As of my last knowledge update in September 2021, flexible sodium-sulfur (Na-S) batteries are still an area of ongoing research and development. While sodium-sulfur batteries themselves have been explored for stationary energy storage applications, I don't have specific information about the voltage rating of flexible Na-S batteries, as this technology might have evolved or progressed since then.
In general, the voltage of a battery cell depends on its specific chemistry and design. Traditional sodium-sulfur batteries typically operate at elevated temperatures and can have a voltage range of around 2.1 to 2.6 volts per cell. Flexible battery technologies often aim to maintain similar voltage levels to their rigid counterparts while enabling flexibility and conformability.
For the most up-to-date information on the voltage rating of flexible sodium-sulfur batteries, I recommend checking recent research papers, scientific journals, or news articles from reputable sources that cover advancements in battery technology.