Structural Origins of the Enhancement in Ionic Conductivity of a Chalcogenide Compound by Adding AgI

Through a chemo-mechanical milling process, we prepared a highly conductive (1.1×10⁻³ S ⋅ cm⁻¹) amorphous 0.5AgI ⋅ 0.5 Ag₃PS₄ conductor, which is much higher than that of pure amorphous Ag₃PS₄ (8.5×10⁻⁴ S ⋅ cm⁻¹). Detailed structural characterizations indicate that compared to the ionic conductivity of the amorphous Ag₃PS₄ conductor, the enhancement can be ascribed to the formation of mixed polymeric anions {[PS₄]_mI_n} around Ag⁺ ions. Through heat-treatment at 370 °C for 20 minutes, the room temperature ionic conductivity of the 0.5AgI ⋅ 0.5 Ag₃PS₄ conductor is further enhanced by about 4 times. This enhancement can be ascribed to the following two aspects: 1) the existence of residual amorphous phase with higher ionic conductivity; 2) the connection of the fast ionic conductive interfaces between the deposited Ag₇PS₆ nano-crystals and the residual amorphous phase. This work reveals the key roles of both disorder and interface in improving the ionic conductivity of solid-state electrolytes.