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 Li1+xTi2-xInx(PO4)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. R3̅c); 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. P21/n) phases. At intermediate compositions, the secondary LiTiPO5 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 In3+, deduced from structural refinements of three phases, was confirmed by 31P MAS NMR spectroscopy. The Li mobility was investigated by 7Li 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 (σb = 1.9 × 10-3 S·cm-1), with an activation energy Eb = 0.27 eV. In this composition, the overall Li conductivity was σov = 1.7 × 10-4 S·cm-1 and Eov = 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 (σov = 1.4 × 10-5 S·cm-1), with an activation energy Eov = 0.39 eV. Structural models deduced with the Rietveld technique were used to analyze the conduction channels and justify the transport properties of different Li1+xTi2-x Inx(PO4)3 phases.

Cation Miscibility and Lithium Mobility in NASICON Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) Series: A Combined NMR and Impedance Study.

Rhombohedral NASICON compounds with general formula Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti4+ by Sc3+ and Li+ in pristine LiTi2(PO4)3 increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and 6Li/7Li, 31P, and 45Sc MAS NMR spectra confirms the presence of secondary LiScO2 and LiScP2O7 phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x < 0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10-3 S·cm-1) and low activation energy (Ea = 0.25 eV) measured at room temperature make Li1.2Ti1.8Sc0.2(PO4)3 one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.

Modeling Ti/Ge Distribution in LiTi2-xGex(PO4)3 NASICON Series by (31)P MAS NMR and First-Principles DFT Calculations.

Ti/Ge distribution in rhombohedral LiTi2-xGex(PO4)3 NASICON series has been analyzed by (31)P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and first-principles density functional theory (DFT) calculations. Nuclear magnetic resonance is an excellent probe to follow Ti/Ge disorder, as it is sensitive to the atomic scale environment without long-range periodicity requirements. In the samples considered here, PO4 units are surrounded by four Ti/Ge octahedra, and then, five different components ascribed to P(OTi)4, P(OTi)3(OGe), P(OTi)2(OGe)2, P(OTi)(OGe)3, and P(OGe)4 environments are expected in (31)P MAS NMR spectra of R3̅c NASICON samples. However, (31)P MAS NMR spectra of analyzed series display a higher number of signals, suggesting that, although the overall symmetry remains R3̅c, partial substitution causes a local decrement in symmetry. With the aid of first-principles DFT calculations, 10 detected (31)P NMR signals have been assigned to different Ti4-nGen arrangements in the R3 subgroup symmetry. In this assignment, the influence of octahedra of the same or different R2(PO4)3 structural units has been considered. The influence of bond distances, angles and atom charges on (31)P NMR chemical shieldings has been discussed. Simulation of the LiTi2-xGex(PO4)3 series suggests that detection of 10 P environments is mainly due to the existence of two oxygen types, O1 and O2, whose charges are differently affected by Ge and Ti occupation of octahedra. From the quantitative analysis of detected components, a random Ti/Ge distribution has been deduced in next nearest neighbor (NNN) sites that surround tetrahedral PO4 units. This random distribution was supported by XRD data displaying Vegard's law.

Near constant loss regime in fast ionic conductors analyzed by impedance and NMR spectroscopies.

Universal dielectric response (UDR) and nearly constant loss (NCL) dispersive regimes have been investigated in fast ion conductors with perovskite and NASICON structure by using NMR and impedance spectroscopy (IS). In this study, the electrical behavior of La(0.5)Li(0.5)TiO3 (LLTO-05) perovskite and Li(1.2)Ti(1.8)Al(0.2)(PO4)3 (LTAP0-02) NASICON compounds was investigated. In both systems a three-dimensional network of conduction paths is present. In the Li-rich LLTO-05 sample, lithium and La are randomly distributed on A-sites of perovskites, but in LTAP0-02 Li and cation vacancies are preferentially disposed at M1 and M2 sites. In perovskite compounds, local motions produced inside unit cells are responsible for the large "near constant loss" regime detected at low temperatures, however, in the case of NASICON compounds, local motions not participating in long-range charge transport were not detected. In both analyzed systems long-range correlated motions are responsible for dc-conductivity values of ceramic grains near 10(-3) S cm(-1) at room temperature, indicating that low-temperature local motions, producing large NCL contribution, are not required to achieve the highest ionic conductivities.