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 nearing their limit. Most researchers think that improvements to Li-ion cells can squeeze in at most 30% more energy by weight (see ‘Powering up’). That means that Li-ion cells will never give electric cars the 800-kilometre range of a petrol tank, or supply power-hungry smartphones with many days of juice.

In 2012, the JCESR hub won US$120 million 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.

Crabtree calls the goal “very aggressive”; veteran battery researcher Jeff Dahn at Dalhousie University in Halifax, Canada, calls it “impossible”. The energy density of rechargeable 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.