Lithium-7 NMR and ionic conductivity studies of lanthanum lithium titanate electrolytes

Abstract

Lanthanum lithium titanate compounds belonging to the solid solution (La 2/3− x Li 3 x □ 1/3−2 x )TiO 3 have been investigated by powder X-ray diffraction analysis, electrical conductivity and 7Li nuclear magnetic resonance (NMR) spectroscopy. These materials are purely ionic conductors with a maximum ionic conductivity of 3.9×10 −4 S cm −1 at room temperature found for x=0.08. 7Li NMR and room temperature conductivity measurements indicate that there are two kinds of lithium ions with different environments and different mobilities present in these materials. This result is in agreement with the crystal structure established by powder X-ray diffraction analysis. Different values of the activation energy have been obtained from spin–lattice relaxation time T 1 measurements at low temperatures and from conductivity measurements (0.12 eV and 0.37 eV respectively). Moreover, the characteristic vibration frequency for the ionic hopping motion obtained from T 1 measurements ( ν 0≈10 11 Hz) is an order of magnitude lower than that of typical phonon frequency, namely, 10 12–10 13 Hz. These results may suggest that the motion of the mobile lithium ions is highly correlated and/or that the dimensionality of the motion has a 2D character.