锂离子电池电极表面钝化膜的扫描探针研究

Solid electrolyte interphase (SEI) is known as an electronic insulating but ionic conducting film formed on the surface of the anode and cathode in Li-ion batteries. It is widely accepted that the physical and chemical properties of the SEI film have significant impacts on the safety, power capability, morphology of lithium deposits, shelf life, and cycle life of lithium-ion battery. It is very important that there be uniform morphology and chemical composition in order to ensure homogeneous current distribution. The SEI must be both mechanically stable and flexible and good adhesion to the electrode. However, the distribution of structure and chemical composition in SEI films are not uniform, which make it difficult to study the structure and properties of SEI films. In our recently work, we have proposed a force spectroscopy method to study the structure and mechanical properties of SEI films in nanometer scale. By this method, we firstly elucidate the double-layers structure of SEI film on MnO electrode, where an inner-hard inorganic layer and outer-soft organic layer. Furthermore, we also observed the evolution of SEI thickness/structure with voltage. There is not significant SEI film formed at voltage above 0.3 V, when the electrode was discharged to 0.1 V, the hard inorganic film start to form. The thickness of SEI reached its maximum when the electrode was discharged to 0.01 V, where there are thick soft SEI layer on the top. However. the soft SEI film dissappear when the electrode is charged up to 3.0 V. In this proposal, we are going to study the carriers transport properties of SEI film, and study the relationship between SEI's electroinic and ionic conductivity and its structure.

锂离子电池电极表面的钝化层（SEI膜）的结构与性能对电极材料的化学及电化学稳定性、锂离子的传输能力起到决定性的作用，进而会影响锂离子电池的循环效率、容量、高低温特性、倍率特性等关键性能。但是，因为SEI膜的生长过程与结构复杂，其生长机理、结构模型以及输运性能等方面仍然存在许多有待解决的基础科学问题。申请人在国际上首创利用扫描探针技术的力谱模式，直观地测得SEI膜的介观结构与成分分布情况，并通过对SEI膜杨氏模量的测量与统计，证明了SEI膜中存在着无机/有机的双层SEI膜结构，为揭示SEI膜的介观结构信息提供了有力的检测手段。在将来的工作中，申请人拟将力谱、电子电导、离子电导等模式应用于SEI膜研究，采用原位与非原位结合的方法重点研究石墨、硅以及过渡金属氧化物负极材料在不同电解液体系、充放电条件下表面SEI膜的结构成分分布以及机械强度、电子电导、离子电导，稳定性等关键性能。

经过四年的研究，项目负责人与研究团队发展和使用了大量先进的表征测试技术，对SEI膜的结构与成分对锂离子电池电化学性能的影响进行了深入的研究，在此基础上对电极材料进行表/界面调控，改善SEI膜的结构稳定性，大幅提高锂电池的电化学性能，取得了大量成果。本项目在扫描探针显微镜技术的基础上，发展了原位电化学扫描探针显微镜技术，结合聚焦离子束刻蚀-扫描电子显微镜技术、离子逐层减薄-XPS技术、超快扫描探针成像、变温扫描探针成像技术等，实现对对锂离子电池电极表面钝化层（SEI膜）结构与成分的表征；并研究了SEI膜结构与性能对电池循环稳定性、高电压稳定性、长循环寿命、库伦效率等关键电化学性能的影响；在以上对SEI膜结构与性能研究的基础上，进一步发展电极材料表面钝化技术、电解液添加剂技术等对SEI膜进行调控，在高电压正极材料、金属锂负极材料、锂硫电池、超级电容器等电化学材料与器件研究中获得有效的性能提升：使钴酸锂正极材料获得在4.5V下稳定的高电压循环，金属锂负极的枝晶生长获得抑制，同时具有更高的安全性与库伦效率；通过表面包覆抑制了锂硫电池中多硫化锂的溶出，提高循环稳定性，并获得高的比容量等。