As of my last knowledge update in September 2021, flexible magnesium-ion batteries were an emerging technology and specific voltage ratings for typical flexible magnesium-ion batteries might not have been widely established or standardized.
Battery technology is a rapidly evolving field, and new developments may have occurred since then. I recommend checking the latest research papers, articles, or manufacturer specifications for the most up-to-date information on the voltage ratings of flexible magnesium-ion batteries.