Salt-in-presalt electrolyte solutions for high-potential non-aqueous sodium metal batteries

Room-temperature non-aqueous sodium metal batteries are viable candidates for cost-effective and safe electrochemical energy storage. However, they show low specific energy and poor cycle life as the use of conventional organic-based non-aqueous electrolyte solutions enables the formation of interphases that cannot prevent degradations at the positive and negative electrodes. Here, to promote the formation of inorganic NaF-rich interphases on both negative and positive electrodes, we propose the salt-in-presalt (SIPS) electrolyte formulation strategy. In SIPS, sodium bis(fluorosulfonyl)imide (NaFSI) salt is dissolved in the liquid precursor of the sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salt, that is, N,N-dimethyltrifluoromethane-sulfonamide, called PreTFSI. The prepared 0.5 M NaFSI in PreTFSI (SIPS5) electrolyte solution shows an electrochemical stability up to 6.7 V versus Na|Na⁺ and enables a Na stripping/plating average Coulombic efficiency of 99.7% at 2.0 mA cm^-2 and 4.0 mAh cm^-2 in Na||Al cell configuration. By testing SIPS5 in Na metal and 'anode-less' coin and pouch cell configurations using NaNi_0.6Mn_0.2Co_0.2O₂ or sulfurized polyacrylonitrile as positive electrode active materials, we demonstrate the ability of the SIPS strategy to deliver improved specific discharge capacity and capacity retentions at high cell potentials and moderate applied specific currents for cell cycle life up to 1,000 cycles.