Understanding Enhanced Ionic Conductivity in Composite Solid-State Electrolyte in a Wide Frequency Range of 10^–2–10¹⁰ Hz

The ionic conductivity of composite solid-state electrolytes (SSEs) can be tuned by introducing inorganic fillers, of which the mechanism remains elusive. Herein, ion conductivity of composite SSEs is characterized in an unprecedentedly wide frequency range of 10^–2–10¹⁰ Hz by combining chronoamperometry, electrochemical impedance spectrum, and dielectric spectrum. Using this method, it is unraveled that how the volume fraction v and surface fluorine content x_F of TiO₂ fillers tune the ionic conductivity of composite SSEs. It is identified that activation energy E_a is more important than carrier concentration c in this game. Specifically, c increases with v while E_a has the minimum value at v = 10% and increases at larger v. Moreover, E_a is further correlated with the dielectric constant of the SSE via the Marcus theory. A conductivity of 3.1×10^–5 S cm⁻¹ is obtained at 30 °C by tuning v and x_F, which is 15 times higher than that of the original SSE. The present method can be used to understand ion conduction in various SSEs for solid-state batteries.