With electric aviation still in its infancy, eVTOL developers have largely been relying on the same lithium-ion battery cells found in electric ground vehicles (EVs) to power their aircraft. Although the performance of lithium-ion batteries in EVs is well understood, their application in the aviation industry is relatively uncharted, and it’s not yet clear how these batteries will withstand the harsh conditions they will endure during eVTOL air taxi operations.
To gain a better understanding, researchers at Oak Ridge National Laboratory (ORNL) in Tennessee conducted a study into the effects that an eVTOL aircraft’s flight profile will have on EV batteries after repeated cycling, simulating typical air taxi operations. The research team found that the power and performance demands for eVTOL flight reduce battery performance and longevity, which could potentially pose a safety threat. It could also increase the cost of aircraft maintenance, as batteries will need frequent replacing.
When eVTOLs take off vertically, “the amount of peril that these batteries can experience is going to be like nothing we’ve seen before,” Ilias Belharouak, an ORNL corporate fellow who participated in the study, told AIN. Because eVTOL batteries will be subject to “very harsh conditions,” he explained, they may be prone to premature and unforeseen damage and corrosion.
Belharouak and his team aim to mitigate this problem by advancing lithium-ion battery technology and optimizing battery cells for eVTOL flights. But to find the best solutions, they first needed to thoroughly define the problem. With this study, the team sought to determine exactly what happens to the batteries at a sub-cell level when subjected to the high power demands of eVTOL flights with repeated cycling. The findings will help to inform their search for new materials, particularly for the cell’s electrolytes, which could lead to better performance and endurance.
High Power Demands
Although lithium-ion batteries have been thoroughly tested and validated for EVs, the conditions they experience during eVTOL operations are not comparable to normal driving conditions. EV batteries generally drain at a steady rate, whereas eVTOLs require short bursts of high power, especially during takeoff and landing.
Electric batteries for air taxis will also endure more frequent and rapid charging and discharging than ground vehicles. For example, a Tesla can drive for several hours on a full battery and takes around 45 minutes to recharge using a Level 3 DC fast charger—and most users don’t need to recharge more than once per day. Meanwhile, the first eVTOL air taxis are expected to fly for about 10 minutes, then charge for about 10 minutes, and repeat. “You really need to charge them very fast and discharge them very fast…which puts a lot of strain on these batteries,” Belharouak said.
Belharouak and his team at ORNL conducted simulated eVTOL battery tests using representative batteries they built on-site at the Department of Energy's Battery Manufacturing Facility. They monitored battery performance during cycling and then assessed the battery’s components afterward to check for corrosion and other chemical or structural changes using a scanning electron microscope.
“Your battery is not just capacity at the end of 1,000 cycles. It’s what’s happening within a cycle that tells you whether your system is going to work or crash,” Marm Dixit, the lead researcher for the study, said in an ORNL statement. “And the stakes are much higher here because you’re asking how safe it is to go up in the air. This is a question we don’t know the answer to yet.”
For the simulation, the researchers employed a high discharge rate of 15C for 45 seconds—which they said represents the average high-power demand for typical eVTOL takeoffs—followed by a low-rate discharge to simulate cruise flight. A discharge rate of 15C means the battery would discharge its entire capacity 15 times in one hour at that rate.
“Remarkably, during this low-rate evaluation, the cell exhibited promising behavior, achieving its original capacity and showing good retention. This observation suggests that the capacity loss experienced after the 15C pulse test was not irreversible, and the cell’s electrochemical capabilities were partially restored under low-rate conditions,” the researchers said in the study, which was published February 12 in ACS Energy Letters.
However, when subjected to subsequent high-power pulse tests after the low-rate recovery period, the battery cell exhibited a “clearly diminished resilience.” Using the scanning electron microscope, the researchers discovered evidence of residual plating—a buildup of lithium atoms that turn into metal—on the anode. This finding indicates a breakdown in the electrolyte material, according to the study.
“The presence of lithium plating can lead to the formation of dendrites—needlelike structures that grow from the anode and can pierce the separator—causing short circuits and compromising the overall safety and lifespan of the battery,” they wrote in the study. “The detection of anode plating underscores the challenges associated with the rapid power surges typical of eVTOL operations. The extreme demands placed on the anode during high-rate discharges could trigger these plating phenomena, highlighting the need for advanced anode materials or innovative design approaches to mitigate this issue and enhance the battery’s cycling durability in eVTOL applications.”
While the electrolyte and anode components of the battery cell showed signs of degradation, the researchers found that the cell’s cathode remained resilient to repeated high-power discharges. “The preserved integrity of the cathode material suggests that, even under intense high-rate discharges, the cathode’s structural stability and electrochemical activity remain relatively unaffected.”
Electrolyte Solutions
Ultimately, the development of an alternative battery chemistry to lithium ions—one that provides both higher energy density and power density—could prove to be the holy grail for electric aviation. But until someone makes that Nobel Prize-worthy discovery, lithium-ion batteries will remain ubiquitous.
In the meantime, there’s still plenty of room for improvement in lithium-ion battery technology. Researchers are continuously looking for ways to make the batteries perform better and last longer using different materials for their components, including anodes, cathodes, and electrolytes. For example, battery maker Amprius is using silicon nanowire anodes in the batteries it is offering for electric aviation applications.
Belharouak and his team believe that the solution for making lithium-ion batteries more suitable to eVTOL operations lies within the electrolyte, the medium between a battery’s cathode and anode that lithium ions travel through during charging and discharging. “Most of these problems can be mitigated through electrolyte solutions,” said Belharouak, “and those electrolytes will just carry lithium ions in a very fast way compared to the conventional electrolytes.”
The ORNL team has been testing new electrolyte solutions developed at the laboratory as part of their eVTOL battery research. One solution is a high-conduction media they created by modifying the salts within the electrolyte. They’re also exploring solutions using a type of gel for the electrolyte material, which is normally a liquid. According to the researchers, ORNL-developed electrolytes performed better than the current state-of-the art batteries, and they retained more capacity in the most power-intensive phases of flight.
While the ORNL team is focusing on electrolyte solutions for now, the ultimate goal of the research program is to eventually develop a whole new battery chemistry that could replace lithium-ion batteries for electric aircraft.
“We started this program by assessing the state of the art, what exists today. From there, we move to solutions in electrolyte development, where we can see a really neat improvement compared to the state of the art. The third portfolio in this program is completely changing the chemistry—meaning changing the cathode, the anode, and the electrolyte at the same time while keeping in mind the power density and energy density balance that needs to be realized for this application,” Belharouak explained.
Examples of new battery chemistries that could be promising for aviation applications include solid-state batteries, which replace the liquid or gel electrolyte with a solid material, or lithium-sulfur batteries, both of which can offer the higher energy densities needed to enable longer-range flights.
“But these are still in the infancy stage, and it will take several years before we can even judge how they’re going to behave under these harsh conditions,” he said. “It has to be tested under these very specialized protocols, or the strain conditions, and then we have to judge whether they’re going to be valuable or not.”
Belharouak stressed that any type of batteries intended for eVTOL applications “will have to be understood and comprehended based on the set of protocols they are going to be subjected to, not based on just the energy density and power density.”