| Summary: | A solid-state electrolyte with an ionic conductivity comparable to that of a liquid electrolyte is demanded of all-solid-state lithium-ion batteries. Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO) is considered to be a promising candidate due to its good thermal stability, high ionic conductivity, and wide electrochemical window. However, the synthesis of a stable cubic-phase LLZO thin film with enhanced densification at a relatively low thermal treatment temperature is yet to be developed. Indium is predicted to be a possible dopant to stabilize the cubic-phase LLZO (c-LLZO). Herein, via a nanolayer stacking process, a LLZO–Li<sub>2</sub>CO<sub>3</sub>–In<sub>2</sub>O<sub>3</sub> multilayer solid electrolyte precursor was obtained. After thermal annealing at different temperatures, the effects of indium doping on the formation of c-LLZO and the ionic conductivities of the prepared LLZO–LZO thin film were systematically investigated. The highest ionic conductivity of 9.6 × 10<sup>−6</sup> S·cm<sup>–1</sup> was obtained at an annealing temperature of 800 °C because the incorporation of indium promoted the formation of c-LLZO and the highly conductive LLZO–LZO interfaces. At the end, a model of LLZO–LZO interface-enhancing ionic conductivity was proposed. This work provides a new approach for the development of low-temperature LLZO-based, solid-state thin-film batteries.
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