With electric trainer airplanes already in service and the first eVTOL models on the brink of certification, lithium-ion batteries have staked their claim on yet another electrifying industry. Lithium-ion batteries may not offer the kind of energy and power that the aviation industry yearns for, but that hasn’t stopped aircraft developers from pursuing their electric aviation dreams and building battery-powered, emissions-free aircraft.
For the advanced air mobility industry, batteries available today are sufficient for small aircraft operating short flights, but scaling the technology to larger, longer-range aircraft will require a breakthrough in battery technology. Exactly what that breakthrough might entail remains to be seen, but in the meantime, new-and-improved batteries could be closer than you think—and they may not be that different from the batteries we already use today.
Power vs. Energy
One of the key challenges that battery developers face is the interplay between power and energy. Higher energy density means a battery can store more energy per unit of volume, whereas power density refers to how quickly a battery can discharge its energy. Ideally, batteries powering electric aircraft would be able to provide both high energy and power densities, but unfortunately, those two qualities don’t go hand-in-hand.
Electric aircraft, particularly new eVTOL models, need batteries with high power densities to provide enough lift during takeoff and landing. At the same time, they need enough energy density to support the desired range plus energy reserves. The FAA has not decided on energy reserve requirements for eVTOL aircraft, but current regulations require commercial airplanes to carry 30 or 45 minutes of energy reserves in day or night VFR conditions, respectively, whereas helicopters are required to have 20 minutes.
Given that most eVTOL aircraft in development today are intended to operate on short inter-city hops of around 20 miles or less, existing energy reserve requirements would double or even triple the planned weight of the batteries. The eVTOL sector has been lobbying for a performance-based approach, rather than the traditional time-based requirement, to help maximize the already limited range of eVTOL aircraft and make the technology more economically viable.
While energy density is important for range, power density is especially critical for eVTOL aircraft during takeoff and landing. Electric car batteries discharge at a relatively steady rate, but eVTOL aircraft require short bursts of high power to take off and land. EV batteries are not optimized for the varying power output of eVTOL aircraft. Aircraft also have much stricter weight limitations, which is another reason that EV batteries are not ideal for eVTOL applications.
Alternatives around the Corner?
Lithium-ion battery technology may not yet be advanced enough to support long-distance flights but for now, they remain the best solution. However, that could soon change; novel approaches to battery chemistry have already emerged from research institutions and made their way into some commercial products.
Examples of new battery chemistries that could be promising for aviation applications include solid-state batteries and lithium-sulfur batteries, both of which can offer the higher energy densities needed to enable longer-range flights. Scientists and engineers have already demonstrated that such alternative battery chemistries are technologically feasible, but they’re a long way from becoming economically viable and certified for use in electric aircraft.
Solid-state batteries have emerged as potentially the most promising alternative to lithium-ion batteries when it comes to aviation applications, and the automotive industry is already testing the technology in electric vehicle (EV) batteries. Samsung announced in August that it had begun pilot production and testing of a solid-state EV battery that it said would provide 600 miles of driving range, ultra-fast charging times, and longer battery lifespans.
“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 [eVTOL operating] conditions,” Ilias Belharouak, a battery scientist with the U.S. Department of Energy’s Oak Ridge National Laboratory in Tennessee, told AIN. As head of ORNL’s electrification section, Belharouak leads a team of researchers focused on advancing battery technology as well as battery manufacturing processes.
Earlier this year, Belharouak and his colleagues published a study that assessed how various EV batteries would perform under eVTOL operating conditions. They found that the power and performance demands for eVTOL flight reduce battery performance and longevity, highlighting the need for tailored, performance-based battery solutions. Before any new type of battery is certified for use in electric aircraft propulsion systems, “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 said.
Solid-state Batteries
Considered by many to be the holy grail of energy storage solutions, solid-state batteries are slowly making their way into the EV market and appear to be the most likely contender for the next generation of aircraft batteries. Whereas lithium-ion batteries typically contain liquid or gel polymer electrolytes, solid-state batteries have solid electrolytes. They have much higher energy densities than traditional lithium-ion batteries and are generally considered to be safer, which makes them an ideal candidate for aviation applications.
Lithium-ion batteries are prone to thermal runaway—uncontrollable overheating that may result in fire or explosion—partly because they contain flammable liquid electrolytes, typically consisting of organic solvents mixed with lithium salts and other additives. In the event of a short circuit or other physical damage, flammable electrolytes in lithium-ion batteries can ignite, potentially making an already bad situation much worse.
In solid-state batteries, the flammable liquid electrolytes are replaced with solid ionic conductors that aren’t flammable. Compared with lithium-ion batteries, solid-state batteries have better thermal stability and can operate efficiently in a wider range of temperatures. With a higher energy density, they also reduce the total weight of the batteries, enabling longer flights with larger aircraft and heavier payloads.
