Even versions that run on pure oxygen, however, have been plagued with problems. Using and recharging existing lithium-air batteries wastes a huge amount of energy because the process involves changing the oxygen from a gaseous state into what is, in essence, a solid, and then back again. Such phase changes require a lot of energy and may thus waste more than 30% of the input electricity. Moreover, the changes in volume that accompany the shift from gas to solid to gas put a strain on the battery’s electrodes. This means they rapidly degenerate to the point where the battery can no longer be recharged.

The crucial difference between Dr Li’s design and previous attempts is that no actual air is involved. Instead, the cell is hermetically sealed and uses oxygen stored inside the battery itself, in a chemical called lithium superoxide (LiO2). Because this compound is unstable, it is easily induced to surrender some of its oxygen.

To stop the superoxide disintegrating spontaneously, Dr Li embed it in the voids of a matrix made of cobalt oxide (yellow, in the artist’s imagining below left, in which white spheres represent lithium ions, red ones oxygen ions and blue streaks the crackle of electricity). This gives the superoxide’s structure stability.

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