As useful as they have proven to be in powering our phones, tablets and electric cars, lithium-ion batteries still have their weaknesses. Chief among them is the very real danger of fire when subjected to high temperatures, a risk we have seen materialize recently through blazing hoverboards and Galaxy Note 7s. Replacing the combustible electrolytes and integrating smart chips are a couple of ways researchers might avert the danger, and another, as Stanford scientists have now found, is by building a flame retardant into the battery that releases itself when things start to heat up.
Integrating flame-retarding capabilities into a battery might sound like an out-there concept, but researchers have actually been chipping away at this idea for a while. Different approaches have included making the battery's separator, the polymer membrane that rests between the cathode and anode, from advanced flame-retardant composite materials or incorporating ceramic. But until now, such efforts have tended to compromise the battery's performance, and have been unable to completely eradicate the risk anyhow.
Scientists at Stanford University seem to have found a better way forward. Their battery design packs a common flame retardant called triphenyl phosphate (TPP) inside a shell of polymer microfibers, which is crafted with a technique called electrospinning.
"It is the 'core-shell' structure of the fibers constituting the separators that makes it unique and efficient," the study's first author, Kai Liu, explains to New Atlas. "And the electrospinning is crucial for the fabrication of core shell structured fibers."
Electrospinning is a fabrication technique that involves pushing fluid out of a syringe and through an electric field in order to break it down into microscopic fibers. This method has been used in laboratories to produce dissolving tampons that release HIV drugs and capsules that release food supplements only after they have passed through the digestive system.
In this case, when the battery is operating normally, the electrospun microbfiber shell serves to encase the TPP and stop it from leaking into the battery's electrolyte and ruining its electrochemical performance. But when the temperature reaches 160 degrees Celsius (320 ° F), the polymer shell melts and releases the retardant into the electrolyte, stopping the combustion process in its tracks. While 160 degrees Celsius might seem hot, the researchers say that relatively speaking, this is a low melting point that allows the material to dissolve "before or at the early stage of combustion."
The researchers have tested this design in a coin cell battery and found that in the event of combustion, the TPP quickly extinguishes the flame. They anticipate that this kind of thermally-triggered, fire-extinguishing separator could be worked into larger batteries, though testing would be required to see if it can withstand stresses like mechanical pressure or overcharging on a larger scale.