Interfaces in Lithium ion batteries: Advanced chemical and morphological characterization of the Solid Electrolyte Interphase

Abstract

As the market share of electric vehicles (EVs) and mobility is expected to rise immensely over the next decades, the demand for improved energy storage technologies is currently at an all-time high. Li-ion batteries attract most attention due to their high stability alongside excellent volumetric and gravimetric energy density. One of the most important factors influencing liquid-electrolyte battery performance and lifetime is the solid-electrolyte interphase (SEI), formed at the interface between anode and electrolyte. A stable SEI is critical towards the cyclability of the cell, as its formation process leads to the (irreversible) immobilization of lithium ions inside this layer. This is especially the case for Si-based anodes, as their large volume changes during cycling will damage the integrity of the SEI layer. SEI reformation after each cycle eventually leads to undesired capacity fading. Beyond silicon-based anodes, metallic Li could be considered as the ultimate anode material due to its exceptional theoretical capacity and lowest electrochemical potential. However, many challenges surround its practical application in both liquid electrolyte and all-solid-state batteries. Due to the high reactivity of Li metal, reactions and degradation processes taking place at the interface often have a strongly negative impact on the battery stability and performance. In addition, dendrite formation issues are well known, even for solid-state electrolyte systems.

In-depth study of these interfaces often relies on advanced characterization methods for the investigation of their morphology, mechanical properties and chemical composition. However, due to the complex nature and reactivity of these interfaces, this is often challenging. Alongside real electrode samples, planar model systems with a well-structured interfacial region could provide supporting information and a better understanding of the processes taking place at the surface. By combining cross-sectional electron microscopy (FIB-SEM) with surface sensitive imaging (AFM), the morphology of the interface region can be investigated in detail. Compositional analysis of the SEI and (buried) electrode surface can be achieved with XPS/HAXPES. By combining multiple x-ray energies, the chemical environment can be probed at various depths, which allows non-destructive profiling. The ultimate goal of this study is to integrate these results, providing a deeper look into the early stages of SEI formation and reduction processes taking place at Si-based materials (Si/SiOX or Si/C composites) and metallic Li anode interfaces. This knowledge could lead to advancements of current state-of-the-art battery systems and even provide further insights towards new generation Li-ion battery technologies.