As of my last knowledge update in September 2021, flexible sodium-ion batteries were still in the research and development phase, and specific voltage ratings might not have been standardized or widely reported. The voltage of a sodium-ion battery can vary depending on the materials used for its electrodes and electrolyte.
Typically, sodium-ion batteries operate at lower voltages compared to lithium-ion batteries. Lithium-ion batteries usually have a nominal voltage around 3.7 to 3.8 volts per cell. Sodium-ion batteries, on the other hand, might have lower nominal voltages, potentially in the range of 2 to 2.5 volts per cell.
Keep in mind that these values are based on general trends, and the actual voltage ratings can vary depending on the specific design and materials of the battery. For the most accurate and up-to-date information on flexible sodium-ion batteries, I recommend checking recent research papers, scientific literature, or manufacturer specifications.