Revisiting Lithium-Sulfur Batteries

Advances could at last make the high-energy batteries practical.

Lithium-sulfur batteries, which can potentially store several times more energy than lithium-ion batteries, have historically been too costly, unsafe, and unreliable to make commercially. But they're getting a fresh look now, due to some recent advances. Improvements to the design of these batteries have led the chemical giant BASF of Ludwigshafen, Germany, to team up with Sion Power, a company in Tucson, AZ, that has already developed prototype lithium-sulfur battery cells.

Sulfur power: This prototype lithium-sulfur battery was developed by Sion Power. In cooperation with BASF, the company intends to improve the battery design for use in electric vehicles. Credit: Sion Power

"Compared to existing technologies used in electric vehicles, the plan is to increase driving distance at least 5 to 10 times," for a given-size battery, says Thomas Weber, CEO of a subsidiary of BASF called BASF Future Business. Other experts say that a threefold improvement is a more reasonable estimate, but that would still be an impressive jump in performance. Weber says that BASF's expertise in materials will help Sion Power further improve its technology and bring it to market faster. He declined to provide details of the arrangement, however, including how much money is involved and how the companies will share any profits.

Lithium-sulfur batteries have one electrode made of lithium and another made of sulfur that is typically paired with carbon. As with lithium-ion batteries, charging and discharging the battery involves the movement of lithium ions between the two electrodes. But the theoretical capacity of lithium-sulfur batteries is higher than that of lithium-ion batteries because of the way the ions are assimilated at the electrodes. For example, at the sulfur electrode, each sulfur atom can host two lithium ions. Typically, in lithium-ion batteries, for every host atom, only 0.5 to 0.7 lithium ions can be accommodated, says Linda Nazar, a professor of chemistry at the University of Waterloo.

Making materials that take advantage of this higher theoretical capacity has been a challenge. One big issue has been that sulfur is an insulating material, making it difficult for electrons and ions to move in and out. So while each sulfur atom may in theory be able to host two lithium ions, in fact often only those atoms of sulfur near the surface of the material accept lithium ions.

Another problem is that as the sulfur binds to lithium ions, eventually forming dilithium sulfide, it forms a number of intermediate products called polysulfides. These dissolve in the battery's liquid electrolyte and eventually can settle in other areas of the battery, where they can block charging and discharging. Because of this, the battery can stop working altogether after only a few dozen cycles.

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What's more, the lithium metal electrode presents potential safety problems. For example, during use, the lithium electrode can grow branchlike structures that increase the impedance of the cell, causing it to heat up. Eventually these structures can cause a short circuit. If the battery heats up, the metal can melt. If the molten lithium leaks out of the cell and comes into contact with water, it can start a fire. The battery's electrolyte can also catch fire.

Although he declined to give specifics, Weber says these safety issues have been solved. BASF's goal is to further improve the materials to access more of their theoretical capacity, something he says the company has a clear plan for doing.