The microstructure of the Li7La3Zr2O12 (LLZO) garnet solid electrolyte is critical for its performance in all-solid-state lithium-ion battery. During conventional high-temperature sintering, second phases are generated at the grain boundaries due to the reaction between sintering aids and LLZO, which have an enormous effect on the performances of LLZO. However, a detailed structure study of the second phases and their impact on physical properties is lacking. Here, crystal structures of the second phases in LLZO pellets are studied in detail by transmission electron microscopy. Three different crystal structures of Li–Al–O second phases, γ-LiAlO2, α-Li5AlO4, and β-Li5AlO4 were identified, and atomic-scale lattice information was obtained by applying low-dose high-resolution imaging for these electron-beam-sensitive second phases. On this basis, the structure–property relationship of these structures was explored. It was found that sintering aids with a higher Li/Al ratio are beneficial to form Li-rich second phases, which result in more highly ionic conductive LLZO.

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Yttrium-doped barium zirconate (BZY) has been considered as a potential electrolyte candidate for intermediate-tolow temperature protonic ceramic fuel cell applications. However, the transport properties of BZY are often limited by the formation of highly resistive space charge zones at lattice discontinuities, such as lattice defects and surfaces. Unlike lattice defects, how to reduce the space charge effects at surfaces remains less explored. In this regard, surface defect engineering can be a meaningful way to regulate the proton transport of BZY by tailoring the space charge distribution close to the surface. Here, the Ar and/or O 2 plasma was used to prepare BZY thin films with different levels of surface defects. The results of electrochemical impedance spectroscopy and detailed structural characterization suggest that the plasma treatment is effective in improving the proton conductivities and lowering the activation energy of BZY thin films through the generation of negatively charged barium vacancy defects and the enrichment of yttrium dopants on the surface.

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