As of my last knowledge update in September 2021, sodium-ion batteries were still in the research and development phase, and commercial products might not have been widely available yet. However, I can provide you with some general information.
The voltage rating of a sodium-ion battery, like any other battery, depends on its specific chemistry and design. Sodium-ion batteries typically use sodium ions as the charge carriers, instead of lithium ions as in lithium-ion batteries. The voltage of a battery cell depends on the electrochemical potential difference between the anode and cathode materials.
The voltage of a sodium-ion battery can vary, but it's generally expected to be lower than that of lithium-ion batteries. Lithium-ion batteries typically operate around 3.6 to 4.2 volts per cell, depending on the specific chemistry. Sodium-ion batteries may operate at lower voltages, potentially in the range of 2 to 3 volts per cell.
Keep in mind that battery technologies are rapidly evolving, and new developments might have occurred after September 2021. To get the most accurate and up-to-date information about the voltage rating of a specific flexible sodium-ion battery, I recommend checking recent research papers, manufacturer specifications, or news articles related to sodium-ion battery technology.