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MOBILE phones looked like bricks in the 1980s. Indeed, they were so cumbersome that most were installed in cars. That was because the batteries required to power them were so hefty. When lithium-ion batteries were invented, mobile phones became small enough to be carried in a pocket or slipped into a handbag. Now a new design of battery that uses oxygen from ambient air to power devices could provide even smaller and lighter sources of power. Not only that, such batteries would be cheaper and last for longer between charges.

Lithium-ion batteries have two electrodes immersed in an electrically conductive solution, called an electrolyte. One of the electrodes, the cathode, is made of lithium cobalt oxide; the other, the anode, is composed of carbon. When the battery is being charged, positively charged lithium ions break away from the cathode and travel in the electrolyte to the anode, where they meet electrons brought there by a charging device. When electricity is needed, the anode releases the lithium ions, which rapidly move back to the cathode. As they do so, the electrons that were paired with them in the anode during the charging process are released. These electrons travel around an external circuit to which the battery is attached and power it.

Peter Bruce and his colleagues at the University of St Andrews in Scotland came up with the idea of replacing the lithium cobalt oxide electrode with a cheaper and lighter alternative. They designed an electrode made from porous carbon and lithium oxide. They knew that lithium oxide forms naturally from lithium ions, electrons and oxygen, but, to their surprise, they found that it could also be made to separate easily when an electric current passed through it. They exposed one side of their porous carbon electrode to an electrolyte rich in lithium ions and put a mesh window on the other side of the electrode through which air could be drawn. Oxygen from the air took the place of the cobalt oxide.

When they charged their battery, the lithium ions migrated to the anode where they combined with electrons from the charging device. When they discharged it, lithium ions and electrons were released from the anode. The ions crossed the electrolyte and the electrons travelled round the external circuit. The ions and electrons then met at the cathode, where they combined with the oxygen to form lithium oxide that filled the pores in the carbon.

Because the oxygen being used by the battery comes from the surrounding air rather than having to be carried around inside the battery, the device that Dr Bruce's team has designed can be a mere one-eighth to one-tenth the size and weight of modern batteries, while still carrying the same charge. Alternatively, batteries using the new technology would last eight to ten times longer than modern batteries of the same size.

Performance and size are not the only expected improvements. Although they are not yet commercially available, making such a battery is expected to be cheaper. Lithium cobalt oxide accounts for 30% of the cost of a lithium-ion battery. Air, however, is free.