Crystallization behavior of Li1+xAlxGe2-x(PO4)3 glass-ceramics: Effect of composition and thermal treatment

Abstract

Lithium aluminum germanium phosphate (LAGP) glass-ceramics are promising solid electrolytes for all solid-state lithium ion batteries and other electrochemical applications. LAGP glass-ceramics (Li1+xAlxGe2-x(PO4)3, x = 0.5–0.8) were synthesized using a two-step heat treatment based on characteristic temperatures from Differential scanning calorimeter (DSC) analysis. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to characterize the crystal phases and their morphologies. Our results show that high purity ion conductive phase can be obtained for the compositions with P/Ge = 2.3 and 2.4 by a two-step heat treatment, instead of the commonly adopted single step heat treatment for composition with P/Ge = 2.0. Electrochemical impedance spectroscopy (EIS) measurements show that Tg + 60%∆T (∆T = Tc-Tg) is the optimal nucleation temperature to obtain high ionic conductivity. Crystallization energies were calculated from the modified Kissinger equation based on DSC measurements and it was concluded that the crystal phase form mainly through internal nucleation. We have found in this paper that, by using a two-step heat treatment, high phase purity LAGP glass-ceramics with lower GeO2 concentration can be obtained to achieve high ionic conductivity.