A whiff of brimstone:Adding sulphur to electrical cells may quintuple their performance

January 3, 2015 - via The Economist

BUILD a better battery, to paraphrase Ralph Waldo Emerson, and the world will beat a path to your door. For consumer goods, from computers to cars, “better” means “better than lithium-ion”. And several groups of engineers think they have one: it is based on lithium and sulphur.

A lithium-ion (Li-ion) battery works by shuttling the eponymous ions, which are positively charged, through an electrolyte that links two electrodes, one made of carbon and the other of a substance containing a heavy metal such as cobalt, manganese or nickel. Such metals have multiple oxidation states, meaning they can lose or gain different numbers of electrons in different circumstances. To balance the movement of lithium ions, electrons (which are negatively charged) move to or from the heavy metal through an external circuit that also links the electrodes, changing the metal’s oxidation state as they do so. When the battery is discharging, both ions and electrons travel spontaneously in one direction, creating a current and releasing energy. When it is being recharged they are forced, by the application of a voltage, to go the other way and thus to store energy.

Lithium-sulphur batteries work in a similar fashion, but dispense with the heavy metal. Instead, they use sulphur, which also has multiple oxidation states—more of them, indeed, than many metals do. This fact, combined with sulphur’s lightness, means lithium-sulphur batteries can, in principle, store four or five times as much energy per gram as lithium-ion ones manage. And, since sulphur is cheap, they can do so at lower cost.

Turning that principle into practice, though, has been a hard slog. Experimental lithium-sulphur cells tend to wear out, because the sulphur in their electrodes gradually dissolves into the electrolyte. There are also questions about their safety. Part of the cycle of a lithium-sulphur battery involves some lithium ions turning into metallic lithium. This metallic form of the element may grow into filaments called dendrites that cause short circuits, and thus overheating and fires.

Various academic groups are working on these problems. Chengdu Liang, at the Oak Ridge National Laboratory, in Tennessee, has developed a solid electrolyte that stops the sulphur dissolving. The dendrite problem, meanwhile, can be ameliorated by adding carbon to the electrode where the lithium is deposited. At Stanford University, Yi Cui employs buckminsterfullerene, a form of the element in which the atoms are organised into spheres, to do this. Cheng Huang at Pacific Northwest National Laboratory use lithiated graphite, which traps lithium atoms within its structure. OXIS Energy, of Oxford, Britain, meanwhile, is experimenting with a lithiated version of graphene, a recently discovered form of carbon that consists of sheets a single atom thick.

Huw Hampson-Jones, OXIS’s boss, hopes the firm’s batteries will go into production in 2015. If they do, they may be the first to reach the market. But they are unlikely to be alone for long. Larger companies, less in need of publicity, are more secretive about their battery programmes. But an unguarded remark by Carlos Ghosn, in a television interview he gave in November, suggests Nissan, the carmaker that he runs, also has a lithium-sulphur battery that is close to production, and will soon be installed in the firm’s LEAF electric cars, doubling the vehicles’ ranges.

At the moment, a LEAF’s Li-ion batteries can store about 140 watt-hours per kilogram. OXIS’s store more than 300 Whr/kg and Dr Hampson-Jones hopes to raise this to 500 Whr/kg by 2018. Whether that will be enough to fend off the likes of Nissan remains to be seen. But whoever wins the race, whale or minnow, the days of the Li-ion look numbered.

External link: http://www.economist.com/news/science-and-technology/21637344-adding-sulphur-electrical-cells-may-quintuple-their-performance-whiff

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