Revisiting Lithium-Sulfur Batteries  

Technology Review reports that new advances may make high energy lithium-sulphur batteries practical at last - Revisiting Lithium-Sulfur Batteries.

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.

"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.