Rechargeable (secondary) all-solid-state lithium batteries are considered to be the next-generation high-performance power sources and are believed to have remarkable advantages over already commercialized lithium ion batteries utilizing aprotic-solution, gel, or polymeric electrolytes with regard to battery miniaturization, high-temperature stability, energy density, and battery safety. Solid electrolytes with high Li ion conductivity but negligible electronic conductivity, with stability against chemical reactions with elemental Li (or Limetal alloys) as the negative electrode (anode) and Co-, Ni-, or Mn-containing oxides as the positive electrode (cathode), and with decomposition voltages higher than 5.5 V against elemental Li are especially useful to achieve high energy and power densities as well as long-term stability.Lithium ion conduction has been reported for a wide range of crystalline metal oxides and halides with different types of structures. [1,2] In general, oxide materials are believed to be superior to non-oxide materials for reasons of handling and mechanical, chemical, and electrochemical stability.[1] So far, most of the discovered inorganic lithium ion conductors have had either high ionic conductivity or high electrochemical stability, but not both. Some oxides are excellent lithium ion conductors; for example, Li 3x La (2/3)Àx & (1/3)À2x TiO 3 (0 < x < 0.16; "LLT"; & represents a vacancy) exhibits a bulk conductivity of 10 À3 S cm À1 and a total (bulk + grain-boundary) conductivity of 7 10 À5 S cm À1 at 27 8C and x % 0.1. However, this compound becomes predominantly electronically conducting within the lithium activity range given by the two electrodes.[3] It has been attempted to replace the transition metal Ti in LLT with Zr, which is fixed-valent and more stable (against chemical reaction with elemental lithium); however, this attempt was unsuccessful owing to the ready formation of the pyrochlore phase La 2 Zr 2 O 7 . [4] Although a large number of possible lithium electrolytes have been reported for the Li 2 O-ZrO 2 system, none of them is suitable for battery applications because of their low conductivity and sensitivity to air. [5] A novel class of fast lithium ion conducting metal oxides with the nominal chemical composition Li 5 La 3 M 2 O 12 (M = Nb, Ta), possessing a garnet-related structure, has been reported from our laboratory.[6] The bond-valence analysis of Li + ion distribution confirms transport pathways which relate to the experimentally observed high Li + ion conductivity, and the Li + ions are predicted to move in a 3D network of energetically equivalent, partially occupied sites. [7] Li 5 La 3 M 2 O 12 (M = Nb, Ta) were the first examples of fast lithium ion conductors possessing garnet-like structures and gave rise to further investigations of conductivity optimization by chemical substitutions and structural modifications. [8, 9] Among the investigated compounds with garnet-related structures, Li 6 BaLa 2 Ta 2 O 12 exhibited the highest Li + ion conductivity of 4 ...