Structure, phase separation and Li dynamics in sol–gel-derived Li1 + xAlxGe2 − x(PO4)3

• Sol–gel processed super-ion conducting Li1 + xAlxGe2-x(PO4)3 (LAGP, 0 ≤ x ≤ 1.5 are investigated.
• The limits of Al and Li solubility have been established employing XRD and solid-state NMR.
• At high x values, crystalline Li9Al8(P2O7)3(PO4)2 is replacing LAGP as the dominant crystalline phase.

In our study, various fast lithium-ion conductors of composition Li1 + xAlxGe2 − xP3O12 (LAGP, 0 ≤ x ≤ 1.2) have been synthesized following a sol–gel process with subsequent annealing. For the annealed, crystallized samples, the subtle structural changes upon exchange of Ge4 + for Al3 + and Li+ in LiGe2P3O12 were followed employing X-ray diffraction and solid-state NMR. Among the samples studied, Li1.4Al0.4Ge1.6P3O12 (x = 0.4) shows the highest Li mobility at room temperature with an activation energy of approx. 30 kJ mol− 1. Employing heteronuclear dipolar NMR techniques, i.e., 27Al{31P}-REDOR NMR and 31P{27Al}-REAPDOR NMR, allowed us to clearly assign the different 31P-MAS-NMR signals to P(OAl)n(OGe)4 − n species with 0 ≤ n ≤ 3 and to show that the tetrahedrally coordinated Al is incorporated in an extra AlPO4 phase, thus not participating in the NASICON structure. For Al-rich samples with x ≥ 0.7, apart from crystalline impurity phases such as AlPO4, Li4P2O7 and GeO2, an additional phase Li9Al3(P2O7)3(PO4)2 is observed, which replaces the LAGP phase as the dominant phase for x > 1.0, giving rise to a compositional limit of Al content as x ≈ 0.6 in LAGP by the sol–gel route. In addition, the effect of the annealing temperature on the nature of the resulting phases was examined.