  In this study, we propose 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). To optimize this
coupling, we conducted experiments on the sodiation of hard carbon materials
(HC) using two different setups: in situ ec-TEM holder (operating in an "anode
free" configuration, referred to as $\mu$-battery) and ex-situ setup (Swagelok
battery configuration). In the in situ TEM experiments, we intentionally
degraded the electrolyte (NP30) using cyclic voltammetry (CV) and analyzed the
recovered liquid product using GC/MS, while the solid product ($\mu$-chip) was
analyzed using TEM techniques in a post-mortem analysis. The ex-situ
experiments served as a reference to observe and detect the insertion of Na+
ions in the HC, SEI size (389 nm), SEI composition (P, Na, F, and O), and Na
plating. Furthermore, the TEM analysis revealed a cyclability limitation in our
in situ TEM system. This issue appears to be caused by the deposition 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
transforms into a second SEI, is electrochemically inactive and reduce the
cyclability of the battery. Overall, our results demonstrate the powerful
synergy achieved by coupling in situ ec-TEM and GC/MS techniques, which
provides a deeper understanding of the dynamics and behavior of SEI.
Consequently, this knowledge contributes to the advancement of the new
generation of NIB.
