An Exploration of the Sodium Solid Electrolyte Interphase in Glyme Electrolytes with Carbonate Additives

Abstract

The utilization of alkali metal anodes is hindered by an inherent instability in organic electrolytes. Sodium (Na) is of growing interest due to its high natural abundance, but the carbonate electrolytes that are popular in lithium systems are unable to form a stable solid electrolyte interphase (SEI) with a sodium metal electrode. However, the glyme (chain ether) electrolytes produce thin, predominantly inorganic SEI at sodium metal interfaces. Using half-cell and symmetric cell analysis, we identify diglyme (G2) as the best performing of the glymes, balancing the high nucleation barrier of the short glyme (G1) and the high plateau overpotential of the long glyme (G4). Based on mesoscale modeling, we capture the critical dependence of the nucleation and early-stage growth morphology on a competing set of mechanisms, including the reduction kinetics on the substrate/newly deposited Na and the surface mobility of these deposits. Through in situ optical microscopy, the onset and growth of Na dendrites is revealed in glyme electrolytes, and the addition of small quantities (~10% volume/volume) of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) to G2 is shown to facilitate uniform sodium plating characteristics in the optical cell, presumably through alterations to SEI composition. X-ray photoelectron spectroscopy (XPS) analysis reveals that the FEC additive results in an SEI with similar atomic composition to that formed in G2 alone, whereas addition of EC to G2 results in an entirely different SEI composition, despite the molecular similarity of the carbonate additives. We have determined that the SEI formed by glyme alone may not support extensive or extreme cycling conditions, but the addition of FEC provides a much more robust SEI at the Na metal surface to facilitate numerous consistent sodium plating and stripping cycles.