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 Li_1.2Ni_0.2Mn_0.6O₂ 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.

石榴石型固体电解质已引起固态锂（Li）电池的广泛关注。然而，高达 1,200°C 的高加工温度限制了大规模生产。在这里，我们报告了一种通过使用保形纳米级无定形氧化铝涂层来降低烧结温度的简单方法。在 980°C 下烧结的陶瓷显示出 0.13 mS cm ^-1的高室温离子电导率。这表明在晶界处分离的第二相具有离子导电性和电子阻隔性，可以阻滞电子传导并提高机械性能。Li 对称电池具有 2,500 次的长期循环寿命和 0.52 mA cm ^-2的高临界电流密度。石榴石电解质使使用锂的高压电池成为可能_1.2 Ni _0.2 Mn _0.6 O ₂在0.05 C下提供248 mAh g ^-1的高比容量。这项工作为降低石榴石电解质的烧结温度提供了新的线索，可以扩展到其他陶瓷的实际应用。