Investigations on Electrolyte Additives and Formation Mechanism of the Solid Electrolyte Interphase for Sodium Ion Batteries

Sodium-ion batteries (NIBs) are promising alternatives to lithium-ion batteries (LIBs) for energy storage. However, NIB technology still faces disadvantages compared to LIBs, one of which is a less-understood solid-electrolyte interphase. Fluoroethylene carbonate (FEC) is the most common electrolyte additive for NIBs owing to its success in LIBs. Here, we report a difluoro-substituted additive, trans-4,5-difluoroethylene carbonate (DFEC), that promotes cycling performance beyond those of FEC-added NIBs. Coin cells with 5% DFEC demonstrated improved capacity retention at C rates as high as 20C while retaining 93.84% of their initial capacity (7000 cycles). Electroanalytical studies demonstrated that the solid-electrolyte interphase (SEI) formed from DFEC is more stable upon cycling. Surface analysis, including in situ Raman, TOF-SIMS, and XPS, demonstrated that the SEI composition for DFEC is richer in inorganic component concentration, promoting Na ion mobility and preventing further SEI growth. Insights from this study could provide alternative solutions for building NIBs with better cycling stability.