## 1 Introduction  
All-solid-state lithium ion batteries (ASS LIBs) have been recognized as a next-generation battery [1, 2]. An essential component to realize ASS LIBs is solid state electrolytes (SEs), and different kinds of materials have been developed such as dry polymer electrolytes [3], plastic crystal electrolytes [4], and inorganic glass-ceramic electrolytes [5]. The use of SEs effectively eliminates safety issues that have always been a concern in organic solvent-based liquid electrolytes including risk of fire or explosion and battery leakage. Also, the SEs are electrically insulating and can act as a separator of batteries, which allows a thinner and more lightweight design, and thus a higher energy density per volume or mass [6]. Among these kinds of SEs, the former two, consisting of organic components, are favored for the purpose of enhancing in a structural flexibility, while the last one based on inorganic components is acknowledged to have a high thermal stability superior to the organic SEs [7]. The inorganic SEs also have the lithium transport number of unity in contrast to the liquid electrolyte having the value between 0.2 and 0.4 [8]. By virtue of these advantages, inorganic SEs are garnering attention intensively from both the research community and industry. Inorganic SEs are generally based on oxides or sulfide, and classified into three types: crystal, glass, and glass-ceramic [9]. The oxide-based SEs have been acknowledged to be air-stable and low toxic as compared with sulfide-based ones [10]. The main challenge for a practical use of the oxide-based SEs is the improvement of their ionic conductivity, which is low due to grain boundary resistances between ceramic particles. The principal requisite for battery electrolytes is high ionic conductivity of preferably higher than \(10^{-3}\) S cm\({}^{-1}\), and this value needs to be achieved within feasible conditions for commercialization. In order to achieve this conductivity value, sintering at high temperatures, conventionally at around 1000 \({}^{\circ}\)C, is necessary in the case of oxide-based SEs [11]. In addition, for practical uses, the sintered pellet of SEs must have a good mechanical robustness so that it does not form any cracks and its conductivity is also stable. With the aim of improvement in inherent robustness and flexibility of the SEs, hybrid materials of inorganic and organic components such as polymer-SE [12; 13] and ionic liquids (ILs)-SE [14; 15; 16] have been proposed. ILs are molten salts composed of organic ions. Since they possess a high thermal stability as well as desirable ionic conductivity, ILs have been used for development of battery electrolytes not only as an additive but also as a main component [17]. By using organic ILs, it is possible to improve the flexibility of inorganic SEs in three different aspects. First, a flexibility, i.e., enhanced multifariousness, in the formation of ion conduction path is improved through coating of SEs with IL. The voids among SE particles are filled with conductive liquids, which allows ion conduction even among isolated SE particles and reduces the grain boundary resistance of SEs, such as those based on Li\({}_{7}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\)[15; 18; 19] and Li\({}_{1.5}\)Al\({}_{0.5}\)Ge\({}_{1.5}\)(PO\({}_{4}\))\({}_{3}\) (LAGP) [20]. Second, the composites of SEs and ILs are known to result in soft solid or gel, and therefore, the flexibility of shape formation is augmented [15; 19]. Finally, flexibility in terms of chemical structure can be improved since ILs have wide-ranging possibilities of chemical structure. In spite of the last characteristic, to the best of our knowledge, solely conventional or commercially available ILs with bis(trifluoromethanesulfonyl)imide ([TFSI]) and bis(fluorosulfonyl)imide ([FSI]) have been used as the binder for the SEs. In our past research, ILs functionalized with an ether oxygen in the cation structure to suppress their crystallization tendency, were synthesized and used as battery electrolytes [21].