J. M. Rojo

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.

Conductivity relaxation parameters of some Ag+ conducting tellurite glasses containing AgI or the (AgI)0.75 (T1I)0.25 eutectic mixture

Abstract The conductivity relaxation parameters of some Ag + conducting tellurite glasses containing either AgI or the (AgI) 0.75 -(T1I) 0.25 eutectic mixture have been determined from an analysis of ac conductivity data measured in a wide temperature range. Transport properties in these materials appear as due to an Ag + ion hopping mechanism. The stretched exponential function exp-( t /τ σ ) β has been used to describe the conductivity relaxation. The relaxation parameters have been investigated as a function of the AgI rate. The results obtained are shown to be in good agreement with the predictions of the Ngai coupling model. A correlation has been also established for the glasses investigated between the stretched exponent β of conductivity relaxation function with the decoupling index R τ ( T g ), proposed by Angell, where R τ ( T g ) is the τ s /τ σ ratio of the average structural and conductivity relaxation times at the glass transition temperature T g .

Antimonic acid and sulfonated polystyrene proton-conducting polymeric composites

Abstract Polymer composites formed of a proton conductor and polyvinylidene fluoride (PVDF) are prepared. Sulfonated polystyrene (SPS) and antimonic acid (AAc) are used as organic and inorganic proton conductor, respectively. PVDF is the insulating matrix used as a binder to improve the mechanical stability of the composites compared with the pure proton conductors. The effect of the SPS and/or AAc on both the glass transition temperature, T g , and the melting temperature, T m , of the PVDF is studied. The ionic conductivity of the composites, after water vapor exposure at r.t., and after water immersion at 50°C, is measured at room temperature. The overall conductivity of the composite increases with time up to saturation, which is attained: (i) after 17 h for the water vapor exposure experiment and (ii) after ≈10 min, for the water immersion experiment. The ionic conductivity of the composites is smaller than that of the pure protonic conductors, but the composites show high-dimensional stability. The shape of the conductivity curve depends on the proton conductor chosen.

Ionic conductivity and structural characterization of Na1.5Nb0.3Zr1.5(PO4)3 with NASICON-type structure

Abstract The NASICON-type Na1.5Nb0.3Zr1.5(PO4)3 was prepared by solid state reaction of Nb2O5 and the precursor γ-NaHZr(PO4)2 at 700 °C. The EPR spectra showed a signal with a g factor of 1.984 assigned to Nb (IV) species in octahedral oxygen environments. The X-ray powder diffraction pattern obtained with monochromatic radiation was indexed on the basis of a rhombohedral cell, the hexagonal parameters being aH = 8.8061(2) and c H = 22.7638(7) A . The Na+ ion conduction was measured by the complex impedance method (frequency range: 0.1–105 Hz; temperature range: 20–500 °C) on four pellets previously sintered at 450, 750, 900 and 1000 °C. The conductivity data are discussed in relation to the sintering temperature. An activation energy of 0.60 eV for the movement of Na+ ions in the NASICON framework has been found.

Effect of the Thermal Treatment on the Particle Size and Electrochemical Response of LiCr0.2Mn1.8 O 4 Spinel

Spinel of composition LiCr 0 . 2 Mn 1 . 8 O 4 has been synthesized by a combustion-aided procedure. The particles of the sample are single crystals of ca. 10 nm size as deduced from X-ray diffraction and transmission electron microscopy. Thermal treatment of the as-prepared sample at increasing temperature (400, 600, 700, 750, 900, 1000, and 1100°C) for different times (1, 3, or 24 h) leads to an increase of the particle size, which attains a value as high as 1560 nm after heating at 1100°C for 1 h. Electrochemical response of the heated samples as positive electrodes in a lithium cell has been studied at three rates (C/24, C, and 3C; where C = 2.9 mA). At C/24 the discharge capacity measured at 4 and 4.8 V is not appreciably affected by the particle size. At C and 3C the discharge capacity measured at those voltages decreases as the particle size increases; a linear dependence has been found for particle sizes above 50 nm. On cycling we have found that the capacity fade also depends on the particle size; the larger the particles the less the capacity fade at the mentioned voltages.

Li+ ion mobility in TeO2LiO0.5LiX (X F, Cl) glasses determined by 7Li NMR and impedance spectroscopy

NMR and impedance spectroscopies have been used to study the Li+ ion mobility in the TeO2LiO0.5LiX (X = F, Cl) glasses. Activation energies determined from the temperature dependence of dc conductivity data decrease as the lithium or halogen content increases. Activation energies deduced from the analysis of the ac conductivity data in the complex modulus formalism and corresponding to the true one-particle energy barrier of ionic motion according to the coupling model, have been compared to those deduced from NMR data (T1−1 relaxation time): a good agreement is obtained. Finally, the influence of the glass composition and ion distribution on Li+ ion mobility has been discussed.

Stabilization of the rhombohedral phase in LiZr2(PO4)3 by thermal quenching

Abstract Two LiZr2(PO4)3 samples, which show rhombohedral and monoclinic symmetries, have been studied by powder XRD and MAS-NMR (31P and 7Li) techniques. In the monoclinic sample phosphorus occupies three crystallographic sites and lithium is preferentially placed in M2 environment. When the samples are subjected to successive heating-quenching treatments, the rhombohedral phase is not appreciably modified while the monoclinic phase is transformed into the rhombohedral one. In the quenched samples only one site for phosphorus has been detected and occupancy of lithium in the more symmetric M1 site is favoured. After the thermal treatments, when the samples are stored at room temperature for one year, the framework does not relax to that of the monoclinic phase but distribution of lithium changes towards a statistical occupancy of M1 and M2 sites.

Ionic conductivity of NASICON-type LiHf2(PO4)3: a reexamination

Abstract The ionic conductivity of two pellets of LiHf 2 (PO 4 ) 3 calcined at 1000 and 1100°C has been measured. Grain interior and grain boundary responses are distinguished in the impedance plots as well as in the real part of conductivity versus frequency plots. The activation energy associated with the motion of Li + ions inside the grains is 0.33 eV while the activation energy corresponding to the total d.c. conductivity changes from 0.36 to 0.47 eV, depending on the relative contribution of grain interior and grain boundary to that parameter.

109Ag NMR investigation of tellurite glasses containing AgI or the (AgI)0.75 (TlI)0.25 eutectic mixture

A 109Ag NMR investigation of [TeO2R AgO0.5](1 − x)[AgI](x) and [TeO2R TlO0.5](1 − x)[AgI](x) glasses, involving either AgI or the (AgI)0.75(TlI)0.25 eutectic mixture, has been carried out. The data analysis suggests a progressive crossing from an oxy-iodide environment of Ag+ ions to a more and more iodide environment as AgI content increases. A quasi-linear dependence of the chemical shift of the 109Ag NMR line (δ) versus the r = (xAg+)AgI(xAg+)tot ratio indicates that a fast exchange of Ag+ ions over different sites is operating at room temperature. The decrease of the 109Ag linewidth observed as xAgI increases agrees with the increase of the Ag+ ion mobility deduced from conductivity data. In the [TeO2 R TlO0.5](1 − x)[(AgI)0.75(TlI)0.25](x) series containing the eutectic mixture, the value of δ (δ = 680 ppm) is independent of xAgI and close to those of α-AgI and β-AgI phases.