MATERIALS DESIGN OF LITHIUM IONIC CONDUCTOR WITH THE SPINEL STRUCTURE. Ryoji Kanno, Department of Chemistry, Faculty of Science, Kobe University, Kobe 657 Japan

Materials, so-called "ionic conductors" or "solid electrolytes", show high ionic diffusion in solids and are promising electrolyte systems for future lithium batteries. Materials design of the ionic conductors was initially based on the classical ionic conductor, alpha-AgI; high ionic conductivity at room-temperature has been obtained for silver and copper electrolytes. However, lithium ionic conductivity is still low at room-temperature and it is still necessary to search for new lithium system. Ionic conductors usually show a transformation from low to high ionic conduction state. By considering the phenomena, we might have an idea for designing new materials with high ionic conductivity. In the present paper, materials syntheses and the transformation from low to high ionic conduction states are discussed for lithium halide spinels, oxide spinels, and other materials based on a cubic close packed anion array.

The close packed anion array may be considered as a framework structure in which mobile species can diffuse. The halide spinels, Li₂MX₄ (M=transition metal), oxide spinels, LiM₂O₄, and other layered rocksalt-type oxides, LiMO₂ may have similar diffusion pathways. Among these, the halide system is more suitable for high lithium ionic diffusion because of the larger ionic radii and larger polarizabilities of anions. High-temperature behavior of these materials is studied using conductivity measurements, and X-ray and neutron diffraction methods.

The halide spinels show a gradual slope change in the Arrhenius conductivity plots which corresponds to a transition from low to high ionic conducting state. The lithium ions have gradually left their normal site to the interstitial site, which may be common phenomena for famous high ionic conductors such as PbF₂ or Rb₄Cu₁₆I₇Cl₁₃. Further, the transition to high ionic conducting states are discussed in comparison with other materials showing first order phase transition.