Quantifying the interfacial contact of active material-solid electrolyte interfaces in all-solid-state lithium-ion batteries.

All-solid-state batteries (ASSBs) are being actively researched worldwide as promising next-generation alternatives to lithium-ion batteries (LIBs). To further enhance the performance of ASSBs, it is essential to quantitatively understand the contact interface between the active material and the solid electrolyte (SE) in the electrode. However, there is no established method to quantitatively evaluate this contact interface. In this study, we assessed the contact interface between the active material and the SE-which is challenging to quantify under constrained test fixture conditions-using electrochemical impedance spectroscopy (EIS). The capacitance obtained from EIS measurements served as a quantifiable indicator. We demonstrated that approximation methods enable accurate capacitance quantification under practical EIS measurement conditions (up to approximately 10 mHz) for battery development. An electrode composite, prepared using a cathode active material and a sulfide-based SE, exhibited increased capacitance with longer mixing times, confirming an improved contact interface between the active material and the SE. Capacitance was highlighted as a critical parameter, exhibiting a correlation with the forming and stacked pressures in ASSBs, and was strongly linked to cell performance. To the best of our knowledge, this study is the first to quantify the contact interface between the active material and the SE in solid-state batteries using capacitance as an indicator.