Post mortem and operando XPEEM: a surface-sensitive tool for studying single particles in Li-ion battery composite electrodes.

X-ray photoemission electron microscopy (XPEEM), with its excellent spatial resolution, is a well-suited technique for elucidating the complex-electrode electrolyte interface reactions in Li-ion batteries. It provides element-specific contrast images that allows the study of the surface morphology and the identification of the various components of the composite electrode. It also enables the acquisition of local X-ray absorption spectra (XAS) on single particles of the electrode, such as the C and O K-edges to track the stability of carbonate-based electrolytes, F K-edge to study the electrolyte salt and binder stability, and the transition metal L-edges to gain insights into the oxidation/reduction processes of positive and negative active materials. Here we discuss the optimal measurement conditions for XPEEM studies of Li-ion battery systems, including (i) electrode preparation through mechanical pressing to reduce surface roughness for improved spatial resolution; (ii) corrections of the XAS spectra at the C K-edge to remove the carbon signal contribution originating from the X-ray optics; and (iii) procedures for minimizing the effect of beam damage. Examples from our recent work are provided to demonstrate the strength of XPEEM to solve challenging interface reaction mechanisms via post mortem measurements. Finally, we present a first XPEEM cell dedicated to operando/in situ experiments in all-solid-state batteries. Representative measurements were carried out on a graphite electrode cycled with LiI-incorporated sulfide-based electrolyte. This measurement demonstrates the strong competitive reactions between the lithiated graphite surface and the Li2O formation caused by the reaction of the intercalated lithium with the residual oxygen in the vacuum chamber. Moreover, we show the versatility of the operando XPEEM cell to investigate other active materials, for example, Li4Ti5O12.