Title: Using Time Constants of Li-Ion Batteries for Safety Evaluation
Authors: Mohsen Derakhshan, Damoon Soudbakhsh
Abstract: Lithium-ion batteries (LiBs) are the preferred choice of energy storage in many aspects of modern life from cell phones to electric vehicles (EVs), because of many desired properties such as high energy and low self-discharge. However, they pose severe hazards if their safety is compromised such as after sustaining mechanical damage. Prior work on evaluating the safety of LiBs following substantial damage was not conclusive as no detectable voltage or capacity changes were observed in them. The current study proposes a powerful, efficient, and reliable tool that can provide a solution to this problem.
Therefore, we aim at quantifying the safety of LiBs following a mechanical load or impact. We created a method to detect mechanical damage to Li-ion cells from their electrical response. The method is based on measuring their impedance spectra, determining their distribution of relaxation times (DRT), and analyzing them. We Modeled a battery impedance based on the DRT by solving a ridge regression optimization that involves a series of inductors, resistors, and capacitors as passive electrical components. New criteria for determining the optimal value of ridge regression optimization is developed in a way that the number of output peaks in the DRT be connected to the physics of the system. We tested five cylindrical cells 18650 with graphite/LiFePO4, while four cylindrical batteries are used to study the effect of mechanical damage on Lithium-Ion battery time constants. In the mechanical damage experiment, two cells were used as controls, and two cells were subjected to the indentation experiment. Using a 12.7 mm hemispherical punch, two cells are indented 6.5mm before short-circuiting. The indentation was held after each 1 mm displacement to measure the impedance spectrum of the cells (in the last step, only 0.5 mm displacement is present). To account for the changes in the EIS measurements and decouple the effect of punch indentation from the order of the experiment and the OCV drops after each EIS measurement, we conducted similar EIS experiments on the control cells. Therefore, control cells were tested with the exact timing of the tests on two indented cells. One cylindrical cell is used to study the effect of temperature and SOC on the time constant of the cell’s internal processes. We measured the impedance spectra of LIBs utilizing an EIS instrument at different cell temperatures (-20 oC to +40 oC) and State-of-charge (0% and 100% SOC). Using the introduced approach results in 5-6 dominant peaks in the 0.01 Hz to 45 kHz range, with 4-5 peaks in the medium and low frequency and only one peak in the high-frequency part of the impedance. The number of dominant peaks agrees with the expected number of internal processes determined experimentally. We use the dependency of the peaks on temperature and SOC (State of Charge) to assign them to major processes (diffusion, charge transfer, SEI (Solid Electrolyte Interphase), and the changes in the properties of the electrodes and separator). Using our DRT formulation and criteria, we show that the indented cells have substantially different high-frequency characteristics than the control group (the changes of the height of high frequency peak is 2.5% for control cells and 36.0% for indented cells). This non-invasive method can detect hazardous mechanical damage to the EV batteries after a road crash or impact landings of drones. Other applications of the proposed approach include 1) EVs evaluation during standard crash tests, 2) planned impact and shock applications, and 3) regular safety checks.
Related Publications:
1- Derakhshan, M., Sahraei, E., & Soudbakhsh, D. (2022). Detecting mechanical indentation from the time constants of Li-ion batteries. Cell Reports Physical Science, 3(11).
2- Derakhshan, M., & Soudbakhsh, D. (2021). Temperature-dependent time constants of li-ion batteries. IEEE Control Systems Letters, 6, 2012-2017.