As of my last knowledge update in September 2021, sodium-sulfur (Na-S) batteries typically have a voltage rating in the range of 2.0 to 2.6 volts per cell. These batteries operate at elevated temperatures (around 300-350°C) to maintain the proper ionic conductivity of the sodium and sulfur materials used in the battery's electrodes. The specific voltage of a Na-S battery can depend on various factors including the materials used, cell design, and operating conditions.
Please note that battery technologies and their specifications can evolve over time, so I recommend checking with more recent sources or manufacturers for the most up-to-date information on flexible sodium-sulfur battery voltage ratings.