To address the safety concern for room-temperature sodiumsulfur batteries, such as fires easily triggered by organic electrolytes, an applicable non-flammable electrolyte formula was developed by introducing trimethyl phosphate (TMP) into 1.0 M NaClO 4 -ethylene carbonate/propylene carbonate electrolyte. It is demonstrated that the electrolyte containing 15 wt.% TMP was a optimized formula, exhibiting nonflammability, thermal stability, electrochemical compatibility and lasting cell performance. Using this optimized electrolyte formula, the sulfur cathode exhibited a stable capacity of 788 mAh g À 1 at a rate of 0.1 C and excellent rate capability of 441 and 177 mAh g À 1 for 200 cycles at rates of 1 C and 5 C, respectively. When the TMP content further increases to 25 wt.%, the specific capacity of batteries declined significantly due to unstable SEI. Our study can shed light on the development of flameretardant electrolyte for room-temperature sodium-sulfur or other batteries.Rechargeable Li-S batteries have attracted tremendous attention over the last decade, for the extremely high capacity (1675 mAh g À 1 ), natural abundance and environmental friendliness of elemental sulfur. However, due to insufficient industrial supply limited by extraction difficulty from natural lithium resources, it is meaningful to develop sodium-sulfur (Na-S) secondary batteries by utilizing the much cheaper and more available sodium as the charge carrier. Actually, the most classic Na-S battery, operated at around 300°C, has been investigated and successfully commercialized since 1960s. [1] However, its extensive use is restricted by serious potential security risks and low utilization of S caused by the high operating temperature. [2] To improve these deficiencies, room-temperature Na-S (RT Na-S) batteries have been developed since 2006. [3] One obstacle to develop high performance RT Na-S battery is the insulating nature of sulfur and Na 2 S, thus various highlyconducting materials, including multiwall carbon nanotubes (MWCNT), [4] carbon hollow sphere, [5] porous carbon, [6] carbonized metal-organic framework [7] and sulfurized polyacrylonitrile (PAN) [8] have been used as substrates. In addition, thanks to the complex three-dimensional structure, these hosts are also able to prevent loss of the soluble sodium polysulfides (NaPS), further suppressing the 'shuttle effect' and improving the electrochemical performance. Porous carbon become one of the best hosts because its high surface area and pore volume. [7b] Another decisive factor of electrochemical performance is electrolyte. Polymer electrolytes were used in early RT Na-S batteries, however, only low energy density and short cycle life were obtained due to the low ionic conductivity of polymer electrolyte at room temperature. [3,9] Ether-based electrolytes, which showed excellent electrochemical performance in the Li-S system, [10] have been demonstrated has higher solubility for NaPS than LiPS, [7b,11] causing serious 'shuttle effect' and mediocre capacities in RT...
