## 3 Results and Discussion
### Ionic Conductivity of LLZO-IL Composites  
An error of our set up of ionic conductivity measurement was estimated to be 2.9% from repeated measurements using three cells having LLZO-15 wt% FSI IL as a standard material. This value was used to provide error bars in the figures in this section. Ionic conductivity was measured by changing three different factors: pressure for the pellet preparation (1-4 tons and also hand press), exposure time (1-24 h), and temperatures (room temperature-100 \({}^{\circ}\)C). Thickness of pellets prepared at different pressures is summarized in Table S1. Figure 5 reports the ionic conductivity of LLZO-IL composites pressed with different pressure, and that of Al\({}_{2}\)O\({}_{3}\)-FSI IL composites is shown as a reference. The concentration of the ILs was fixed to be 15 wt%. The Nyquist plot of pristine  
Figure 3: (**a**) DSC and (**b**) TGA curves of LLZO–15 wt% FSI IL (red), LLZO–15 wt% DFOB IL (blue), pristine FSI IL (orange), and pristine DFOB IL (green). LLZO, pressed with 1 ton, is shown in the supplementary information (Figure S3). One large semicircle was observed and the ionic conductivity estimated from the diameter of the semicircle was \(1.1\times 10^{-9}\) S cm\({}^{-1}\). When LLZO was integrated with the ILs, the conductivity values increased to the order of \(10^{-4}\) S cm\({}^{-1}\) as shown in Figure 5 even when the pellets were prepared by hand pressing. In these cases, semicircle was not observed in the Nyquist plots (Figure S4), confirming that the grain boundary resistances were eliminated by ILs. The ionic conductivity of the LLZO composites was slightly lower compared with that of the pristine ILs (3.5 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\) for FSI IL and 0.7 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\) for DFOB IL). The conductivity values of our LLZO-IL composites are comparable to other LLZO composites with conventional ILs such as \(N\)-butyl-N-methylpyrrolidinium bis(trifluoromethylomentanesulfonyl)imide ([Py\({}_{14}\)][TFSI]) as summarized in Table 1 and even with the composites based on LAGP containing 50 wt% IL. When FSI IL was added to LLZO, the conductivity increased by making the pellet with 1 ton instead of hand press, and the increased value was maintained until the pressure was increased to 4 ton. In the case of LLZO-DFOB IL composites, the conductivity values did not increase even when the pressure was increased to 2 ton, and instead they rather decreased above 3 ton, which suggests the bleed out of DFOB IL. This was not observed in the case of FSI IL, which is probably because of the presence of physical interaction among the components. In the DSC curve of LLZO-FSI IL (Figure 3a), the melting peak  
Figure 4: ATR-FTIR spectra of (**a**) pristine FSI IL, (**b**) LLZO–15 wt% FSI IL, (**c**) pristine DFOB IL, and (**d**) LLZO–15 wt% DFOB IL. Figure 5: Ionic conductivity of LLZO–15 wt% FSI IL composite (red) and LLZO–15 wt% DFOB IL composite (blue) as a function of the pressure to make electrolyte pellets. The conductivity of Al\({}_{2}\)O\({}_{3}\)-15 wt% FSI IL composites is also shown as a reference (black). arising from FSI IL was intense compared with that in the DSC curve of pristine ILs, suggesting enhanced structural order of the IL in the composites. In addition, in the infrared spectroscopy, the dominant conformer of FSI anion was found to change from trans to cis with the presence of LLZO. Taking all these results into account, we consider that FSI anion interacts more with LLZO than DFOB anion. The reference sample based on a non-ion conductive ceramic, specifically Al\({}_{2}\)O\({}_{3}\), and 15 wt% FSI IL exhibited ionic conductivity around 10\({}^{-6}\) S cm\({}^{-1}\), which suggests not all combinations of ceramic and ILs are suitable as hybrid electrolytes.