Prospective view of battery degradation diagnostic systems

In recent years, the reuse and recycling of large-scale batteries have gained momentum, driven by concerns over material shortages and cost efficiency. As this trend continues, a critical focus has emerged on systems that can accurately diagnose battery degradation, particularly in terms of safety and long-term performance. While several battery condition monitoring systems have already been proposed and are in practical use, this paper focuses on technologies applicable across a wide range of battery systems — not only for electric vehicles (EVs) but also for stationary energy storage — with particular emphasis on Electrochemical Impedance Spectroscopy (EIS). EIS is a powerful diagnostic technique that involves applying a very small alternating current (AC) signal to an electrochemical system, small enough not to interfere with its operation, and analyzing the frequency-dependent response of the resulting current. Because various electrochemical processes — such as electron transfer, ion migration, charge transfer reactions, and diffusion of reactive species exhibit unique characteristic response frequencies, EIS enables the decomposition of these reactions into their fundamental processes. As a result, EIS proves to be an exceptionally versatile and effective method for evaluating not only lithium-ion secondary batteries but also a wide range of electrochemical devices. These include other storage batteries like lead-acid, lithium-ion, and sodium-sulfur (NAS) batteries, as well as energy-generating systems such as solar cells and fuel cells.