Electrifying Aviation: The Path to Decarbonizing the Skies
After decades of tantalizing breakthroughs in battery technology, the last decade witnessed the emergence of energy storage as a challenger to fossil fuels for powering vehicles. We are now in the midst of a once-in-a-lifetime opportunity to change the energy landscape and electrify all forms of transportation: light duty passenger cars, heavy duty commercial vehicles, as well as various forms of transportation such as trains, ships, and aircraft.
Such a dramatic transition will require a multifaceted approach that takes into consideration technology needs, infrastructure support, workforce transitions, safety and regulations, and energy justice. The U.S. Department of Energy’s (DOE) Argonne National Laboratory, with numerous public and private sector collaborators, has been strategizing about this transition to ensure the lessons from the past are applied to the future.
The transition to the electrification of everything has been spurred by a confluence of three factors that has led to profound changes, for example, to the light duty passenger car market (so-called electric cars or EVs).
Factors Driving the Transition
First, the cost of lithium-ion batteries has been reduced by more than an order of magnitude in the last decade, approaching parity with gasoline engines. Cost reduction, combined with performance improvement such as enhanced driving range and fast recharge, has led consumers to increasingly see EVs as a credible and compelling alternative to gasoline cars. Argonne’s discovery of the nickel cobalt manganese oxide cathode—used in many EVs and a triumph of decades of R&D investments from the DOE—has been integral to improvements in the technology.
Second, consumers have become increasingly aware of the devastating effects of climate change and are willing to pay for technologies that provide credible solutions, thus spurring demand and driving companies to provide solutions.
Third, governments across the world have increasingly provided both supply-side and demand-side incentives for moving towards electrification, further spurring the market. The impact of these megatrends is the increasing view that battery technology is now mature enough to impact other transportation sectors previously thought to be too hard to decarbonize, such as aviation.
Assessing R&D Needs and Roadmaps
In 2019, Argonne collaborated with DOE and NASA to convene a panel of experts from academia, industry, national labs, and government. Their objective was to assess the research and development requirements for electric aviation, explore the similarities and differences between electric aviation and electric vehicles (EVs), and generate recommendations to accelerate the growth of the market.
The workshop highlighted a roadmap aimed at decarbonizing the aviation sector by leveraging existing lithium-ion cells, with appropriate modifications, to target relatively easier goals such as electrifying vertical takeoff and landing aircraft. The roadmap also encompasses the development of next-generation batteries tailored for commuter aircraft or regional jets, which present more complex challenges in terms of electrification. The ultimate challenge is electrifying 737-class aircraft which require energy density more than five times current lithium-ion technology. In contrast to EVs, electric aviation also requires significantly higher power demand, especially during take-off and landing with the need for reserves to ensure safety.
The 2019 assessment made clear that the world of electric aviation resembled that of EVs from two decades ago, with many unknowns related to the integration of storage into vehicle designs, establishing targets for different markets, and developing standards to ensure a smooth transition.
A Simulation Tool for 21st Century Flight
Argonne has been developing an aircraft simulation tool, named Aeronomie, to support the transition to 21st Century flight. It allows users to simulate entire flights and experiment with different systems to optimize energy and performance for all kinds of aircraft — including electric and hybrid airplanes, vehicles for urban air mobility, and unmanned aerial vehicles. With Aeronomie, users can automatically build virtual aircraft using models — which capture the flying environment, the aircraft’s motion and aerodynamics, and how it is powered and flown. The program is fully customizable, allowing users to add models from an existing library or their own files.
Aeronomie has been critical in generating targets for batteries for different aviation applications. Similar tools for EVs were integral in driving battery research, allowing the community to focus on the most critical challenges and develop solutions that improved performance and cost.
Insights gained from the 2019 workshop, along with tools such as Aeronomie, are currently driving research and development efforts towards enhancing battery materials and devices. The battery community has developed roadmaps to improve battery energy densities and reduce costs. These road-maps encompass both short-term advancements achieved through innovation with next-generation lithium-ion batteries, as well as long-term challenges that require a more fundamental approach.
Unlocking Higher Energy Densities
Batteries consist of two active electrodes, anodes and cathodes, with electrolytes that move ions between the two. The two electrodes are prevented from shorting using thin polymeric separators. Building better batteries requires innovation in the materials that comprise these four components, along with innovative processing and manufacturing techniques that enable cost-effective integration of these materials into devices.
At Argonne, the research focus is on developing new cathode materials that enhance the performance of existing lithium-ion cathodes, which are typically composed of transition metal oxides and phosphates. The aim is to unlock higher energy densities by adjusting these cathode materials. These include the use of higher nickel content materials and operating the battery at high voltages. Changing the anode from the current graphite to silicon allows a 30 percent boost in energy density, but comes with calendar life challenges, a focus of the R&D.
A significant effort is underway to explore the utilization of lithium metal as the anode in batteries. When combined with current cathodes, this approach has the potential to provide a substantial increase in energy density. Lithium metal has been the “holy grail” for lithium-ion battery anodes for decades but has suffered from the formation of needle-like structures, called dendrites, during charge. Dendrites penetrate the soft separators and short the cell. Recent advances in solid electrolytes, where the electrolyte and the separator are combined into a mechanically hard material, have shown promise in preventing dendrites. Such batteries are now referred to as solid-state batteries.
Lithium metal anodes are crucial for the long-term targets for aviation. This is because enabling lithium metal will open the door to new cathodes, beyond the currently used transition metal oxides/phosphates. These include sulfur cathodes that, in theory, could lead to a tripling of current lithium-ion batteries, and air cathodes that could be even more energetic. Lithium-air batteries have attracted attention for aviation applications because their theoretical energy density approaches that of gasoline engines. However, the system is plagued by numerous fundamental challenges that severely limit its practicality. Solving these challenges requires sustained R&D to discover compelling solutions.
A long-term R&D program like this needs to consider challenges associated with material supply and end-of-life recycling. These considerations are now front and center in the present push to build lithium-ion batteries for the growing electrification market, with concerns related to resiliency of supply of nickel, cobalt, and lithium in the U.S. While significant resources are now being devoted to this challenge, future innovations need to bring this consideration into the discovery stage to avoid similar consequences in the future. Further, end-of-life disposal of these batteries remains a challenge that, if solved, could result in a truly circular battery ecosystem. Such considerations for sustainability should be given more prominence in the future.
Electric aviation is in its early stages and resembles the initial stages of the EV revolution. Like the EV industry’s early days, electric aviation involves various vehicle types, hybridization levels, and different designs and chemistries being considered. It took two decades for the EV space to settle into an equilibrium and for the market to mature. Looking at the growing climate emergency, two decades is not an acceptable timeframe.
Drawing lessons from the evolution of EVs, it is essential to bring together all stakeholders involved in electric aviation. This includes battery researchers, battery component manufacturers, battery companies, system integrators, aircraft companies, regulators, and funding agencies. Such a concerted effort will bring the promise of electrification to this hard-to-decarbonize sector and jumpstart a revolution in aviation.
This article was written by Venkat Srinivasan, Director of the Argonne National Laboratory’s Collaborative Center for Energy Storage Science and the Joint Center for Energy Storage Research. For more information, visit here .