A Simple Approach for Operando Interface Probing for Batteries: Combining Scanning APXPS with Spectroscopic Recognition

Probing the solid/liquid interface of batteries operando/in situ with ambient pressure X-ray photoelectron spectroscopy (APXPS) using the dip-and-pull method remains a challenging endeavor due to spatial and temporal variations in liquid layer shape, thickness, and composition. Monitoring the electrochemical and topographical nature of the liquid edge where the interface is accessed is essential to correctly interpret interfacial spectra. In this work, a methodology combining experimental design and software-based data processing for interface probing is reported. This experimental methodology utilizes continuous motion during fixed-mode APXPS measurements by periodically scanning across the dry electrode and thick electrolyte regions to capture the transitional interface. Two software-based approaches for retrieving the interface spectra are evaluated. In an analysis of the intensity attenuation pattern of a unique electrode signal, interface spectra are recognized at the edge of the intensity transition from electrode to electrolyte. The second method utilizes peak positions for interface identification. Selected spectra with the same peak energies also exhibit the same chemical features, indicating the close correlations between the interface energetics and local chemical compositions. Further, topographical information can be extracted using scanning APXPS by translating spectral intensities into liquid thickness, creating a spectro-microscopic 3D image of the liquid edge region. In the examined systems, the thickness of a propylene carbonate electrolyte edge on both lithium cobalt oxide and gold WE surfaces exhibits a step-jump transition from the thin to thick liquid region. The liquid distribution is also shown to depend on the morphological and chemical nature of the electrode. The imaging provides a better understanding of the relationship between liquid distribution and probed interface features while validating the functionality of the setup.