The electrolyte is the material between the cathode and anode, or the positive and negative electrodes at opposite ends of the battery cell. It facilitates the movement of lithium atoms between the two electrodes as a battery charges and discharges.
When a battery is discharging, or outputting electricity, lithium atoms are released from the negatively charged anode and flow toward the positively charged cathode. During this process, the lithium atoms shed their outer electrons and become positively charged lithium ions. Those freed electrons, which have a negative charge, then move toward the cathode. The opposite happens when a battery recharges; lithium ions are released from the cathode and move back toward the anode.
Researchers have been experimenting with different types of solid electrolytes for decades. All have generally run into the same problems: low ionic conductivity, high surface resistance at the electrode-electrolyte interface, and poor mechanical stability with brittle solids. Higher resistance and lower conductivity both hinder the flow of electricity through the battery, limiting its performance and reducing the overall energy efficiency.
For these reasons, solid-state batteries on the commercial market have so far been limited to small electronic devices, including some hearing aids, pacemakers, and wearable fitness trackers. However, just as lithium-ion batteries have evolved over the years, the technology behind solid-state batteries is ever-improving and becoming more relevant for a wider range of applications.
Unfortunately, solid-state batteries are also more expensive than lithium-ion batteries, both in terms of raw material prices and the cost of the more complex manufacturing processes. To compete with lithium-ion batteries in any market and make them worth the extra cost, solid-state batteries will need to deliver a pretty dramatic improvement in performance over their ubiquitous liquid-state counterparts. With the state of battery technology today, the most advanced lithium-ion batteries for aviation applications are just about on par with solid-state battery performance numbers.
Li-ion Battery Breakthroughs
While the automotive industry eagerly awaits the arrival of solid-state EV batteries, the aviation industry is praying for a Nobel Prize-worthy breakthrough that could someday make long-distance travel on battery-electric airplanes possible. In the meantime, pragmatic scientists are still working to improve the lithium-ion batteries that so many have long been eager to replace.
In addition to increasing power and energy, researchers are looking to make lithium-ion batteries more resilient and increase their lifespans, thereby reducing the frequency of battery replacements.
Belharouak believes that the solution to optimizing lithium-ion batteries for eVTOL applications all boils down to the electrolyte. He and his team at ORNL have been developing and testing new liquid and gel electrolyte materials that can conduct lithium ions more quickly and efficiently than traditional electrolytes found in today’s batteries. Belhaourak and other battery researchers are also looking into alternative materials for cathodes and anodes.
Whereas the most common material used for anodes in lithium-ion batteries today is graphite, silicon has recently emerged as a promising alternative anode material, particularly when it comes to electric aircraft. Silicon can store up to 10 times more charge than graphite. However, the material swells during charging, which causes it to crack and degrade over time. To solve that problem, researchers are looking at ways to protect and reinforce the silicon anodes.
For example, a team of researchers at the Gwangju Institute of Science and Technology (GIST) in South Korea has devised a solution that slows down degradation in anodes using a chemical additive. They injected silicon anodes with a graphene oxide solution that forms a “web-like structure,” holding the anode’s particles together without impeding their capacity to store and release lithium atoms.
Amprius, an aircraft battery manufacturer based in California, appears to have cracked the code in silicon anode technology with its family of high-performance lithium-ion battery cells featuring patented silicon nanowire anodes. Implementing a nanowire structure for the anode creates more surface area in contact with the electrolyte, thereby enabling faster charge/discharge rates and increasing the power density.
According to Amprius, its proprietary silicon nanowire anodes are configured in a way that tolerates swelling and resists cracking. That secret sauce has enabled Amprius to produce what it claims are the most energy-dense lithium-ion batteries available to the aviation industry today. It is also offering the technology for energy storage applications in the defense sector. The company offers several versions of its silicon nanowire batteries for different uses, boasting specific energies up to 500 watt-hours per kilogram (Wh/kg) and energy densities up to 1,300 watt-hours per liter (Wh/L).
Electric propulsion systems pioneer MagniX recently stepped into the energy storage sector with plans to produce batteries specifically for aircraft. Announcing the move on June 24, MagniX said its new Samson line of batteries will deliver 300 Wh/kg and have a service life of more than 1,000 full-depth discharge cycles. The Everett, Washington-based company intends to implement the Samson batteries with its family of electric propulsion systems with power ratings ranging from 350 to 650 kilowatts. MagniX is offering the propulsion systems as retrofits for legacy aircraft including the Cessna Caravan and DHC-2 Beaver.