“[These] batteries will make UAM ubiquitous.”

Chao-Yang Wang, director of the Electrochemical Engine Center at Penn State
lift off

Flying cars are "imminent" new study finds

Science fiction has been promising us flying cars for decades, but now a new super-heated battery design could finally make this a reality for eVTOL.

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It’s the morning of the big pitch and like any self-respecting main character, you’ve rolled out of bed well after your alarm in a panic to rush to your car — or closest subway station — only to arrive at your office late and covered in sweat.

This is a tale as old as time, but transportation may be finally ready to solve it in the form of a sleek, Jetsons-like flying car (or rather, electric vertical take-off and landing — eVTOL) that can whisk you away above rush hour traffic and place you down at the office without breaking a sweat. This notion is called UAM, or urban air mobility.

But while this future may be fast approaching, one key lagging part of this technology could stand in its way: slow to charge lithium-ion batteries. This is a problem that Chao-Yang Wang, director of the Electrochemical Engine Center at Penn State, tells Inverse he and colleagues may have just solved in a new paper published Monday in the journal Joule.

The solution? Turning up the heat for these batteries to nearly 150 degrees Fahrenheit.

“We believe that commercial production of the thermally modulated battery is imminent,” Wang says.

What’s new — “Only a handful of papers” have been published on the topic of eVTOL batteries, write the researchers, making this subject fertile ground for innovation. In their work, they look at how standard lithium-ion batteries can be modified to improve fast charging. Fast charging is a necessity for eVTOLs that complete many stop-and-go trips in a day, as well as the overall charging cycle life.

“(Thermally modulated batteries) offer fast charging capability through rapid preheating to 60 deg Celsius and charging at this elevated temperature,” says Wang.

By contrast, traditional lithium-ion batteries would operate at ambient temperature or about 68 degrees Fahrenheit and are plagued by a type of battery degradation called “plating” — a kind of metallic build-up in the battery that affects its performance over time. Thermal stimulation, aka heating, is one of the easiest ways to solve this problem, the researchers write.

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Why it matters — The United Nations predicts that 68 percent of the world’s population will live in cities by 2050, making innovation in urban transportation more salient than ever. Offering UAM as an alternative to congested traffic or cramped public transportation could help control the carbon footprint of these growing urban populations. Designing an improved battery for these crafts is a big part of that puzzle.

Here’s the background — While flying cars may be the most easily accessible comparison for eVTOL craft, it may be more accurate instead to compare them to zippy, electrically powered helicopters — or human-sized drones. As their name suggests, these crafts are designed to take off and land vertically, which makes them good candidates for urban air mobility.

The dream for UAM is that you can catch a flight from the suburbs, through the city, or to the airport by hopping on an eVTOL at centrally located vertiports (a mix between a helipad and an airport, but for city-cruising eVTOL.) In theory, these crafts would clear up traffic congestion by zipping passengers to and from their destinations by air and in a fraction of the time it would take on the ground.

This reality, however, hasn’t quite come to fruition. Partially because it would require huge infrastructure and policy changes to implement, but also because eVTOL isn’t quite ready yet to pick up and drop off passengers on a dime.

A major aspect of this technical problem, Wang says, is that eVTOL will need better batteries than those currently being designed for other electric craft, like EVs.

“eVTOL batteries must have ~3x more power density than EV batteries due to the need of vertical takeoff and landing [and] must be recharged within 5-10 minutes concurrently with passenger swapping in order to make commercial operation economical,” says Wang.

The high number of trips per day — especially during rush hour — will also require ultralong cycle life for the batteries.

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What they did — To deliver on this tall order of fast-charging and long-lasting batteries, Wang says the team found that all they needed to do was added a tiny heating element to existing batteries in the form of a nickel foil thinner than a strand of human hair.

“The on-demand thermal modulation is achieved by energy contained in a battery itself and a 10-micrometer thin heating element made of nickel foils,” explains Wang.

The researchers achieved this in just a couple simple steps:

  • They embedded the nickel foil, less than 1.5 percent of the battery’s weight, into the battery itself
  • Controlled thermal modulation brought the heat up to 140 degrees Fahrenheit for plating-free recharging

In their trials, the team determined that this small modification made it possible to charge a 270 Wh/kg eVTOL battery in less than 10-minutes — a reasonable amount of time for passengers to hop on and off — with enough energy to travel up to 50-miles and recharge fully over 2,000 times with little loss of capacity.

An ambient charging battery, on the other hand, loses 20 percent capacity after only 150 charge cycles.

What’s next — Other challenges still lie ahead for UAM and eVTOLs, but Wang says that continuing to improve this new battery design could help bring the industry one step closer to the flying cars science fiction promised us.

“We are continuing to develop the next generation of thermally modulated batteries for eVTOLs with further increase in energy density to 350-400 Wh/kg and reduction in cost to $50/kWh,” said Wang. “Such Gen2 eVTOL batteries will make UAM ubiquitous.”

Abstract: Electric vertical takeoff and landing (eVTOL) aircraft have attracted considerable interest as a disruptive technology to transform future transportation systems. Their unique operating profiles and requirements present grand challenges to batteries. This work identifies the primary battery requirements for eVTOL in terms of specific energy and power, fast charging, cycle life, and safety, revealing that eVTOL batteries have more stringent requirements than electric vehicle batteries in all aspects. Notably, we find that fast charging is essential for downsizing aircraft and batteries for low cost while achieving high vehicle utilization rates to maximize revenues. We experimentally demonstrate two energy-dense Li-ion battery designs that can recharge adequate energy for 80 km eVTOL trips in 5–10 min and sustain over 2,000 fast-charge cycles, laying a foundation for eVTOL batteries.

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