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For transportation applications, the safety of lithium ion batteries is an important problem demanding further progress. A promising approach is the replacement of flammable liquid electrolytes with non-flammable solid electrolytes. Solid electrolytes can also block dendrites, solving another important problem in Li metal batteries. Solid lithium electrolytes have been studied extensively and many classes of promising materials have been identified, including sulfides, oxides, ceramics, inorganic glasses, and polymers. Several reviews thoroughly discuss these classes of materials. [1] Recently, the Li 7 La 3 Zr 2 O 12 (LLZO) ceramic electrolyte with a garnet crystal structure has emerged as a promising solid electrolyte material. Favorable properties include high ionic conductivity (ca. 4 10 À4 S cm À1 at 25 8C), low electronic conductivity (less than or equal to 10 À8 S cm À1 ), electrochemical stability with Li metal, and thermal and chemical stability. [2] Stability with Li metal is an especially important feature, which combined with the high lithium conductivities, sets this material apart from other solid Li-ion conductors.The total conductivity of polycrystalline oxide conductors is a function of both lattice and grain boundary contributions. Thus, it is important to have knowledge of both the lattice and grain boundary behavior. Several groups report consistent total Li + conductivities for cubic polycrystalline LLZO samples; [2b, 3] however, there is less clarity on the relative contributions of the grains and grain boundaries. For instance, Murugan et al. show comparable grain and grain boundary resistances over a limited temperature range. [2b] In other studies, it is difficult to resolve impedance responses due to grains and grain boundaries. [3c,e] Given the importance of this material, it is critical that these conduction processes be thoroughly characterized. This paper reports thorough characterization of LLZO by broadband impedance spectroscopy. Impedance spectroscopy was performed at temperatures from À100 to + 60 8C with the frequency of the applied sinusoidal voltage varied from 10 9 to 10 À2 Hz. This frequency range is two to three orders magnitude broader than other reports, and the temperature window is larger as well. By controlling these two experimental variables, the contributions to total conductivity have been elucidated.LLZO is synthesized by hot isotactic pressing, yielding dense membranes with high-fidelity grain boundaries as previously reported. [3e] Aluminum is intentionally doped into the crystal lattice to stabilize the cubic phase; the nominal composition is Li 6.28 Al 0.24 La 3 Zr 2 O 12. The XRD diffraction pattern for this composition is presented in Figure 1. The absence of doublet peak reflections is indicative of the pure phase cubic structure. [3b] A SEM micrograph of the LLZO fracture surface is provided in the Figure 1 inset. The solid-state reaction and hot isotactic pressing densification protocols yielded dense LLZO membranes with few observable voids. The...

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