A New La_2/3Li_xTi_1-xAl_xO₃ Solid Solution:  Structure, Microstructure, and Li⁺ Conductivity

Abstract

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.

Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

“…No impurity phase is found to exit along with the perovskite phase in the studied composition range of y≤0.10 except for y=0, with TiO 2 exiting as a second phase. Cubic superstructure reflection lines, stemming from the ordering of La 3+ , Li + , and vacancies or alternate arrangement of Larich and La-poor layers along C-axis, which nearly doubles c-axis [1,[10][11][12], are observed in all the studied samples and denoted as asterisks in Fig. 1.…”

“…The high-to medium-frequency semicircle can be assigned to grain boundary effect, as the capacitance for this semicircle is in the order of nF [5,14]. It is obvious that grain boundary conductivity is at least two orders magnitude lower than bulk conductivity, which is a characteristic of such perovskite ionic conductors that hurdles its use for its practical applications [5,10,11]. With the use of equivalent circuit to fit the data, bulk conductivity and grain boundary conductivity are determined.…”

“…Of these materials, Li 3x La 2=3Àx □ 1=3À2x TiO 3 (LLT), where □ represents vacancy with a perovskite-related structure, has attracted peculiar attention for its high lithium ion conductivity of 1.0×10 −3 S/cm at room temperature when x≈0.10 [1]. To further improve lithium ion conductivity, much effort has been dedicated to substitute A-site La by other cations, such as other rare earth cations or alkaline earth cations, and/or substitute B-site Ti by other trivalent, tetravalent, pentavalent or hexavalent cations [2][3][4][5][6][7][8][9][10][11][12][13]. So far, only doping Sr up to 5% [12] and partial substitution of Al for Ti [10] are reported to increase the ionic conductivity slightly.…”

Beyond the Structure–Property Relationship Paradigm: Influence of the Crystal Structure and Microstructure on the Li+ Conductivity of La2/3LixTi1−xAlxO3 Oxides

“…[8] However, the size of the domains depends on the composition and the thermal history of the sample. It appears that the intensity of the superlattice reflections in the PXRD patterns is related to the size of the domains in such a way that, broad (almost invisible) peaks are due to a microdomain microstructure and sharp peaks correspond to relatively large domains.…”

“…In this sense, substitution of La and/or Ti by other metal ions has lead to the discovery of a multitude of new systems with different properties according to the type and degree of substitution. [7] Among the systems obtained by substitution of Ti in the lithium-lanthanide-titanates is the La 2/3 Li x Ti 1Àx Al x O 3 (0.06 x 0.3) system, [8] which also has a very high lithium conductivity, though significantly lower than the La 2/3Àx Li 3x TiO 3 oxides. We believe that the reason for having lower conductivity is that the optimum charge carriers/A-cation vacancies ratio is not achieved in the La 2/3 Li x Ti 1Àx Al x O 3 system, but rather that of La 0.56 Li 0.33 + x Ti 1Àx Al x O 3 , which has the highest lithium conductivity reported to date in the literature for a Abstract: The crystal structures of several oxides of the La 2/3 Li x Ti 1Àx Al x O 3 system have been studied by selectedarea electron diffraction, high-resolution transmission electron microscopy, and powder neutron diffraction, and their lithium conductivity has been by complex impedance spectroscopy.…”