Stanford researchers have developed a rechargeable battery with a built-in fire extinguisher. It’s not a new concept, but this looks to be the first time it would work without affecting battery performance.

As with previous efforts, the researchers used triphenyl phosphate, a flame retardant. When exposed to heat, it forms phosphoric acid and in turn pyrophosphoric acid, which blocks the transfer of heat. The problem to date has been that putting triphenyl phosphate directly into the electrolytes in a battery have compromised conductivity and thus affected performance.

The Stanford tactic was to house the triphenyl phosphate inside a custom-made microfiber shell built from poly(vinylidene fluoride–hexafluoropropylene). That’s a material with two key characteristics: it won’t be affected by the electrolytes, but will dissolve at around 160 degrees Celsius.

That means in the event of a battery fire it can melt and unleashes the flame retardant before the battery moves past burning to full-on explosion. In testing, the flames were extinguished within 0.4 seconds.

While the system has some obvious uses, most notably for batteries in phones and tablets, the researchers note more testing is needed. In particular they want to check if the shell would have any problems when exposed to physical stress or problems such as the battery being overcharged.