Summary: Garnet-type solid electrolytes have attracted extensive attention for solid-state lithium (Li) batteries. However, the high processing temperature of up to 1,200°C limits large-scale production. Here, we report a simple approach to reduce the sintering temperature by using a conformal nanoscale amorphous alumina coating. The ceramic sintered at 980°C shows a high room temperature ionic conductivity of 0.13 mS cm−1. It reveals that the second phases segregated at grain boundaries are ionically conductive but electronically blocking, which can block electronic conduction and improve mechanical property. The Li symmetry cells indicate a long-term 2,500-cycle life and a high critical current density of 0.52 mA cm−2. The garnet electrolyte enables the high-voltage cells using Li1.2Ni0.2Mn0.6O2 to deliver a high specific capacity of 248 mAh g−1 at 0.05 C. This work provides a new clue for lowering sintering temperature for garnet electrolytes, which can extend to other ceramics toward practical applications.
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