A cheaper way to make a battery’s solid electrolyte

A sulfur-based solid electrolyte (SSE) can be prepared for about one-sixth the price of previous materials, paving the way toward safer lithium-ion batteries (ACS Appl. Energy Mater., 2024, DOI: 10.1021/acsaem.4c02595).

Lithium-ion batteries currently depend on liquid electrolytes, solutions that contain flammable organic solvents. These solvents are responsible for fires and pose significant safety risks in vehicular, robotic, and large-scale grid-storage applications. Solid-state batteries, which use a solid electrolyte rather than a liquid solution, avoid this problem, but so far SSEs have been too expensive to be commercially viable, says Lei Zhou from the University of Electronic Science and Technology of China (UESTC). “For a mass-produced battery, the most sensitive factor is cost,” he says.

Much of the cost arises from expensive lithium sulfide salt, Zhou says. Top-performing SSEs cost about $170 per kilogram.

Zhou, Bin Wang, and Furong Zhang at UESTC and their colleagues noticed that when lithium sulfide is made into an electrolyte, the salt’s crystal structure is destroyed as the substance is thoroughly ground in a ball mill. So the researchers wondered whether they could avoid using the pricey salt by directly grinding elemental lithium and sulfur and using the amorphous mixture as an SSE. But the mixture had poor ionic conductivity, a property that would lead to batteries that charge slowly and hold less energy, Wang says.

The researchers kept tweaking the material, eventually adding a lithium iodide salt to the ball mill. That addition boosted the electrolyte’s performance to 2.73 mS/cm. Top-performing SSEs made with lithium sulfide or germanium still have conductivities that are several times higher, but they cost multiple times more than the new electrolyte, which comes with a price tag of about $28 per kilogram.

Besides further improving the electrolyte’s ionic conductivity, the researchers hope to test it in a small portable battery, Wang says.