Stress-Optimized Cathode for Advanced All-Solid-State Batteries via Quantitative Analysis of Microstructure and Resistance

Among the fire-safe solid-state electrolytes, sulfide electrolytes have contributed significantly to the development of all-solid-state batteries (ASSBs) due to high ionic conductivity above 10 mS cm⁻¹ and superior ductility. However, ASSBs face issues such as pore formation, high resistance of thick electrolytes, and mechanical degradation at the interface due to the inevitable point contact nature of the solid phase. Therefore, the ideal electrode design for ASSBs requires optimization of the fabrication process and feedback based on quantitative analysis. In this work, we optimized the fabrication process of ASSBs by controlling the stresses applied to the cathode and quantitative analysis of microstructure and resistance. We controlled the compressive- and shear-stress applied to the cathode by employing uniaxial- and roll-press process. Compared to the free-to-uniaxial pressed cell (FUPC), the roll-to-uniaxial pressed cell (RUPC) had a 3.02% lower porosity and a 26.77% higher discharge of 162.69 mAh g⁻¹ at 0.1C. RUPC exhibited higher capacity and over 30% lower cathode active material-electrolyte interfacial resistance than FUPC, even during cycling of 0.2C. These results indicate that optimally controlled stresses are effective in forming intimate interfacial contacts and dense packing of electrodes. We conducted image segmentation of 20 × 20 μm² area to quantitatively analyze the differences in microstructure under different fabrication processes or cycling conditions. Before cycling, RUPC exhibited a 3.38% lower porosity, similar electronic connectivity, and 23.57% higher electrochemical active area compared to FUPC. After cycling, RUPC exhibited a 0.44% lower porosity, 23.88% higher electronic connectivity, and 9.24% higher electrochemical active area compared to FUPC. These results indicate that RUPCs prepared under optimal stress conditions have a beneficial microstructure for electrochemical evaluation. We expect these results to be an important milestone in the development of next-generation batteries with ideal electrode microstructures.