## 2 Materials and Methods  
Two types of ILs, namely \(N\)-ethoxy-\(N\)-methylpiperidinium bis(fluorosulfonyl)imide (FSI IL) and \(N\)-ethoxy-\(N\)-methylpiperidinium difluoro(oxalato)borate (DFOB IL) (Figure 1), were synthesized as reported in our previous papers [23; 24]. Solid electrolyte, Al-doped Li\({}_{7}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\), more specifically Li\({}_{6.24}\)La\({}_{3}\)Zr\({}_{2}\)Al\({}_{0.24}\)O\({}_{11.98}\) (hereafter LLZO), was purchased from NEI Corporation (Somerset, New Jersey, USA). Dried ILs were added to LLZO and mixed by manual grinding using agate mortar in an Ar-filled glove box (Jacomex, Dagneux, France, O\({}_{2}\) and H\({}_{2}\)O < 10 ppm). According to the literature [15], concentration of ILs was fixed to 15 wt% to keep the hybrid materials in a solid state. For several electrochemical analyses, ILs containing a lithium salt were also used. Specifically, LiFSI and FSI IL, as well as LiDFOB and DFOB IL, both mixed at a ratio of 1:9 mol/mol [25; 26], were individually added to LLZO. X-ray diffraction (XRD) analysis was carried out using a Rigaku d-max Ultima + diffractometer equipped with a CuK\(\alpha\) source (Rigaku, Tokyo, Japan). The samples were prepared in the Ar-filled dry box and covered with a Kapton tape. The spectra were recorded from 20\({}^{\circ}\) to 60\({}^{\circ}\) with a scan rate of 0.5\({}^{\circ}\) min\({}^{-1}\). Differential scanning calorimetry (DSC) was performed using a Mettler-Toledo DSC 821 under a N\({}_{2}\) flow (Mettler-Toledo, Columbus, OH, USA). The samples were cooled from room temperature to \(-120\)\({}^{\circ}\)C with a rate of \(-10\)\({}^{\circ}\)C min\({}^{-1}\) and kept for 7 min to equilibrate the temperature. The heating scan was then taken from \(-120\) to 120 \({}^{\circ}\)C with a rate of 3 \({}^{\circ}\)C min\({}^{-1}\). Thermogravimetric analysis (TGA) was also performed using Mettler-Toledo TGA 2 (LF) under a N\({}_{2}\) flow from room temperature to 700 \({}^{\circ}\)C with a rate of 5 \({}^{\circ}\)C min\({}^{-1}\). Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra were recorded using a Bruker Alpha spectrometer (Bruker, Billerica, MA, USA) employing a germanium crystal in the range of 4000 to 400 cm\({}^{-1}\) with a resolution of 4 cm\({}^{-1}\). The analysis was conducted in the Ar-filled glove box. A series of electrochemical measurements, except for galvanostatic charge-discharge cycling, was performed by using a Biologic VSP instrument (Biologic, Seyssinet-Pariset, France). For these analyses, home-made bulk-type cells, having a pair of stainless steel cylinders (\(\mathcal{O}\) = 10 mm) as current collectors nested in an insulating tube of polyoxymethylene (\(\mathcal{O}\) = 10 mm), were used. For the ionic conductivity measurements, 120 mg of the composite was put between the current collectors and pressed by a hydraulic press (Specac, Orpington, UK) or by hand for 10 s. A whole cell was put in a housing made of stainless and then closed by using a hex key to keep constant pressure during the analysis. After the cell assembly in the Ar-filled glove box, the measurements were carried out in ambient atmosphere. An AC signal with an amplitude of 10 mV was applied to the cell at OCV. The frequency range was 1 MHz to 1 Hz with 6 points per decade. The conductivity (\(\sigma\)) was calculated by using the following equation:  
\[\sigma\ =\ \frac{1}{R}.\frac{l}{a}\]  
where \(R\) is a resistance value at the intersection of a Nyquist plot with the Z'real axis, \(l\) is the thickness of SE pellets (see Table S1), and \(a\) is the area of electrodes (0.785 cm\({}^{2}\)). The impedance spectra were recorded with respect to the cells by changing three different factors: pressure for the pellet preparation (1-4 tons and hand press), exposure of time (1-24 h), and temperatures (room temperature\(-\)100 \({}^{\circ}\)C). For the temperature dependency analysis, the cells were put in an oven and stored at a set temperature for 6 h prior to the analysis.