Physicochemical investigation on the hard carbon interface in ionic liquid electrolyte

An investigation of the solid electrolyte interphase (SEI) on hard carbon (HC) anodes in aprotic sodium batteries was carried out by post-mortem analyses. Electrodes cycled in 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI), and N-trimethyl-N-butylammonium bis(fluorosulfonyl)imide (N₁₁₁₄FSI) ionic liquid electrolytes have been collected and studied: benchmark electrodes cycled in commercial organic solutions were examined for comparison purpose. The SEI composition and morphology of post-mortem HC electrodes were analyzed by X-ray Photoelectron Spectroscopy (XPS) and focused ion beam scanning electron microscopy (FIB-SEM) analysis. Overall, the HC electrodes cycled in carbonate-based electrolyte have shown thicker SEI mainly composed of organic compounds, whereas in ILs electrolytes they have shown thinner layer richer in inorganic species, such as NaF, Na₂CO₃, N-containing species, and “small” sulfide-based compounds, these improving the SEI interfacial properties towards the Na⁺ migration kinetics. Focusing on ILs based cell formulations, anodes cycled in the EMIFSI-based electrolyte shows several cracks on its surface.