BATTERIES are notoriously hard to improve. Nowhere was this more apparent than at the opening last week, on July 29th, of Tesla’s Gigafactory, a massive battery plant in Nevada. According to its boss Elon Musk, Tesla built the factory because wringing more efficiency out of batteries is far more difficult than optimising the process by which they are made.

It is an ironic coincidence, therefore, that last week also saw the publication, in Nature Energy, of a paper outlining a way of making a battery whose prototype stores twice as much juice as the lithium-ion cells the Gigafactory will turn out, and which could eventually do better than that. The new battery, brainchild of Ju Li of the Massachusetts Institute of Technology, is some way from commercialisation, but its design is such that commercialising it should not be hard.

The fundamental idea behind Dr Li’s device is not new. It is a version of what is known as a lithium-air battery, something that has been a desideratum of energy-storage research since the 1970s. In theory, such batteries could hold more than four times the energy per kilogram of lithium-ion batteries. Building them, though, has proved taxing. As their name suggests, they draw in air. The part they need is the oxygen, but other atmospheric components—water vapour and carbon dioxide in particular—often damage them.

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