Electrochemical impedance spectroscopy for all‐solid‐state batteries: theory, methods and future outlook

Electrochemical impedance spectroscopy (EIS) is widely used to probe the physical and chemical processes in lithium (Li)-ion batteries (LiBs). The key parameters include state-of-charge, rate capacity or power fade, degradation and temperature dependence, which are needed to inform battery management systems as well as for quality assurance and monitoring. All-solid-state batteries using a solid-state electrolyte (SE), promise greater energy densities via a Li metal anode as well as enhanced safety, but their development is in its nascent stages and the EIS measurement, cell set-up and modelling approach can be vastly different for various SE chemistries and cell configurations. This review aims to condense the current knowledge of EIS in the context of state-of-the-art solid-state electrolytes and batteries, with a view to advancing their scale-up from the laboratory to commercial deployment. Experimental and modelling best practices are highlighted, as well as emerging impedance methods for conventional LiBs as a guide for opportunities in the solid-state.