The stoichiometry range, crystal chemistry, ionic conductivity, and electrochemical window of the La 2/3 Li x Ti 1-x Al x O 3 system with perovskite-related structure have been studied. The range of the existence of the solid solution is (0.06 ) x ) 0.3). Powder X-ray diffraction and transmission electron microscopy results show that these oxides have a unit cell multiple of the perovskite cell with dimensions a ≈ 2a p , b ≈ 2a p , c ≈ 2a p . The ionic conductivity of the materials and its dependence on composition and temperature have been examined by complex impedance spectroscopy. Lithium ion conductivity increases with increasing lithium content up to a value (7.6 × 10 -5 S cm -1 at room temperature) corresponding to x ) 0.25. Electrochemical experiments indicate that these materials can be used as solid electrolytes in secondary batteries having Li metal as the anode down to 1.6 V.

We have found that for the lanthanum/lithium titanates and related materials with a perovskite-related structure, ABO3, optimum lithium conductivity takes place for a particular concentration of A-cation vacancies (□) equal to 8%. We have prepared a new oxide with the highest lithium conductivity reported to date for a crystalline material (σ295 K = 2.95 × 10-3 Scm-1). This is La0.56Li0.36□0.08Ti0.97Al0.03O3, the x = 0.03 composition in the solid solution of general formula La0.56Li0.33+ x □0.11 - x Ti1 - x Al x O3 (0 ≤ x ≤ 0.06). A similar situation is obtained for x = 0.03 in the related system La0.56 - x Sr x Li0.33+ x □0.11 - x TiO3, where σ295 K = 2.54 × 10-3 Scm-1. We have also prepared a material with the highest known conductivity (σ295K = 1.9 × 10-3 Scm-1) in the “classical” La2/3 - x Li3 x □1/3 - 2 x TiO3 system and it also corresponds to the same A-cation deficiency. Optimization of Li conductivity requires then both the control of the charge carriers/vacancies ratio as well as shortening of the B−O distances or increasing of the A−O distances. 3-D microtwinning is observed in the crystals of all these oxides, which have a ≈√2a p × ≈√2a p × ≈2a p perovskite superstructure.

The very high value of ionic conductivity at room temperature reported for La0.51Li0.34TiO2.94 some years ago did originate a great deal of interest in the study of materials of general formula La2/3–xLi3xTiO3 (LaLiTiO) and related systems. These oxides have shown to be potential solid electrolytes for lithium secondary batteries, but other aspects, apart from the conducting properties, such as their dielectric behaviour and crystal structure, have been the main focus of important studies in the area of inorganic solid‐state chemistry. LaLiTiO‐related compounds have the perovskite‐type structure (ABO3) with A‐cation ordering. However, essential details of their crystal structure, as for instance the location of the lithium atoms, are still under discussion and are the subject of current work. We show in this short review of mainly our own work that the complex microstructure of these materials, which has been studied in detail by transmission electron microscopy, is the cause of the difficulties on the precise determination of their crystal structure. We have developed a new approach for crystal structure refinement, which takes into account the microstructural effects, obtaining significantly better results than conventional methods of refinement. The microstructure of these oxides also affects their conducting and dielectric properties. Therefore, different parameters such as composition and microstructure must be considered to understand and, eventually, optimise, as we have done, the conducting properties of LaLiTiO‐related systems.

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The stoichiometry range, crystal chemistry, ionic conductivity, and electrochemical window of the La2/3 - x Sr x Li x TiO3 system with a perovskite-related structure have been studied. The range of existence of the solid solution is (0.06 ≤ x ≤ 0.3). Powder X-ray diffraction and transmission electron microscopy results show that these oxides have a unit cell multiple of the perovskite cell with dimensions a ≈ √2 ap, b ≈ √2 ap, and c ≈ 2ap. The ionic conductivity of the material and its dependence on composition and temperature have been examined by complex impedance spectroscopy. Lithium ion conductivity increases with increasing lithium content up to the value 7.12 10-5 S·cm-1 at room temperature, corresponding to x = 0.25. Electrochemical experiments indicate that these materials can be used as solid electrolytes in secondary batteries having Li metal as the anode down to 1.75 V.