Investigation of Local Structures and Dynamics of Li and Na Ions in Nasicon-Type Solid Electrolytes by NMR Spectroscopy

Abstract

We investigated the local structure and the dynamics of Li and Na ions in the solid electrolytes Li1.2Al0.6Ti1.4(PO4)3, Li1.6Al0.6Ge1.4(PO4)3, and Na3.4Sc2(SiO4)0.4(PO4)2.6 by NMR spectroscopy. While 27Al MAS NMR spectroscopy reveals the presence of only octahedral [AlO6] environments for Li1.6Al0.6Ge1.4(PO4)3, Li1.2Al0.6Ti1.4(PO4)3 exhibits large amounts of Al located in tetrahedral [AlO4] environments. 31P MAS NMR measurements reveal multiple environments with different Al/Ge next nearest neighbor composition around P and the relative intensities are in good agreement with a statistical arrangement of Ge/Al on the octahedral sites for Li1.6Al0.6Ge1.4(PO4)3. For Li1.2Al0.6Ti1.4(PO4)3, the 31P MAS NMR results indicate the presence of structural disorder. These results are confirmed by X-ray diffraction and Rietveld refinements. The dynamics of Li ions have been investigated by temperature-dependent static 7Li NMR lineshape analysis and 7Li NMR relaxometry. Jump rates of about 7∙108 s-1 can be estimated for Li1.6Al0.6Ge1.4(PO4)3 at 400 K and the activation barrier for a single Li ion jump is 0.14 eV. A Li diffusion coefficient of 10-11 m2/s and a Li ion conductivity of 3.6 mS/cm can be estimated at 400 K. For Na3.4Sc2(SiO4)0.4(PO4)2.6, NMR spectroscopy reveals that Si and P are arranged statistically on the tetrahedral sites and Sc is exclusively located on octahedral sites. Temperature-dependent 23Na NMR measurements show a strong increase of the asymmetry of the local environment, consistent with XRD measurements which yield a strongly anisotropic thermal expansion. Jump rates of higher than 5∙103 s-1 can be estimated for the Na ions for temperatures above 300 K.