Air Power

Energy: Batteries that draw oxygen from the air could provide a cheaper, lighter and longer-lasting alternative to existing designs

Economist.com, Sep 3rd 2009, (From The Economist print edition)

MOBILE phones looked like bricks in the 1980s. That was largely because the batteries needed to power them were so hefty. When lithium-ion batteries were invented, mobile phones became small enough to be slipped into a pocket. Now a new design of battery, which uses oxygen from ambient air to power devices, could provide even an smaller and lighter source of power. Not only that, such batteries would be cheaper and would run for longer between charges.

Illustration by Belle Mellor

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 power an external circuit.

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 met at the cathode, and 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, 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. Making such a battery is also expected to be cheaper. Lithium cobalt oxide accounts for 30% of the cost of a lithium-ion battery. Air, however, is free.

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