Solid-state batteries with desirable advantages, including high-energy density, wide temperature tolerance, and fewer safety-concerns, have been considered as a promising energy storage technology to replace organic liquid electrolyte-dominated Li-ion batteries. Solid-state electrolytes (SSEs) as the most critical component in solid-state batteries largely lead the future battery development. Among different types of solid-state electrolytes, garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolytes have particularly high ionic conductivity (10–3 to 10–4 S/cm) and good chemical stability against Li metal, offering a great opportunity for solid-state Li-metal batteries. Since the discovery of garnet-type LLZO in 2007, there has been an increasing interest in the development of garnet-type solid-state electrolytes and all solid-state batteries. Garnet-type electrolyte has been considered one of the most promising and important solid-state electrolytes for batteries with potential benefits in energy density, electrochemical stability, high temperature stability, and safety. In this Review, we will survey recent development of garnet-type LLZO electrolytes with discussions of experimental studies and theoretical results in parallel, LLZO electrolyte synthesis strategies and modifications, stability of garnet solid electrolytes/electrodes, emerging nanostructure designs, degradation mechanisms and mitigations, and battery architectures and integrations. We will also provide a target-oriented research overview of garnet-type LLZO electrolyte and its application in various types of solid-state battery concepts (e.g., Li-ion, Li–S, and Li–air), and we will show opportunities and perspectives as guides for future development of solid electrolytes and solid-state batteries.

Chemical stability of garnet-type lithium ion conductors is one of the critical issues in their application in all-solid-state batteries. Here, we conducted quantitative analysis of impurity layers on the garnet-type solid electrolytes, Li6.5La3–x AE x Zr1.5–x Ta0.5+x O12 (x = 0 and 0.1; AE = Ca, Sr, and Ba), by means of X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Two complimentary XPS techniques were employed: (i) background analyses by Tougaard’s method and (ii) relative intensity analyses of La 3d/La 4d spectra to determine the surface chemical composition. XPS revealed that even after cleaning by annealing and polishing, the surface is covered by LiOH- and Li2CO3-based compounds with a thickness of 4–6 nm within 30 min as a result of the reaction with traces of H2O (<0.5 ppm) and CO2 (<5 ppm) in an Ar-filled glovebox. The sensitivity to H2O and CO2 depends on the basicity of dopants. Ba-doped solid electrolytes exhibited the thickest impurity layers compared to Sr- and Ca-doped compounds. A surface cleaning process, consisting of annealing and polishing, effectively reduces the charge-transfer resistance to 10–15 Ω cm2 because of negligible impurity layers. Highest short-circuit tolerance is obtained for a 700 °C annealed specimen (critical current density: 0.5 mA cm–2), which is possibly due to the strengthened grain boundaries by Li2CO3 among grains around its melting point.

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Schematic representation of Li growth during DC polarization experiments in the investigated garnet-type metal oxides.