Kyle Proffitt
July 31, 2024 | In the surprisingly near future, we may see greater adoption and standardization of not only battery-powered electric vehicles (EVs) that traverse roads but also EVs that take to the air.
New research from Lawrence Berkeley National Laboratory with collaborators from UC Berkeley, 24M, And Battery Aero, University of Michigan, and Carnegie Mellon University has moved the needle closer to this future through a somewhat surprising omics-style analysis to identify superior electrolyte blends. Ultimately, their discoveries allowed for the creation of electric aircraft-ready battery cells that meet heavy power-draining demands for more than 100 cycles. The research was published in June in Joule (DOI: 10.1016/j.joule.2024.05.013).
Flying Cars
First we must address where flying vehicles fit in the market. Battery Power spoke with Dr. Brett Helms, corresponding author, to understand this context as well as the details of electrolyte improvement for helping us get there. “You’re going to see air taxis within the next five years,” he predicted, referring to the first most likely use case for these electric vertical take-off and landing (eVTOL) vehicles. In fact, battery-powered electric air taxis have been cleared to fly at the Paris Olympics, and an air taxi service is slated to launch in Dubai by early 2026.
The motivation here is straightforward: a new axis for travel, aerial transportation to offset congestion, and 2- to 6-fold faster transit time. For several reasons, including overall efficiency, greenhouse gas emissions, and quieter operation, the current push is to accomplish this with distributed electric motors and propellers.
Harsh Battery Conditions
The precise situations where air taxis can be most beneficial are still being worked out, and various designs are being explored. The vertical takeoff and landing approach is attractive for more urban environments, as no runway is needed. In every case, the greatest benefit and utility rely on improvements in batteries. In addition to the concerns common to batteries for terrestrial EVs—energy density, charging speed, safety, longevity—batteries designed for eVTOL vehicles have at least one extra requirement.
“Getting off the ground and landing is about power,” Helms said. “Where the aviation industry has struggled thus far is designing batteries with the right power-to-energy ratio to accommodate the high power requirements on takeoff and landing while also being able to ‘drive’ the distance.” This power is a measure of the energy output over time. In other words, it’s not just about whether the vehicle can carry you 100 miles before it needs a recharge, it’s about whether it can quickly deliver a pulse of high energy (a high power discharge) to get off the ground, then cruise at a lower energy drain for the distance, and yet retain sufficient capacity—and again, power—to safely land.
The power requirements during takeoff and landing can be as high as 15C—meaning in just 4 minutes if held at this high rate of discharge. By comparison, a Tesla Model 3 operates at an average 0.3C in regular highway driving. This high energy usage in takeoff and landing makes short trips less economical, and the damage inflicted on a battery by repeatedly discharging at high current is significantly greater than lower-speed discharges. Off-the-shelf EV battery chemistries just won’t hold up. Terrestrial EV batteries are “,” Helms said. When translated to eVTOL applications, “these demanding power requirements just cause them to experience an unacceptable amount of power fade.”
The Cathode and the Interphase
Curious about the underlying mechanisms causing power fade, Helms “We essentially sliced through the battery and started looking at the components, and we realized that the cathode particles were corroding and fracturing over time.”
Normally, the interactions between electrolyte and cathode lead to the formation of a cathode-electrolyte interphase (CEI). This interphase results from redox reactions between the electrolyte and the cathode, and the byproducts that form actually create a critical barrier that enables ion conductivity while limiting further decomposition, providing battery stability. Accordingly, “our team began to revisit the electrolyte that’s used in the lithium metal batteries,” Helms said.
The researchers created cells using lithium-nickel-manganese-cobalt oxide (LiNi0.8Mn0.1Co0.1O2; NMC811) cathodes, liquid electrolyte, and lithium metal anodes. They then varied just 10% of the total lithium salt composition in the electrolyte by choosing from 7 different options and assessed various battery characteristics. Two of these materials, LiClO4 and lithium difluoro(oxalate)borate (LiDFOB), enabled superior performance in terms of current leakage and electrolyte degradation, and importantly, in capacity retention when undergoing repeated cycles of high-power (6C) discharge.
