As of my last knowledge update in September 2021, flexible magnesium-ion batteries were still a developing technology and not yet widely commercialized or standardized. Therefore, I might not have information on the most current developments or specific voltage ratings of flexible magnesium-ion batteries.
However, traditional lithium-ion batteries typically have a voltage rating of around 3.7 to 3.8 volts per cell. Magnesium-ion batteries, being a potential alternative to lithium-ion batteries, might have similar voltage ratings. It's important to note that the actual voltage rating can depend on the specific chemistry and design of the battery.
For the most up-to-date and accurate information about the voltage rating of flexible magnesium-ion batteries, I recommend checking recent research papers, news articles, or official announcements from relevant research institutions or companies in the battery technology field.