Study of the glass-to-crystal transformation of the NASICON-type solid electrolyte Li1 + xAlxGe2 − x(PO4)3

• Melt-quench and sol-gel processed super-ion conducting Li1+xAlxGe2 − x(PO4)3 (LAGP, 0.4 ≤ x ≤ 1.0) are investigated.
• Glass to crystal conversion and formation of different crystalline phases has been established by XRD and solid state NMR.
• The findings allow for the formation of virtually phase pure LAGP in the extended compositional range 0.4 ≤ x ≤ 0.8.

In this study, we present a detailed investigation of the glass to crystal transformation in the Li1 + xAlxGe2 − xP3O12 (LAGP) system. Precursor glasses were synthesized following melt-quench and sol-gel procedures. The glass to crystal transition was studied employing X-ray diffraction and advanced solid state nuclear magnetic resonance (solid state NMR) techniques including 7Li-, 27Al-, 31P NMR, 27Al{7Li}- and 27Al{31P}-REDOR-NMR as well as 31P{27Al}-REAPDOR-NMR. Whereas Al-poor LAGP precursor glasses exhibit a direct glass-to-LAGP crystal transition, without any extended change in the Al content of the LAGP crystals and without the formation of side phases, for glasses richer in Al the crystallization of LAGP is accompanied by the formation of AlPO4, which for x = 1.0 is the first crystalline phase to precipitate. For these compositions, the first crystalline LAGP phase precipitating contains an Al content lower than the nominal x-value, however, with annealing, more Al is continuously incorporated into the LAGP structure, accompanied by a decrease in the relative amount of the AlPO4 phase. After surpassing a threshold temperature – which decreases from T = 900 °C for x = 0.4 to 720 °C for x = 0.8 – at which the LAGP phases reaches its highest Al content, close to the nominal composition, Al is gradually expelled from the LAGP structure, forming LGP and AlPO4.The findings enable the formation of virtually phase pure LAGP over an extended compositional range until x = 0.8.