Heterogeneous lithium diffusion in nanocrystalline Li₂O: Al₂O₃ composites

Li diffusion in the nanocrystalline composite system (1 - x)Li₂O:xAl₂O₃ was studied by NMR measurements. Heterogeneous, biexponential ⁷Li NMR relaxation is found and two Li species with different dynamic behaviour can be discriminated via their individual spin-spin and spin-lattice relaxation times. The heterogeneous dynamics of the two Li species which also shows up in the ⁷Li NMR spectra reflects the heterogeneous structure of the nanocrystalline composites consisting of crystalline grains and a high volume fraction of interracial regions. The number fraction of fast Li ions located in the interfacial regions increases with temperature and insulator content x. Accordingly biexponential relaxation becomes more pronounced. From the temperature dependence of the two spin-lattice relaxation rates activation energies of about 0.3 eV for both the fast and the slow Li species are obtained, independent of the composition of the composites.