E. Morán

Lithium-ion conductivity in the novel La1/3-xLi3xNbO3 solid solution with perovskite-related structure

Abstract The stoichiometric range, crystal chemistry, ionic conductivity and electrochemical window of the La 1/3− x Li 3 x NbO 3 solid solution with a perovskite-related structure have been studied. The range of existence of the solid solution appears to be 0≤x≤0.06. These niobates have a basic diagonal unit cell a ≈√2 a p b √2 a p c ≈2 a p . Ionic conductivity of the materials and its dependence with the composition and temperature have been examined. We have found that the highest conductivity value is 4.3×1O −5 S cm −1 at 300 K for x =0.04. The electrochemical window of the compounds has been investigated by potentiostatic discharge and charge. Electrochemical experiments show that the use of the materials as solid electrolytes in secondary batteries is limited down to 1.75 V using Li metal as anode.

Electrochemical lithium intercalation in Li2Ti3O7-Ramsdellite structure

Electrochemical lithium intercalation has been carried out in the Ramsdellite-type structure of Li2Ti3O7. The intercalation of 2.24 lithium ions at a very low potential in this compound leads to the formation of a new phase Li4.24Ti3O7. From powder X-ray diffraction we deduce that the structure of Li4.24Ti3O7 is closely related to the parent Ramsdellite. Both the large reversibility and the relatively high specific capacity (235 Ah Kg−1) make this compound a promising material as the negative electrode for secondary lithium cells.

Phonons in Nd2−xCexCuO4

We have measured far infrared reflection and Raman spectra on high quality single crystals and ceramic samples of the Nd2−xCexCuO4 system. The TO and LO frequencies of all infrared-active phonons were determined from a Kramers-Kronig analysis of the infrared reflection spectra. The frequencies of the detected Raman-active phonons are 228 cm−1 (A1g Nd), 328 cm−1 (B1g O2) and 480 cm−1 (Eg 02). A resonantly enhanced peak at 580 cm−1 is observed for Ce-doped samples only with polarization properties like an A1g phonon. We have also performed lattice-dynamical calculations with parameters extracted from those of perovskites and metallic oxides. The frequencies obtained agree well with our experimental findings. We present the eigenvectors of all zone center phonons. Finally, we also detected a feature at 165 cm−1 in the ab-polarized infrared spectra in Nd2CuO4 which might be of magnetic origin.

Probing thermodynamic fluctuations in high temperature superconductors

Abstract We probe thermodynamic fluctuations in HTSC by measuring the excess electrical conductivity, Δσ, abovr T c in single-phase (within 4%) Ba 2 LnCu 3 O 7−δ compounds, with LnY, Ho and Sm. As expected, the measured relative effect, Δσ / σ (300 K), is much more important in HTSC than for low-temperature superconductors (at least one order of magnitude). In the reduced temperature region −5 ϵ ≡ ln [ (T-T c )/T c ] Δσ / σ ( 300 K ) = Aϵ x , where A is a temperature-independent amplitude. x is found to be similar for all compounds, with average value =−0.47 ± 0.06. This result confirms an universal critical behaviour of Δσ in HTSC, and the value of agrees with that predicted by the Aslamazov-Larkin (AL) theory for three-dimensional BCS superconductivity. However, A shows a normal conductivity dependence which is not accounted for by the AL theory.

α-NaFeO2: ionic conductivity and sodium extraction

Abstract Complex admittance measurements have been performed on α-NaFeO 2 and on the product Na 0.9 FeO 2 obtained by sodium extraction, both being polycrystalline powders. The electrical conductivity in both samples is proposed to be due to sodium ions hopping between octahedral positions through the adjacent, empty, tetrahedral ones characteristic of a rock-salt structural framework. The parent material shows an activation energy in the dc conductivity of 0.31 eV between 306 and 498 K. Removal of sodium, in spite of creating octahedral vacancies, results in lower conductivity values, and a higher activation energy of 0.58 eV in a similar temperature range.

