Stack-pressure-dependent interfacial capacitance governs electrochemical contact in all-solid-state batteries

All-solid-state batteries (ASSB) are regarded as next-generation lithium-ion batteries capable of simultaneously delivering high energy density and enhanced safety. For the practical realization of ASSB, a quantitative understanding of solid–solid interfacial contact at the active material/solid electrolyte (AM/SE) interface is essential, particularly under practically relevant low stack pressure conditions. In this study, we systematically investigate how the correlation between solid–solid interfacial contact and battery performance evolves as a function of stack pressure. Using layered oxide cathodes (NCM) and natural graphite (Gr), we directly quantify that Gr exhibits a significantly smaller capacitance at the AM/SE interface than NCM, indicating a reduced effective electrochemically active contact area. Moreover, this interfacial capacitance shows a strong dependence on stack pressure and exerts a stronger influence on battery performance than bulk ionic conductivity, a conventional descriptor of solid electrolyte performance. These findings reveal that stack-pressure-dependent interfacial capacitance is a key descriptor governing the electrochemically active contact area in ASSB, establishing solid–solid interfacial contact as a critical design parameter for ASSB operation under practically relevant low stack pressure conditions.