Sites and Mobility of Lithium along the Li1+xTi2–xInx(PO4)3 (0 ≤ x ≤ 2) Series Deduced by XRD, NMR, and Impedance Spectroscopy

The structure and Li conductivity has been investigated in the Li 1+ x Ti 2– x In x (PO 4 ) 3 (0 ≤ x ≤ 2) series prepared by the ceramic route at 900 °C. The XRD patterns of 0 ≤ x ≤ 0.2 samples show the presence of rhombohedral (S.G. R 3̅ c ); those of 0.2 ≤ x ≤ 1 samples display both rhombohedral and orthorhombic (S.G. Pbca ), and 1 ≤ x ≤ 2 samples exhibit only monoclinic (S.G. P 2 1 / n ) phases. At intermediate compositions, the secondary LiTiPO 5 phase was detected. The Rietveld analysis of XRD patterns was used to deduce unit-cell parameters, chemical composition, and percentage of phases. The amount of In 3+, deduced from structural refinements of three phases, was confirmed by 31 P MAS NMR spectroscopy. The Li mobility was investigated by 7 Li MAS NMR and impedance spectroscopies. The Li conductivity increased with the Li content in rhombohedral but decreased in orthorhombic, increasing again in monoclinic samples. The maximum conductivity was obtained in the rhombohedral x = 0.2 sample (σ b = 1.9 × 10 –3 S·cm –1 ), with an activation energy E b = 0.27 eV. In this composition, the overall Li conductivity was σ ov = 1.7 × 10 –4 S·cm –1 and E ov = 0.32 eV, making this composition a potential solid electrolyte for all-solid-state batteries. Another maximum conductivity was detected in the monoclinic x ∼ 1.25 sample (σ ov = 1.4 × 10 –5 S·cm –1 ), with an activation energy E ov = 0.39 eV. Structural models deduced with the Rietveld technique were used to analyze the conduction channels and justify the transport properties of different Li 1+ x Ti 2– x In x (PO 4 ) 3 phases.