As of my last knowledge update in September 2021, flexible lithium-titanate (Li4Ti5O12) batteries were not as commonly discussed or commercially available as other types of lithium-ion batteries. However, the voltage rating of a typical lithium-titanate battery is similar to that of other lithium-ion batteries, which is around 3.6 to 3.7 volts per cell.
It's important to note that battery technology is rapidly evolving, and new developments may have occurred since my last update. I recommend checking with recent and reputable sources or battery manufacturers for the most up-to-date information on the voltage rating of flexible lithium-titanate batteries.