Yi-Chen Weng: Photoelectron Spectroscopy of Buried Battery Interfaces: From Ex Situ to Operando

Abstract

The continued development of lithium-ion batteries critically relies on understanding the reactions occurring at electrode-electrolyte interfaces. This thesis applies and advances Photoelectron Spectroscopy (PES) methodologies – ranging from ex situ to operando techniques – to investigate these reactions, in particular electrolyte decomposition, at buried interfaces underneath the surface.

Before diving into electrochemistry, it is important to establish the probing depth for such surface-sensitive studies. Therefore, the thesis begins by experimentally determining the inelastic mean free path for polymers and solid polymer electrolytes (SPEs) using the overlayer method. The measured values are compared with current predictive models, and key considerations for using this method are discussed.

The investigation of electrolyte decomposition then proceeds with an ex situ study on the solid electrolyte interphase (SEI), comparing how fluorinated and fluorine-free liquid electrolytes decompose on various anode materials. Here, a preferential decomposition mechanism is revealed: the fluorinated electrolyte tends to degrade on silicon, whereas the fluorine-free one degrades preferentially on graphite.

To access early-stage decomposition under controlled conditions, an in situ lithium deposition that intends to mimic the first plating process is used. This approach enables direct observation of SPE decomposition when it is brought in contact with lithium, and allows us to identify differences in stability among various lithium salts. Using this as a stepping stone, I advance to operando measurements in an anode-free cell configuration with SPEs. By tracking spectral changes during cell operation, the dynamic evolution of SEI, i.e. the polymer and lithium salt decomposition pathways, is resolved. The influence of different applied electrochemical protocols on decomposition behavior, as well as the evolution of the copper working electrode, are also examined. Importantly, these operando experiments capture reactions that cannot be discovered by the in situ lithium deposition study alone.

Finally, the feasibility of conducting these complex operando experiments in laboratory-scale instruments is assessed. The initial results, technical challenges, and potential for broader accessibility are outlined. By applying and advancing PES across various systems, my thesis contributes to a more well-defined picture of interfacial evolution, offering insights for the development of next-generation batteries.