Highly anticipated: Rechargeable lithium-ion batteries are increasing in power density at a rate of around 8% per year, and if that continues then they’ll be approaching gasoline densities in around ten years. Aluminum-air batteries, on the other hand, almost match gasoline, while being substantially lighter and cheaper than lithium-ion. There’s one big problem though: corrosion.
A new team of researchers claim to have the solution, however. According to their research published in Science, lithium-ion batteries can lose 5% of their charge in a month if left unused while aluminum-air batteries can lose 80% in a month due to corrosion of the aluminum. The researchers’ aluminum-air batteries lose only 0.02% in a month, which is over a thousandfold improvement.
A standard battery contains a liquid electrolyte between an anode and a cathode, and it generates its power by collecting the electrons freed when an ion travels from the anode to the cathode. In aluminum-air batteries, the anode is aluminum and the cathode is air. Because the battery can source the air from its environment and because aluminum is abundant, light and cheap, aluminum-air batteries could very well be the battery technology of the future.
Unfortunately, however, the electrolyte will start corroding the aluminum as soon as they’re mixed, which is when the battery is first used. Using a non-corrosive electrolyte substantially reduces performance and extracting the electrolyte once the battery goes back into standby doesn’t work because aluminum is hydrophilic (water attracting). The researchers’ solution is to flush out the electrolyte with oil.
The new batteries have a thin membrane between the anodes and cathodes, and while the battery is in use both sides are filled with the electrolyte. Once the battery is switched to standby, the side closest to the aluminum is flushed with oil which protects the aluminum because the water-based electrolyte won’t mix with the oil. When the battery is needed again the oil is replaced by the electrolyte. There’s no chance of any oil staying behind because aluminum repels oil when submerged.
To test these batteries in a semi-realistic scenario, the researchers used up a small portion of the batteries' power, then let it sit for a day or two before repeating the process. This mimics what an electric car might be required to do in a city. Their battery lasted twenty-four days, eight times longer than a traditional aluminum-air battery.
But, will aluminum-air batteries be replacing lithium-ion ones in the next few years? No, they won’t. There are still complicated issues with recharging the batteries, so the first few generations may be non-rechargeable or only have a few recharges. Or perhaps they’ll be swap out batteries that robotic arms extract from your car and replace with charged batteries at service stations, so that they can be recharged using more complicated mechanical methods.
Though that doesn’t mean that aluminum-air batteries can’t work in tandem with lithium-ion ones. Like gasoline in hybrid vehicles, aluminum-air batteries can be used as range extenders or for when a user forgets to recharge their main battery. As one of the researchers explains, this has a lot of potential in the market, “you could use it and then pull into your driveway and park it for a month, and then come back and still expect it to have a usable battery. ...I really think this is a game-changer in terms of the use of these batteries.”
Battery technology is one of the largest bottlenecks in terms of technological advancements – we all dream about smartphones that won't require recharging more than once a week, just like we’re all hoping for electric cars that can go across the country and back without major recharging hurdles. Even electric planes could be made possible by lighter and smaller batteries. That future is now just a little bit closer.