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

Abstract In our study, various fast lithium-ion conductors of composition Li 1 +  x Al x Ge 2 −  x P 3 O 12 (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 Ge 4 + for Al 3 + and Li + in LiGe 2 P 3 O 12 were followed employing X-ray diffraction and solid-state NMR. Among the samples studied, Li 1.4 Al 0.4 Ge 1.6 P 3 O 12 ( 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., 27 Al{ 31 P}-REDOR NMR and 31 P{ 27 Al}-REAPDOR NMR, allowed us to clearly assign the different 31 P-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 AlPO 4 phase, thus not participating in the NASICON structure. For Al-rich samples with x  ≥ 0.7, apart from crystalline impurity phases such as AlPO 4 , Li 4 P 2 O 7 and GeO 2 , an additional phase Li 9 Al 3 (P 2 O 7 ) 3 (PO 4 ) 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.