Novel Fast Lithium Ion Conduction in Garnet‐Type Li 5 La 3 M 2 O 12 (M = Nb, Ta)

Lithium metal oxides with the nominal composition Li 5 La 3 M 2 O 12 (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid‐state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid‐state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10 −6 S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li 5 La 3 Nb 2 O 12 and Li 5 La 3 Ta 2 O 12 , respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li 14 ZnGe 4 O 16 . Among the investigated materials, the tantalum compound Li 5 La 3 Ta 2 O 12 is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain‐boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.