Operando Differential Electrochemical Pressiometry for Probing Electrochemo‐Mechanics in All‐Solid‐State Batteries

Abstract Owing to an absence or lack of soft (and/or liquid) components, electrochemo‐mechanical effects are imperative for all‐solid‐state batteries (ASBs) based on inorganic solid electrolytes (SEs). As this aspect has been overlooked, relevant investigation has remained scarce. In this work, the development of a new operando differential electrochemical pressiometry (DEP) for ASBs is reported. The time‐ (or capacity‐) derivative differential pressure signals ( dP/dt or dP/dQ ) reflecting corresponding differential volume changes of electrode active materials feature the specific state of charges (SOCs). This finding leads to a precise estimation of the SOCs of graphite (Gr) electrodes in LiNi 0.70 Co 0.15 Mn 0.15 O 2 (NCM)/Gr all‐solid‐state full cells using sulfide SEs with varying capacity ratios of negative to positive electrodes (n/p ratios); this is corroborated by complementary analysis using a three‐electrode electrochemical cell and ex situ X‐ray diffraction measurements. Furthermore, electrochemo‐mechanical behaviors of NCM/Gr full cells with Gr electrodes employing SEs excluding or including reductively unstable Li 10 GeP 2 S 12 are investigated. Notable volume changes caused by lithiation of Li 10 GeP 2 S 12 are detected. Importantly, significantly delayed SOC for Gr caused by a severe side reaction with Li 10 GeP 2 S 12 is disclosed by the operando DEP result.