Halogenated fire retardants are some of the worst persistent organic pollutants, so like you I looked up triphenyl phosphate to see if it's likely to become a problem in its own right. It looks pretty safe to me when you consider all factors.

In active batteries triphenyl phosphate remains separated from humans. In proper recycling processes it remains separated from humans. In improperly landfilled batteries it can escape containment, but it has negligible vapor pressure at ambient temperatures so it won't spread by air. It rapidly biodegrades in the presence of water so it won't lead to human exposure through waterways or groundwater plumes if a landfill leaks.