As of my last knowledge update in September 2021, flexible sodium-ion batteries were an emerging technology, and specific voltage ratings might not have been standardized or widely reported. The voltage of a battery depends on its chemistry and design, and it can vary depending on the materials used for the anode, cathode, and electrolyte.
Traditional lithium-ion batteries typically have a nominal voltage of around 3.7 to 3.8 volts per cell. Sodium-ion batteries, being an alternative to lithium-ion batteries, might have a similar voltage range, but this can differ based on the specific materials used and the overall design of the battery.
For the most accurate and up-to-date information, I recommend checking recent scientific literature, manufacturers' specifications, or news articles related to flexible sodium-ion battery technology.