As of my last knowledge update in September 2021, flexible sodium-sulfur (Na-S) batteries were still an emerging technology, and specific voltage ratings might not have been standardized or widely reported. Sodium-sulfur batteries typically operate at elevated temperatures to facilitate the necessary chemical reactions, which can make them less suitable for flexible applications compared to other battery chemistries.
Voltage ratings of batteries are determined by the chemistry and configuration of the cells. Traditional Na-S batteries have a nominal voltage of around 2.1 to 2.4 volts per cell. However, if there have been recent developments in flexible Na-S batteries since then, I might not have that information.
I recommend checking the latest research papers, articles, or manufacturers' documentation to get the most up-to-date information on the voltage rating of flexible sodium-sulfur batteries, especially if developments have occurred after September 2021.