A MALDI–TIMS–TOF Method Tailored for Low-Mass and High-Ion-Mobility Ions Enables the Molecular Characterization of Passivation Layers in Lithium-Ion Batteries

During the initial charge of a lithium-ion battery, electrolyte components undergo reductive decomposition at the surface of the negative electrode, leading to the formation of a nanometric passivation layer known as the solid electrolyte interphase (SEI). This interphase, composed of inorganic and organic species, plays a critical role in ensuring ionic conductivity while preventing continuous electrolyte degradation upon cycling. Despite its importance for cell performance and lifetime, comprehensive molecular-level characterization of the SEI remains a major analytical challenge. Matrix-assisted laser desorption/ionization high-resolution mass spectrometry (MALDI-HRMS) affords a direct analysis of the passivation layers of LiB electrodes. However, for such complex samples containing molecules composed of a wide variety of chemical elements (H, Li, C, N, O, F, Al, P, and S) without a specific isotopic pattern, MALDI–HRMS alone may not provide sufficient molecular information. Therefore, coupling it with ion mobility spectrometry (IMS) can offer a more comprehensive visualization of the SEI composition. This work presents a trapped ion mobility spectrometry–HRMS method dedicated to the study of small molecules with low m / z and very high ion mobility like the SEI’s inorganic lithium salts. This method was successfully applied on electrolytes in electrospray ionization (ESI) and on negative electrodes in MALDI. When applied to cycled electrodes of various electrolyte compositions, this method enabled the generation of two-dimensional heatmaps. The additional mobility dimension allowed the separation of organic vs inorganic small molecules, facilitating the molecular assignment and offering additional structural information on the composition of SEI.