Probing the Properties of Locally Formed Solid Electrolyte Interphases on Hard Carbon Anodes

Abstract The solid electrolyte interphase (SEI) formation on hard carbon (HC), as one of the most widely used anode materials in sodium (Na)‐ion batteries, is still not fully understood compared to the SEI formation on anodes used in lithium (Li)‐ion batteries, in terms of passivation properties and stability, which strongly depends on various factors such as experimental parameters and the electrolyte composition. Herein, we report the localized formation of SEI microspots on HC using cyclic voltammetry in combination with scanning electrochemical cell microscopy (SECCM) in non‐aqueous ether‐ and carbonate‐based electrolytes. Using the same instrumental setup for SECCM and for atomic force microscopy (AFM), the locally formed SEI spots could be directly characterized with respect to the morphology, height, passivation and nanomechanical properties in dependence of the experimental deposition parameters such as scan rate and cycling number. In addition, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) in combination with AFM revealed the chemical composition of the SEI layer by recording spatially resolved 3D mass maps of the SEI spots. This combination of high‐resolution microscopic and spectrometric methods provides new insights into the dynamics of SEI formation as a function of the electrolyte and the experimental parameters.