ORNL study highlights need for tailored battery designs for eVTOL applications
07 March 2024
A study by a team from Oak Ridge National Laboratory (ORNL) highlights the need for tailored battery chemistry designs for eVTOL applications to address both anode plating and cathode instability resulting from the demands of the application.
In addition, the study found that innovative second-use strategies would be paramount upon completion of the eVTOL services. The open-access study appears in the journal ACS Energy Letters.
High power is a critical requirement of lithium-ion batteries designed to satisfy the load profiles of advanced air mobility. Here, we simulate the initial takeoff step of electric vertical takeoff and landing (eVTOL) vehicles powered by a lithium-ion battery that is subjected to an intense 15C discharge pulse at the beginning of the discharge cycle followed by a subsequent low-rate discharge. We conducted extensive electrochemical testing to assess the long-term stability of a lithium-ion battery under these high-strain conditions. The main finding is that despite the performance recovery observed at low rates, the reapplication of high rates leads to drastic cell failure.
—Dixit et al.
High power requirements in eVTOL load profiles and electrochemical behavior of lithium-ion batteries under simulated takeoff step. (A) Schematic diagram showing major segments of an eVTOL mission profile and the corresponding power requirements. (B) Current profile for the testing carried out in this study. Cells are charged at a nominal 1C rate until a full state-of-charge is achieved (4.2 V cutoff). At the beginning of discharge, a current pulse equivalent to 15C is applied for 45 s. Subsequent discharge is carried out at a nominal C/3 current. For the batteries investigated, 1C corresponds to ∼0.08 A, and 15C corresponds to roughly 1.2 A. (C) First charge–discharge cycle polarization curve with the dotted line showing the charge cycle and solid line depicting the discharge cycle. (D) Capacity retention of the cell over extended cycling under the simulated climb step discharge protocol. (E) Complete polarization curves and (F) zoomed in plot of the polarizations within the 15C discharge pulse segment for the 1st, 25th, 50th, 75th, and 100th cycles. The cut-off voltage of 3 V is identified by a dashed line in (E) and (F). Dixit et al.
To better understand how high strain events such as liftoff can affect LiB stability, Ilias Belharouak, Marm Dixit and colleagues stressed out a set of LiBs and investigated how their performance changed.
The researchers manufactured a set of LiB cells containing a specially designed, fast-charging and discharging electrolyte. Then, they drained 15 times the battery’s optimal capacity for 45 seconds. This process simulated the rapid, high-power discharge needed during vertical takeoff.
After the initial discharge pulse, the cells were further drained at a more normal discharge rate and then recharged. The team found that none of the tested cells lasted more than 100 cycles under these high-stress conditions, with most starting to show decreased performance around 85 cycles.
After being stressed, the researchers subjected the LiB cells to a more normal, lower rate power draw. In this experiment, they observed that the cells partially retained their capacities under low-rate conditions, but failed quickly when put under rapid current drain conditions again.
These results indicate that the LiBs typically used in drones might not have the characteristics necessary for long-term, high-stress usages, but they could be retired and meet more typical power demands in other applications, such as battery back-ups for power supplies and energy-grid storage. The researchers say that more work is needed to develop alternative battery technologies that are better suited for vertical takeoff and other high-power-demand applications.
The authors acknowledge funding from the US Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory.
Resources
Marm Dixit, Anuj Bisht, Rachid Essehli, Ruhul Amin, Chol-Bum M. Kweon, and Ilias Belharouak (2024) “Lithium-Ion Battery Power Performance Assessment for the Climb Step of an Electric Vertical Takeoff and Landing (eVTOL) Application” ACS Energy Letters 0, 9 doi: 10.1021/acsenergylett.3c02385
Although this is not directly about how you charge the eVTOL battery, if you are also fast charging it routinely that surely can't help the cycle life of a battery already under severe discharge stress.
That is why I fancy designs which swap out modular batteries after each trip, so that they can be charged slowly.
Posted by: Davemart | 07 March 2024 at 02:30 AM
eVTOL batteries are going to be right at the forefront of battery development, as a premium there for energy density is well worth it.
Here is a solid state electrolyte made from earth abundant materials:
https://newenergyandfuel.com/http:/newenergyandfuel/com/2024/03/07/research-discovers-a-solid-conductor-to-replace-electrolyte/
Looking at the summary of the research paper behind it:
' Han et al. designed electrolytes based on a Li7Si2S7I chemistry with ion arrangements similar to those in intermetallic systems. This leads to anion packing that alternates between hexagonal close packed structures and sheared face-centered cubic-like motifs to accommodate sulfur and iodine complexes, analogous to the structure of nickel-zirconium. '
Which is pretty darn good for cost.
Posted by: Davemart | 07 March 2024 at 02:53 AM
Or a small rocket motor - just to get you going.
I assume supercaps don't have the capacity for a lift off.
Posted by: mahonj | 07 March 2024 at 11:19 AM
The best battery for e-VTOL would be high-performance Li-Polymer with 60 C discharge rating for take-off and landing, PLUS a FC stack and Liquid H2 tank for reasonable-distance cruising and for recharging the battery. Nothing can beat the lightness of Liquid H2 for aviation, NOTHING.
Agree with Davemart: Too much trouble re-charging the e-VTOL if quick turn-around is desirable for productivity, because even 20-minute charging at 3 C rate wold shorten the battery life and limit the productivity of the short-trip vehicle, so battery swapping is the next best option after the use of Liquid H2. for "self-charging" capability.
Posted by: Roger Pham | 07 March 2024 at 01:23 PM
Roger you're absolutely right lithium polymer with a range extender is the way to go
Posted by: SJC | 07 March 2024 at 01:29 PM
Hi Roger.
