Effects of Heat Treatment and Additive LiF on the Properties of Solid-State Electrolyte of Li_1.5Al_0.5Ge_1.5(PO₄)₃

A solid electrolyte of LAGP [Li_1.5Al_0.5Ge_1.5(PO₄)₃] contained 0.5 wt% LiF was prepared by using low-volatile raw materials. The effects of different heat treatment conditions (750–900 °C, 4–10 h) and additive (LiF) on the ionic conductivity, structural morphology, and crystal transformation process were investigated in detail. EIS (electrochemical impedance spectroscopies) showed that the ionic conductivity of LAGP contained 0.5 wt% LiF had a highest value of 3.17 × 10⁻⁴ S cm⁻¹ with low activation energy (0.31 eV) after treating 825 °C for 6 h, more than LAGP of 2.45 × 10⁻⁴ S cm⁻¹. DSC analysis and SEM images indicated that adding a small amount of LiF to LAGP not only can lower the glass transition temperature (from 513 °C to 507 °C) and crystallization temperature (from 622 °C to 605 °C), but also can modify the grain boundary and increase the relative density of LAGP (from 95.8% to 97.7%).