A stable electrode/electrolyte interface is an essential requirement to ensuring safe operation and long-life span of Li-ion batteries. The challenge of assessing such stability relates to the difficulty of determining the exact thermodynamic stability window of the electrolytes. Indeed, the strong mismatch between the theoretical and experimental values and their dependence on the investigated electrode remains a major hurdle in the field. Surprisingly, the presence onf a surface layer, as a result of electrolyte decomposition, has been reported on Li₄Ti₅O₁₂ (LTO) electrodes operating at a voltage plateau where the electrolyte components are expected to remain stable. Despite the considerable work aimed at elucidating the origin, thickness and homogeneities of such surface layer, there is still a lack of knowledge on the reaction mechanism behind its formation. Here we propose the use of a spatially-resolved technique, the X-Ray PhotoEmission Electron Microscopy (XPEEM), to gain information on single particles of the composite electrode at different stages of cycling. By combining nanoscale microscopy capabilities with local spectroscopic features within the first 3-4 nm from the surface, we are able to gain unique information on the different particles of the LTO surface. The high lateral resolution achieved with the XPEEM on a LTO electrode is attested in Figure 1.A showing the elemental distribution of the active material (blue), carbon (red) and binder (green) particles. The local XAS spectra acquired at the C K-edges on carbon (Figure 1.B) and LTO (Figure 1.C) areas provide a straightforward signature of their different reactivity upon cycling. Indeed, we demonstrate the presence of a surface layer solely on top of the LTO particles and only on lithiated samples. Thus, we provide an innovative solution to study the complex electrode/electrolyte interaction and propose a mechanism that explains the surface layer formation and the distribution across the LTO electrode, opening the way to achieving an optimized electrode and electrolyte engineering.