High Lithium Ionic Conductivity in the Li_1+xAl_xGe_yTi_2-x-y(PO₄)₃ NASICON Series

Two Li 1+ x Al x Ge y Ti 2 - x - y (PO 4 ) 3 (0.2 ≤ x ≤ 0.8; y = 0.8, 1.0) solid solutions have been prepared as polycrystalline powders. These compounds crystallize in the NASICON-type structure, R 3̄ c space group, and the crystal structures have been characterized by the Rietveld method with laboratory X-ray powder diffraction data. The cell parameters evolution along the two series agrees with the substitution of larger Ti 4+ by smaller Ge 4+ and Al 3+ cations. The electrical properties have been characterized by an impedance study. Bulk conductivity values at room temperature are close to 10 - 3 S·cm - 1 with low activation energies (≈0.35 eV). The trajectories of the Li + cations have been simulated from the bond valence sum calculation. Structural keys, which justify the high ionic conductivity and the low activation energy, are discussed.