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To investigate the effects of sodium salt in a poly(ethylene oxide) (PEO) electrolyte on the initial discharge properties of a Na/S cell, various concentrations of NaCF3SO3 from 0.1 to 2.0 m were added to the electrolyte. Scanning electron microscopy anddifferential scanning calorimetry results showed a continuous decrease in the crystallinity of the PEO electrolyte withincreasing amounts of sodium salt. The ionic conductivity of the PEO electrolyte at 70 oC increased with increasing sodiumsalt concentration up to 0.33 m, at which point the highest ionic conductivity of 1.732 × 10-4 S cm−1was obtained. However, theionic conductivity decreased at 2.0 m sodium salt. The same trend was seen in the initial discharge capacity, in that the highestvalue of 435 mAh g−1was achieved at a sodium salt concentration of 0.33 m. These results demonstrate that 0.33 m of sodiumsalt is the optimum composition for Na/S batteries.