When ions are in charge: Generalized ionic impedance spectroscopy for characterizing energy materials and devices

Abstract

Electrochemical Impedance Spectroscopy (EIS) is the conventional technique for studying the electrical response of individual materials or complete energy devices such as batteries, fuel cells, and supercapacitors. However, EIS has several limitations, including its spatial resolution, the description of ion insertion phenomena (especially when multiple ion species are involved), and the presence of porous electrodes. In this paper, Generalized Ionic Impedance Spectroscopy (GIIS) is proposed to address these issues by complementing traditional EIS to analyze ionic concentration changes under an AC voltage stimulus. A broad range of characterization techniques can be employed to analyze such ionic concentration variations, as these significantly modify the functional properties of the material, such as optical, magnetic, and electrical behavior. Some of these techniques also offer high spatial resolution, enabling lateral and depth profiling analysis. This study provides a theoretical framework for the development of GIIS in the field of energy, analyzing battery-like and fuel cell-like devices while resolving the major limitations of EIS mentioned above. The proven versatility of GIIS opens new pathways for the detailed characterization of energy materials and devices, advancing the understanding of low-frequency fundamental electrochemical processes and broadening the scope of their applications. While many of the discussed cases are experimentally validated, others are presented as perspectives of GIIS applications.