The reason why lithium ion batteries can start burning so easily is that the electrolytes necessary to enable the exchange of electrons between cathode and anode are flammable and highly reactive. Though battery manufacturers have tried to minimize this risk through internal protective covers or by adding flame retardants, the risk persists, acknowledged Stanford researcher Kai Liu. In addition, these measures have side effects: They reduce the energy density and ion mobility which in turns reduces the battery performances.
The Stanford research team added a separating layer to the battery. Consisting of two normally separated components, this additional component releases flame retardant agents if the battery is overheated. During normal operation, the flame retardant is encapsulated and does not affect battery performance. The component replaces conventional separators, typically a layer of microporous membranes. The technology developed by Kai Liu and his colleagues is also based on microporous material; however, it is made of microfiber compound balance tissue. Each fiber of this tissue contains a core of triphenylphosphate (TPP), a widespread flame retardant. Around this core there is a polymer layer that separates it from the electrolyte. This polymer features a low melting point; however, during normal battery operating temperatures it is stable. During this normal operation, the new separator exhibits similar electric characteristics as conventional polyethylene-based separators. As soon as the polymer melting temperature is reached, the fibers melt, releasing the encapsulated TPP.
In the tests carried out by the Californian researchers, temperatures of 160 degrees Celsius triggered a complete release of the flame-retardant agent into the electrolyte. The researchers even tried to set fire on the mixture of flame retardant and electrolyte. As a result, the flames went out immediately. “Within 0.4 seconds, they were completely quenched”, the scientists reported.
Related news: https://advances.sciencemag.org/content/3/1/e1601978