Aiming at unraveling the microscopi

Aiming at unraveling the microscopic origin of the degraded performances, various structural and chemical characterizations were performed to the cold sintered LLZO pellets. To start with, X-ray diffraction was performed to investigate whether LLZO remained phase-pure after densification. This behavior was found to depend heavily on how the powder was synthesized. If LLZO was synthesized directly by high-energy planetary mill (Fig. S1), it would decompose significantly after CSP (Fig. S2). This phenomenon may be attributed to the fact that the mechanochemically synthesized phases, especially the amorphous ones at the particle surfaces, are frequently metastable [30]. Nevertheless, the powders used for the experiments in Fig. 1 and Fig. 2 were all synthesized by the conventional solid-state reaction. The acquired phases may largely be considered thermodynamically stable, and no severe decomposition was observed after CSP. As shown in Fig. 3, the X-ray diffraction pattern of the LLZO powder calcined at 1200 °C not only matched very well with the cubic garnet structure, but also showed sharp peaks, indicating good crystallinity. With such powder being used, the X-ray diffraction pattern remained phase-pure after CSP, no matter which transient liquid phase was employed to aid the densification.