Purpose-built eVTOL battery promises 50-mile trips on a 10-minute charge

Lithium-ion batteries do a great job of powering smartphones, laptops and even cars, but electric flight presents some unique challenges for this go-to solution for energy storage. Scientists have now demonstrated a new type of lithium-battery that leverages an innovative high-temperature charging technology to give it enough juice for meaningful aerial trips in just five to 10 minutes.

The reason using electrical powertrains and batteries for flight is so difficult as opposed to, say, powering a car down a highway, is because all of that heavy gear needs to be hauled into the air. There are limitations to how much energy a battery can store and still be light enough for the aircraft to take off, which pales in comparison to the energy density offered by traditional, kerosene-based jet fuel.

Making these batteries able to carry more energy per kilogram is one of the key challenges facing the electric aviation industry, though there are other factors to consider, too. These include fast charging times to keep the vehicles on the move and avoid inefficient battery swaps, and equipping those batteries with the ability to deliver the required amounts of power.

"Batteries for flying cars need very high energy density so that you can stay in the air," says Chao-Yang Wang, a mechanical engineer at Pennsylvania State University and author of the study. "And they also need very high power during take-off and landing. It requires a lot of power to go vertically up and down."

The new technology demonstrated by Wang and his colleagues actually builds on an experimental battery they developed a couple of years ago. Traditionally, lithium batteries can only operate safely within a certain temperature range. Too cold, and spikes will form on the anode in a process known as lithium plating, while too hot and the battery will quickly degrade.

The researchers were able to find a way around this with a unique design that uses a thin nickel foil attached to the negative terminal, which can rapidly heat up the battery to 60 °C (140 °F) in 30 seconds. The battery only remains at this temperature for 10 minutes, before being quickly cooled again.

This is enough to take advantage of the higher charging efficiencies offered by higher operating temperatures, but avoids degradation and lithium plating. In 2019, the scientists demonstrated this by building a prototype battery that could charge an electric car in 10 minutes to offer a range of up to 300 miles (480 km), and they've now tailored the technology for eVTOL (electric vertical takeoff and landing) aircraft.

The team says the new experimental batteries have the required density to power an eVTOL aircraft over a 50-mile (80-km) journey, but can be recharged in five to 10 minutes thanks to the high-temperature charging technology. Demonstrating the longevity of the design, the team also showed that this performance can be sustained over 2,000 fast-charging cycles.

"Under normal circumstances, the three attributes necessary for an eVTOL battery work against each other," says Wang. "High energy density reduces fast charging and fast charging usually reduces the number of possible recharge cycles. But we are able to do all three in a single battery."

The battery built by the team is not about to be dropped into a flying taxi ready for take-off. Rather, its development was a way of gauging the unique battery requirements for this type of transport, which will involve frequent take-offs and landings and a whole lot of recharging, and lay the groundwork for technologies that combine them in a feasible way.

"I hope that the work we have done in this paper will give people a solid idea that we don't need another 20 years to finally get these vehicles," says Wang. "I believe we have demonstrated that the eVTOL is commercially viable."

The research was published in the journal Joule.

Source: Pennsylvania State University