Liquid-phase electron microscopy of electrochemical processes: Microcells configuration, corrosion of aluminium current collector and lithium-ion battery cycling

Electrochemical liquid-phase electron microscopy (ec-LPEM) has been seen as a promising technique for revealing new insights into electrochemical phenomena that occur at the micro- and nanoscale levels, particularly at the solid-liquid interface. Specifically, monitoring the dynamic evolution of both the surface and internal structure of lithium-ion battery (LiB) cathode materials, along with evaluating the stability of the aluminum current collector during operation, could enhance our understanding of degradation mechanisms by linking material alterations to a decline in electrochemical performance. Since the cathode side of the LiB accounts for the highest weight fraction and significantly contributes to the key properties of the LiB, even small enhancements in its performance would greatly benefit the overall performance of the entire cell. In particular, Ni-rich LiNixMnyCozO2 (NMC) has gained attention because of its good electrochemical properties. Additionally, the corrosion of the Al current collector and its impact on the LiB performance decay is often negligate. In that sense, ec-LPEM could serve as a diagnostic method for observing structural and chemical degradation during battery operation; however, integrating the liquid electrolyte and electrode system into the EM holders for consistent electrochemical measurements remains a major challenge.

This thesis aims to optimize an experimental ec-LPEM platform featuring an Al electrode to enable the dynamic study of the evolution of the active cathode material-current collector pair during LiB cycling. Integrating an Al electrode will enhance the practical relevance of the developed ec-LPEM experimental setup compared to previous studies. First, the configuration of the coplanar electrodes that make up the electrochemical microcell must be optimized. To achieve this, a three-dimensional finite element model simulating the electrochemical behavior of Pt coplanar thin-film electrodes is developed and reliably applied to the oxygen evolution reaction. The findings indicate that symmetrical geometries of the coplanar electrodes should be favored when designing electrochemical microcells. Based on this, an electrochemical chip dedicated to ec-LPEM and incorporating an Al electrode is developed and applied to the localized pitting corrosion of aluminum occurring in saline solution, which is a system that has been widely studied over the past decades and is simpler than the LiB one. The developed experimental platform allows for the real-time and in-situ study of localized anodic corrosion of pure Al films, revealing varying corrosion mechanisms depending on the corrosion kinetics and indicating the formation of gas bubbles at the corrosion front, which were experimentally confirmed to be molecular hydrogen. Finally, the developed Al chip demonstrated its ability to conduct accelerated ageing tests of cathode-side LiB systems using ec-LPEM. Galvanostatic charge-discharge cycles were performed on