## 4 Conclusions  
Organic-inorganic hybrid materials based on LLZO and either FSI IL or DFOB IL were prepared. From TGA analysis, both composites were confirmed to be stable in the temperature range where batteries are expected to work. In DSC and ATR-FTIR analyses, it was suggested that FSI IL formed a physical interaction with LLZO, while evidence of interactions was not observed in the case of DFOB IL. In the latter case, its ionic conductivity decreased when the pellet was pressed with a pressure higher than 2 ton, which is probably due to a bleed out of DFOB IL. With respect to the electrolyte pellets prepared with 1 ton, the higher conductivities were found for LLZO-FSI IL (4.8 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) at 15 \({}^{\circ}\)C) and the conductivity of 10\({}^{-3}\) S cm\({}^{-1}\) was achieved at room temperature, which cannot be achieved through the cold pressing of pristine LLZO. In addition to this, a constant lithium intercalation/de-intercalation behavior was observed during the CV of LLZO-FSI IL doped with LiFSI. For this reason, the ASS lithium metal cells, having the composition LiFePO4 | LLZO-FSI IL-LiFSI | Li, were assembled. The cell exhibited a specific capacity higher than 100 mAh g\({}^{-1}\) throughout 10 cycles when the cell was cycled at C/20. These results confirm that the addition of ILs effectively reduces the grain boundary resistance between LLZO particles. By using ILs as a conductive binder for LLZO, a battery with favorable performance can be prepared through a simple cold-pressing procedure.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12147318/s1](https://www.mdpi.com/article/10.3390/app12147318/s1)]([https://www.mdpi.com/article/10.3390/app12147318/s1](https://www.mdpi.com/article/10.3390/app12147318/s1)), Figure S1: Enlarged TGA curve of pristine LLZO; Table S1: Thickness of the pellet of LLZO-FSI IL and LLZO-DFOB IL; Figure S2: Gauss curve fitting of ATR-FTIR spectra of pure FSI IL and LLZO-FSI IL composite, Figure S3: Impedance spectrum of pure LLZO pressed with 1 ton; Figure S4: Nyquist plot of impedance spectra; Figure S5: Change of the ionic conductivity of LLZO-FSI IL and LLZO-DFOB IL composites versus time; Figure S6: CV of LLZO-FSI IL and LLZO-DFOB IL composites with and without a corresponding lithium salt.  
Figure 8: (**a**) Specific capacity during the charge–discharge cycle of LiFePO4 | Li cell having LLZO–FSI IL–LiFSI as the electrolyte and (**b**) its voltage profile during the 2 nd and 5 th cycles.  
Author Contributions:Conceptualization, A.T. and M.A.N.; methodology, A.T.; investigation, R.R. and L.M.; data curation, A.T. and R.R.; writing--original draft preparation, A.T.; writing--review and editing, L.M. and M.A.N.; supervision, M.A.N.; funding acquisition, A.T. and M.A.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Sapienza University of Rome, Avvio alla Ricerca 2020 (AR220172B8 AEAEAE) for \"Design of multifunctional surface on inorganic ceramic solid electrolytes by using ionic liquids\" (A.T.) and Ateneo 2020 (RM120172B3B81486) for \"Ionic Liquid-based electrolytes for Energy Storage devices (ILES)\" (M.A.N.).  
Authors acknowledge [PERSON], the Department of Physics, Sapienza University of Rome for his technical support during fabrication of home-made ASS cells.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.