A β"-Alumina/Inorganic Ionic Liquid Dual Electrolyte for Intermediate-Temperature Sodium-Sulfur Batteries

Although sodium-sulfur (Na-S) batteries present the great prospects of high energy density, long cyclability, and sustainability, their deployment is heavily encumbered by safety, practicality, and versatility issues engendered by their high operating temperatures above 300 degrees C. Lowering the operating temperatures impedes the performance of Na-S batteries due to the formation of insulating S/polysulfides, diminished Na ion conduction in the beta"-alumina solid electrolyte (BASE), the Na metal dendrite growth at temperatures below its melting point, and the shuttle effect occurring in the absence of the BASE. Herein, a Na-S battery that integrates a dual electrolyte consisting of the BASE and a novel inorganic ionic liquid is proposed for intermediate-temperature operations of 150 degrees C. Investigations reveal the ionic liquid to have high ionic conductivity, wide electrochemical window, and excellent thermal and chemical stability, making it propitious for intermediate-temperature operations. The high reversible capacity of 795 mAh (g-S)(-1) at 0.1 mA (electrode area: 0.785 cm(2)) and an average capacity of 381 mAh (g-S)(-1) achieved over 1000 cycles at 0.5 mA validate the use of ionic liquids in dual electrolyte systems to improve Na-S performance.