By Alex Scerri

Alex Scerri started his aviation career in 1994 and initially flew for Air Malta. He joined Emirates in 2006, flying several types including the Airbus A380 as captain and worked in the fleet technical office. He is passionate about aviation safety and is looking forward to seeing urban aviation as the new mode of safe and clean city mobility.

q-and-a

A look at Zenlabs’ eVTOL battery technology that caught Lilium’s attention

Lithium-ion batteries are one the cornerstone technologies that are required for the successful introduction of advanced air mobility (AAM). Zenlabs Energy Inc. is one of the companies that is working to provide these batteries for eVTOL aircraft and already counts Lilium as one of its partners.

Lilium
Zenlabs Energy Inc. is one of the companies that is working to provide these batteries for eVTOL aircraft and already counts Lilium as one of its partners. Lilium Image

We spoke to Michael Sinkula, co-founder and chief business officer at Zenlabs, who gave us his candid view on the state of play and future for this technology.

Alex Scerri: Michael, can you give us a brief background on your work with Zenlabs Energy Inc.?

Michael Sinkula: I’ve been in the lithium-ion battery business for over 15 years now, mostly working with ground vehicle batteries, but also for consumer electronics and some military applications.

I was one of the founders of Zenlabs Energy Inc. five years ago, where we purchased the technology from another company. That gave us a good head start. The technology we are working with today is the result of about 10 years of constant development. It was not something that happened overnight, and now our cells are ready to be productized and deployed.

Alex Scerri: Quite a few eVTOL leaders still describe battery energy capacity as one of their main challenges. Do you share this view?

Michael Sinkula: In general, developing new lithium battery technology is very complicated. For example, today’s smartphones utilize a cell chemistry that is essentially a couple decades old. The industry normally sees incremental progress rather than big step changes.

Of course, everyone wants more energy density, more flight range, etc., as quickly as possible. However, there are many other boxes to tick, such as safety, and specifically for eVTOL aircraft, power output and cycle life for the economics.

Alex Scerri: It seems that the industry is looking, at least initially, for an energy density of around 300 to 350 watt-hours per kilogram. How far are we from an aircraft battery with that capacity?

Michael Sinkula: That is where we are at this point, and with the cell we are developing with Lilium, we are at ~330 Wh/kg. The crucial distinction is usable energy and available power at any state of charge (SOC) that must be available for take-off and at the end of the flight.

We often compare our batteries to those used by Tesla, as they are generally well known. Although Tesla’s batteries have good energy density, the power output drops significantly at low SOC, which is not suitable for eVTOL applications.

Alex Scerri: Is there an industry standard when quoting lithium-ion battery energy densities, for example, what ancillary items are included in the mass?

Michael Sinkula: There are two numbers. One would just be the energy density capacity at the cell level. Then of course, there is the other figure which would include the containment box or pack, the battery management system (BMS), any cooling system, etc., so that second number will decrease with more inactive material you have around the cell. In our case, since we are cell manufacturers, our datasheets represent the first value.

Zenlabs Lilium
An image of Zenlabs Energy Inc.’s battery cell. Zenlabs Image

Alex Scerri: Briefly, what is your part in the process when certifying batteries for aviation?

Michael Sinkula: It is a continuous learning process. The consensus is that certification will be done at the pack and system level rather than for the individual cells. I think the regulating authorities are still evaluating how they want to handle battery certification.

There is RTCA DO-311 Minimum Operational Performance Standards for Rechargeable Lithium Batteries and Battery Systems and other standards in Europe. Historically, rechargeable batteries have been an auxiliary electrical power source in aviation. Now, there is this big shift where the battery is the main power source for flight as well, which requires a rethink on the whole process.

Alex Scerri: What do you think is the most challenging part of the certification process?

Michael Sinkula: Irrespective of the chemistry used, the more energy you pack into a specific volume, the greater the exothermic event when the cell goes into thermal runaway. Battery safety comes down to how we package the cell. Cell-to-cell propagation must be prevented and if you still do have such an event, you must have the means to contain it, as well as means to vent any expanding gases and liquids. The certification must therefore consider the system as a pack, rather than individual cells.

Alex Scerri: Do you see predictive maintenance as part of the solution?

