Original article: Evtol.com, Author: Elan Head
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 GmbH, Munich, Germany, 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.
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