The mobile world is dependent upon lithium-ion batteries – today’s ultimate rechargeable energy store. Just last year, consumers bought five billion Li-ion cells to provide power-hungry laptops, cameras, cell phones and electric cars. “It is the Li-Polymer equipment battery packs technology anyone has experienced,” says George Crabtree, director of your US Joint Center for Energy Storage Research (JCESR), which happens to be based on the Argonne National Laboratory near Chicago, Illinois. But Crabtree desires to do much, much better.
Modern Li-ion batteries hold greater than twice as much energy by weight as the first commercial versions sold by Sony in 1991 – and therefore are ten times cheaper. However they are nearing their limit. Most researchers think that improvements to Li-ion cells can squeeze in at most of the 30% more energy by weight (see ‘Powering up’). This means that Li-ion cells will never give electric cars the 800-kilometre range of a petrol tank, or supply power-hungry smartphones with lots of days of juice.
In 2012, the JCESR hub won US$120 million in the US Department of Energy to take a leap beyond Li-ion technology. Its stated goal was to make cells that, when scaled approximately the sort of commercial battery packs utilized in electric cars, would be 5 times more energy dense in comparison to the standard through the day, and five times cheaper, in only five years. It means hitting a target of 400 watt-hours per kilogram (Wh kg-1) by 2017.
Crabtree calls the aim “very aggressive”; veteran battery researcher Jeff Dahn at Dalhousie University in Halifax, Canada, calls it “impossible”. The electricity density of rechargeable batteries has risen only sixfold because the early lead-nickel rechargeables of your 1900s. But, says Dahn, the JCESR’s target focuses attention on technologies that will be crucial to help the world to switch to alternative energy sources – storing up solar technology for night-time or even a rainy day, for instance. As well as the US hub is much from alone. Many research teams and firms in Asia, the Americas and Europe are searching beyond Li-ion, and are pursuing strategies which could topple it by reviewing the throne.
Chemical engineer Elton Cairns suspected he had tamed Custom medical equipment batteries chemistry early last year, when his coin-sized cells were still going strong even after a few months of continual draining and recharging. By July, his cells on the Lawrence Berkeley National Laboratory in Berkeley, California, had cycled 1,500 times and had lost only 1 / 2 of their capacity1 – a performance roughly over a par with the best Li-ion batteries.
His batteries are derived from lithium-sulphur (Li-S) technology, which uses extremely cheap materials and in theory can pack in 5 times more energy by weight than Li-ion (in practice, researchers suspect, it will probably be only twice as much). Li-S batteries were first posited 4 decades ago, but researchers could not get them to outlive past about 100 cycles. Now, many believe that the products are the technology closest to learning to be a commercially viable successor to Li-ion.
One among Li-S’s main advantages, says Cairns, is it gets rid of the “dead weight” in the Li-ion battery. Within a typical Li-ion cell, space is taken up from a layered graphite electrode that does nothing more than host lithium ions. These ions flow using a charge-carrying liquid electrolyte in to a layered metal oxide electrode. As with all batteries, current is generated because electrons must flow around an outside circuit to balance the costs (see ‘Radical redesigns’). To recharge battery, a voltage is applied to turn back the electron flow, which also drives the lithium ions back.
Inside a Li-S battery, the graphite is replaced with a sliver of pure lithium metal that does double duty as the electrode along with the supplier of lithium ions: it shrinks since the battery runs, and reforms if the battery is recharged. Along with the metal oxide is replaced by cheaper, lighter sulphur that could really pack the lithium in: each sulphur atom bonds to 2 lithium atoms, whereas it will require several metal atom to bond to simply one lithium. All of that produces a distinct 23dexjpky and cost advantage for Li-S technology.
Although the reaction between lithium and sulphur results in a problem. As the Electronic devices Li-Polymer batteries is charged and discharged, soluble Li-S compounds can seep into the electrolyte, degrading the electrodes so the battery loses charge along with the cell gums up. In order to avoid this, Cairns uses tricks made possible by advances in nanotechnology and electrolyte chemistry – including adulterating his sulphur electrode with graphene oxide binders, and using engineered electrolytes which do not dissolve lithium and sulphur a whole lot. Cairns predicts that the commercial-sized cell could achieve an energy-density of around 500 Wh kg-1. Other labs are reporting similar results, he says.