Weakly Polar Ether-Aided Ionic Liquid Electrolyte Enables High-Performance Sodium Metal Batteries over Wide Temperature Range

Sodium (Na) metal batteries (SMBs) have attracted many attentions because of high specific capacity and low redox potential. However, SMBs encounter several crucial challenges including Na dendrites growth, severe interfacial side reactions and safe issues from liquid organic electrolyte. In this study, a non-flammable 1,2-bis (1,1,2,2-tetrafluoroethoxy) ethane (TFEE) aided ionic liquid (IL) electrolyte (TILE) is reported with sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) salt and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([Py13][FSI]) IL solvent. The IL solvent contains plentiful organic cations and anions, which helps to provide a cationic shielding effect by [Py13]+ cations and regulate the Na+ solvation structure with more FSI- and TFSI- anions. The unique solvation structure of Na+ facilitates a stable inorganic-rich solid electrolyte interphase (SEI), effectively inhibiting unstable Na deposition and suppressing side reactions between the electrolyte and Na-metal in SMBs. Therefore, the Na3V2(PO4)3 (NVP)||Na cells with TILE can deliver good rate capability and superior cycling performance with capacity retention of 95.5% and Coulombic efficiency of approximate to 99.9% after 2000 cycles at 5C rate and 25 degrees C. The high-voltage NaNi0.33Fe0.33Mn0.33O2(NFM)||Na cells with TILE can also deliver good cycling performance with capacity retention of 92% after 150 cycles in the voltage range of 2.0-4.1 V at 0.5C rate and 25 degrees C. A weakly polar ether-assisted ionic liquid electrolyte with non-flammability is reported to enable high-performance sodium metal batteries over a wide temperature range, which provides a cationic shielding effect to facilitate Na+ uniform deposition and regulate the Na+ solvation structure with more FSI- and TFSI- anions to generate an anion-derived inorganic-dominated SEI layer. image