## 3 Results and Discussion  
### Fundamental Properties of LLZO-IL Composites  
Figure 2 is the XRD pattern of LLZO and its composites containing 15 wt% ILs. The diffraction pattern of Al-doped LLZO, taken from the reference [27], is also shown as a reference. The slope in the background at lower angles is due to Kapton. In the pattern of pristine LLZO, typical signals of a cubic phase of Al-doped LLZO are observed. It is known that the cubic structure of LLZO needs to be maintained to realize its high ionic conductivity [28]. The XRD patterns of LLZO-IL composites are identical to that of the pristine LLZO, confirming the crystalline structure of LLZO is stable and not affected by the presence of the ILs. This suggests that unfavorable reactions, such as dissolution of metal cations by the ILs, did not occur during the synthesis of the composites. Figure 3 shows thermal properties of the LLZO-IL composites. As shown in the DSC curve of FSI IL (orange) and its LLZO composite (red), a glass transition temperature (\(T_{\text{g}}\)) of pure FSI IL was around \(-95\)\({}^{\circ}\)C and remained the same even when the IL was mixed with LLZO. A melting temperature (\(T_{\text{m}}\)) of FSI IL was observed at a constant temperature of c.a. \(-15\)\({}^{\circ}\)C regardless of the presence of LLZO. With the \(T_{\text{m}}\), however, the peak intensity was more pronounced with LLZO. The change in peak intensity but not in peak position suggests that the degree of structural order of the IL is enhanced in the LLZO composites, which is most likely due to physical interactions between LLZO and FSI IL (will be discussed subsequently), but this interaction does not affect the dissociation rate of the IL. This increase was not observed when DFOB IL was used. Both \(T_{\text{g}}\) and \(T_{\text{m}}\) assigned to DFOB IL were observed at the same temperatures and became minor when the IL was mixed with LLZO. Results of TGA are shown in Figure 3b. The curve in the range of 500-700 \({}^{\circ}\)C is not shown because the weight change in this range was negligible (<0.01 wt% per min). As reported in the literature, LLZO is thermally stable and its decomposition temperature is above 1000 \({}^{\circ}\)C [7]. Similarly, a huge weight loss was not observed in the curve of the pristine LLZO (black), but slight weight losses were visible at 200 and 400 \({}^{\circ}\)C (see Figure S1), which are expected to be assigned to the decomposition of impurities such as La(NO\({}_{3}\))\({}_{3}\)[29]. For the composites with 15 wt% FSI IL and DFOB IL, weight losses at 500 \({}^{\circ}\)C were found to be 9.8% and 12.7%, respectively. These losses are expected to be due to the decomposition of the organic components, i.e., ILs. However, the weight losses were observed at 200 and 400 \({}^{\circ}\)C (see red and blue curves), which were different from the decomposition temperatures of pure ILs. They are rather similar to those of LLZO. This fact suggests that reactive gases, such as HNO\({}_{3}\) and NO\({}_{x}\)[30], are formed due to the presence of  
Figure 2: XRD curves of (**a**) LLZO and its composites with (**b**) 15 wt% FSI IL and (**c**) 15 wt% DFOB IL. The reference pattern (bar) is taken from the reference [27]. a trace of metal nitrate in LLZO and cause the decomposition of ILs at lower temperatures. Although the decomposition temperatures of LLZO-IL composites are lower compared to that of pure LLZO, the composites can be confirmed to be stable in the temperature range of battery operation. In Figure 4, ATR-FTIR spectra of the composites are shown. The samples are pristine FSI IL, LLZO-15 wt% FSI IL, pristine DFOB IL, and LLZO-15 wt% DFOB IL. In the upper two curves for the materials with FSI anion, strong signals are attributable as follows: 1383, 1364, 1218, and 1180 cm\({}^{-1}\) for SO\({}_{2}\) str; 1106 cm\({}^{-1}\) for S-N-S str; 831 and 750 cm\({}^{-1}\) for S-F str [31,32]. The minor signals at 1474 and 1457 cm\({}^{-1}\) (broken red lines) as well as those in the red square are assigned to the cation structure [33].