J.L. Fourquet

Mechanism of ionic conduction and electrochemical intercalation of lithium into the perovskite lanthanum lithium titanate

Abstract The ionic conductivity and electrochemical intercalation properties of La 2 3−x Li 3 x TiO 3 solid solutions (for 0.07 ≤ x ≤0.13) have been studied. These compounds present a perovskite-type structure (ABO 3 ) with cation deficiency at the A-sites. The purely ionic conductivity was confirmed and the mechanism of ionic conduction investigated using impedance spectroscopy techniques. We find that the temperature dependence of conductivity can be modelized by a Vogel-Tamman-Fulcher (VTF)-type relationship. In these materials, where the high ionic conductivity may originate from the presence of vacancies in the A-sites of the perovskite structure, the VTF behavior would suggest a mechanism of conduction involving the tilting of the TiO 6 octahedra. The lithium intercalation was also investigated in LiClO 4 (M)-PC electrolyte using galvanostatic discharge and charge at very low rates (one Li/250 and /1500 h) in order to approach the equilibrium. It was shown that the lithium intercalation leads to the presence of a plateau around 1.5 V/Li in the discharge curve, it is partly reversible and the capacity of the electrode is not very high. A maximum lithium uptake of 0.15 was found. The diffusion coefficient of lithium in the intercalated material was determined by impedance spectroscopy at room temperature and found to range from 10 −8 cm 2 s −1 to 10 −9 cm 2 s −1 as intercalation proceeds. Since the experimental impedance spectroscopy data performed at room temperature follow a Warburg behavior at low frequency, the intercalation seems to proceed in a single-phase process although a plateau is observable in the discharge curve.

Crystal structure of the metastable form of aluminum trifluoride β-AlF3 and the gallium and indium homologs

Abstract The crystal structure of the metastable phase β-AlF3, which is related to the hexagonal tungsten bronze structure, has been solved by X-ray powder and single-crystal diffraction methods. The crystal habit is pseudo-hexagonal with systematic twinning (rotation of 120° around the c axis), but the true symmetry is orthorhombic with space group Cmcm, Z = 12, a = 6.931(3), b = 12.002(6), c = 7.134(2), A (R = 0.044 and Rw = 0.051) from 929 independent reflections). The network is built from very regular AlF6 octahedra rotated by approximately 7.2° from the positions of the ideal HTB structure. A similar network, with the same propagation of the tilting, was observed in the compound (H2O)0.33FeF3 and in the metastable polymorphs of CrF3 and of VF3. Our reinvestigation of the structures of β-GaF3 and β-InF3 using powder data shows that they are isotypic with the aluminum compound, with a = 7.210(1), b = 12.398(2), c = 7.333(1) and a = 7.875(2), b = 13.499(4), c = 7.956(2), A, respectively.

Anomalies in Li+ ion dynamics observed by impedance spectroscopy and 7Li NMR in the perovskite fast ion conductor (Li3xLa2/3-x□1/3-2x)TiO3

Abstract A previous study, performed on the fast ionic conductor Li 3 x La 2/3- x TiO 3 by 7 Li and 6 Li Nuclear Magnetic Resonance (NMR), has shown that Li + ions undergo two different motions: a fast motion ( τ c ≈10 −9 s) inside the A-cage of the perovskite structure and a slower one ( τ c ≈10 −6 s) from one A-cage to a next vacant one. Furthermore, a change of these two motion mechanisms is observed around 200 K. Apart from NMR, impedance spectroscopy may also afford information on the ionic motion mechanism. Lithium motion in Li 3 x La 2/3- x TiO 3 is then studied by impedance spectroscopy in the 1 Hz–10 MHz frequency range and in the 140–500 K temperature range. The results obtained by these two techniques, i.e. 7 Li NMR and impedance spectroscopy, are then compared in the 140–270 K temperature range. As observed in NMR, the dc conductivity shows a change in the mechanism of ionic motion around 200 K. Apart from the dc plateau, the real part of conductivity ( σ ′) displays a dispersive behavior at high frequencies. Plotting the ac data in terms of impedance and modulus reveals the presence, in the mechanism of conduction, of both a nonlocalized conduction (long-range motion of the mobile ions) and a localized one (dipolar relaxation). According to these experimental observations, an equivalent electrical circuit is proposed, taking into account the physical processes assumed to be present when a small electrical signal is applied to the oxide. Both dipole polarization and long-range motion of the mobile ions are included in the electrical circuit of the conductive pathways. A complex nonlinear least squares fitting procedure (CNLS) is used to fit this electrical model to the experimental conductivity vs. frequency response ( σ ′ and σ ″). This procedure shows that all the parameters linked to the conductive pathways undergo a sudden change around 200 K, suggesting that a change in the ionic motion mechanism occurs at this temperature. This result is discussed in relation to both the crystallographic structure of the ionic conductor and the results previously obtained by 7 Li NMR.

