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c-LLZO) is a promising Li + ion conductor for applications as a ceramic solid electrolyte in next generation high safety lithium batteries. The sintering temperature of c-LLZO is usually higher than 1100 °C, where Li-loss is severe, especially in conventional air ambient sintering method. Covering the green body with "mother powder" is often adopted for compensating the Li-loss. The mother powder having the same composition as the green body cannot be repeatedly use, which raises the cost of the c-LLZO ceramics. A self-compensating Li-loss method without mother powder is proposed and investigated to prepare high-quality c-LLZO ceramics. In this method, excess lithium is added to c-LLZO green pellets to self-compensate Li-loss at high temperature. The impact of different amounts of excess Li and crucible material, such as Pt, MgO, Al 2 O 3 , and ZrO 2 is studied. With optimized such sintering method, Ta doped LLZO pellets with 10% excess Li can be well sintered inside low-cost MgO crucible without mother powder at 1250 °C for only 40 min and laboratory scale production is demonstrated. The ceramics have relative densities of ∼96%, conductivities of ∼6.47 × 10 −4 S cm −1 and critical current density of 1.15 mA cm −2 at 25 °C, which is fundamental for further researches on solid-state batteries.

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Solid‐state batteries (SSBs) promise high energy density and strong safety due to using nonflammable solid‐state electrolytes (SSEs) and high‐capacity Li metal anode. Ta‐substituted Li7La3Zr2O12 (LLZT) SSE possesses superior ionic conductivity and stability with Li metal, yet the interfacial compatibility and lithium dendrite hazards still hinder its applications. Herein, an interfacial engineering is demonstrated by facile acid‐salt (AS) treatment on LLZT, constructing a 3D cross‐linking LiF‐LiCl (CF) network. Such structure facilitates Li wetting via capillary permeation. Notably, CF as electronically insulting phases block the electrons through the interface and ulteriorly suppress the dendrite formation. The assembled Li symmetric cell exhibited a low interfacial impedance (11.6 Ω cm2) and high critical current densities (CCDs) in the time‐constant mode, 1.8 mA cm−2 at 25 °C and 3.6 mA cm−2 at 60 °C, respectively. Meanwhile, by exploring the capacity‐constant mode of CCD measurement, the concept of critical areal capacity (CAC) is first proposed, obtaining its values of ≈0.5 mAh cm−2 at 25 °C and 1.2 mAh cm−2 at 60 °C. Moreover, the safety‐enhanced hybrid SSBs matched with LiFePO4 and LiNi0.6Co0.2Mn0.2O2 deliver a remarkable rate and cycling performances, validating the feasibility of this interfacial engineering in various SSB systems.