2014-Nature-更好的电池.pdf
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1、The mobile world depends on lithium-ion batteries todays 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,
2、 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 S
3、ony 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,
4、 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, wou
5、ld 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 (Whkg1) by 2017.Crabtree calls the goal “very aggressive”; veteran battery researcher Jeff Dahn at Dal-housie University in Halifax,
6、 Canada, calls it “impossible”. The energy density of recharge-able batteries has risen only sixfold since the early leadnickel rechargeables of the 1900s. But, says Dahn, the JCESRs target focuses attention on technologies that will be crucial in helping the world to switch to renewable energy sour
7、ces 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 enginee
8、r 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
9、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 lithiumsulphur (LiS) technology, which uses extremely cheap materials and in theory can pack in five times more energy by weight than Li-ion (in prac
10、-tice, researchers suspect, it will probably be only twice as much). LiS batteries were first posited 40years ago, but researchers could not get them to survive past about 100cycles. Now, many think that the devices are the technology closest to becoming a commercially viable suc-cessor to Li-ion. O
11、ne of LiSs 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 lay
12、ered 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 LiS
13、 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, A BETTER BATTERYChemists are rein
14、venting rechargeable cells to drive down costs and boost capacity.BY RICHARD VAN NOORDEN2 6 | N A T U R E | V O L 5 0 7 | 6 M A R C H 2 0 1 4FEATURENEWS 2014 Macmillan Publishers Limited. All rights reservedlighter sulphur that can really pack the lithium in: each sulphur atom bonds to two lithium a
15、toms, 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 LiS technology.But the reaction between lithium and sulphur causes a problem. As the battery is charged and discharged, soluble LiS com-pounds can seep into the e
16、lectrolyte, 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 design
17、ed 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 Whkg1. Other labs are reporting similar results, he says. Some researchers doubt that the academic cheer will translate into commercial suc-ces
18、s. 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 y
19、ears working on LiS at battery firm PolyPlus in Berkeley, just 5kilometres 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 LiSs pros-pects: Oxis Energy in Abingdon, UK. It says it has run large cell
20、s for an impressive 900cycles, 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 kg1 by 2016 for an electric vehicle. PACK MORE PUNCH PER IONAs the worlds lightest metal, lithium provides a
21、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 mag-nesium ions shuttle two at a time instantly multiplying the electrical ene
22、rgy that can be released for the same volume. Magnesium comes with its own challenge, however: whereas lithium zips through elec-trolytes 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 t
23、he JCESR, is shooting high-energy X-rays at magnesium in various electrolytes to investi-gate 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 sur-rounding solvent, attracting clusters of solvent molecules that make
24、it bulkier. That kind of basic research is key to creating a better bat-tery, 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 bring-ing an advantage to the field.Materi
25、als scientist Kristin Persson at Law-rence Berkeley is using a supercomputer to simulate the innards of possible new batter-ies, 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 differ
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