Metal-air batteries are one of the lightest and most compact types of batteries, but when not in use, they degrade quickly, as corrosion eats away at their metal electrodes. While typical rechargeable lithium-ion batteries only lose about 5 percent of their charge after a month of storage, they are too costly, bulky, or heavy for many applications. Primary (non-rechargeable) aluminum-air batteries are much less expensive and more compact and lightweight, but they can lose 80 percent of their charge a month.
A new design overcomes the problem of corrosion in aluminum-air batteries by introducing an oil barrier between the aluminum electrode and the electrolyte — the fluid between the two battery electrodes that eats away at the aluminum when the battery is on standby. The oil is rapidly pumped away and replaced with electrolyte as soon as the battery is used. As a result, the energy loss is cut to just 0.02 percent a month — more than a thousand-fold improvement.
The system uses a thin membrane placed between the battery electrodes. When the battery is in use, both sides of the membrane are filled with a liquid electrolyte, but when the battery is put on standby, oil is pumped into the side closest to the aluminum electrode, protecting the aluminum surface from the electrolyte on the other side of the membrane.
The battery system also takes advantage of a property of aluminum called “underwater oleophobicity” — that is, when aluminum is immersed in water, it repels oil from its surface. As a result, when the battery is reactivated and electrolyte is pumped back in, the electrolyte easily displaces the oil from the aluminum surface, which restores the power capabilities of the battery. The result is an aluminum-air prototype with a much longer shelf-life than that of conventional aluminum-air batteries.
When the battery was repeatedly used and then put on standby for one to two days, the new design lasted 24 days, while the conventional design lasted for only three. Aluminum, in addition to being very inexpensive, can store and deliver more energy per pound than almost anything else.
Aluminum-air batteries have been used as range extenders for electric vehicles to supplement built-in rechargeable batteries and are sometimes used as power sources in remote locations or for some underwater vehicles. With a longer shelf-life, the use of aluminum-air batteries could extend to other applications.