High-Frequency Electrochemical Impedance Spectroscopy for Detecting Li Deposition Toward Assessing Thermal Stability in LiBs

Li-deposition is a critical factor in one of the degradation modes of lithium-ion batteries (LiB). The Li-deposition increases the risk of internal short circuits and lowers thermal runaway onset temperature, compromising safety 1 . Detecting the Li-deposition is possible through disassembly or analysis using large-scale equipment such as muonic X-ray visualization 2 . On the other hand, a non-destructive and convenient evaluation technique is required to assess batteries’ safety in commercial use. A novel inspection technique using high-frequency electrochemical impedance spectroscopy (HF-IS) has been developed to detect the presence of Li-deposition 3 . Electrochemical impedance spectroscopy (EIS), which nondestructively observes ionic diffusion, reaction, and charge transfer in 10 mHz to 10 kHz response, is commonly used for diagnosing the state of battery degradation 4 . Compared to the conventional EIS, the HF-EIS uses the real part of battery impedance Re( Z bat ) above 100 kHz, allowing observation of only the electronic behavior of the battery in high-frequency regions where ion behavior cannot be tracked. It utilizes the phenomenon of current concentration at interfaces of materials with different conductivities, known as the "skin effect," which is caused by a high-frequency electromagnetic field. Since the Li-deposition occurs at the interface between the negative electrode and the electrolyte, the interface within the high-frequency current path changes from the negative electrode to the Li-metal. This change induces alterations in the electromagnetic field and the high-frequency current path, subsequently resulting in a variation in Re( Z bat ). Figure 1 displays a representative result illustrating the relationship between the thermal runaway onset temperature and the change in Re( Z bat )| freq =1MHz . We conducted a degradation test to deposit Li-metal and a thermal runaway test using two types of 18650 cylindrical cells (NCA-3400 mAh, NCM-2650 mAh). Two batteries subjected to excessive charging C-rates demonstrated the proportional relationship between the decrease in thermal runaway onset temperature and the reduction in Re( Z bat )| freq =1MHz in individual cases. The results indicate that different battery types have different thermal stability of LiB against the Li-deposition. The HF-EIS is not only a safety diagnostic tool but can also contribute to developing battery materials by reducing destructive testing costs. This method of on-the-spot safety quantification diagnosis is expected to contribute to the safer proliferation of the LiBs. Reference 1 Li, Y. L. et al. Thermal Runaway Triggered by Plated Lithium on the Anode after Fast Charging. Acs Applied Materials & Interfaces 11, 46839-46850 (2019). https://doi.org/10.1021/acsami.9b16589 2 Umegaki, I. et al. Nondestructive High-Sensitivity Detections of Metallic Lithium Deposited on a Battery Anode Using Muonic X-rays. Analytical Chemistry 92, 8194-8200 (2020). https://doi.org/10.1021/acs.analchem.0c00370 3 Ishigaki, M. et al. Operando Li metal plating diagnostics via MHz band electromagnetics. Nature Communications 14, 7275 (2023). https://doi.org/10.1038/s41467-023-43138-w 4 Barsoukov, E. Impedance Spectroscopy Theory, Experiment, and Applications Second Edition Preface. Impedance Spectroscopy: Theory, Experiment, and Applications, 2nd Edition, XII-+ (2005). Figure 1