Michael Sinkula: Yes. The BMS continuously measures voltage and temperature, and usually, those two metrics will give you a very good indication on the cell’s state of health (SOH). Voltage is the primary indicator. You can compare the evolution of voltage to known behaviors and patterns associated with specific failure modes, to detect a problem cell before a critical failure occurs.

Alex Scerri: Which battery chemistry do you think holds the most promise for sustainable aviation applications?

Michael Sinkula: Of course, I have a little bias in this, but I truly believe that the lithium-silicon based chemistry is presently the best solution for eVTOL applications. The primary reason is that it offers both energy and power density which is unique among current battery technologies currently available.

This chart shows how Zenlabs Energy Inc. cells deliver both high energy and high power compared to other state-of-the-art cells. Courtesy of Zenlabs

Alex Scerri: What are the strengths of your technology?

Michael Sinkula: Our technology uses silicon-based anodes with silicon oxide as the active material, and the main strength is our electrode design. We also use a step called pre-lithiation where we apply a layer of lithium on the anode prior to assembling the cell which helps to compensate for irreversible capacity loss and helps increase cycle life.

Alex Scerri: What can you tell us about battery performance challenges in cold and hot ambient conditions?

Michael Sinkula: The cells are designed to work optimally at room temperature, and this is where you will get the maximum cycle life. With higher temperatures, we can see a slight improvement in capacity retention, as in simple terms, the ions can move faster. With lower temperatures, you could see some limitation on the discharge capacity. As an example, at 0°C (32°F), you could reach up to 89% of the nominal discharge capacity at room temperature. If you constantly operate at the low or high temperature extremes, you will also see a deterioration of the cycle life. Similarly, fast charging can also decrease cycle life, although we have shown that our chemistry is quite resilient under repeated fast charge conditions.

Zenlabs Lilium
This chart shows the temperature effect on discharge capacity. Courtesy of Zenlabs
Zenlabs Lilium
This chart shows the effect of charge rate on cycle life (C/3 = 3 hrs & 4C = 15 min). Courtesy of Zenlabs

Alex Scerri: Lithium hydroxide/carbonate prices have been increasing, for several reasons. What is the biggest cost driver for lithium-ion battery production?

Michael Sinkula: Traditionally, for nickel manganese cobalt (NMC)/graphite or lithium cobalt oxide (LCO)/graphite-based systems, lithium wasn’t the main cost driver, with cobalt being the more significant part of the raw material fraction. For most cathode materials, excluding lithium ferrophosphate (LFP), cobalt remains the biggest cost driver but you’re right in saying that in the past few years, there has been a rise in the cost of lithium as well.

I think lithium prices will stabilize again but cobalt remains an issue simply because of the limited supplies and geopolitical issues at these locations. That is why you see the effort from cathode makers to move away from this element due to the cost, but also the other undesirable effects associated with its mining and processing.

Alex Scerri: How are you looking at the full life cycle of lithium-ion batteries, in other words, from the raw material, to design, to production, and eventual disposal and/or recycling?

Michael Sinkula: One issue we have is that once a cell is charged and discharged so many times over its life, new compounds are formed that are not straightforward to reverse back to reusable raw materials. Where we have done some positive breakthroughs is that in the past, N-Methyl-2-pyrrolidone (NMP) solvents were used in the production of the electrodes and we have replaced these with water-based solutions as much as possible to reduce the environmental impact.

Alex Scerri: As a leader in what is one of the industries that will be a cornerstone in electrification, is there something that keeps you up at night?

Michael Sinkula: Internally, we are pushing the envelope on the technology and we do rely on some very specialized suppliers for specific components and in some cases, there is just a single supplier which of course is a concern.

Regarding the end-users of our products, we are keeping an eye on the certification timelines from the Federal Aviation Administration (FAA) and other agencies. Recently, there seems to have been some changes with how the FAA will tackle certification of these aircraft which makes me think that timelines could slide. Of course, this is something which could affect our business.

Alex Scerri: Michael, thank you for your time and we will be following the progress of Zenlabs and your cells with great interest.

Alastair McIntosh, chief technology officer at Lilium, provides an update on the company’s aircraft development. McIntosh discusses the aircraft’s battery technology at 5:55 in the video.

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