As you know I support hydrogen where its lower weight increment for energy delivered beats batteries.
And I agree, hydrogen will probably have a part to play as VTOL technology matures.
However the problem right now is getting them off the ground and landing them again, we can worry about increasing cruising range later.
Since fuel cells have low power density as opposed to the high energy density inherent in hydrogen, then the conservative route to advancement is to make short range battery only VTOL to start with, as for instance Joby are doing although a major investor is Toyota with its penchant for fuel cells.
Sirius are trying to go straight to hydrogen supported by batteries, but have not had any test flights as yet, and several elements of their design are ambitious, to say the least.
Posted by: Davemart | 07 March 2024 at 02:02 PM
Rolls-Royce and Honeywell have megawatt turbine alternators ready to go right off the shelf Joby, Beta and others can just plug them in and have range no problem.
Posted by: SJC | 07 March 2024 at 02:14 PM
Great point, SJC!
Nothing can beat the power-to-weight ratio of MegaWatt aero-derived gas turbine generators. Forget about the fragile and short-lived LiB for high-drain e-VTOL application with pitiful range. Just use one of them MegaWatt turbine alternator for take-off and landing mega-power requirement, while use a fuel cell for ultra-efficient cruise at much lower power, and of course Liquid H2 to maximize payload and near-zero emission.
For even faster cruise speeds and power redundancy, we can have 3 smaller turbine alternators providing power for lifting off, and only 1 turbine necessary for cruise,, at maximum efficiency and maximum compactness with cryo-pre-cooled intake air thanks to the LH2.
With this latter combination, we can do roof-top to roof-top inter-city travel even as far as a thousand mile away, while cruising at 300 mph in pressurized cabin. Very convenient and even safer than business jets due to the extra power redundancy and the VTOL capability that can land anywhere in case of emergency and not being restricted solely to airport landing, and just as fast as business jet door-to-door time due to the capability of roof-top to roof-top, and bypassing all the traffic hassles of getting to and from the airports. A perfect All-Terrain Vehicle and all-weather aircraft for all kinds of wilderness trips and vacations as well.
Posted by: Roger Pham | 07 March 2024 at 07:42 PM
Well, SJC, actually only 2 turbine alternators would be needed, and just for power redundancy, because the MegaWatt turbine alternator is also very efficient at low power during high altitude cruise.
For smaller e-VTOL aircraft, even 1 single MegaWatt alternator is sufficient if parachute-equipped in case of failure of the single engine. A 1,000-hp gas turbine can provide its full output efficiently at sea-level, then at 30,000 ft cruise, it will maintain the same efficiency when delivering only 330 hp needed for fast cruise due to the much thinner air at high altitudes.
Posted by: Roger Pham | 07 March 2024 at 07:50 PM
Well, actually, a MW alternator should have a minimum of 1,340 hp electrical output, so should be good for an e-VTOL aircraft that can carry as much load as a business jet when fueled with LH2, and not 1,000 hp. This will be a wonderful advancement for business and general aviation with the MW aero-turbine alternators powering e-VTOL. Thanks, SJC, for the brilliant idea!
Posted by: Roger Pham | 07 March 2024 at 07:56 PM
Lithium sulphur batteries can hit twice the energy density of conventional lithium batteries so would be dead handy for VTOL.
For some reason these guys here have chosen to announce an incremental improvement in conductivity:
https://www.sciencedaily.com/releases/2024/03/240306150616.htm
which is all very nice, but 100 billion times as much conductivity is hardly something to shout about! ;-)
Apparently these batteries heal by moderate heating from damage caused by expansion.
Posted by: Davemart | 08 March 2024 at 02:46 AM
You are welcome Roger it seemed like an obvious match.
You mentioning lithium polymer was exactly what I've been working on for years now
I came to the conclusion it's a natural for evtol.
Posted by: SJC | 09 March 2024 at 11:15 AM
Charging challenges for eVTOL airports:
https://techxplore.com/news/2024-03-electric-aircraft-grid-site-generation.html
'It might seem that any 50-square-foot surface would work as a vertiport, but electrical and airspace requirements set hard constraints. From talking with aircraft manufacturers, the research team determined that for consistent operations, an average vertiport would need 1-megawatt or greater charging capacity. That amount of electricity powers around 800 homes, an amount that typically requires major grid upgrades and years of planning. The NREL team sought to clarify the extent of that prep work for eVTOL charge sites.
"We studied three interrelated pieces of the puzzle for eVTOL vertiports: evaluating in-flight demand, estimating charging demand, and determining grid capability to meet this demand," Solanki said.'
I think that a good option may be to use fuel cell power generation, either from hydrogen or methanol, which is now more or less a plug and play solution.
There are of course energy efficiency losses, but looking at one metric in isolation is often to miss the point.
Posted by: Davemart | 10 March 2024 at 03:47 AM
Thank you, Davemart, for this latest insight. It may be far easier to deliver Liquid H2 via VTOL to rooftop heliports. A larger size LH2-VTOL can carry 1 metric ton of payload. Each VTOL flight will consume about 5-10 kg of LH2, depending on distance, so one metric ton of LH2 will last all day and will cover 100-200 VTOL flights. Much easier than re-configure the fast-charging infrastructure for battery-only e-VTOL. The incredible lightness of LH2 is again a great advantage for the logistic in this regard, in comparison to petroleum fuel. Easy to deliver the LH2 and very quick and easy to refuel in comparison to much longer times for charging of battery-only e-VTOL..
Posted by: Roger Pham | 10 March 2024 at 04:59 PM