DRT Characterization of Lithium-Ion Batteries for eVTOL Applications
Authors: Young Preston , Roy Anirban , Bera Bapi , Aaron Douglas S , Mench Matthew M 
Published: December 2024

The demand for electric vehicles (EV) and electric vertical take-off and landing (eVTOL) aircraft is growing exponentially. The lithium-ion battery (LIB) is commonly selected for such applications due to its promising relatively high energy and power density. However, range limitations, capacity fade, and durability of Li-ion systems limit EVs. These questions become even more vital when discussing aerospace applications of eVTOL, which have more stringent requirements for safety, power, and durability. Additionally, cell-to-cell-variations (CtCv) play a major role in pack performance. To address the current needs, this work employs a characterization method combining the techniques of electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT), allowing for better characterization of cell performance and degradation at relevant high C-rates. In this work, cells are cycled at high-rate discharge (10C and 25C) at various temperatures to mimic eVTOL applications, with characterization tests being performed at beginning of life and every 50 cycles for a total of 500 cycles.

By applying DRT to the EIS data, the electrochemical processes inside the cell can be deconvoluted and examined [1-3]. Gaussian peaks are fit to the DRT distribution function, and the resistance of each process can be predicted, thereby monitoring changes in HFR, interfacial particle-particle interactions, SEI/CEI interphase layers, and charge transfer/intercalation kinetics at various states of charge (SOC) and cycle number. Furthermore, diffusion related processes can also be identified and estimated from DC current-interrupt/application methods.

Degradation tests were performed to simulate various thermal boundary conditions faced by eVTOLs ranging from 20°C to -20°C. The results reveal that, at high discharge rates operating at room temperature, cells reached up to 90°C and signatures of degradation in the SEI layer are seen in the DRT data. Other cells cycled at 0°C show evidence of lithium plating due to the cold environment. Active cooling at 20°C showed the least degradation as identified from the various internal processes. This method of testing provides an approach to comprehensively analyze individual internal degradation modes of a Li-ion battery.

References:

[1] Wan, Ting Hei et al. “Influence of the Discretization Methods on the Distribution of Relaxation Times Deconvolution: Implementing Radial Basis Functions with DRTtools” Electrochimica Acta, Vol. 1842015, http://dx.doi.org/10.1016/j.electacta.2015.09.097

[2] Iurilli, Pietro et al. “Detection of Lithium-Ion Cells’ Degradation through Deconvolution of Electrochemical Impedance Spectroscopy with Distribution of Relaxation Time” Energy Technology, Vol. 10, 2022, https://doi.org/10.1002/ente.202200547

[3] Chen, Xiang et al. “Detection of lithium plating in lithium-ion batteries by distribution of relaxation times” Journal of Power Sources, Vol. 496. 2021. https://doi.org/10.1016/j.jpowsour.2021.229867

Figure 1