Relationship Between Contact Resistance in e − ‐Conductive Pathways and Mechanical Properties of Active Materials in Microstructurally‐Controlled Composite Electrodes for Sulfide All‐Solid‐State Batteries

The microstructures of composites and mechanical properties of solid electrolytes (SEs) have been extensively studied for all‐solid‐state batteries (ASSBs). However, the mechanical properties of active materials (AMs) have received little attention despite the use of hard oxide materials, primarily because soft sulfide SEs are typically employed. The relationship between the contact resistance in e − ‐conductive pathways and the mechanical properties of AMs in microstructurally‐controlled composite electrodes was clarified by combining scanning spread resistance microscopy (SSRM) and scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (SEM‐EDX). SEM‐EDX observations revealed that the LiCoO 2 regions in LiCoO 2 –Li 10 GeP 2 S 12 composites contained numerous interfaces and voids. SSRM imaging confirmed both high and low contact resistances within the LiCoO 2 regions. Conversely, continuous connections without interfaces or voids were established in the graphite regions of graphite–Li 10 P 3 S 12 Br composites, resulting in low contact resistances and uniform e − ‐conductive pathways. Furthermore, e − ‐conductive pathways were readily formed in the graphite regions even when the graphite/Li 10 P 3 S 12 Br ratios were decreased to levels causing apparent graphite localization in the cross‐sections. These differences in contact resistance mainly arise from differences in deformability associated with material hardness, suggesting that AMs and SEs require high deformability to construct ASSBs composite electrodes with highly e − ‐conductive pathways.