Omics for Batteries
But what was happening at the interphase? The same two salt additives that showed improvements in general cell performance also protected the cathodes from fracturing. The researchers sought the source of this protection using a powerful technique, x-ray photoelectron spectroscopy (XPS), to interrogate the specific elemental signatures in the cathode-electrolyte interphase. The technique reveals a sort of fingerprint of the specific different elements and chemical groups present in the interphase, and the group likens this approach to omics, with the chemical fingerprint referred to as the “interphase-ome”. You might think of it as analogous to a genotype (interphase signature) and its resulting phenotype (battery performance).
Surprisingly, despite having dissimilar structures (and only accounting for 10% of total salt), the two cathode-protective additives produced very similar profiles at the cathode-electrolyte interphase. “The successes that we see are similar because they do the same thing with respect to what they form on the interphases,” Helms explained.
He went on to detail some of the understanding gained from the analysis. “Most electrolytes form interphases that are very rich with inorganic compounds; those tend to promote cathode degradation over time.” In contrast, these two dissimilar additives both “substantially decreased the inorganic content in favor of organofluoroethers.” Helms described these organofluoroethers as effectively corrosion-resistant coatings.
Onward to Practical Batteries
With this knowledge, the researchers went on to test their batteries in a realistic simulated flight protocol. For this, the cells used LiDFOB as the salt additive and achieved total energy density of 440 Wh/kg. A simulated protocol included an ~2C discharge (1.9 W/Wh) for 30 seconds and then cruising for nearly 2 hours (covering 200 simulated miles) at a low 0.4C discharge. At landing, the voltage and state-of-charge are lower, and the same 1.9 W/Wh discharge power corresponds to a slightly increased ~2.3C rate. The high energy density of the cells helps offset the need for a high C-rate (much lower here than the 15C required in some prior calculations). With this protocol, the batteries retained sufficient capacity for landing and experienced less than 20% power fade over at least 120 cycles, significantly outperforming batteries with other electrolytes.
Gratifyingly, these batteries are moving right into the next stage. With their commercialization partner, 24M, the research team is making electrolytes at 100-kg scale and developing 20-Ah multilayer pouch cells that “are going to be integrated into packs for testing in the modules that go into the air taxis,” Helms said. A test flight is projected for 2025.
Other Avenues
Helms expects the omics approach to be applicable in other use cases including EV batteries. They are looking at electrolyte design for silicon-based EV batteries, lithium metal batteries, sodium-ion chemistries, sodium-metal batteries, and solid-state batteries. “These techniques will apply in all those cases,” he said, though he admitted interphase analysis in solid-state cells presented some challenges.
Future iterations may gain throughput, automation, and computational assistance. “It’s relatively easy to use high-throughput DFT (density functional theory) methods, some of which are in the Materials Project at Berkeley Lab, to predict what electrolytes would give you the right voltage stability or ion conductivity to use in a battery,” Helms said. From there, robotics could be used to formulate different electrolytes, create and test cells, and subject them to the XPS omics screening. “We are leveraging a lot of recent upgrades to XPS analytical capability to go from iterative one-at-a-time to high throughput analyses,” Helms said, mentioning research capabilities available at labs including the Molecular Foundry and the Advanced Light Source facilities at Lawrence Berkeley National Laboratory and the Advanced Photon Source at Argonne National Laboratory.
“On the basis of the analysis and coupling that analysis to the cell performance characteristics, then you begin to make those omics-related touchpoints between the useful interphases that are generated with each electrolyte and what types of cell performance they produce on the back end,” Helms said. “If you’re a machine learning enthusiast, you would train your models on that data and use generative AI approaches to make the next series of hypotheses to test.”
Currently, electrolyte optimization can make a big difference for eVTOL batteries. However, it isn’t all electrolytes for Helms, as higher energy density batteries will help offset the need for exceptionally high power. “Where we are moving in the future is pushing to higher voltage cathode materials to get out even more energy density,” he said.
