Recent advances in electrochemical impedance spectroscopy for solid-state batteries

Electrochemical impedance spectroscopy (EIS) is a powerful technique widely used for characterizing electrochemical systems, especially in the investigation of ion diffusion, electrochemical reactions, and charge transfer within lithium-ion batteries. Solid-state batteries (SSBs), envisioned for their potential to achieve high energy density and enhanced safety, comprise essential components-namely, a high-voltage cathode, a solid-state electrolyte (SSE), and a Li metal anode. While SSBs hold great promise, the utilization of EIS in studying SSBs is still in its infancy. Since various SSB systems exhibit different electrolyte properties (e.g., ionic conductivity, ionic transport activation energy, and ionic transference number) and interface properties (e.g., interface impedance and charge transfer impedance), the assignment of EIS features is highly controversial. Herein, this review provides an overview of the various SSE types and introduces the applied fundamentals of EIS in SSBs, including EIS basic theory, cell design and modeling methods. Subsequently, a common physical model for SSBs is summarized based on EIS research examples in SSBs. The assignment of each EIS features is discussed, and the corresponding equivalent circuit is also offered. Lastly, the review introduces emerging EIS-related technologies, laying the groundwork for future application in SSBs investigations. This review aims to enhance the understanding of SSBs and EIS, fostering the acceleration of SSBs from laboratory-scale experimentation to practical commercial applications.