固态电解质/电极材料界面问题的原位透射电镜研究

The high energy density and long cycle life of lithium-ion batteries has led to their adoption in all manner of technologies, but serious safety concerns still exist due to their use of flammable organic solvent electrolytes. Development of high conductivity solid-state electrolytes for lithium ion batteries has proceeded rapidly in recent years, whereas the rate capability of most all-solid-state cells, particularly those employing high-voltage oxide cathodes, remains poor. This is typically ascribed to high internal resistance at the interfaces between electrodes and solid electrolytes. Space charges at the interface, atomic diffusion and/or chemical reactions during battery cycling, and mechanical issues may all play a role in degrading battery performance, but the exact mechanisms of the interfacial impedance have not been fully explained - in part because experimental evaluation of the interface can be very difficult. .Since the inception of in situ transmission electron microscopy (TEM) techniques for battery research in 2010, in situ TEM has been able to make exciting contributions to the fields that shed light on the understanding of material dynamics especially the charge transfer mechanism within interfaces during electrochemical reactions. This project aims to address this challenging interface problems of solid-state battery under the framework of in situ TEM, to explore the interface evolution during battery operation and how the above factors influence the interfacial impedance. By incorporating in situ scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) as well as energy dispersion spectroscopy (EDS), a detailed characterization of the interface between electrode and solid electrolytes, including the accurate composition and structure, the dispersion of lithium and the mechanical stress within the interface, will be vividly presented and the unique interfacial phenomena related to lithium ion transport and its corresponding charge transfer will be uncovered. This project is expected to attain an important new insight for fundamental understanding of the interfacial phenomena and provide principle solutions for the critical problems encountered in the development of high capacity solid-state lithium ion battery.

随着电动汽车和大规模储能等领域对高容量、高安全性锂离子电池需求的不断增长，开发以固态电解质为基础的全固态锂电池已迫在眉睫。然而，目前全固态锂电池的性能仍远远落后于商业化液体电解质锂电池，其中一个关键的瓶颈问题在于电极材料和固体电解质的界面处较大的阻抗严重制约了Li+的传输。在本项目中，我们将利用原位电学透射电子显微镜（TEM）技术，在原子尺度来深入研究固态电解质/电极材料界面问题。通过在TEM中原位构筑固态纳米电池器件，借助于TEM强大的成像功能与高空间解析度的微区分析能力，来原位实时地表征和研究固态电解质/电极材料界面处的空间电荷层效应与界面电荷输运机制、界面元素扩散与界面相形成、以及界面应力与稳定性等问题，以期从基础研究层面获得对固态电解质/电极材料界面问题的较为全面透彻的研究和认知，从而为全固态锂离子电池的界面结构优化设计与界面阻抗问题的解决，提供基础性的关键实验和理论依据。

本项目的主要研究内容，是基于电学探针原位透射电镜（TEM）技术，通过在TEM中原位构筑固态纳米电池模型器件，借助于TEM的即时成像功能与高空间解析度的微区分析能力，来原位实时地表征和研究与固态电解质/电极材料界面相关的科学问题，以期为全固态锂电池与其它相关电池器件中界面结构优化与性能提升，提供基础性的实验和理论依据。在本项目的支持下，我们主要在以下几个方面开展了深入的研究工作，并取得了一系列较为重要的进展：1）利用原位TEM研究了基于固态电解质离子传输过程的金属锂沉积与生长行为，发现了碳纳米管（CNTs）模型电极的表面特性与锂沉积行为之间的对应关系，通过调控CNTs表面缺陷密度与亲锂性，可以有效调节金属锂纳米结构的成核生长过程，实现致密锂沉积，为抑制锂枝晶的生长提供了新见解和思路，另外，通过对锂生长动力学过程的细致调控，还实现了二维碱金属锂纳米片的控制生长；2）分别以三种代表性金属氟化物为前驱体，对固态电解质界面膜（SEI）的关键组分——氟化锂（LiF）——的生成过程进行了细致的原位TEM研究，发现具有半金属特性和较低锂离子扩散势垒的氟化钛（TiF3），其电化学锂化反应动力过程更为平稳可控，所生成的纳米LiF更为细密均匀，非常适宜用于固态电池中金属锂负极表面SEI膜的人工构筑；3）利用原位TEM深入研究了氟化石墨（CFx）纳米片的锂化反应动力学过程，揭示了CFx电化学锂化反应的一个新机理：发现锂化反应过程具有明显的两阶段反应特征，在逾渗导电通道形成前后，CFx纳米片经历了一个从高阻态到低阻态的转变过程；4）针对二次铝-硫电池中多电子转移过程带来的动力学缓慢、循环稳定性差等问题，发展了石墨烯负载的高分散纳米氮化钛（TiN）催化剂，有效降低了硫正极充放电反应过程的动力学势垒，并对多硫化物中间产物表现出了明显的吸附锚定作用，从而大幅提升了铝硫电池的电荷转换效率与循环稳定性。本项目共发表标注资助的SCI论文13篇，申请发明专利1项，共培养博士研究生7名，其中4名已毕业，3名在读。