Formulating Interfacial Impedances for Designing High-Energy and High-Power All-Solid-State Battery Cathodes

All-solid-state batteries (ASSBs) are safe, high-energy-storage systems. However, despite the progress achieved in the development of high-ionic-conductivity solid electrolytes (SEs), the power performance of ASSBs remains low because of the high interfacial impedances in composite cathodes. Therefore, understanding the interfacial factors is crucial for obtaining high power ASSBs. This study provides a quantitative analysis of the influence of these factors using impedance spectroscopy measurements, which enables the elucidation of the interfacial impedance values of two key parameters, the grain-boundary resistance (r_i,gb) and charge-transfer resistance (r_i/e). Systematic investigation revealed an unexpected increase in the cathodic resistance with the decrease in the size of the cathode active material (CAM) particles, indicating that even high-reaction-surface-area CAMs yield low r_i/e but high r_i,gb values owing to their high porosity, resulting in a trade-off relationship. In contrast, this phenomenon is unlikely to occur in liquid-electrolyte-based batteries. Notably, we discuss how composite cathode design impacts performances of stable, high-power, and high-energy ASSBs.