Today’s lithium-ion batteries use liquid electrolytes that can catch fire if a battery overheats. Solid electrolytes made of nonflammable polymers and ceramics are safer, but their performance isn’t good enough for practical use yet. Researchers now report a new liquid-state polymer electrolyte that is both nonflammable and just as conductive as today’s electrolytes (Matter 2022, DOI: 10.1016/j.matt.2022.11.003).
Conventional electrolytes become unsafe beyond 60 °C. The novel electrolyte works at temperatures of up to 100 °C. Because it uses the same salts and solvents used in today’s electrolytes, it should be relatively straightforward to incorporate into existing batteries. It also shows promise for use in emerging lithium metal battery technology, says Zhenan Bao, a chemical engineer at Stanford University.
Today’s liquid electrolytes are made of lithium salts dissolved in volatile organic solvents. These solvents help shuttle ions between the two electrodes. Polymer and ceramic electrolytes cannot do that as well, and they also do not form a high-contact interface with the electrodes, which further hinders the flow of ions and affects performance. Part of what holds back the performance of these polymers, in particular, is their rigid backbones, which restrict the movement of lithium ions.
So Bao, materials scientist and engineer Yi Cui, and their colleagues chose relatively gooey polysiloxane, which has a flexible backbone, for their electrolyte. Bao and Cui’s team started by adding lithium salt to a viscous solution of the polysiloxane monomers and dimethoxyethane solvent. At first they found little success. The lithium ions interacted with the anions on the monomers, cross-linking them and making the polymer rigid, reducing ionic conductivity, Bao explains. But to their surprise, when they added extra salt, increasing the salt-to-polymer ratio to 8:1, the mixture became gooey again.
Detailed studies using spectroscopy and magnetic resonance showed that in the salty electrolyte, the solvent molecules were tethered to both lithium ions and anions on the polymer side chains. This reduced the crosslinking of the polymer, making it softer and more conductive, while also dampening the solvent’s flammability.
Glass wool soaked with the new electrolyte did not catch fire when exposed to a direct flame for a few seconds, while conventional electrolyte ignited. Small battery cells made with the electrolyte and commercial lithium-ion battery anodes and cathodes could light an LED even when placed on a hot plate. The cells could be recharged 400 times; batteries today typically last for 1,000 recharge cycles. “Additional testing will be needed to see if it meets other requirements such as manufacturability and long-term industrial level testing,” Bao says.
Polysiloxane has other properties that could make it a good choice for lithium-metal batteries, in particular. It is chemically inert, which should reduce its interactions with the lithium electrode. Lithium’s reactivity with electrolyte chemicals tends to shorten these batteries’ useful lifetime.
Other researchers have made liquid electrolytes nonflammable by using additives, but these accelerate the degradation of graphite anode, shortening the battery’s useful lifetime, says Pacific Northwest National Laboratory battery researcher Jiguang (Jason) Zhang. He says that the new electrolyte “has great potential for practical application.”