This proposal presents a novel hybrid electrolyte based on oxide-based solid electrolytes (OSEs) and ionic liquids (ILs). The aim of this hybridization is to reduce the grain boundary resistances of OSEs. Solid electrolytes are promising materials which no doubt improve safety of lithium ion batteries. Especially OSEs are recognized as air-stable electrolytes compared to sulfide-based solid electrolytes. However, their high ionic conductivity is achieved solely after sintering processes under high temperatures. In this study, ILs composed of organic ions are chosen as a partner for OSEs. The integration with organic materials will improve the flexibility of OSEs in terms of the multifariousness of ionic conductivity, the diversity of material design, and the variation in the structure of resulting batteries.
Taking into account the advantages of organic chemistry, ILs will be suitably functionalized and added to OSEs in two ways: (1) as macro-binders between OSE particles and (2) as nano-binders in OSE glasses. In the (1) case, we intent to form IL coatings on OSE particles to suppress their grain boundary resistance and achieve the ionic conductivity higher than 1.0 mS/cm. The coatings will be immobilized by improving the compatibility between OSEs and ILs, which will be undertaken through the insertion of functional moieties to the ILs such as halide anions, polar groups, and lithium salts. In the (2) case, ILs will be included in the glass structures of OSEs, thus the possibility of IL bleed-out will be minimized. This is a heretofore untried novel approach for the design of solid lithium ion conductors. The ceramic formations of inorganic-organic hybrids will be attempted by taking both the contributions of Van der Waals interactions of organic moieties and Coulombic interactions of ionic moieties. To this end, we intend to propose stem materials which are designed and can be functionalized based on the dual concept of organic and inorganic chemistry.
The concept of our hybrid materials based on OSEs and ILs is unattempted and innovative. Some achievements in the development of inorganic-organic hybrid materials, such as amine containing perovskite light harvesters and metal¿organic frameworks, have been reported. However, in the field of solid electrolytes for lithium ion batteries, the combination of organic and inorganic concept is not well discovered except for polymer electrolytes. In this research, the synthesis of inorganic-based solid electrolytes, which is supported by interactions that are particular to organic molecules, will be attempted. This will be a new approach for the synthesis of solid electrolytes. Also, the resulting combinations of inorganic chemistry and organic chemistry will exhibit some advantages which are characterized by the term ¿Flexible¿ because of following reasons.
(1) The flexibility in the formation of the ion conduction pathways will be enhanced. As mentioned before, the first foremost important property of the electrolytes is high ionic conductivity. By modifying OSE particle surfaces amorphous with ILs, the grain boundary resistances will be suppressed, and the ionic conductivity of the OSEs is expected to be improved. In general, the grain boundary resistance is controlled by the heat treatments around 1000 oC. Although the cold sintering process has been reported, this procedure still requires heating at around 300 oC [J. Am. Ceram. Soc., 2017, 100, 2123]. In this study, we propose the use of ILs as an alternative approach to control the unfavorable coarse interfaces, which does not require high temperature treatments. This will be beneficial not only in terms of the improvement of ionic conductivity but also in terms of the reduction of the energy consumption.
(2) The intrinsic flexibility of material structures will be improved. The solid electrolytes are beneficial for minimizing the possibility of liquid leakage. However, some restrictions, i.e. difficulty in the preparation of films, the creation of fine structures, and the formation of smooth interfaces between each materials, are added by the solid state features. In order to form electrolyte layers between electrodes, chemical vapor deposition techniques, which require careful control, have been applied. By improving the flexibility of OSEs with the addition of ILs, the processability of OSEs is expected to be enhanced. This means thin film structures can be obtained without the chemical vapor deposition techniques. Moreover, because the application of ILs will improve the intrinsic flexibility of the materials, the geometry of the resulting batteries can be easily modified and will be shock-resistive.
(3) The flexibility will be added also in terms of functional design of OSEs. The most significant advantages of the use of organic compounds are the diversification of their structures and functions. In the case of hybrid materials based of OSEs and ILs, further functions can be added to the hybrid materials through the modifications of ILs. As an example, the cations of ILs will be customized with allyl- and ionic-group. The former modification enables the polymerization of ILs, which contributes to the cancelation of the IL bleed-out. Through the latter modification, we intend to obtain zwitterionic structure and allocate functions to each ion moieties. Specifically, one part will be functionalized so as to interact with the solid electrolytes and another part will used to dissociate lithium salts. This functionalization is expected to increase the concentration of lithium ion in the solid electrolytes. The materials proposed in this study will be a stem material for functionalized solid electrolytes at the end. The combination of inorganic-organic concept will exponentially widen the variety of the functionalization.
By taking the advantages explained above, we would like to conclude that inorganic-organic hybrid electrolytes proposed here are innovative and beneficial. We strongly deem that this electrolyte will be next generation electrolytes and in turn will be followed by other research in the future.