Lithium-Sulfur Batteries
Lithium-Sulfur batteries (LSBs) use a lithium metal anode and a soluble polysulfide cathode. Lithium ions are stripped from the anode during discharge and form lithium polysulfides in the cathode. Li2S in the cathode is the result of complete discharge.
On recharge, the lithium ions are plated back onto the anode as the lithium polysulfides in the cathode move towards S8. High order Li-polysulfides (Li2S3 to Li2S8) are soluble in the electrolyte and migrate to the anode, scrubbing off any dendrite growth.
Lithium-sulfur battery. Click to enlarge.
The theoretical specific energy of a lithium-sulfur battery chemistry is in excess of 2,500 Wh/kg with a theoretical energy density of 2,600 Wh/L.
LSBs have a number of issues, including cycle life and operation at higher temperatures. Among the limiting mechanisms, according to Sion, are the rough lithium surface on the anode during cycling and Li/electrolyte depletion.

In 2010, Sion, a Brookhaven National Laboratories spin-off, received a three-year research grant worth up to $5 million from the United States Department of Energy Advanced Research Projects Agency - Energy (ARPA-E) (earlier post) for the development of practical, economical and safe lithium-sulfur (Li-S) batteries for powering electric vehicles. Sion’s award was one of 10 made to advanced battery projects by ARPA-E during this second round of project funding.

Performance targets for this program are to exceed 500 Wh/kg and 500 cycles at commercially viable recharge rates. By 2016, the goal is to produce a cell with 600 Wh/kg and 1,000 cycles. Sion Power believes that by utilizing Li-S technology, a battery pack weighing less than 700 lbs (318 kg) can power a 3,500 lb (1,588 kg) five-passenger vehicle more than 300 miles (483 km).

In 2009, Sion Power Corporation and BASF SE signed a Joint Development Agreement (JDA) to accelerate the commercialization of Sion Power’s proprietary lithium-sulfur (Li-S) battery technology for the electric vehicle (EV) market and other high-energy applications. (Earlier post.)

Privately held Sion Power Corporation was established initially as Moltech Corporation in 1994, and holds more than 100 US and international patents on its technology.