Previous attempts at building mini fire extinguishers inside batteries significantly reduced their performance. Yi Cui, this project's lead scientist, says this doesn't have the same effect and that the chemical won't be released under normal temperatures:

"Using our 'smart' separators, battery electrochemical performance will not be affected by the flame retardant under normal conditions. However, once there is a potential thermal runaway, the flame retardant will be activated and nip the fire or explosion in the bud."

According to IEEE Spectrum, future experiments could include ensuring overcharging and deep discharging don't activate the retardant. You can read up on what the researchers have done thus far in their paper published by Science.