## 2 Preparation of IL-Based Gel
### Classification of Preparation Methods
#### 2.1.1 Sol-Gel Method  
A wide variety of IL-based gels, including chemical gels and physical gels, has been successfully synthesized via the sol-gel process to date [24; 25; 26]. The sol-gel process is a simple and low-toxic synthesis technique that generally involves hydrolysis and poly condensation of metal alkoxide precursor in IL [24]. Typically, the sol-gel process can be divided into stages as sol formation, gelation, aging, and drying. The resulting material produced by sol-gel reaction in IL, termed as ionogel or IL-based gel, comprises a continuous mesoporous matrix with an interstitial liquid phase, the IL, that fills the pores. While drying the gel under ambient conditions, the volatile species produced from hydrolysis and condensation is removed. In 2017, [PERSON] et al. prepared an IL-based gel by the sol-gel process from 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTFSI) IL containing LiTFSI and silica (SiO\({}_{2}\)) sources as a mixture of tetramethylsilane (TMOS) and triethylvinyl-lorthosilicate (VTEOS) with formic acid catalyst in cyclohexane [20]. The ionic conductivity of the prepared IL-based gel was reported to be 2 mS cm\({}^{-1}\), which was comparable to that of neat IL (3 mS cm\({}^{-1}\)). Later in 2019, they developed a LiOH-modified IL-based gel employing a two-step sol-gel process with TMOS, VTEOS, and formic acid as SiO\({}_{2}\) precursor solution and 0.5 M LiTFSI in BMIMTFSI IL at -20 \({}^{\circ}\)C [27]. This study showed that the LiOH-modified IL-based gel has slightly lower ionic conductivity than that of original IL-based gel due to a decreased surface area and size of the pore of modified gel with the attendance of LiOH. In 2018, [PERSON]'s group reported a one-step sol-gel method to synthesize an organosilon-functionalized IL-based gel by in situ immobilization of a TFSI-based IL composed of 1-trimethylsilylmethyl-3-butylimidazolium (SiMC\({}_{4}\)Im) containing 0.6 mol kg\({}^{-1}\) LiTFSI within a nanoporous SiO\({}_{2}\) matrix [28]. The as-prepared IL-based gel exhibited a wide electrochemical potential window of 4.87 V (vs. Li/Li) at room temperature and a conductivity of 0.18 mS cm\({}^{-1}\). The IL-based gel showed significantly lower ionic conductivity than the pristine IL. The trends in the values of IL-based gels and neat ionic IL observed in this study were similar to those of previously reported data [20; 27]. Meanwhile, [PERSON] et al. developed a thick, self-standing IL-based gel from a sol made of TMOS, dimethyl-dimethoxysilane (DMDMS), formic acid, and the IL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIMTFSI) where the [TMOS/DMDMS]/(EMIMTFSI)- formic acid molar ratio was [70/30]/50:1 at room temperature [29]. Recently, [PERSON] and coworkers prepared a flower-like structured IL-based gel using lithium trifluoromenthanesulfonate (LiOTf) or LiTFSI salt mixed with tetramethyl-lorthosilicate (TEOS) in 1-butyl-3-methylimidazolium tetralfluoroborate (BMIMBF\({}_{4}\)) IL with dilute hydrochloric acid solution as a catalyst [30]. The two gels obtained from LiOTf and LiTFSI salt were named as IE-T and IE-B, respectively. Field emission scanning electron microscopy (FESEM) images of gel electrolytes are displayed in Figure 1a-d (IE-T) and Figure 1e-f (IE-B). The IE-T exhibits a regular and flower-like sheet structure, whereas, for IE-B, the flower-like sheets were not regular and complete in comparison with IE-T. A schematic representation of the three dimensional structure of synthesized IL-based gel electrolyte is shown in Figure 1g. From the electrochemical measurements, the conductivity of IL-based gels with LiOTf and LiTFSI salts was estimated to be 1.21 mS cm\({}^{-1}\) and 1.19 mS cm\({}^{-1}\), respectively, at 30 \({}^{\circ}\)C. Earlier, a research work by [PERSON] et al.