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.
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.
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.
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.”
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