Solid-State NMR Investigations on the Structure and Dynamics of the Ionic Conductor Li1-xAlxTi2-x(PO4)3 (0.0 ≤ x ≤ 1.0)

The local structure and mobility of lithium ions of the NASICON-type ionic conductor Li1-xAlxTi2-x(PO4)(3) (with x = 0.0, 0.1, 0.2, 0.35, 0.5, 0.7 and 1.0), synthesized using conventional solid-state reaction route have been studied with solid-state nuclear magnetic resonance (NMR) techniques. Li-6, Li-7, Al-27, and P-31 solid-state NMR experiments have been employed to trace the structural changes with varying cation concentration. The structural evolution and the creation of new Al and P environments with changing cation contents were studied by magic-angle spinning (MAS) NMR measurements. Li-6 MAS NMR and Al-27 triple-quantum MAS (3QMAS) show high-resolution spectra enabling site assignments and phase-purity inspections. The temperature dependences of Li-7 NMR spin-lattice relaxation (SLR) rates for different compositions yield important information on the lithium ion mobility in the systems. Li ion jump rates, the activation energies, and the dimensionality of Li diffusion were deduced from the SLR experiments. A vacancy migration model has been proposed for the Li+ ionic diffusion process in pure-phase Li1-xAlxTi2-x(PO4)(3) prepared by solid-state reaction. Above a certain threshold value of x (0.5) additional phosphate phases appear that slows down diffusion. This phenomenon can be observed from Li-6 exchange spectroscopy. The optimum cation concentration for maximum ionic mobility in the phase-pure Li1-xAlxTi2-x(PO4)(3) system can be read directly from the solid-state NMR results.