Calculational Clarification of the Reduction Factors Against Ionic Conductivity of Solid Electrolyte in All-Solid-State Battery

All-solid-state batteries are desired to be used especially for electric vehicles due to the expected features for rapid charging, safety, and unnecessity of battery cooling systems. In recent years, solid electrolytes with high ionic conductivity have been developed. 1 However, it is known that the effective ionic conductivity in the electrode is significantly lower than that expected from the conductivity of the solid electrolytes. 2,3 The conductivity reduction is possibly caused by the tortuosity of active materials as well as the resistance of the grain boundaries. In this study, the quantitative clarification of these factors has been carried out by numerical analysis. As a preliminary study, the conductivity of a simple solid electrolyte layer was investigated. The reduction of the ionic conductivity is possibly caused by the three factors shown in Fig. 1. The first factor is the tortuosity of the ion-conduction path in the layer. The tortuosity is the ratio of the path length to the layer thickness, and the presence of vacancy in the layer increases the tortuosity. The second factor is the contact area of the solid electrolyte particles. The small contact area causes the bottleneck structure of the particles which reduces the conductivity. The third factor is the contact resistance of the particles. The increase in the resistance is probably caused by the mismatch of the crystal lattice at the contact area. Since the shape of ionic conduction path governs the first and second factors, we named these factors as path resistances. The second factor can be regarded as an apparent increase in the path length. Hence, the product of the path resistances was defined as path resistance coefficient. The experimental conductivity was measured with the layer of an argyrodite-type sulfide-based solid electrolyte. The porosity was controlled by changing the confining and molding pressure and the effective ionic conductivity was obtained from the electrochemical impedance measurement. In the calculation, the ionic conductivity was evaluated only from the path resistances using the random walk method 4 and the effect of the third factor was not considered. The structural information was obtained from the cross-sectional SEM images of the solid electrolyte layers with different porosities. Although the calculation excludes the third factor, i.e ., contact resistance, the calculated conductivity corresponds well to the experimental values as in Fig. 2. This suggests