Structures and Electrochemistry of γ‑Butyrolactone Solvates of Na Salts

Phase behaviors, solvate structures, Na+ transport properties, electrochemical stability, and battery performance of binary Na salt and γ-butyrolactone (GBL) mixtures have been studied systematically. A concentrated [sodium bis­(fluorosulfonyl)­amide (NaFSA)]/[GBL] = 1/1 mixture remains in the liquid state at room temperature, and a single crystal of the same composition is obtained below room temperature. The Na metal/electrolyte interface is stabilized, and continuous electrolyte decomposition is suppressed in molten [NaFSA]/[GBL] = 1/1, whereas GBL solvates of NaN­(SO2CF3)2 (NaTFSA) and NaPF6 exhibit a continual increase in interfacial resistance at the Na metal electrode. Solvent-bridged (Na+–GBL–Na+) and anion-bridged (Na+–FSA––Na+) structures are observed in the [NaFSA]/[GBL] = 1/1 single crystal. These ligand-bridged structures produce a remarkably high Na+ transference number of 0.84 due to dynamic ligand-exchange conduction of Na+ in the electrolyte. Although the ionic conductivity of [NaFSA]/[GBL] = 1/1 is less than that of typical Na+-based electrolytes, the rate capability of a Na0.44MnO2 electrode in molten [NaFSA]/[GBL] = 1/1 is slightly greater than that in 1 mol dm–3 NaPF6/propylene carbonate. This observation is attributed to a high Na+ transference number, which suppresses the development of a concentration gradient in Na+ batteries under anion-blocking conditions. These observations indicate that not only ionic conductivity but also Na+ transference number are essential electrolyte properties for achieving high rate capabilities in Na batteries.