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Batteries are a central aspect of eVTOL technology, and in many places around the world where it’s cold for a substantial part of the year, how battery technologies will perform is a top concern.
Charging time, mission duration and ambient temperatures all impact battery operation. Conversely, batteries obviously have a direct impact on flight time.
For all that concern, however, eVTOL companies like Jaunt Air Mobility are not bothered. Jaunt has solid plans to launch operations in colder locations like Canada.
“The Jaunt Journey will meet the certification requirements of -40 F to 130 F [-40 C to 54 C], allowing the aircraft to operate anywhere in the world,” said chief communications officer Nancy McGrory Richardson.
Headquartered in Dallas, Texas, Jaunt is also planning to do its design work and manufacturing in Montreal, Quebec. In addition, during spring 2022, Jaunt agreed to invest over $100 million with L&T Technology Services over multiple years, enabling the engineering firm to open a center in Montreal and provide Jaunt with end-to-end engineering support in structural design analysis and certification.
For its battery system, battery management software, flight controls and electrification, Jaunt is working with BAE Systems.
The cell selection and arrangement of this battery system are specific to the Jaunt Journey. McGrory Richardson added that BAE’s modeling and simulation capabilities will maximize performance across the broadest environmental conditions and mission sets.
With more than 40 years of experience designing and certifying critical aerospace systems, BAE’s work also includes electrifying public transportation buses and hybrid-electric vehicles.
“We are confident in their rich legacy with high voltage energy storage systems,” she said, “and specifically in the battery selection and optimizing performance.”
Jaunt’s strong focus on battery technology that will perform over a large temperature range is a main reason why Canadian vertiport developer Vertiko Mobility has plans to initially operate a fleet of 70 Jaunt Journey aircraft by 2027 within a vertiport network in the Montreal and Quebec City areas.
Vertiko president John Valley and executive vice president Yannick Richard said that beyond the fact that a 25% increase in battery capacity is expected over the next five years, new battery chemistries are currently being studied.
“These technological advances will bring a significant increase in stored power but also, in some cases, a strong decrease in the sensitivity of cells to low temperatures,” they stated.
And while it’s currently very difficult, they added, to anticipate what the reality of batteries will be in 2027 when they launch operations, “the only thing we can do is to use an aircraft that is consistent with current technology and then take advantage of each technological advancement that will improve our operational capabilities.”
Several factors at play
Dr. Constantin Ionel Stefan, chief technology officer at battery developer Amprius, explained that there are several factors with operating batteries at low temperatures. In short, any materials that reduce the internal resistance of the cells will result in better low-temperature operation.
“Silicon brings advantages from the anode side, high conductivity electrolytes would improve the low-temperature performance as well, and low-impedance cathode materials would improve the cathode performance,” he said. “Any of these components and all components together can benefit low-temperature operation. In general, the electrolyte conductivity is the limiting factor, and that has the highest room for improvement.”
Lithium-ion cells are limited in terms of their low-temperature performance by quite a few factors. These relate to cell design, electrolyte properties and electrode characteristics.
Stefan said that of these factors, electrolyte properties have the most dramatic impact on low temperature performance of Li-ion batteries — that is, if the electrolyte is frozen, the Li-ion cell will not operate.
However, he also explained that sufficient electrolyte conductivity at low temperature is not enough to ensure efficient operation.
“To reduce the loss at low-temperature operation,” he said, “high diffusivity electrode materials must be identified coupled with improved cell design.”
In its battery, Amprius uses a silicon nanowire anode instead of a Li/Li+ electrode.
Silicon materials have already demonstrated better performance compared to graphite at temperatures lower than -20 C (-4 F). This finding was published in a paper in the Journal of the Electrochemical Society in 2016.
Graphite cells can lose up to 40% of their energy at low temperatures. Stefan also explained that at low temperatures, graphite can easily suffer lithium plating on particle surfaces.
“Silicon ‘potential’ versus Li/Li+ is higher than that of graphite and, thus, is much less prone to lithium plating at low temperature or during fast charge,” he explained. “A higher surface area also increases the reaction rate, further reducing the losses in energy at low temperatures.”
Stefan noted that their cells, designed for electric land vehicles, appear to lose only about 20% of the energy at low temperature versus 40% for existing electric vehicles (EVs).
“That is also from a much higher power capability level, which means that even at low temperature, the power level meets the EV target of standard cells at room temperature,” he said. “I would imagine that power and energy loss is even more critical for eVTOLs, as their range will be severely impacted by cold weather.”
eVTOL battery maker Cuberg released its design earlier in 2022 that swaps the anode from traditional graphite to lithium metal, which is vastly lighter and more energy dense. They have also developed a novel liquid electrolyte that doesn’t introduce any new critical materials supply requirements to battery manufacturing.
Stefan added that the design of the aircraft holds potential to positively impact low-temperature battery operation in a few ways.
One of these is pre-conditioning/heating the batteries during charging and starting the flight soon after. Another is recovering heat during operation.
He also noted that “I would expect that the temperature impact will be different across the wide variety of eVTOL designs currently in development.”