Understanding the Evolution of the Structure and Electrical Properties during Crystallization of Li_1.5Al_0.5Ge_1.5(PO₄)₃ and Li_1.5Sc_0.17Al_0.33Ge_1.5(PO₄)₃ NASICON -Type Glass Ceramics

In this paper, the effects of crystallization advance on the material structure and electrical properties of lithium-ion Na+ super ionic conductor (NASICON) glass ceramics were investigated. Glasses with Li1.5Al0.5Ge1.5(PO4)3 and Li1.5Sc0.17Al0.33Ge1.5(PO4)3 compositions were crystallized in controlled conditions to obtain gradual increment of the volume crystallized fraction. The glass-to-crystal transformation was then monitored by differential scanning calorimetry analysis (DSC), X-ray diffractometry (XRD), Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy (MAS NMR), and electron microscopy, along with chemical analyses. Finally, the electrical properties of the specimens were evaluated by impedance spectroscopy to observe the changes in electrical properties according to the crystallization advance. Results revealed that glasses containing scandium are more stable against crystallization than their neat counterparts. Crystallization led to the formation of single-phase NASICON glass ceramics. Scandium induced a lattice expansion of the NASICON structure. Furthermore, crystallization induces remarkable structural changes in the materials as a whole, either in local order or in medium to long order. No important increase in conductivity was observed in earlier stages of crystallization. After the percolation of crystals, conductivity increases sharply and the remaining glassy phase has little impact on the total conductivity of the material. Scandium expands the rhombohedral structure but increases the glass stability and reduces the sizes of crystals for the fully crystallized glass ceramics. Glass ceramics with larger grains are more propitious for conductivity than the more refined ones. Therefore, this paper offers key information about the understanding of NASICON crystallization and its structural evolution, providing important insights into the crystallization of these electrolytes.