Local structure and lithium mobility in intercalated Li3AlxTi2−x(PO4)3 NASICON type materials: a combined neutron diffraction and NMR study

Abstract

The structural features of intercalated Li₃Al_xTi_2−x(PO₄)₃ compounds, with x = 0 and 0.2, have been deduced by Rietveld analysis of neutron diffraction (ND) patterns recorded between 100 and 500 K. The Li insertion decreases the symmetry from Rc to R in analyzed compounds. In pristine Li_1+xAl_xTi_2−x(PO₄)₃ samples, Li occupies mainly six-fold M₁ sites at ternary axes; but in lithiated Li₃Al_xTi_2−x(PO₄)₃ samples, Li is located near M₂ positions at M₃/M₃′ four-fold coordinated sites. In both cases, Li arrangement minimizes electrostatic Li–Li repulsions. The insertion of lithium resulted in the reduction of Ti⁴⁺ to Ti³⁺ that shifts ⁷Li, ²⁷Al and ³¹P MAS-NMR resonances towards more positive chemical shifts, improving the resolution of different sites. The detection of twelve components in ⁷Li MAS-NMR spectra recorded at room temperature suggests the location of Li⁺ ions at three-oxygen faces that define M₂ cavities. From ⁷Li MAS-NMR spectra, the occupancy of sites and mobility of lithium were investigated in the temperature range 100–500 K. The correlation between structural information, deduced by neutron diffraction, and lithium mobility, deduced by NMR spectroscopy, provides new insights into structural factors that affect lithium mobility in materials with NASICON structure.