Capturing Nano‐Scale Inhomogeneity of the Electrode Electrolyte Interface in Sodium‐Ion Batteries Through Tip‐Enhanced Raman Spectroscopy

A prime challenge in the development of new battery chemistries is the fundamental understanding of the generation of the electrode–electrolyte interface (EEI) and its evolution upon cycling. Tip-enhanced Raman spectroscopy (TERS) under an inert gas atmosphere is employed to study the chemical components of the anode/cathode electrolyte interface in a sodium-ion battery. After the first cycle, TERS reveals that the EEI mostly consists of organic carbonate/dicarbonate, oligoethylene oxides, α,β-unsaturated vinyl ketones/acetates, and inorganic species ClO$_4$$^−$, ClO$_3$$^−$, and Na$_2$CO$_3$. Whereas after 5× cycling, the EEI composition has evolved to contain long chain monodentate or bridging/bidentate carboxylates and alkoxides. The TERS map reveals the nano-scale heterogeneity present in the EEI layers and elucidates a multilayered nano-mosaic coating structure. The sheer volume of Raman signature present in the TERS signal can completely unravel the mysteries regarding the chemical composition and may shed light to the physicochemical behavior of the EEI.