Coupling Liquid Electrochemical TEM and Mass‐Spectrometry to Investigate Electrochemical Reactions Occurring in a Na‐Ion Battery Anode

A novel approach for investigating the formation of solid electrolyte interphase (SEI) in Na‐ion batteries (NIB) through the coupling of in situ liquid electrochemical transmission electron microscopy (ec‐TEM) and gas‐chromatography mass‐spectrometry (GC/MS) is proposed. To optimize this coupling, experiments are conducted on the sodiation of hard carbon materials (HC) using two setups: in situ ec‐TEM holder and ex situ setup. Electrolyte (NP30) is intentionally degraded using cyclic voltammetry (CV), and the recovered liquid product is analyzed using GC/MS. Solid product (µ‐chip) is analyzed using TEM techniques in a post‐mortem analysis. The ex situ experiments served as a reference to for insertion of Na+ ions in the HC, SEI size (389 nm), SEI composition (P, Na, F, and O), and Na plating. The in situ TEM analysis reveals a cyclability limitation, this issue appears to be caused by the plating of Na in the form of a “foam” structure, resulting from the gas release during the reaction of Na with DMC/EC electrolyte. The foam structure, subsequently transformes into a second SEI, is electrochemically inactive and reduces the cyclability of the battery. Overall, the results demonstrate the powerful synergy achieved by coupling in situ ec‐TEM and GC/MS techniques. In this study, a novel approach for investigating the formation of solid electrolyte interphase (SEI) in Na‐ion batteries through the coupling of in situ liquid electrochemical TEM and gas‐chromatography mass‐spectrometry is proposed. To optimize this coupling, experiments are conducted on the sodiation of hard carbon materials using two different setups: in situ ec‐TEM holder and ex situ setup.