A novel “126” phase of the family of Y2Ba4Cu6+nO14+n high-temperature superconducting materials

Abstract Following our work on lithium insertion into YBa 2 Cu 3 O 7− δ (commonly referred to as “123”) in which we observed the room-temperature formation of YBa 2 Cu 4 O 8 (“124”) [1,2], we have now found, by the same procedure using “124” as starting material, a novel phase with Y:Cu:Ba in the ratio 1:2:6 (“126”), intergrown in different proportions with YBa 2 Cu 4 O 8 and YBa 2 Cu 5 O 9 (“125”). Powder X-ray diffraction patterns and electron microscopy and diffraction suggest a unit cell of ≈ 3.80 × 3.86 × 33.4 A containing four [CuO 4 ] chain layers instead of the two observed in the original “124”.

New electrode materials for lithium rechargeable batteries

Abstract In this paper, a contribution to the search of new electrode materials for lithium batteries is reported. First, we will present the electrochemical behavior of an Aurivillius-type phase with the composition Bi 4 V 2 O 11 . This oxide may be used as cathode material (390 A h/kg) if the voltage is not so low (average voltage 1.6 V). On the other hand, if Bi 4 V 2 O 11 is reduced down to 0.5 V, it reacts with 28 Li ions per formula unit. Considering only the low voltage region, Li 28 Bi 4 V 2 O 11 could be a candidate to anode material (360 A h/kg at 0.7 V). Our research has also been directed towards the applications of several types of titanium oxides in lithium batteries. Among these compounds, we present the results for K x Ti 8 O 16 and Li 2 Ti 3 O 7 . The best results are those obtained from the ramsdellite Li 2 Ti 3 O 7 : the large reversibility, low polarization and relatively high capacity (235 A h/kg) make this compound a promising material as negative electrode for lithium ion cells. However, the relatively high average potential, close to 1.4 V, would reduce considerably the performance of a rocking-chair battery using this ramsdellite instead of carbon.

Lanthanide substitution by high pressure in the RuSr2GdCu2O8 magnetic superconductor

Abstract A systematic study of the structural and magnetic properties of the family RuSr2RECu2O8 (RE=Er, Ho, Y, Dy, Tb, Gd and Eu) has been performed. All these materials appear to be tetragonal (P4/mmm) and the unit cell volume decreases along with the lanthanide ion dimension. Differences are observed in the magnetic behaviour of these compounds. High pressure and high temperature are needed to synthesize most of the members of this family.

Bi4V2O11 and related compounds as positive electrode materials for lithium rechargeable batteries

Abstract In the search for new intercalation electrode materials, several phases related to the compound Bi 4 V 2 O 11 have been tested as positive electrodes in room temperature electrochemical lithium cells. Bi 4 V 2 O 11 , Bi 3.6 Pb 0.4 V 2 O 11− y and Bi 4 V 1.8 Cu 0.2 O 11− y are structurally similar compounds differing only from a microstructural point of view. Electrochemical lithium intercalation is not affected by such structural and compositional differences, since the performance of all these phases is equivalent. The surprising amount of 8 lithium ions per vanadium atom inserted in Bi 4 V 2 O 11 during the first discharge at an average potential of 1.7 V implies a theoretical specific energy of 655 W h/Kg. In spite of this promising value, the irreversibility found after the first discharge slightly reduces the possibilities of these materials as positive electrodes in room temperature rechargeable lithium batteries. Even so, the energy density is high enough to consider the materials for future improvements.

Lithium insertion in Ba2YCu3O7-y

Novel materials, LixBa2YCu3O7-y(0≤x≤2), have been obtained by reaction of Ba2YCu3O7-y with n-butyl lithium. The most striking feature of these materials is the coexistence of both ionic conductivity and superconductivity for 0<x<0.5. Transmission electron microscopy (TEM) and electron diffraction show the lithiation to occur anisotropically.