The mobile world is determined by 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 phone devices and electric cars. “It is the Custom Lithium Ion Batteries technology anyone has experienced,” says George Crabtree, director of the US Joint Center for Energy Storage Research (JCESR), which can be based on the Argonne National Laboratory near Chicago, Illinois. But Crabtree wishes to do much, a lot better.
Modern Li-ion batteries hold greater than twice as much energy by weight as being the first commercial versions sold by Sony in 1991 – and therefore are 10 times cheaper. But are nearing their limit. Most researchers feel that improvements to Li-ion cells can squeeze in at most of the 30% more energy by weight (see ‘Powering up’). Which means that Li-ion cells will never give electric cars the 800-kilometre variety of a petrol tank, or supply power-hungry smartphones with many events of juice.
In 2012, the JCESR hub won US$120 million through the US Department of Energy to adopt a leap beyond Li-ion technology. Its stated goal was to make cells that, when scaled as much as the kind of commercial battery packs used in electric cars, can be five times more energy dense in comparison to the standard of the day, and five times cheaper, within just 5 years. That means hitting a target of 400 watt-hours per kilogram (Wh kg-1) by 2017.
Crabtree calls the target “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 ever since the early lead-nickel rechargeables of your 1900s. But, says Dahn, the JCESR’s target focuses attention on technologies that can be crucial in assisting the planet to change to sustainable energy sources – storing up solar energy for night-time or perhaps a rainy day, as an example. Along with the US hub is far from alone. Many research teams and companies in Asia, the Americas and Europe are seeking beyond Li-ion, and so are pursuing strategies which may topple it looking at the throne.
Chemical engineer Elton Cairns suspected he had tamed Rechargeable 18650 Li-ion battery packs chemistry early last year, when his coin-sized cells were still going strong even after a couple of months of continual draining and recharging. By July, his cells at the Lawrence Berkeley National Laboratory in Berkeley, California, had cycled 1,500 times and had lost only one half of their capacity1 – a performance roughly on a par with all the best Li-ion batteries.
His batteries are based on lithium-sulphur (Li-S) technology, which utilizes extremely cheap materials and also in theory can pack in five times more energy by weight than Li-ion (in reality, researchers suspect, it might be only twice as much). Li-S batteries were first posited forty years ago, but researchers could not buy them to thrive past about 100 cycles. Now, many think that the devices are the technology closest to becoming a commercially viable successor to Li-ion.
Certainly one of Li-S’s main advantages, says Cairns, is it gets rid of the “dead weight” within a Li-ion battery. Inside a typical Li-ion cell, space is taken up by 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 most 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 used to turn back electron flow, that drives the lithium ions back.
In a Li-S battery, the graphite is replaced from a sliver of pure lithium metal that does dual purpose as both the electrode and the supplier of lithium ions: it shrinks because the battery runs, and reforms as soon as 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 will take more than one metal atom to bond to merely one lithium. All of that creates a distinct 23dexjpky and cost advantage for Li-S technology.
Although the reaction between lithium and sulphur creates a problem. As being the Rechargeable custom Li-Polymer batteries is charged and discharged, soluble Li-S compounds can seep in the electrolyte, degrading the electrodes so the battery loses charge as well as 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 taking advantage of specially designed electrolytes that do not dissolve lithium and sulphur a whole lot. Cairns predicts a commercial-sized cell could achieve a power-density of about 500 Wh kg-1. Other labs are reporting similar results, he says.