Li₆ALa₂Nb₂O₁₂ (A=Ca, Sr, Ba): A New Class of Fast Lithium Ion Conductors with Garnet‐Like Structure

Abstract

Garnet‐like structured metal oxides with the general formula Li6ALa2Nb2O12 (A=Ca, Sr, Ba) have been prepared by solid‐state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X‐ray diffraction data reveal that Li6ALa2Nb2O12 compounds are isostructural with the parent compound Li5La3Nb2O12. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca‐substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr‐ and Ba‐substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (∼14% of the total resistance at 20°C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba‐compound Li6BaLa2Nb2O12 shows the highest ionic conductivity of 6 × 10−6 S/cm at room temperature (22°C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca‐ and Sr‐ compounds, respectively. The ionic conductivity is comparable with that of parent Li5La3Nb2O12 and other fast lithium ion conductors known so far.

Structural limitations for optimizing garnet-type solid electrolytes: a perspective

“…In general, researchers have focused on two main approaches to increase the ionic conductivity in a material: through an increase of the Li + concentration or an increase of the mobility of the cations. Commonly, aliovalent substitution is used to increase the Li + content in order to optimize the ionic performance, for instance in Li 7 14,[33][34][35][36][37][38] While this substitution leads to an increased ionic conductivity due to a higher lithium content, the nature of the alkaline earth cations seem to have an effect on the conductivity as well. An increase in the ionic radius of the 8-fold coordinated site increases the ionic conductivity.…”

Li+ transport properties of W substituted Li7La3Zr2O12 cubic lithium garnets

“…A series of garnet-like solid electrolytes have been reported as a novel family of fast lithium ion conductors by Weppner and his group. [1][2][3][4][5][6][7][8][9][10] Among them, Li 7 La 3 Zr 2 O 12 (LLZ) sintered at 1230 • C reported by Murugan et al has received considerable importance in recent times as result of its high total (bulk + grain boundary) ionic conductivity of 5 × 10 −4 S cm −1 at 25 • C, combined with good chemical stability against lithium metal and commercial electrodes. 5 Li 7 La 3 Zr 2 O 12 (LLZ) exists in tetragonal and cubic phase.…”

Ionics—a key technology for our energy and environmental needs on the rise

“…Furthermore, these compounds have decomposition voltages of >6 V. The nominal chemical composition of the garnet-like structured compounds is Li 5 La 3 M 2 O 12 (M=Nb, Ta) [9]. Partial substitution of the lanthanum by alkaline earth elements with a lower valency results in the composition Li 6 ALa 2 M 2 O 12 (A=Ca, Sr, Ba; M=Nb, Ta) [10]. The member Li 6 BaLa 2 Ta 2 O 12 provides the highest conductivity among all lithium ion conductors known so far with a conductivity of 4×10 −5 S/ cm and an activation energy of 0.40 eV [11].…”