Non-flammable electrolyte for dendrite-free sodium-sulfur battery

Room temperature (RT) sodium-sulfur (Na-S) batteries are a promising technology for stationary energy storage thanks to their high energy density of 1274 Wh kg−1 and low cost. However, RT Na-S batteries are hazardous because they use highly volatile and flammable electrolytes. Here, we develop a new nonflammable electrolyte for RT Na-S batteries, consisting of sodium trifluoromethanesulfonimide (NaTFSI) in a mixture of trimethyl phosphate (TMP) and fluoroethylene carbonate (FEC). The nonflammable electrolyte facilitates highly stable and reversible Na plating/stripping during cycles. The dendrite-free Na-S battery with the NaTFSI/TMP+FEC electrolyte delivers a remarkable reversible capacity of 788 ​mAh g−1 after 300 cycles at 1C, corresponding to a negligible capacity decay below 0.04% per cycle. The ab initio molecular dynamics simulations and surface analysis reveal the formation of a NaF-rich solid electrolyte interphase (SEI) film with reduced interfacial resistance thanks to the introduction of FEC into the electrolyte. The formed NaF SEI layer suppresses the growth of Na dendrites on the anode, enhancing the electrochemical performance of the RT Na-S batteries. The new findings reported here will shed new light on dendrite-free RT Na-S batteries by the rational design of nonflammable electrolytes.