Modeling Li-ion conductivity in fast ionic conductor La2/3−xLi3xTiO3

Abstract Monophasic samples of fast ionic conductors La2/3−xLi3xTiO3 (LLTO), with x varying from 0.06 to 0.13, are prepared by solid-state reaction. The total dc-conductivity is measured by complex impedance spectroscopy in the 10 MHz –1 Hz frequency range. Considering only the resulting location of oxygen atoms and employing bond valence equations, the conduction geometry and dc ionic conductivity are modeled. An averaged pathway for the Li+ conduction is proposed in this paper, assuming that the time-averaged position for Li+ is the geometrical centre of the A-cage. The saddle point of this pathway (Vumax) can be related to the activation energy for the ionic jump. Moreover, in order to model dc-ionic conductivity, not only activation energy, but also number of carriers and site occupancies have been considered. We propose three possibilities for the Li+ location in the structure in order to predict bulk conductivity in LLTO phase. Experimental evidence allows the exclusion of one of the three possibilities, while the other two are both in agreement with experimental values.

A distribution of activation energies for the local and long-range ionic motion is consistent with the disordered structure of the perovskite Li3xLa2/3-xTiO3

Abstract 7 Li nuclear magnetic resonance spin–lattice relaxation time T1 versus temperature is reported in the 150 K–900 K temperature range on lithium lanthanum titanate fast ionic conductors. Because of the presence of disorder in the distribution of the lanthanum ions in the crystalline structure of this oxide and consequently in the conduction pathways of the lithium ions we propose to explain the strong asymmetry shown by these T1 versus 1/T curves by assuming independent ionic hops over a distribution of activation energies for the thermally activated Li+ ion hops. According to this assumed model the spin–lattice relaxation times T1 and the DC conductivity are fitted consistently in the 200–600 K and 300–400 K temperature ranges respectively. For both lower and higher temperatures a departure of the experimental data from the model is observed and explained. The use of this model to fit both T1 and DC conductivity data ruled out the possibility that different forms of the distribution would lead to a reasonable representation of T1. The physical meaning of the obtained parameters is discussed in accordance with the structure of the compounds.

Electrochemical intercalation of lithium into the ramsdellite-type structure of Li2Ti3O7

The chemical and electrochemical intercalation of lithium ions into the ramsdellite-type structure of Li2Ti3O7 has been carried out by chemical reduction with n-BuLi or by galvanostatic discharge of an electrochemical cell. A maximum lithium uptake has been found to be of 0.60 and 1 per Li2Ti3O7 respectively at room temperature. During this intercalation process the ramsdellite network is preserved as shown either by electrochemical galvanostatic discharge or by X-ray powder diffraction analysis. However it is clearly shown by X-ray diffraction analysis and by electrochemical spectroscopy that two intercalation sites of very close energy are involved in this process. The potential difference between the two peaks observed in the voltammogram is 350 ± 20 mV. The variations of both the charge transfer resistance and the diffusion coefficient observed by a.c. impedance spectroscopy as intercalation proceeds can be explained by the presence of these two different intercalation sites.

t-AlF3: Crystal structure determination from X-ray powder diffraction data. A new MX3 corner-sharing octahedra 3D network

Abstract t-AlF3 is obtained from the thermolysis of [(CH3)4N]AlF4 · H2O or amorphous AlF3 · xH2O (x P4 nmm ; a = 10.1843 A, c = 7.1738 A; Z = 16. t-AlF3 belongs to the family of 3D corner-sharing octahedra networks, the Al atoms lying at the points of a 3D six-connected net. A relation with Na4Ca4Al7F33 is found which may be considered as a stuffed polytype.

