As of my last knowledge update in September 2021, flexible sodium-ion batteries were still a relatively emerging technology and hadn't yet gained widespread commercial adoption. At that time, the specific voltage ratings of flexible sodium-ion batteries might have varied based on the particular research and development efforts.
However, in general, sodium-ion batteries tend to have lower voltages compared to traditional lithium-ion batteries. Lithium-ion batteries typically have a cell voltage of around 3.7 to 3.8 volts. Sodium-ion batteries, due to the larger size of sodium ions compared to lithium ions, usually have lower cell voltages in the range of 2.0 to 2.5 volts.
Please note that developments in battery technology can change rapidly, and newer information might be available beyond September 2021. If flexible sodium-ion batteries have made significant progress since then, I recommend checking the latest scientific literature or news from reputable sources for the most up-to-date information on their specifications.