As of my last knowledge update in September 2021, flexible lithium-titanate (Li4Ti5O12) batteries are not as common as other types of lithium-ion batteries, such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4) batteries. Therefore, I don't have specific information about their voltage ratings at that time.
However, lithium-titanate batteries are known for having a nominal voltage of around 2.4 to 2.5 volts per cell, which is lower than the typical 3.7 to 3.8 volts per cell of other lithium-ion batteries. This is due to the unique characteristics of the lithium-titanate chemistry.
Please note that developments in battery technology can occur rapidly, and new information might be available beyond my last update in September 2021. I recommend checking with manufacturers or reliable sources for the most up-to-date information regarding the voltage rating of flexible lithium-titanate batteries.