These fragile and futuristic batteries last longer than a little oil

New design could help solve a problem that has long caused chaos for air-conditioned power cells

Batteries that use aluminum and oxygen generally live quickly and die young. However, a new design can help electrical devices last longer.

Aluminum-air batteries promise candidates for a new generation of non-rechargeable batteries because they are extremely light and dense. However, batteries are not widely used because their internal components deteriorate quickly. In the new aluminum-to-air configuration described in Science November 9, the oil acts as a buffer between the corrosive parts of the battery to dramatically extend the life of the device. These improved batteries that can be used can provide back-up power to electric cars or provide power to remote areas off the grid.

“It’s a very smart design,” said Yiying Wu, a chemist at Ohio State University who was not involved in the work. The petroleum buffer system could also improve other types of metal-air batteries that are prone to self-crash, such as zinc-air devices, he said (SN: 1/21/17, p . 22).

Each aluminum-air battery cell contains two electrodes, an aluminum anode and a cathode, separated by a liquid called an electrolyte. Oxygen molecules sucked in from the air reach the cathode, where they respond to electrons and aluminum particles that flow from the anode through the electrolyte and provide energy to the electronics. When the battery is in standby mode, the aqueous electrolyte unfortunately feeds on the aluminum anode.

“It’s basically like a rusty tool,” said co-author Brandon Hopkins, a mechanical engineer at the United States Naval Research Laboratory in Washington, DC who worked at MIT. “If you put aluminum – or really any metal – in water, it starts to rust and break down.” As a result, aluminum-air batteries can lose almost 80% of their stored charge if left on a shelf in just one month.

Hopkins and his colleagues built a more durable aluminum-air device by inserting a polymer membrane between the electrodes of the cell. When the battery activates a device, the electrolyte is pumped from a reservoir to areas on both sides of the membrane. When the cell is not in use, the electrolyte is drained from the edge of the membrane next to the aluminum anode and oil flows out to replace it. This oil protects the aluminum from the electrolyte on the other side of the membrane. Once the battery is reactivated, the oil is pumped and stored, and the electrolyte returns.

In lab experiments, the Hopkins team used the new battery cell and an oil-buffered aluminum air cell for five minutes, with 24- or 72-hour breaks between intervals. The electrolyte from the conventional battery eats up through the aluminum anode in just a few days and the cell dies. However, the electrolyte in the oil-charged battery caused the aluminum anode to explode at a slower rate, and the battery operated for several weeks before the energy in the cell was completely drained.

The power output of a single-cell prototype can potentially increase either by making a larger version or by placing multiple cells in a battery. “Researchers have yet to examine the performance, cost, and reliability of a full-size battery,” Wu said.

These light cells serve not only as a backup power source for electric vehicles, but also as power for remote drones, Hopkins said. Aluminum-air batteries can also provide off-grid power to military and civilian personnel in remote areas.

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