Some researchers doubt that the academic cheer will translate into commercial success. Laboratories often use low proportions of sulphur and lots of electrolyte, which is relatively easy to work with but does not create an energy-dense battery. Bumping up the sulphur and decreasing the electrolyte makes the cell more likely to gum up, says Steve Visco, who has spent more than 20 years working on Li–S at battery firm PolyPlus in Berkeley, just 5 kilometres west of Cairns' lab. Making a cheap commercial cell that works over a range of temperatures will also be hard, he says.
At least one company stands by Li–S's prospects: Oxis Energy in Abingdon, UK. It says it has run large cells for an impressive 900 cycles, at energy densities that match current Li-ion cells. Oxis is working with Lotus Engineering, headquartered in Ann Arbor, Michigan, on a project to reach 400 Wh kg-1 by 2016 for an electric vehicle.
PACK MORE PUNCH PER ION
As the world's lightest metal, lithium provides a huge weight advantage. But some researchers argue that the next generation of cells should switch to heavier elements such as magnesium. Unlike lithium ions, which can carry only one electrical charge each, doubly charged magnesium ions shuttle two at a time — instantly multiplying the electrical energy that can be released for the same volume.
Magnesium comes with its own challenge, however: whereas lithium zips through electrolytes and electrodes, magnesium with its two charges moves as if through treacle.
Peter Chupas, a battery researcher at Argonne National Laboratory who is working with the JCESR, is shooting high-energy X-rays at magnesium in various electrolytes to investigate why it experiences so much drag. So far, he and his colleagues have found that magnesium exerts a strong pull on oxygen atoms in any surrounding solvent, attracting clusters of solvent molecules that make it bulkier. That kind of basic research is key to creating a better battery, but it is not usually done by industry, says Crabtree. “The typical R&D operation operates on trial and error, not fundamental research,” he says. This, he says, is where JCESR is bringing an advantage to the field.
Materials scientist Kristin Persson at Lawrence Berkeley is using a supercomputer to simulate the innards of possible new batteries, trying to find a combination of electrodes and electrolytes that will allow magnesium to pass through more easily. “Right now, we are crunching through around 2,000 different electrolytes,” she says.
“Five times more energy dense, and five times cheaper, in just five years: an “impossible” goal?”
Persson and Gerbrand Ceder, a materials scientist at the Massachusetts Institute of Technology in Cambridge, founded a company to develop these higher-charge-carrying batteries. Pellion Technologies, based in Cambridge, is tight-lipped about its results; it has published only one paper about electrolytes2. A spate of patents published in late 2013 hint that the company is developing more-open electrode structures to help the magnesium ions to flow. Major electronics firms such as Toyota, LG, Samsung and Hitachi are also working on such cells, releasing little information beyond occasional teasers.
As companies jostle in secret, Persson continues to run through what she calls the “electrolyte genome”. The sifting-by-supercomputer approach could also help the search for batteries made with other multiple-charge-carrying (or 'multivalent') metals, such as aluminium and calcium. Ceder urges patience, pointing out that research into Li-ion battery chemistry has enjoyed a 40-year head start. “We have so little information about multivalent ions,” he says.