An Electrochemically Rechargeable Fuel for Continuous Operation of Electric Aircraft and Vehicles
Authors: Gao Yue , Chen Shu , Han Chenyang , Wu Guanbin , Wang Shengfei , Wang Ganyu , Wang Min , Johnson Alissa Claire , Zhu Mengyao , Ye Lei , Jolly Jason , Chen Peining , Wang Bingjie , Yang Shu , Peng Huisheng , Pikul James H. 
Published: December 2024

Aviation and long-distance transport represent more than 6% of global CO2 emissions, but electrifying these fleets requires significant improvements in battery specific energy and power. Regional and narrow-body all-electric aircraft require 600-1,000 Wh kg-1 while mass-produced rechargeable batteries have energy densities below 300 Wh kg-1. Furthermore, batteries should be able to recharge instantly to near-continuously operate the machines they power and offset their high sunk costs, yet fully charging current batteries still takes 1-16 hours.

Overcoming these technical bottlenecks requires entirely new approaches for powering electric vehicles that go beyond the pioneering intercalation materials that have dominated modern advances in batteries. For example, there are many energy-dense materials that can improve battery specific energy but do not work well in an intercalation battery architecture, especially small molecules such as organic compounds, organosulfurs, halides, and gases. Batteries with energy-dense small molecules suffer from active material loss by dissolution, irreversible reactions due to material crossover between electrodes, slow ion transport in the bulk material, and the use of excessive carbon to provide electronic conduction. These challenges cause current batteries to fall short of their high theoretical energy and power densities and suffer from poor reversibility and slow recharging.

In this talk, we explore the potential for electrochemical fuels that are filled with energy-dense small molecules that can be electrically discharged in a battery cell, not a fuel cell, at the high rates needed for electric aircraft, and can be ejected from the vehicle and recharged in an external electrochemical cell so that new fuel can be rapidly filled into the vehicle for continuous operation.

Our solution uses CH3S3CH3 small molecules stored in a supporting electrolyte as a catholyte paired with Li metal anodes, which we discharge in our extractor cell. The active material ratio in the catholyte is 65-80 wt.% to maximize cell specific energy. The discharge reaction occurs on an engineered porous current collector and the products are made soluble in the catholyte, which is critical to removing them for recycling and recharging. The liquid-to-liquid conversion enables fast kinetics, minimal polarization, promotes ion, electron, and mass transport and enables full conversion of the small molecules, which has been challenging in prior work.

We demonstrate this concept in a toy robot dog and a commercial DJI quadrotor helicopter. A 10.4 Ah extractor cell provided up to 192 A kg-1 and achieved a specific energy of up to 850 Wh kg-1 (counting the full cell mass), which doubled the flight time of the Li-ion battery powered helicopter from 20 to 41 minutes. The fuel was mechanically recharged in 77 seconds, so that multiple rapid refills allowed a helicopter to operate for 23 hours in a 24-hour period. The fuel had a 3.2-year storage life and delivered 98% of its room temperature capacity at -20°C. Different from the fuels used in engines and fuel cells, the discharged liquid is rechargeable (99.97% chemical efficiency) with electrical input and the regenerated Li and catholyte can be used again in the stomach in what we refer to as a mechagenic cycle. This recycling makes the stomach more cost-efficient than batteries and gasoline after 41 flights in an electric aircraft. We also demonstrate fuels that have up to 1,447 Wh kg-1 or are completely aqueous for fire-safe applications. Overall, the use of abundant small molecules as electrochemical fuels could lead to a more sustainable and better performing energy cycle than current batteries.

Figure caption: A sustainable energy cycle based on electrochemically rechargeable fuels. a, Envisioned mechagenic energy cycle for electric vehicles and robots that are rapidly refilled and return the discharged fuels to a recycler where it is converted back to a charged catholyte and metal using sustainable electricity. b, Photographs of a DJI Mini 2 helicopter powered by an extractor cell (a) and the recorded flight time compared to its factory-pack Li-ion battery (b). The fuel-powered helicopter had a 41-minute flight time, which is two times longer than the helicopter with the factory-pack Li-ion battery. c, The conversion of CH3S3CH3 monitored by 1H NMR. d, Voltage profile of the extractor cell operated at room temperature and -20°C. e, The voltage profile of the catholyte stored after 3.2 years. f, Rate performance of the extractor cell. With current densities of 32 to 192 A kg-1, the cell delivered at least 1 V output. g, A recycler separated by a LiSICON solid electrolyte plate. The two split chambers were filled with a CNT foam and a liquid electrolyte, respectively, to recycle the catholyte and Li metal at high efficiency.

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