Lithium Tracer Diffusion in Lithium-Metal-Oxide Compounds for Battery Cathodes

本文研究了锂离子电池正极材料锂金属氧化物中锂离子的扩散现象。在这种情况下，扩散对于理解电极的动力学过程和电池的基本性能（充电/放电时间、最大容量、副反应）是很重要的。本文的目的是在层状（LiNi0.33Mn0.33Co0.33O2; LiCoO2）和尖晶石（LiNi0.5Mn1.5O4; LiMn2O4）结构类型模型系统中进行首次系统的Li示踪剂扩散研究。研究了烧结体材料、薄膜和单晶形式的锂金属氧化物（仅限LiCoO2）。示踪剂扩散实验将通过在具有天然同位素组成（例如6LiCoO2/LiCoO2）的样品上沉积一层薄薄的6Li同位素富集层来完成。然后对样品进行扩散退火，导致6Li渗透到所研究的薄膜中。用二次离子质谱法记录了同位素深度剖面。通过比较退火前后的6Li深度分布，得到扩散方程的充分解，可以推导出扩散系数。实验将作为温度、结构状态和Li含量（例如Li1-xCoO2）的函数进行。如果扩散系数遵循阿伦尼乌斯定律，则在文献的框架内确定和分析扩散的激活焓。结果将与电化学方法（恒流间歇滴定技术）在同一类型材料上的结果进行比较。结果之间的差异将被讨论和解释。电化学方法测定扩散系数的准确性和有效性有待进一步探讨。这项工作也与一种很少用于锂基材料的示踪剂扩散的系统方法的进步有关。

The present proposal deals with the phenomenon of Li diffusion in lithium-metal-oxide compounds as cathode materials for Li-ion batteries. In this context, diffusion is important for a fundamental understanding of kinetic processes at electrodes and of basic battery performance (charging/discharging times, maximum capacities, side reactions). The aim of the proposal is to carry out the first systematic Li tracer diffusion studies in model systems of layered (LiNi0.33Mn0.33Co0.33O2; LiCoO2) and spinel (LiNi0.5Mn1.5O4; LiMn2O4) structure types. Lithium-metal-oxides in form of sintered bulk material, thin films and single crystals (LiCoO2 only) are investigated. Tracer diffusion experiments will be done by depositing a thin 6Li isotope enriched layer on top of the samples with natural isotope composition (e.g. 6LiCoO2/LiCoO2). Afterwards the samples are diffusion annealed, leading to a penetration of 6Li into the film under investigation. Isotope depth profiles are recorded by Secondary Ion Mass Spectrometry. By comparison of the 6Li depth profile before and after annealing to adequate solutions of the diffusion equation, diffusivities can be derived. The experiments will be done as a function of temperature, of the structural state, and of Li content (e. g. Li1-xCoO2). If the diffusivities follow the Arrhenius law, the activation enthalpy of diffusion will be determined and analysed in the framework of literature. The results will be compared to those of electrochemical methods (Galvanostatic Intermittent Titration Technique) on the same type of material. The difference between the results will be discussed and interpreted. The exactness and validity of electrochemical methods for diffusivity determination should be elaborated. The work is also of relevance for an advancement of a methodical approach of tracer diffusion that is rarely used for Li based materials.