As of my last knowledge update in September 2021, flexible magnesium-ion batteries were still an emerging technology, and specific voltage ratings might not have been standardized or widely reported. However, I can provide you with some general information.
Traditional lithium-ion batteries typically have a nominal voltage of around 3.6 to 3.7 volts per cell. Since magnesium-ion batteries are being developed as an alternative to lithium-ion batteries, they would likely have a voltage range that's comparable.
It's important to note that battery technology is rapidly evolving, and new developments might have occurred since my last update. To get the most accurate and up-to-date information on the voltage rating of flexible magnesium-ion batteries, I recommend checking recent research papers, news articles, or official sources from battery manufacturers and research institutions.