The mobile world depends on lithium-ion batteries — today's ultimate rechargeable energy store. Last year, consumers bought five billion Li-ion cells to supply power-hungry laptops, cameras, mobile phones and electric cars. "It is the best battery technology anyone has ever seen," says George Crabtree, director of the US Joint Center for Energy Storage Research (JCESR), which is based at the Argonne National Laboratory near Chicago, Illinois. But Crabtree wants to do much, much better.
Modern Li-ion batteries hold more than twice as much energy by weight as the first commercial versions sold by Sony in 1991 — and are ten times cheaper. But they are near-ing their limit. Most researchers think that improvements to Li-ion cells can squeeze in at most 30% more energy by weight (see 'Power-ing up'). That means that Li-ion cells will never give electric cars the 800-kilometre range of a petrol tank, or supply power-hungry smart-phones with many days of juice.
In 2012, the JCESR hub won US$120 mil-lion from the US Department of Energy to take a leap beyond Li-ion technology. Its stated goal was to make cells that, when scaled up to the sort of commercial battery packs used in electric cars, would be five times more energy dense than the standard of the day, and five times cheaper, in just five years. That means hitting a target of 400 watt-hours per kilogram (Wh kg−1) by 2017.
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Crabtree calls the goal "very aggressive"; veteran battery researcher Jeff Dahn at Dal-housie University in Halifax, Canada, calls it "impossible". The energy density of recharge-able batteries has risen only sixfold since the early lead–nickel rechargeables of the 1900s. But, says Dahn, the JCESR's target focuses attention on technologies that will be crucial in helping the world to switch to renewable energy sources — storing up solar energy for night-time or a rainy day, for example. And the US hub is far from alone. Many research teams and companies in Asia, the Americas and Europe are looking beyond Li-ion, and are pursuing strategies that may topple it from its throne.
LOSE THE DEAD WEIGHTChemical engineer Elton Cairns suspected he had tamed a promising-but-wild battery chemistry early last year, when his coin-sized cells were still going strong even after a few months of continual draining and recharg-ing. By July, his cells at the Lawrence Berkeley National Laboratory in Berkeley, California, had cycled 1,500 times and had lost only half of their capacity1 — a performance roughly on a par with the best Li-ion batteries.
His batteries are based on lithium–sulphur (Li–S) technology, which uses extremely cheap materials and in theory can pack in five times more energy by weight than Li-ion (in prac-tice, researchers suspect, it will probably be only twice as much). Li–S batteries were first posited 40 years ago, but researchers could not get them to survive past about 100 cycles. Now, many think that the devices are the technology closest to becoming a commercially viable suc-cessor to Li-ion.
One of Li–S's main advantages, says Cairns, is that it gets rid of the "dead weight" in a Li-ion battery. Inside a typical Li-ion cell, space is taken up by a layered graphite electrode that does little more than host lithium ions. These ions flow through a charge-carrying liquid electrolyte into a layered metal oxide elec-trode. As with all batteries, current is generated because electrons must flow around an out-side circuit to balance the charges (see 'Radical redesigns'). To recharge the battery, a voltage is applied to reverse the electron flow, which also drives the lithium ions back.
In a Li–S battery, the graphite is replaced by a sliver of pure lithium metal that does dou-ble duty as both the electrode and the supplier of lithium ions: it shrinks as the battery runs, and reforms when the battery is recharged. And the metal oxide is replaced by cheaper, lighter sulphur that can really pack the lithium in: each sulphur atom bonds to two lithium atoms, whereas it takes more than one metal atom to bond to just one lithium. All of that creates a distinct weight and cost advantage for Li–S technology.
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But the reaction between lithium and sulphur causes a problem. As the battery is charged and discharged, soluble Li–S com-pounds can seep into the electrolyte, degrad-ing the electrodes so that the battery loses charge and the cell gums up. To prevent this, Cairns uses tricks made possible by advances in nanotechnology and electrolyte chemistry — including adulterating his sulphur electrode with graphene oxide binders, and using spe-cially designed electrolytes that do not dissolve lithium and sulphur so much. Cairns predicts that a commercial-sized cell could achieve an energy-density of around 500 Wh kg−1. Other labs are reporting similar results, he says.
Some researchers doubt that the academic cheer will translate into commercial suc-cess. Laboratories often use low proportions of sulphur and lots of electrolyte, which is relatively easy to work with but does not cre-ate 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 pros-pects: 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.