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Batteries will be a critical enabling technology for most advanced air mobility aircraft, but especially for Lilium’s seven-seat eVTOL Lilium Jet. Unlike many of its competitors in the air taxi market, Lilium is pursuing a regional air shuttle model that will see its Lilium Jets deployed on longer routes from launch. For example, Joby Aviation and Archer are both predicting average trip lengths of around 25 miles (40 kilometers), but Lilium expects its air taxis to cover 60 to 75 miles (95 to 120 km) at a time — journeys that will deplete its batteries to a much lower state of charge.
At the same time, the Lilium Jet — which uses 36 small ducted fans in its wings for both thrust and control — has a disc loading that is up to 10 times higher than some competing open propeller architectures. That results in a very high power demand during hovering flight, necessitating batteries that can provide that power even at a low state of charge, as when landing after a long flight. Moreover, to enable Lilium’s business model in which each aircraft performs 20 to 25 flights per day, the batteries will need to be capable of fast charging, which can negatively impact cycle life and economics.
All of this adds up to what Venkat Viswanathan, an associate professor of mechanical engineering at Carnegie Mellon University, describes as the “AND” problem. As he told eVTOL.com, “The various metrics taken separately appear feasible with near-term commercially available lithium-ion batteries, but the challenge will be to deliver all of these requirements simultaneously.” Doubts about Lilium’s ability to achieve sufficiently high energy density and high specific power and fast charging and long cycle life with its batteries account for much of the skepticism expressed toward the Lilium Jet design.
Lilium has yet to reveal its third-party battery supplier, with co-founder Patrick Nathen and chief technology officer Alastair McIntosh explaining in an interview last month that Lilium is still sensitive about protecting its core intellectual property. However, with the startup now in the process of going public through a combination with Qell Acquisition Corp., Lilium has disclosed significant details about its battery development work through a technical paper, blog, and form F-4 registration statement recently filed with the U.S. Securities and Exchange Commission.
Notably, Lilium has confirmed that it will be using large-format lithium-ion pouch batteries with a cell chemistry based on a silicon-dominant anode combined with conventional nickel, manganese, and cobalt (NMC) cathodes and liquid electrolytes. According to its form F-4, Lilium believes this combination “offers the best compromise of energy and power density at a low state of charge,” which Nathen and McIntosh have said will be as low as 10 to 15 percent. Lilium claims to have secured exclusive rights for this chemistry in the eVTOL market, and expects to conduct the majority of battery cell production on standard lithium-ion pouch cell production lines.
According to Lilium, laboratory testing of the pouch cells has shown nominal energy density levels of 330 watt-hours per kilogram, which the company projects will enable an aircraft range of 155 miles (250 km) at entry into service. The company is targeting a battery system capable of recharging to 80 percent charge in 15 minutes, and 100 percent within 30 minutes, and is “working with leading suppliers for charging technology using equipment based on chargers for the electric trucking industry,” its form F-4 states.
Meanwhile, it is developing digital platforms “for coordinating and monitoring the rapid charging of a jet’s batteries, with controlled temperature regulation for optimal throughput and battery health and longevity.”
Lilium anticipates that its batteries should provide “a sufficient cycle life,” defined as over 800 standard charge/discharge cycles measured until 80 percent capacity. According to Viswanathan, however, this may be optimistic. As he and his co-authors explained in a 2018 paper, batteries typically experience a power fade alongside capacity fade, which over time reduces their maximum deliverable power and thus their ability to support safe vertical landings, when power requirements are highest.
“Given [Lilium’s] higher specific energy requirements, going with a silicon-dominant anode certainly makes sense,” he told eVTOL.com via email. “However, current silicon-dominant anodes typically cannot meet the extremely large power requirements for the Lilium Jet. Thus, being able to deliver the power density at low state of charge, i.e. landing segment is likely to determine end-of-life, not fade to 80 percent capacity.”
McIntosh confirmed that the seven-seat Lilium Jet will have a total of 72 fully interchangeable battery modules, or two for each of the 36 ducted fans, distributed across 12 packs that will work in parallel to supply the required power. Lilium is developing the battery pack design and energy management in house as part of its core technology, and is designing its battery casing to protect against the effects of multiple-cell thermal runaway.
The packs are positioned low in the body of the aircraft with regard for crash safety and the ability to upgrade as battery technology improves. Based on its projected operational tempo and battery cycle life, Lilium expects to replace the batteries on each aircraft two to three times per year. While replacing them won’t necessarily be a trivial undertaking, Nathen pointed out that the packs are readily accessible from the ground, versus some designs that incorporate batteries into lifting surfaces. Moreover, some other eVTOL developers “are saying that they’ll do battery swaps in service, which would lead to battery logistics on the vertiports, which we are not doing,” he noted.
In February 2020, Lilium suffered a major setback when its first five-seat technology demonstrator, dubbed “Phoenix,” was destroyed in a ground fire. According to its form F-4, the incident occurred during installation of a battery on the ground after maintenance of a battery module.
“The battery involved in this incident was not the battery technology intended for use on the serial aircraft but a basic battery system without fire protection measures used solely to provide power for the technology demonstrator,” the filing states (while elsewhere acknowledging that the system incorporated “many of the technologies of our envisioned and certifiable series solution”).
“There was a known weakness there,” McIntosh told eVTOL.com last month. “The silver lining to the cloud, as it were, has been some really good in-depth analysis and further understanding [of the cause of the fire], and then subsequently some of the redesign work that we’ve brought forward and validated to put into the aircraft that we’re going to be flying again quite soon.”