An aluminum–air battery sandwiches a circulating aqueous electrode between an aluminum anode and a cathode in contact with ambient air. When the battery is connected to an external circuit, two electrochemical reactions take place: At the anode, aluminum reacts with hydroxide ions in the electrolyte to liberate electrons; at the cathode, oxygen from ambient air reacts with water and with the electrons reaching the cathode to form hydroxide ions. Since the aluminum is consumed, the battery can’t be recharged. Still, Al–air batteries offer an appealing combination of light weight, low cost, and high energy density. But they face a major limitation: When they’re not in use, the electrolyte corrodes the aluminum anodes. In a month the batteries can irreversibly lose as much as 80% of their charge, compared with 5% losses for lithium-ion batteries. Various strategies have been devised to reduce the corrosion and extend the battery shelf life, but they typically come at the expense of the power and energy output.

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