The understanding of surface reactions at the electrode–electrolyte interfaces has been a longstanding challenge in Li-ion batteries. X-ray photoemission electron microscopy is used to throw light on the disputed aspects of the surface reactivity of high-energy Li-rich Li1+x (Ni a Co b Mn1–a–b )1–x O2 (HE-NCM) cycled in an aprotic electrolyte against Li4Ti5O12 (LTO). Despite the highly oxidative potential of 5.1 V vs Li+/Li, there is no formation of a layer of oxidized electrolyte byproducts on any of the cathode particles; instead, a homogeneous organic–inorganic layer builds up across the particles of the LTO anode due to the electrolyte and poly­(vinylidene fluoride) binder decomposition on HE-NCM. In addition, such a layer incorporates, already from the first charge, micrometer-sized agglomerates of transition metals (TMs). The presence of TMs on the anode is explained by the instability of the reduced Mn, Co, and Ni formed at the surface of HE-NCM mainly during delithiation. The reduced TMs are unstable and prone to be transported to the LTO, where they get further reduced to metallic-like clusters. These results demonstrate that a dual reaction takes place at the HE-NCM–electrolyte interface if subject to high potential, namely, degradation of the surface structure and decomposition of the electrolyte, affecting directly the anode surface through the migration–diffusion processes.

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.

High‐resolution, surface sensitive soft X‐ray photoemission electron microscopy (XPEEM) reveals the fine interplay between oxygen and transition metal (TM) redox activities on the surface of a Li1.17(Ni0.22Co0.12Mn0.66)0.83O2 (Li‐rich NCM) electrode. We demonstrate that the oxidation of oxygen in the lattice is accompanied by TM reduction already at 4.47 V vs. Li+/Li, as a result of oxygen loss from the surface, the latter process being enhanced at 4.8 V where oxygen gas reaches a maximum release rate. Simultaneously, we find evidence for the chemical oxidation of the electrolyte solvents above 4.8 V induced by the released oxygen, leading to the formation of a carbonate by‐products layer that covers homogeneously the particles of the counter electrode Li4Ti5O12 (LTO). The latter observation demonstrates that migration‐diffusion of such oxidized solvent by‐products occurs only when the solvents are chemically oxidized. We showed also that the Li‐rich NCM is susceptible to oxygen loss during soaking in the electrolyte, which causes TMs reduction and the poisoning of the counter electrode.

Preferential reduction of solvents solely on LTO particles, caused by their adsorption at the outer surface planes during lithiation.