Recently, much attention has been paid to lithium ion conducting solid electrolytes for their possible applications to high energy density batteries, chemical sensors and separators, and for fundamental research of the transport mechanism.
Most of the cathode materials used in rechargeable lithium batteries possess hexagonal-layered structure or spinel structure. Many cathode materials are rather poor electronic conductors, for example, the electronic conductivity of most promising cathode $LiMn_2O_4$ is $1.4×10^{-4}Scm^{-1}$ at room temperature. Usually, in practice 10 to 50 wt.% electronic conductors such as graphite or carbon black have to be mixed with active oxides. As a result the energy density of the batteries is almost reduced. Therfore, cathode materials with both high ionic and high electronic conductivities are required for large energy density rechargeable batteries.
There are several studies on lithium ion conduction in the perovskite-type oxides, $ABO_3$. Latie et al. have investigated the lithium ion conductivity in the A-site deficient perovskite-type $Ln_{1/3}Li_xNb_{1-x}Ti_xO_3$(Ln=Na, Nd and x≤0.1). It has been reported that lanthanum lithium titanates $(La_{2/3-x}Li_{3x}□_{1/3-2x})TiO_3$ with perovskite-type structure show high lithium ion conductivity, as high as $10^{-3}S cm^{-1}$ at room temperature. This high conductivity is considered to originate from the presence of many equivalent sites and vacancies in the A-sites for lithium ions to occupy and migrate among the lattice of A-sites. Accordingly, the lithium and vacancy concentration strongly influences the conductivity, for example, in $(La_{2/3-x}Li_{3x}□_{1/3-2x})TiO_3$, the conductivity is a parabolic function and has a maximum value. In addition, heat capacites and electric moduli for lanthanum lithium titanate were measured and a glass transition was observed at $T_g=10^2$ ±2K due to the freezing-in of positional disorder of the lithium ions. This suggersts that there is a potentially close relationship between the high ion conductivity and the positional disorder of mobile ions. The smallest cross-sectional area of an interstitial passageway, named the "bottleneck" is located in the space between two adjacent A-sites, which is surrounded by four oxygens. The substitution of the other lanthanide ion (Ln = Pr, Nd and Sm) with smaller ionic radius for La in $La_{1/2}Li_{1/2}TiO_3$ decreases the ionic conductivity, while the substitution of Sr with larger ionic radius for $La_{1/2}Li_{1/2}$ in $La_{1/2}Li_{1/2}TiO_3$ increases the ionic conductivity. These results indicate that the contraction of the A-site space reduces the bottleneck size and consequently disturbs the lithium ion migration via A-site vacancies, while the dilatation of the A-site space increases the bottleneck size and consequently promites ion migration. Applying pressure suppresses the ionic conductivity in lanthanum lithium titanate and likewise the substitution of the smaller ion for La.
Therefore this research was progressed for lanthanum lithium titanate system in three directions that follow. First, the change of ionic conductivity which was caused by substitution Zr, Sn, Mn and Ge for B-site(Ti ion). The Zr and Sn which have large ionic radius caused the decrease in ionic conductivity. The Mn and Ge which have small ionic radius caused the increase in ionic conductivity. Second, aging was done at 1423K for the improvement of grain boundary which has small ionic conductivity compared with grain. And this treatment can help the application of this material. Apparent grain boundary conductivity increased as the increase of aging time. Third, Charge/Discharge property was observed for the application as the cathode material. The best charge and discharge charateristics was obtained when Ge ion was substituted.
