Lithium Glasses: Improvements As a Solid Electrolyte,

FRECHERO M. A.

Encuentro; Fourth International Meeting On Dielectric Materials (IMDM’4); 2013

Sciences Faculty Semlalia of Marrakech, the Cadi Ayyad University, the Sciences Faculty of Kenitra, Ibn Tofail University and the laboratory LaMaCoP Sfax, Tunisia

There is no doubt that, in the last decades, the need for a portable power source has increased due to the miniaturization of electronic appliances. Also, the operation of some diminutive electric devices has been possible thanks to the development of the technology for very little power sources which require, in many cases, low power and, because of this, the science of the semiconductor materials has made a remarkable progress. The demand for batteries with high energy densities has inevitably led to the research and development of systems involving thermodynamically more stable than aqueous electrolytes. The opportunities for extreme miniaturization and for very simple production techniques are of obvious importance in applications and reliability, which are key factors. A solid electrolyte is a phase which has an electric conductance wholly due to ionic motion within the solid. Such phases have been known for over a century, and until recently all known materials of this type had high resistivity at room temperatures. Nevertheless, what is needed, in general, is an electrolyte with high conductivity at room temperature and a high decomposition potential to open up a wide range of applications. Many compounds have been studied with these goals in mind and many of these materials in the ionic conductivity field have the spotlights on the lithium cation. This ion has some very attractive properties as a charge carrier; just to mention a few, we can highlight that it is the metallic cation with the lowest atomic weight: solid electrolytes with lithium as the mobile specie have high energy densities, which allows to develop light and compact batteries with high voltages and low discharge level, which in turn permits to extend the lifetime of an electric device as well. In general, the solid electrolytes can be crystalline or amorphous and, at least, an ion in them is very mobile. In order to consider if they are good enough as electrical conductors, they must be compared with liquid strong electrolytes whose conductivities are near to 10-3 -101 S.cm-1 at room temperature. When we consider solid electrolytes, we usually think of crystalline solids, which in general have hard packing structures with well establish conduction paths for the charge carriers. As a consequence of the latter, their conduction activation energies are on the order of 1 eV (i.e. 100 KJ.mol-1) or higher. In the light of these facts, the scientist in this field have long been searching for similar materials with less structural order to make easier the ion movement through the structure. Then, the glassy systems appeared during the second half of the last century as the ideal solution to this problem. Considering now the lithium glasses, it may be said that they are typically made of oxides or sulfides. These kinds of materials have many advantages over the above mentioned, because they are easier to manufacture, their properties are isotropic, and they have a huge range of possibilities in their compositions. But, they still have a problem: their low conductivity at room temperature (10-6 – 10-3 S.cm-1). To make a glassy solid electrolyte we essentially need two or three fundamental components. They are: first, one that is able to give a glassy matrix, as for example, the ancient well-known SiO2. The truth is that there are many other oxides that are able to originate a glassy matrix: B2O3, GeO2, As2O3, TeO2, P2O5, etc. But, in some cases some oxides need a very little quantity of other specie -a different oxide- to find the way to generate the glass, i.e. to avoid the crystalline structure. This would be the second component and its function is in between modification and formation of the glass matrix (e.g. transition metal oxides). And finally as the third one, we need a strictly modifier oxide whose function is to provide the mobile cations in the glass matrix, which will be responsible for the ion conductivity. This description of the composition is similar to sulfides glasses. The most common method used to manufacture glasses is “the quenching”, i.e. the rapid cooling of the appropriate composition of their melting oxides. The temperature jump is decisive to obtain an amorphous solid, i.e. a glass system. This thermal history in the manufactured process is relevant in some thermodynamics glassy properties, and then it is important to know it. We review our results of the last ten years research in tellurite glassy systems where the main ionic carrier is the Lithium cation. The main objectives in our work have been to improve not only the ionic conductivity but also some other properties in the glassy matrix, e.g. glass transition temperature, enthalpy of matrix relaxation, etc. In addition, we have explored the relationship between the ionic conductivity and the main characteristics of the “structure” of the glassy matrix at atomic level, short range order and even at intermediate range order. In order to face this task we have employed different techniques as: Fourier Transform Infrared spectroscopy, X Ray Diffraction, Differential Scanning Calorimetric, Density Measures and Dielectric Spectroscopy. Particularly, we have studied all these properties in glassy systems of formula: x Li2O. (1-x) V2O5. 2 TeO2 ; x Li2O. (1-x) [0.50 V2O5..0.50 MoO3] 2 TeO2. Also, we have studied the partial replacement of the lithium oxide by the silver oxide in order to confirm the influence of the repolimerization of the glassy matrix in the ionic conductivity. To evaluate the relative magnitude of the repolimerization by different modifier oxides, we have measured the systems densities and what we learned from these results was that, as a natural consequence of its small size, the lithium cation involves a high repolimerization in the glassy matrix with a direct influence in the cation mobility. On the other hand, we have also studied the influence in the ionic conductivity when the ratio V/Mo is changed. We have found that it has a moderate influence especially in the activation energy of the conductivity mechanism, but it is almost negligible in the measured values of the conductivity. Furthermore, we have compared the electrical response of the lithium cation as the charge carrier in these glassy matrixes to the electrical responses with other charge carriers like sodium cation and silver cation. Also we have studied, the mixed mobile ion effect in these systems, i.e. when lithium cation is in presence of another cation of one charge, as for example, silver cation. In this case we have found that the behavior was very different compared to the mixed alkaline effect, i.e. when lithium and sodium are together, actually the former was weaker than the latter. Finally, from the microscopic results like FTIR, we have learned that different modifier oxides involve different local order, essentially, in the kind of tellurium polyhedron mainly present in the glassy matrix. We are interested in this kind of knowledge because it will be useful to understand how to control the structure in the middle-range order, which is relevant in the electrical conduction process and as a consequence, in the technological applications of these materials as solid electrolytes.