Lithium-ion batteries commonly used in consumer electronics are notorious for bursting into flame when damaged or improperly packaged. Inspired by the unusual behavior of some liquids that solidify on impact, researchers have developed a practical and inexpensive way to help prevent these fires.
In a lithium-ion battery, a thin piece of plastic separates the two electrodes. If the battery is damaged and the plastic layer fails, the electrodes can come into contact and cause the battery's liquid electrolyte to catch fire. To make these batteries safer, some use a nonflammable, solid electrolyte. These solid-state batteries require significant retooling of the current production process.
As an alternative, the new design mixes an additive into the conventional electrolyte to create an impact-resistant electrolyte. It solidifies when hit, preventing the electrodes from touching if the battery is damaged during a fall or crash. If the electrodes don't touch each other, the battery doesn't catch fire. Incorporating the additive would require only minor adjustments to the conventional battery manufacturing process.
The reversible “shear thickening” behavior depends on a colloid, which is a suspension of tiny, solid particles in a liquid. For the battery colloid, silica suspended in common liquid electrolytes was used for lithium-ion batteries. On impact, the silica particles clump together and block the flow of fluids and ions. Perfectly spherical, 200-nanometer-diameter particles of silica — or essentially a superfine sand — were used. The very uniform particle size enables them to disperse homogeneously in the electrolyte; otherwise, the liquid becomes less viscous on impact.
During manufacture of traditional lithium-ion batteries, an electrolyte is squirted into the battery case at the end of the production process and then the battery is sealed. This cannot be done with a shear-thickening electrolyte because as soon as it is injected, it solidifies. The problem was solved by putting the silica in place before adding the electrolyte.
Future work will involve enhancing the system so the part of the battery that's damaged in a crash would remain solid, while the rest of the battery would continue working.