Nuclear magnetic resonance investigation of Li+-ion dynamics in the perovskite fast-ion conductor Li3xLa2/3-x□1/3-2xTiO3

7Li nuclear magnetic resonance relaxation times T1, T1ρ and T2 versus temperature are reported in the 150-900 K temperature range for the lithium lanthanum titanate Li3xLa2/3-x1/3-2xTiO3 perovskite-type fast-ionic conductors. The presence of Li+ ions of two kinds with slightly differing environments is displayed in these experiments. These ions exhibit two different motions: a fast one with a characteristic frequency around 100 MHz at 350 K and a slow one whose frequency is around 60 kHz at 280 K. These two different Li+ species cannot be differentiated by means of the fast motion (only one T1 is observed from the experiments), but only by means of the slow ones (two T1ρ and two T2 are observed). These motions are respectively attributed to Li+ motion inside the A-cage of the perovskite structure formed by the oxygen ions and to Li+ hops between the cages. T1- and T1ρ-studies also performed on the 6Li nucleus clearly show that just dipolar nuclear interaction is responsible for Li+ relaxation. This result is at variance with what has been previously put forward for the relaxation process in these compounds.

Les pyrochlores AIB2X6: Mise en evidence de l'occupation par le cation AI de nouvelles positions cristallographiques dans le groupe d'espace fd3m

Abstract In A I B 2 X 6 pyrochlores—A I = Pb, Tl; X = O, F— the A I ions occupe 32e positions with a probability of 25% instead of the 8b positions as it was previously described; this results from the observations of very weak X-diffraction peaks provided by planes such as h = 4n, k = 4n, l = 4n + 2, visible on single crystals photographs of RbCoCrF 6 , RbNb 2 O 5 F, TlNb 2 O 5 F; this displacement of A I with respect to the 8b position -about 0,6 A for Tl + and 0,4 A for Rb + - is in agreement with the great size of the 8b cavity -about 1,80 A-. However Cs + , the radius of which is well adapted to this cavity, is not significantly displaced in homologous compounds.

In search of the cubic phase of the Li+ ion-conducting perovskite La2/3−xLi3xTiO3: structure and properties of quenched and in situ heated samples

Abstract In search of the cubic phase of the Li + ion-conducting perovskite La 2/3− x Li 3 x TiO 3 (LLTO), several samples, quenched and heated at different temperatures, are examined by powder X-ray diffraction (XRD). The best structural model (for 0.06 x P 4/ mmm ; a ≈ a p , c ≈2 a p ) in which La 3+ ions and vacancies are always unequally distributed in the two different A sites, centers of the perovskite dodecahedral cages. The quenching experiments bring the evidence of a very fast disordering/ordering of La 3+ ions between their two unequivalent positions, and the in situ thermodiffractometry experiments show that the La 3+ ion thermal diffusion becomes noticeable above 800 °C and depends on the solid solution composition x . Although the thermal diffusion of the La 3+ ions occurs above 800 °C, the disordering of these ions remains limited for low x values (with c /2 a values slightly greater than 1) even at 1200 °C, while a nearly complete disordering is reached at 1200 °C for x =0.11 (with c /2 a very close to 1). As reported by earlier authors, the kinetics of the fast La 3+ ions disordering/ordering in the temperature range 900–1200 °C has to be fully considered during the preparation of the samples, and the thermal history can noticeably change the La 3+ ion partition and hence the electrical conductivity of the prepared phase. The equilibrated samples can be obtained by using proper heating time and temperature.