Evaluation of ionic conduction performance in Li₃PS₄ glass electrolytes using block model theory

Abstract

All-solid-state batteries have shown promise as possible energy-storage devices of the future, with the potential to overcome the limitations of present-day conventional batteries. The curvature of ionic conduction pathways occurs inside solid electrolytes, and investigations into the magnitude of this curvature, i.e., “tortuosity,” enable us to analyze their ionic conduction performance. This study investigated the correlation between the filling ratio and ionic conductivity to evaluate tortuosity using the Bruggeman equation. Our block model theory suggests that the ionic conductivity for a uniform block-shaped void distribution is proportional to the square of the filling ratio. When this relationship is applied to the Bruggeman equation, the Bruggeman exponent, which is an indicator of tortuosity for a solid electrolyte, is determined to be α = 2. This theoretical value of the Bruggeman exponent was different from that of the spherical void model (α = 1.5) proposed previously. Impedance measurements revealed that the Bruggeman exponent of an Li₃PS₄ glass electrolyte is approximately 1.8, which is similar to but slightly lower than that determined by the block model. Cross-sectional scanning electron microscopy images revealed that the electrolyte has a uniform block void distribution, as in the block model. Monte Carlo simulations suggest that the actual, slightly lower Bruggeman exponent (α = 1.8) stems from the electric field. Tortuosity analyses using the block model and impedance measurements enable us to evaluate the macroscopic ionic conduction performance of solid electrolytes.