Researchers at Stanford University, California, have developed a “smart” phosphorus flame retardant solution for lithium ion batteries. The phosphorus flame retardant is encapsulated in electrospun microfibres, held in the electrolyte between the cathode and anode.
The microfibers have a polymer shell (PVDF-HFP poly(vinylidene fluoride – hexafluoroporpylene) is proposed) within which is a phosphorus flame retardant (TPP triphenyl phosphate is proposed). This avoids the need to dissolve the flame retardant in the electrolyte, which tends to deteriorate battery electrical performance. The flame retardant is released in case of battery overheating, because the polymer microfibre shell melts (at around 160°C). Tests show that electrolyte flames can be extinguished in less than half a second.
“Fire-fighting lithium–ion battery doesn’t compromise on performance”, Chemistry World 17th January 2017 https://www.chemistryworld.com/news/fire-fighting-lithiumion-battery-doesnt-compromise-on-performance/2500264.article and “Electrospun core-shell microfiber separator with thermal-triggered flame-retardant properties for lithium-ion batteries”, K. Liu et al., Sci. Adv. 2017 – 3: e1601978 http://advances.sciencemag.org/content/3/1/e1601978