The mobile world is dependent upon lithium-ion batteries – today’s ultimate rechargeable energy store. Last year, consumers bought five billion Li-ion cells to deliver power-hungry laptops, cameras, cellphones and electric cars. “It is definitely the Li-Polymer equipment battery packs technology anyone has experienced,” says George Crabtree, director from the US Joint Center for Energy Storage Research (JCESR), which happens to be based with the Argonne National Laboratory near Chicago, Illinois. But Crabtree would like to do much, much better.
Modern Li-ion batteries hold greater than double the amount energy by weight since the first commercial versions sold by Sony in 1991 – and are ten 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’). Because of this Li-ion cells will never give electric cars the 800-kilometre selection of a petrol tank, or supply power-hungry smartphones with a lot of events of juice.
In 2012, the JCESR hub won US$120 million from the US Department of Energy to consider a leap beyond Li-ion technology. Its stated goal was to make cells that, when scaled approximately the type of commercial battery packs utilized in electric cars, will be 5 times more energy dense than the standard throughout the day, and five times cheaper, within five years. This means hitting a target of 400 watt-hours per kilogram (Wh kg-1) by 2017.
Crabtree calls the goal “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 since the early lead-nickel rechargeables from the 1900s. But, says Dahn, the JCESR’s target focuses attention on technologies that will be crucial in aiding the planet to change to sustainable energy sources – storing up solar technology for night-time or perhaps a rainy day, as an example. As well as the US hub is way from alone. Many research teams and companies in Asia, the Americas and Europe are seeking beyond Li-ion, and are pursuing strategies that may topple it from its throne.
Chemical engineer Elton Cairns suspected he had tamed Custom medical equipment batteries chemistry early this past year, when his coin-sized cells were going strong even after a number of months of continual draining and recharging. By July, his cells with the Lawrence Berkeley National Laboratory in Berkeley, California, had cycled 1,500 times and had lost only 50 % of their capacity1 – a performance roughly with a par with all the best Li-ion batteries.
His batteries derive from lithium-sulphur (Li-S) technology, which uses extremely cheap materials and then in theory can pack in 5 times more energy by weight than Li-ion (in practice, researchers suspect, it will most likely be only twice as much). Li-S batteries were first posited four decades ago, but researchers could not buy them to outlive past about 100 cycles. Now, many believe that the devices are the technology nearest learning to be a commercially viable successor to Li-ion.
Certainly one of Li-S’s main advantages, says Cairns, is that it gets rid of the “dead weight” in the 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 by way of a charge-carrying liquid electrolyte right into a layered metal oxide electrode. As with every batteries, current is generated because electrons must flow around an outside circuit to balance the charges (see ‘Radical redesigns’). To recharge the battery, a voltage is applied to turn back the electron flow, that drives the lithium ions back.
In a Li-S battery, the graphite is replaced by a sliver of pure lithium metal that does double duty as the two electrode along with the supplier of lithium ions: it shrinks as being the battery runs, and reforms once the battery is recharged. And the metal oxide is replaced by cheaper, lighter sulphur that may really pack the lithium in: each sulphur atom bonds to 2 lithium atoms, whereas it will require more than one metal atom to bond to just one lithium. All that creates a distinct 23dexjpky and price advantage for Li-S technology.
Although the reaction between lithium and sulphur creates a problem. Since the Rechargeable custom Li-Polymer batteries is charged and discharged, soluble Li-S compounds can seep into the electrolyte, degrading the electrodes so the battery loses charge and also the cell gums up. To stop 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 engineered electrolytes which do not dissolve lithium and sulphur so much. Cairns predicts a commercial-sized cell could achieve an energy-density of approximately 500 Wh kg-1. Other labs are reporting similar results, he says.