As of my last knowledge update in September 2021, sodium-air (Na-Air) batteries are still in the early stages of research and development, and there isn't a widely accepted or standardized voltage rating for a typical flexible sodium-air battery. The voltage rating of a battery depends on several factors, including the specific materials used for the electrodes and electrolytes, the design of the battery, and the operating conditions.
Sodium-air batteries are a type of metal-air battery that uses sodium as the anode and ambient oxygen as the cathode to generate electricity through a chemical reaction. They have the potential for high energy density and could be used in various applications, including flexible and portable devices. However, there may have been advancements or developments in sodium-air battery technology since my last update.
To get the most accurate and up-to-date information on the voltage rating of a flexible sodium-air battery, I recommend checking recent scientific literature, research papers, or news articles from reputable sources in the field of battery research and energy storage.