As of my last knowledge update in September 2021, flexible sodium-sulfur (Na-S) batteries are still an emerging technology, and I do not have specific information about their voltage ratings. Sodium-sulfur batteries, in general, are known for their high energy density and operating at elevated temperatures, making them suitable for grid energy storage and certain niche applications.
The voltage rating of a sodium-sulfur battery can depend on factors like the number of cells connected in series, the specific electrolyte composition, and the overall design of the battery. Traditional stationary sodium-sulfur batteries used in grid applications typically have a voltage rating in the range of 2.0 to 2.6 volts per cell.
However, I do not have information on the specific voltage ratings of flexible sodium-sulfur batteries or any advancements that might have occurred after September 2021. I recommend checking with recent research papers, scientific articles, or battery manufacturers for the most up-to-date information on flexible sodium-sulfur battery technology.