All-solid-state lithium-ion batteries (ASSBs) using inorganic electrolytes (SEs) are promising candidates for next-generation batteries because of their improved safety and high energy density. However, bringing out the performance of ASSBs beyond conventional lithium-ion batteries (LIBs) is still a challenge. The ASSBs with high power density will be achieved by optimizing ionic and electronic conduction properties in the electrode composites^[1], which depend on the microstructural properties of the composites. The relationship between the conduction and microstructural properties is generally evaluated by macroscopic parameters based on the tortuosity factor and effective conductivity^[2]. However, the relationship should also be evaluated from the microscopic conduction properties of the electrode composites for ASSBs, which is not well understood so far.

The scanning spreading resistance microscopy (SSRM) technique directly measures current distributions at the sample surfaces in the nanoscale. Otoyama et al. analyzed the local resistance using SSRM to demonstrate the presence of electrically isolated cathode active material (CAM) particles in ASSBs after charging, providing information on the electronic contact of CAMs^[3]. The local resistance analysis using SSRM is an effective evaluation technique for optimizing the electrode design for high-performance ASSBs.

Herein, we investigated the microscopic electronic conduction properties of cathode composites blended with LiNbO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) and sulfide SEs at a volume ratio of 22:78, 34:66, 45:55, and 66:34 using the SSRM technique. Some of the NCM domains in the 22NCM-78SE cathode composites were electrically isolated. The presence of the isolated NCM leads to a significant capacity loss. In 45NCM-55SE and 34NCM-66SE cathode composites, all the NCM particles were electrically connected to a current collector. These cathode composites showed bimodal resistance distributions in the NCM domains. The dispersion of the local resistance indicated the existence of NCM particles with poor electronic transport pathways, causing still limited capacity (105 mAh g⁻¹). The 66NCM-34SE cathode composite exhibited unimodal resistance distribution in the NCM domains. This microscopic conduction should result in a homogeneous current distribution, thus allowing for sufficient utilization of NCM (150 mAh g⁻¹). To achieve high-performance ASSBs, all NCM particles in cathode composites are required to be connected to the current collector in contact with each other. The analysis of microscopic conduction properties using the SSRM technique provides insight into an optimized microstructure design to realize ASSBs with high power density.