Although a relatively high density can be reached by CSP, the solid electrolytes also need to show decent ionic conductivity. Considering that the grain boundaries formed by CSP would likely differ from those produced by conventional sintering and influence the total ionic conductivity [20], electrochemical impedance spectroscopy (EIS) measurement was carried out. The densest ceramic, i.e., the one showing 87.7% relative density in Fig.1b, was selected for study. It was found that CSP not only altered the total conductivity significantly, but also introduced unexpected distraction for measurement. If the pellet was stored in ambient air before measurement like the conventionally sintered ceramics [11], the ionic conductivity comparable to such ceramics, i.e., 10-4 S cm-1, can be reached. Nevertheless, preventing the air exposure led to completely different results. CSP typically needs to be followed by a baking step to remove the residual water [25]. If the pellet was stored in Ar atmosphere right after baking, it only showed a very low conductivity of 3.38 × 10-9 S cm-1 at 20 °C. Exposing the sample to ambient atmosphere gradually increased the conductivity. As shown in Fig. 2a, only 3-minute air exposure reduced the resistance significantly. After 24 hours of exposure, the conductivity reached 1.08 × 10-5 S cm-1. Clearly, the high ionic conductivity of 10-5-10-4 S cm-1 was not intrinsic to the CSP pellet, but arose from air exposure. It is possible that CSP had introduced a hygroscopic component into LLZO, and the increased ionic conductivity resulted from the absorbed moisture. This speculation was supported by the temperature-dependent EIS measurement (Fig. 2b). As temperature increased, the pellet after prolonged air exposure showed a decreased conductivity at first, instead of the Arrhenius behavior. Given that the absorbed water may dissociate from the material upon heating and took away the misleading conductivity enhancement, this phenomenon would be understandable. When the temperature was sufficiently high to remove most water, an Arrhenius behavior emerged. In order to eliminate the distraction from moisture, the temperature dependent ionic conductivity measurement was conducted on the CSP pellet that had never been exposed to air after baking. Instead of showing the initial conductivity drop, a typical Arrhenius behavior was observed at all temperatures studied. From such measurement, the room-temperature conductivity (3.38 × 10-9 S cm-1) was found to be far lower than that of the conventionally sintered LLZO (10-4 S cm-1), and the activation energy (0.64 eV) was also higher than the reported value (~ 0.30 eV) [11]. Clearly, CSP had rendered LLZO much less conductive and highly air sensitive, although a relatively high density of 87.7% can be reached at an extremely low temperature of 350 °C.