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.

We report the excellent cycling stability of graphite in combination with two types of sulfide solid electrolytes, 0.75Li2S-0.25P2S5 (LPS) and 0.3LiI-0.7(0.75Li2S-0.25P2S5), and discuss the stability of the graphite-solid electrolyte interface by analyzing the normalized cumulative irreversible charge and the total amount of lithium consumed at the lithiated state. The rate limitations and the influence of the morphology and the size of graphite particles on the utilization of the electrode are studied as well. At higher current densities, the utilization of the graphite is decreasing as a consequence of the poor effective ionic conductivity of the composite electrode, which is also evidenced by the increasing polarization and ohmic resistance across the electrode.

A new solid state electrolyte in the system Li-P-S-O is reported with the composition of Li 3.2 PS 3.7 O 0.3 . This material is isostructural with Li 10 GeP 2 S 12 , LGPS-type structure. A higher conductivity and lower activation energy than other LPSO materials is reported due to the lattice volume slightly higher. In a full solid-state battery, our material exhibits higher performance than Li 10 GeP 2 S 12 thank to the formation of an effective SEI. These results indicate that Li-P-S-O system is very promising to find solid electrolyte and should be explored in order to improve the stability of the family of solid state electrolyte at ambient moisture.

No abstract