich layered film electrode to offer intrinsic cathode/electrolyte interface, combining with multiple investigating techniques, to study the interface development during charge-discharge process. We will use ALD technique to deposit different thickness of ion conductor on film electrode and study its mechanism. Furthermore, we will screen effective film-forming additives by theoretical calculation and experimental confirmation to construct high-voltage stable SEI film on Li-rich layered cathode and research its mechanism. The study will improve the cyclic life of high voltage lithium ion battery by restraining oxidative decomposition of electrolyte.

富锂层状正极材料由于其高比容量和新的充放电机制受到广泛地关注。但是其工作电压高于电解液稳定的电化学窗口，引起电解液在材料表面发生氧化分解，造成电解液的消耗和材料循环性能迅速地恶化。研究富锂层状正极材料/电解液的界面特性、构筑稳定的正极/电解液界面是满足电池长寿命和高比能要求的关键。然而，传统的极片制备方法一般会引入粘结剂，其粘在电极表面很难被彻底地清除，必然对正极表面膜的测试带来干扰。为此，本项目将制备纯的富锂层状正极材料薄膜电极，以提供干净的本征界面，结合多种检测技术，研究正极/电解液界面在高电压下充放电时演变过程。通过原子层沉积对膜电极进行不同厚度的包覆，研究包覆在提供稳定的正极界面中的作用机理。并通过理论计算与实验筛选成膜添加剂，以构筑高电压下稳定的正极SEI膜，研究其作用机理。本课题在抑制电解液在正极表面的分解、提高高电压锂离子电池的循环性能上有重要的意义。

理解正极表面SEI膜的生长过程和机理、构筑稳定的正极/电解液界面对于提高锂离子电池循环性能具有重要的意义。本项目以高电压正极材料薄膜电极为对象，用原子力显微镜结合扫描电镜、X-射线光电子能谱、表面能谱分析、电化学阻抗和循环伏安分析等多种检测技术，研究了高电压循环过程中正极表面的演变过程和电解液的电化学反应，由此，我们提出了合理的电解液在正极表面的电化学反应过程，以理解界面SEI 膜的生长过程和生长机理。我们还用原位电化学原子力显微镜的方法对高电压下钴酸锂正极表面SEI膜的演变过程进行了研究，结果发现LiCoO2材料在高电压下不同晶面的反应活性不同，电解液在棱面上容易发生反应，形成不稳定的SEI膜，正是由于这种不稳定的界面，造成了LiCoO2在高电压工作条件下循环稳定性的降低。用原子层沉积方法对LiCoO2进行表面包覆后发现电解液在棱面上的反应被抑制，获得了稳定的界面。我们还筛选了几种有效的正极成膜添加剂，研究了这些添加剂在构筑高电压下稳定的正极SEI膜中的效果和作用机理。这些研究对理解高电压循环过程中正极表面电解液的电化学反应历程和表面SEI膜的演变过程具有重要意义，并对提高高电压正极材料/电解液界面稳定性给出了有效的解决方案。