Electric vertical take-off and landing (eVTOL) aircraft developer Lilium has come out swinging in a bid to convince sceptics that the batteries it intends to use in its all-electric Lilium Jet can deliver the performance promised.
Speaking on a company webinar devoted to its battery technology on 10 November, founder Daniel Wiegand repeatedly dismissed claims of underperformance and excessive power consumption, calling them “myths” and “obviously wrong”.
He says tests covering key aspects of the batteries – power, energy density, and possible fast-charging degradation – have shown them to be capable of delivering the Lilium Jet’s stated goal of transporting six passengers on routes of at least 95nm (175km).
While acknowledging the aircraft’s ducted-fan design will consume roughly twice as much power during vertical take-offs and landings as its propeller-powered rivals – an estimated 2,147kW versus 1,014kW – Wiegand says that is much less than predicted.
In fact, tests flights of Lilium’s Phoenix scaled demonstrators have shown power consumption to be slightly lower than initially suggested by simple calculations.
Based on those results Wiegand strongly dismisses claims that the Lilium Jet would require five or even 10 times the power of propeller aircraft to hover. “It puts these statements in the land of myths,” he says.
Additionally, its “validated performance model” supports the lower power consumption figures over all mission profiles, he adds.
Besides, he says, any downside from the power consumption in hover is “more than compensated [for] in cruise flight”, where the “engine cross-section is better sized and will be significantly more efficient than propeller-based eVTOLs”.
“On top we are flying significantly faster in long-range missions compared to our peers, hence our aircraft is ideally suited to long-range missions. And the better batteries get, the bigger our advantage becomes.”
Its calculations suggest that a cell specific power of 2.47kW/kg would be sufficient “to provide sufficient power for hover down to the last 20% of charge”.
In contrast, its production-conforming Silicon anode batteries – preferred over standard lithium-ion chemistries to gain additional range – comfortably deliver 5kW/kg at 50% state of charge and have an energy density of 330Wh/kg. Wiegand says it has a “roadmap” to increase this to 350Wh/kg by 2026 and to 400Wh/kg by 2028.
Designed by Ionblox – a battery specialist in which Lilium has invested – the pouch cells promise “higher energy, higher power and higher fast charging capability” than Li-ion cells with graphite anodes.
Although the Ionblox cells will be state of the art when the Lilium Jet comes on the market in 2026, Wiegand dismisses suggestions that its success hinges on next-generation batteries.
“We have heard concerns that our aircraft would be dependent on battery technology half a decade away: this is obviously wrong.
“We have done four years of flight testing on a scaled demonstrator using off-the-shelf batteries with standard lithium-ion chemistries.”
In fact, he says, the initial Phoenix 1 demonstrator used cells more commonly found in e-cigarettes.
Internal tests of the production-conforming cells have shown them capable of delivering a 95nm mission, while retaining the energy reserves required by European regulators.
Additionally, tests conducted by the Idaho National Laboratory for Ionblox showed the cells retained a claimed 88% capacity after 809 full cycles.
Lilium has also carried out internal “heavy-duty tests” of the batteries to check for any potential capacity retention drop-off caused by fast charging and full-weight take-offs. Wiegand says the results show 88% capacity was retained after 1,450 flight cycles. Lilium’s business case calls for 88% capacity after 800 cycles.
Manufacture of the cells is currently being undertaken by CustomCells in Germany using a “state-of-the-art production facility dedicated to Lilium”.
However, the aircraft developer has moved to add a second source for battery production, unveiling a new agreement with InoBat, which will make the cells at two factories in Slovakia starting in 2024.
Lilium is currently building its initial production-conforming Lilium Jet – the first of six test assets for the certification campaign – with the final assembly of the wings and fuselage to take place at its headquarters near Munich in December, says Wiegand.
A maiden sortie with a pilot on board is set for the second half of 2024, he adds.
But Wiegand’s claims about the battery performance are unlikely to satisfy the firm’s biggest sceptics. Financial research house Iceberg Research has been a notable critic of the company, in particular over what it perceives as a lack of openness and transparency from Ionblox, including a refusal to release detailed test result data to investors or journalists.