Revisiting Lithium-Sulfur Batteries

Continued from page 1

In terms of addressing safety issues, three advances could account for Weber's confidence. Methods of chemically treating lithium metal electrodes can prevent at least some dendrite formation, although not all researchers are convinced that this approach will be sufficient. Also, improved polymer and ceramic membranes that separate the two electrodes and resist being pierced by the dendrites could prevent short circuits. The batteries, however, could still be vulnerable to short circuit if they're damaged. To prevent electrolyte fires, Nazar says that less volatile electrolytes could be used with lithium-sulfur batteries because they have lower voltage than lithium-ion batteries.

Other issues, including low conductivity and a limited number of recharge cycles, seem to have been addressed at least in part by Sion Power. The company has produced cells that store more than twice as much energy as lithium-ion batteries available today, something BASF hopes to improve. And Weber says that the batteries can last the lifetime of a car, although this may be based on projections from Sion Power, rather than measured performance.

John Kopera, Sion Power's director of commercial operations, says that the company's current batteries are rated for 50 cycles, and that it has a "comprehensive plan" to reach about 1,000 cycles. (That's enough for as much as 300,000 miles of driving, with a battery pack that provides a 300-mile range.)

Both companies are keeping details of their advances to themselves. But this week, in the journal Nature Materials, Nazar described one possible approach to solving these problems. In the past, researchers have improved conductivity by combining sulfur with carbon. Nazar went a step further by taking electrodes composed of regularly spaced carbon tubes and making them just a few nanometers wide. (Their structure is different from that of carbon nanotubes.) Nazar's team then packed sulfur into the nanoscale spaces between these tubes, so that most of the sulfur atoms sit close to conductive carbon, making them accessible to both electrons and lithium ions.

The carbon tubes also helped solve the issue of polysulfides, which can kill a cell prematurely. The carbon tubes effectively trap the polysulfides in place until they are fully converted to dilithium sulfide, which does not poison the battery. Coating the carbon with a polymer that has an affinity for polysulfides also helps keep them in place. But it's not clear whether BASF might also try a nanostructured electrode to improve Sion's materials. So far, Sion Power has not used nanostructured materials, Kopera says. One challenge with Nazar's approach is that it will be difficult to manufacture the carbon tube electrodes in high volumes.

Some issues likely remain. For one thing, the batteries may be costly--lithium metal is the most expensive form of lithium. Also, firm data isn't yet available on how many recharge cycles the batteries can undergo and how they respond to safety tests. Still, Nazar says, the technology has "certainly come a long way. Our developments and those of a couple of other companies are certainly enabling it to be much closer to reality."