September 28, 2022

Crack the code for eVTOL batteries

One of the keys to unlocking the potential of electric vertical take-off and landing (eVTOL) aircraft is developing lightweight lithium-ion batteries that provide sufficient energy density. Ideally, an efficient battery system would provide eVTOLs with the required range and fast recharge time to make this entirely new form of air travel scalable and commercially viable.

For years, the battery industry has talked about achieving higher gravimetric energy density – a so-called “magic number” of 450 watt-hours per kilogram, which some experts say would open up the sector wide and boost the emerging eVTOL industry.

For comparison, a standard standard electric vehicle battery has an energy density of 250 to 350 watt hours per kilogram. But in November 2021, a company called Amprius announced that it was developing the first commercially available lithium-ion battery offering 450 watt-hours per kilogram. In February, the company sent its first deliveries. Now, the Fremont, Calif.-based battery maker says it’s on the verge of releasing a lithium-ion battery with an even higher energy density: 500 watt-hours per kilogram.

“We have already reached 500 watt-hours per kilogram,” said Ronnie Tao, vice president of business development at Amprius, in an interview with FLYING. “We have already achieved lifetimes in excess of 500 cycles. We are very excited about the potential of our platform.

Tao said these high-energy-density batteries are in late-stage development, meaning they’ve made samples, but Amprius hasn’t technically marketed them yet. “We sample [batteries] for people to test and validate, and to see if they’re interested in integrating it into their apps,” Tao said.

For the electricity sector, autonomy is essential. A battery with greater energy density could theoretically allow eVTOLs to fly longer and farther. With a longer range, an eVTOL tasked with carrying cargo or passengers over short 25-minute hops could now be used to fly between nearby towns and villages.

Several eVTOL developers, including Joby Aviation (NYSE: JOBY), Wisk Aero, Lilium (NASDAQ: LILM), Volocopter, Archer Aviation (New York Stock Exchange: ACHR), and others – are currently testing full-scale prototypes and working toward certification by the FAA or EASA. However, none have yet received certification for their aircraft.

Electric aircraft developers have worked hard to push the limits of range for their aircraft. Last year, Joby Aviation achieved a milestone by flying its S4 prototype a distance of 150nm on a single charge, using off-the-shelf conventional EV batteries. This was a significant moment for the aircraft, which demonstrated that it could achieve its intended range in a flight test environment.

Three months ago, Beta Technologies completed eight legs across six states in a demonstration flight of its Alia electric aircraft totaling 1,219nm.

Faster recharge times

The Beta website states that Alia will be able to fully charge from empty in 50 minutes. By comparison, Amprius claims to have achieved a remarkable recharge rate in its proprietary batteries: 0% to 80% in just six minutes.

Battery recharge times are another important metric that is directly related to the operational success of the eVTOL sector. Time is money, as they say. But also, several air taxi developers, including Joby and Archer in California, are running vertical business models that will require large-scale operations. To operate efficiently on such a scale, battery recharging must be as fast as possible, to minimize runtime and maximize the number of flights per aircraft.

Amprius says it has engaged in strategic discussions with most of the major eVTOL developers who are currently testing full-scale prototypes, hoping to deliver high-performance lithium-ion batteries. [Courtesy: Amprius]

Amprius says its cutting-edge battery technology has the potential to increase range and shorten turnaround times, two factors that could go a long way in helping an unproven industry succeed in a highly skeptical industry.

As you’d expect, the developers of eVTOL are very tight-lipped about their technology, including their battery systems. Several have said FLYING they flight-tested their prototypes with off-the-shelf lithium-ion batteries designed for electric cars. However, eventually, standard EV batteries won’t be good enough, experts say. Manufacturers believe the electric aviation industry will be willing to pay higher prices for certified high-performance batteries that will extend aircraft range and reduce turnaround times.

Amprius aims to conquer this market.

Amprius supplied batteries for an experimental high-performance, high-altitude drone aircraft manufactured by Airbus Defense and Space for the US military. [Courtesy: Airbus Defence and Space]

Trials with Airbus

Already, the company has supplied batteries for an experimental high-performance, high-altitude drone aircraft manufactured by Airbus Defense and Space for the US military. The Airbus Zephyr flew for 64 days in a row, thanks in part to its Amprius batteries. The experimental drone soared to more than 60,000 feet, flying over the southern United States, the Gulf of Mexico, South America and Arizona, before crashing abruptly for reasons the military did not know. has not been revealed.

“Things are different when you’re operating in the stratosphere,” Tao said. “You have very, very different operating conditions. And that required us to be innovative, to figure out how to simulate environments up there, so that what we build translates. There are different elements that need to be considered both at the cell level and at the system level,” he said. “Airbus has allowed us to take those learnings and really start applying them to everything else in aviation.”

Revolutionary technology

So what was the breakthrough technology that allowed Amprius to achieve 450 and now 500 watt hours per kilogram? In a nutshell, the breakthrough was in silicon.

For decades, anodes – the positive and negative poles of a rechargeable battery where electrons are released into an external circuit and where electrons are returned to the battery – have been made from less conductive carbon graphite. Scientists had long known that silicon would be a much more efficient material for anodes, but since silicon tends to swell when loaded with lithium, it could never be used successfully. Swelling can lead to cracked anodes, rendering the batteries useless.

Amprius, building on initial research at Stanford University, has found a way to make a silicon anode that resists swelling and cracking.

Now manufacturers like Enovix (NASDAQ: ENVX) and Sila follow the trend. They have developed silicon anode batteries for cell phones, wearables and laptops. Sila applies this technology to the batteries of electric cars.

Go public

Amprius is on track to go public before the end of this year through a merger with special purpose acquisition company (SPAC) Kensington Capital Acquisition Corp. IV (NYSE: KCA.S).

“The reason we go public is not to solicit more funds for R&D development,” Tao said. “The entire use of funds is dedicated to scaling our mass production solution.” He said production will take place in the United States, possibly in Texas or Georgia.

The merger, announced in May, puts Amprius at an implied value of $939 million, assuming no buyout by public shareholders. Subject to Kensington shareholder approval, the gross proceeds of the merger will be approximately $430 million for Amprius, including $230 million in cash and up to $200 million in additional equity financing.

In the meantime, the company said it has engaged in “strategic discussions” about supporting most of the major eVTOL developers who are currently testing full-scale prototypes.

Nevertheless, it remains to be seen whether the Amprius battery technology will eventually prove itself in the eVTOL field. “By partnering up, we can give them considerations for things like battery performance or temperature control,” Tao said. “All of our lessons learned can be effectively applied to aviation.”