Electrolyte optimization for sodium-sulfur batteries

Abstract

Due to high theoretical capacity, low cost, and high energy density, sodium-sulfur (Na-S) batteries are attractive for next-generation grid-level storage systems. However, the polysulfide shuttle leads to a rapid capacity loss in sodium-sulfur batteries with elemental sulfur as the cathode material. Most previous studies have focused on nanoengineering methods for creating stable Na anodes and S cathodes. A proven strategy to mitigate the shuttle effect is to covalently bond elemental sulfur to a polymeric backbone and use it as the active ingredient instead of elemental sulfur. In this regard, we synthesized sulfurized polyacrylonitrile (SPAN) cathodes. In addition to the electrodes, electrolyte selection is crucial for sodium sulfur batteries with long cycle life, high energy densities, and rate capabilities. Thus, we explored various electrolyte compositions; specifically organic solvents such as propylene carbonate (PC), dioxolane (DOL), dimethoxyethane, and diglyme (DIG) were mixed in different proportions to create electrolyte solvents with both ethers and carbonates to promote the formation of bilateral solid electrolyte interphase (SEI). This bilateral SEI strategy has been employed to prevent polysulfide shuttle and dendrite growth in lithium-sulfur batteries. Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) was chosen as the electrolyte salt. The prepared coin cells were tested for rate capability and capacity retention, and the results have been analyzed. High initial discharge capacity of ∼740 mAh g−1 with ∼66% capacity retention over 100 cycles was observed for 0.8M NaTFSI in PC50DOL50 (v/v). The cell with 0.8M NaTFSI in PC50DIG50 has exhibited strong capacity retention of 74.60% with excellent Coulombic efficiency of 99%. Molecular dynamics (MD) simulations were carried out to further understand these results.