As of my last knowledge update in September 2021, flexible lithium-titanate (Li4Ti5O12) batteries were not as commonly discussed or available as more traditional lithium-ion batteries. Lithium-titanate batteries are known for their fast charging capabilities, long cycle life, and safety features. However, flexible versions of these batteries might not have been as extensively developed or commercialized.
Since developments in battery technology can change rapidly, I recommend checking the latest information from reputable sources or battery manufacturers to get the most accurate and up-to-date information about the voltage rating of flexible lithium-titanate batteries if they have become more prevalent since my last update.