Crystallization kinetics of Li 2 O–Al 2 O 3 –GeO 2 –P 2 O 5 glass–ceramics system

Lithium aluminum germanium phosphate glass–ceramics with NASICON structure find potential application in the field of energy storage device/solid-state battery. Two different glasses with nominal compositions (a) Li1.5Al0.5Ge1.5P2.9Si0.1O12 (LAGP1) and (b) Li1.5Al0.5Ge1.5P2.5Si0.5O12 (LAGP2) were prepared by standard melt-quench technique, and crystallization kinetics phenomenon in these systems was studied using differential thermal analysis technique (DTA). In addition to different conventional methods for crystallization kinetics analysis, model-free kinetics was also applied. The values obtained for activation energy of crystallization (E a) are compared and used for determination of crystallization index, n and m. The E a value obtained for LAGP1 is 375 ± 17 kJ mol−1 which is higher as compared to the E a value of LAGP2, 199 ± 22 kJ mol−1. LAGP2 with higher amount of Si(0.5) causes significant structural modification in the phosphate network and an early phase separation in the silico-phosphate glass. Thus, LAGP2 shows lower activation energy value as compared to LAGP1. The kinetic parameter, n, related to crystal nucleation, was evaluated from crystallized volume fraction (x) at a fixed temperature using predetermined E a value, and other parameter ‘m’ related to the crystal growth was determined using the modified Kissinger equation. The model-free kinetics was used to evaluate the variation of E a, m and n with temperature and suggests a dynamic nucleation and crystallization process with progressive change in kinetic parameters. In LAGP1 sample, an unusual increase in E a value was observed at x value > 0.2 and can be correlated with the existing ‘self-feeding’ process, which is observed in DTA plots. In both of these systems, the values of n and m are found to be equal and more than 3 which suggests three-dimensional growths of Li1.5Ge1.5Al0.5(PO4) crystals on a constant number of already grown nuclei. XRD and micro-Raman spectroscopy were used to identify the crystalline phase formed and various structural units present in the glass and glass–ceramics samples. Minor amount of LiAlPO4 was confirmed from XRD and Raman spectroscopy along with major Li1.5Ge1.5Al0.5(PO4) phase. In addition, using advanced kinetics and technology solution software non-isothermal data were simulated and the isothermal conversion data were extracted for various temperatures which are found to be very close to experimental isothermal data.