Post mortem/operando XPEEM: for studying the surface of single particle in Li-ion battery electrodes

The continuous need for increasing the electrochemical performance and safety of Li-ion batteries requires an incessant development of characterization techniques capable of providing better insights into the physics and chemistry of the various parts of the battery, in particular, the composite electrodes. Both the bulk and the surface structural modifications of the electrodes need to be investigated upon cycling, since they both have a direct impact on the battery performance. Specifically, understanding the different reactions taking place at the interface between the electrolyte and the various particles of the electrode remains crucial for the development of stable, efficient, and safe batteries. However, due to the inherent difficulty of probing the confined interfacial regions, a full picture of the processes remains elusive, despite being the target of extensive investigation in the last three decades. In this context, we explored the potential of synchrotron radiation X-ray photoemission electron microscopy (XPEEM) to study single particles in composite electrodes of Li-ion batteries. We have shown that the simultaneous investigation of the transition metal oxidation states in the lithium oxide particles, the stability of non-active components (i.e., conductive carbon and binder), and of the electrolyte upon cycling can be carried out using XPEEM, at both the microscopic and spectroscopic levels. By varying the energy of the incoming photons, local elemental maps of the sample can be obtained with a spatial resolution better than 70 nm. The limited escape depth of electrons from materials, in the range of 2-3 nm for low energy electrons, makes this technique very surface sensitive and particularly well suited for the study of surfaces and interfaces. Another intrinsic advantage of XPEEM resides in its non-destructive nature: in our case a 13 mm disc of a commercial-like electrode can be transferred to the microscope without prior sample thinning or grinding or changing the working environment of the particles in general, as required for other microscopy techniques. In this study, we discuss in detail the technical aspects that need to be considered in order to use XPEEM for studying Li-ion battery electrodes and our strategy to solve such challenges. First, we outline how we solved several technical problems, such as surface roughness, spectral normalization, and beam damage. Then, several case studies are presented to highlight the usefulness of combining lateral resolution and surface sensitivity with chemical information. Finally, we present a first XPEEM electrochemical cell dedicated for operando/in-situ experiments in all-solid-state batteries. Measurements carried out on a graphite electrode cycled with LiI-incorporated sulfide-based electrolyte. This measurement demonstrate the strong thermodynamic competitive reactions between the lithiated graphite surface and the Li2O formation caused by the reaction of the intercalated lithium with the residual oxygen and water in the ultra-high vacuum environment. Moreover, we show the versatility of the operando XPEEM cell to investigate other active materials e.g. Li4Ti5O12.