Lilium’s eVTOL aircraft, with tiny ducted electric “jets” instead of exposed propellers, looks incredible, but puts a unique strain on the battery. Lilium has invested millions of dollars in companies that make cells that it believes can do the job.
I previously wrote about Lilium’s small fan design and its impact on aircraft efficiency and possible use cases. In summary, other things being equal, using a large number of small-diameter fans instead of a few large-diameter propellers will result in an aircraft that chews about twice as much power during his VTOL and hovering phases of flight . Resistance may be reduced during the cruise phase. As a result, Lilium targets longer inter-city excursions, as opposed to shorter cross-town hops, where this inefficiency becomes more of a disadvantage.
One thing I didn’t delve too much into in that first article is what this means for batteries. For the kind of long-haul flights that Lilium has to handle, the battery needs to have a high specific energy. But that battery must also have very good power density, as it must also be able to handle very high discharge rates to handle these high-power VTOL flight phases.
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Most importantly, Lilium requires a battery capable of high power output even at low state of charge. Imagine that you were flying near the limit of your rated range, and an emergency situation caused the pilot to rush into the reserve battery at his level, and the charge dropped below his 20%. The aircraft must still be able to perform a safe vertical landing. This is an operation that requires up to 45 seconds of high power on the Lilium projection.
It’s just a special battery, capable of sustaining high output and pumping tons of juice even when it’s almost full. (formerly known as Zenlabs) said it was handled thanks to a multi-million dollar investment in a company called Zenlabs.
A key difference between Ionblox and other lithium battery cell manufacturers is that most conventional cells use a silicon-dominant anode where graphite is used. Silicon anodes are capable of ultra-fast charge/discharge rates and solid bumps of 25% of specific energy, but silicon expands as much as 400% upon charging, and their size prematurely disappears after repeated charge cycles. tend to. The changes tend to accumulate and the anode begins to crack and deteriorate.
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Ionblox holds numerous patents in this area. We’re talking about holding small blobs of silicon in a silicon oxide/carbon composite matrix.For electric aviation, the numbers sound like a good step forward.
At the cell level, Tesla’s Model 3 battery tends to be the go-to for certain energy comparisons. Inside the EV They say they currently hold 272-296 Wh/kg. Ionblox says its aviation-specific battery can hold as much as 340 Wh/kg of capacity when slowly charged, providing a useful boost of 15-25%.
In terms of power density, Ionblox says its cells can handle 3.8 kW per kilogram at a giant discharge rate of 12C in a 30-second pulse at 50% state of charge, and about 3 kW/kg even down to 30. can be processed. % capacity, again at a 12C rate for 30 seconds at a hot ambient temperature of 30 °C (86 °F) – so a high power vertical landing at a low charge level should be fine.
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We also tested the lifespan of an automotive grade cell and found that it could retain more than 80% of its capacity after 1,000 charge and discharge cycles at a very fast 4C rate, equivalent to a full charge in 15 minutes. Aviation-grade cells have not yet undergone such rigorous testing, but Ionblox says they can withstand well over 700 cycles of 1C (60 minutes) charge and discharge cycles.
Lilium says it will most likely use the next generation of these batteries before they go into production. The company has pushed back its certification expectations to 2025, so Ionblox still has a few years to fine-tune its formulation.
Source: Ionblox, Yuri
