Characterization and Degradation Analysis of Commercial Lithium-Ion Batteries for EVTOL Applications

Electric vertical take-off and landing (eVTOL) aircraft are gaining more attention for their potential to relieve traffic congestion in urban areas. Lithium-ion batteries are the most common choice for eVTOL systems thanks to their relatively high energy and power densities. These batteries have seen large amounts of research and development over the past decade thanks to their usage in electric vehicles (EVs) and consumer electronics such as cellphones and laptops. However, the demands placed on the lithium-ion batteries that will be used in eVTOL aircraft are much more stringent, requiring further research on how these cells would behave in such an application. The power demands of an electric aircraft during vertical take-off and landing would require fast discharge rates of around 10-25C. The safety demands of batteries used in aircraft would be very stringent with thermal runaway being a major area of concern. Recently, additional concern surrounding EV performance in cold weather has been increasing and eVTOL systems would face similar challenges.

Despite this rising attention, little research has been done to analyze how current state-of-the-art lithium-ion batteries would behave in an eVTOL application. This study tests the performance of three commercial cells chosen for their high discharge power, providing important data on cell performance and degradation under high c-rates. The cells are tested for up to 500 cycles over four different ambient conditions: 20°C with natural convection and 20°C, 0C, and -10°C with forced convection cooling. A characterization test, comprising of a 1C battery capacity determination (BCD) and electrochemical impedance spectroscopy (EIS), was performed at beginning of life and after every 50 cycles to track cell degradation. Distribution of relaxation times (DRT) is employed to deconvolute the over-potentials of different processes in the cell. Changes in the DRT spectra are used to identify degradation mechanisms within the cell, including SEI growth/decomposition, lithium plating, and electrode degradation. The results of these endurance tests provide much needed information on battery performance in eVTOL conditions.

Additionally, accelerating rate calorimetry (ARC) testing was performed on a few of the cells to study their thermal runaway characteristics, and a new battery test stand was developed which rapidly controls the temperature of the cells, laying the groundwork for further study of lithium-ion battery's applicability to eVTOL systems.