Engineering of Electrolyte Solution in Governing the Stability of Sodium Metal Anode for Room Temperature Sodium–Sulfur Batteries

The charge storage properties of room temperature sodium–sulfur (Na–S) batteries are dependent on the electrolyte chemistry, which dictates the interfacial stability between electrode and electrolyte. Here, we introduce a localized high-concentration electrolyte (LHCE) consisting of 6 M sodium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane and diluted with bis(2,2,2-trifluoroethyl)ether in a 1:1.5 molar ratio for Na–S batteries. This LHCE consists of a highly aggregated solvation shell over strongly interacting Na+–FSI– ion pairs and minimal free solvent. Such a solvation structure enables the formation of an inorganic-rich solid electrolyte interphase on the Na metal anode, which suppresses dendrite growth. Moreover, the LHCE shows compatibility with sulfurized polyacrylonitrile (SPAN) cathode without dissolution of soluble Na-polysulfides during cycling. Coupled with a SPAN cathode and 50 μm of Na metal anode, the pouch-type Na–S battery using the LHCE delivers an areal capacity of 2.6 mAh cm–2 and cycling stability after 300 cycles at 1 C.