## 4 Results and Discussion
### Electrode Density (Pressure) Effect  
When manufacturing an electrode for a solid-state battery, a pressing process is included to improve the contact characteristics between the internal solid particles. Therefore, examining how the electrolyte connection characteristics vary depending on the pressurization condition is essential. Figure 9a-c show how the tortuosity, porosity, and Bruggeman coefficient changed with different shapes of the active material, respectively, when the applied pressure changed in the range of 40-660 MPa during the manufacture of electrodes. Figure 8: (**a**) Tortuosities, (**b**) porosities, and (**c**) Bruggeman coefficients of the composite electrodes containing spherical Al\({}_{2}\)O\({}_{3}\) powders with different average particle sizes of 10 \(\upmu\)m (S10), 70 \(\upmu\)m (S70), and 100 \(\upmu\)m (S100). Overall, regardless of the shape of the active material, the degree of tortuosity tended to decrease monotonically as the pressing pressure increased. However, the pattern of the porosity change was somewhat different when it had a fibrous shape compared to those with the other shapes. First, in the case of electrodes containing spherical and plate-shaped active materials, the change in porosity with an increase in the pressing pressure was insignificant (Figure 9b). This means that an increase in pressure did not significantly improve the electrolyte penetration into the pores in the electrode. On the other hand, it was confirmed that the Bruggeman coefficient, which reflects the electrolyte connectivity, decreased significantly as the applied pressure increased (Figure 9c). In other words, the decrease in tortuosity with increasing pressure in electrodes containing spherical and plate-shaped active materials was believed to be due to improved contact between electrolyte particles (reduction in lithium-ion migration resistance at electrolyte grain boundaries) and improved tightness of ion migration paths [46]. In the case of an electrode containing a fibrous material, the porosity changed significantly in the region where the pressing pressure was low, but there was almost no change in the area where the pressing pressure was high (regions A and B in Figure 9b). On the other hand, the Bruggeman coefficient did not differ in the low-pressure region but changed significantly in the high-pressure region (regions A and B in Figure 9c). As previously discussed, there are many micropores in an electrode composed of a fibrous material due to the aggregation of particles. When pressure was applied, the electrolyte penetrated the agglomerate and filled these micropores (region A), which eventually led to a considerable decrease in tortuosity. Then, when electrolyte penetration into the micropores of the ag  
Figure 9: Variations in (**a**) tortuosity, (**b**) porosity, and (**c**) Bruggeman coefficient of the composite electrodes containing different shapes of Al\({}_{2}\)O\({}_{3}\) powders with pressing pressure. The regions A and B in the figures represent variations for fibrous Al\({}_{2}\)O\({}_{3}\) contacting electrode at low- and high-pressure regions, respectively. gregate was no longer possible because of the limited electrolyte ductility (region B), as in the case of using spherical and plate-shaped active materials, the contact characteristics of the electrolyte particles improved as the pressing pressure increased, and the tortuosity further decreased. In summary, as the pressing pressure increased during electrode manufacturing, the degree of tortuosity decreased, resulting from the improved contact between electrolyte particles and the electrolyte filling factor in the electrolyte. This was an exceptional result only in the electrode of an all-solid-state battery, which was not observed in a lithium-ion battery using a liquid electrolyte. In general, in the case of a lithium-ion battery, when the electrode is pressurized, the distortion of the ion transport path filled with liquid electrolyte becomes severe, and the tortuosity tends to increase [47].