The mobile world is dependent upon lithium-ion batteries – today’s ultimate rechargeable energy store. A year ago, consumers bought five billion Li-ion cells to offer power-hungry laptops, cameras, mobile phone devices and electric cars. “It is definitely the Power Tools replacement Batteries technology anyone has ever seen,” 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 wishes to do much, a lot better.
Modern Li-ion batteries hold greater than twice as much energy by weight because the first commercial versions sold by Sony in 1991 – and so are ten times cheaper. But are nearing their limit. Most researchers believe 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 selection 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 consider a leap beyond Li-ion technology. Its stated goal was to make cells that, when scaled around the kind of commercial battery packs found in electric cars, can be five times more energy dense than the standard during the day, and 5 times cheaper, within just 5yrs. It 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 considering that the early lead-nickel rechargeables of your 1900s. But, says Dahn, the JCESR’s target focuses attention on technologies that might be crucial in assisting the world to switch to renewable energy sources – storing up solar energy for night-time or a rainy day, as an example. Along with the US hub is way from alone. Many research teams and corporations in Asia, the Americas and Europe are looking beyond Li-ion, and they are pursuing strategies which may topple it looking at the throne.
Chemical engineer Elton Cairns suspected he had tamed Multi-function digital tester batteries chemistry early a year ago, when his coin-sized cells were 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 along with lost only one half of their capacity1 – a performance roughly on the par together with 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 five times more energy by weight than Li-ion (in practice, researchers suspect, it will most likely be only double the amount). Li-S batteries were first posited four decades ago, but researchers could not get them to live past about 100 cycles. Now, many assume that the products are the technology nearest to becoming a commercially viable successor to Li-ion.
Among Li-S’s main advantages, says Cairns, is that it removes the “dead weight” in the Li-ion battery. Inside a typical Li-ion cell, space is taken up with 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 electrode. Like all batteries, current is generated because electrons must flow around some other circuit to balance the costs (see ‘Radical redesigns’). To recharge the battery, a voltage is applied to reverse the electron flow, that also drives the lithium ions back.
Inside a Li-S battery, the graphite is replaced with a sliver of pure lithium metal that does dual purpose as both 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 will really pack the lithium in: each sulphur atom bonds to 2 lithium atoms, whereas it takes several metal atom to bond to just one lithium. All that produces a distinct 23dexjpky and price advantage for Li-S technology.
Although the reaction between lithium and sulphur creates a problem. Because the ODM RC toys Li-Po battery packs is charged and discharged, soluble Li-S compounds can seep into the electrolyte, degrading the electrodes in order that the battery loses charge and also 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 taking advantage of specially engineered electrolytes that do not dissolve lithium and sulphur a great deal. Cairns predicts which a commercial-sized cell could achieve an energy-density of approximately 500 Wh kg-1. Other labs are reporting similar results, he says.