With boost in the energy density delivered by Li-ion batteries (LIBs), there is an increasing demand on improving their safety. Electrolytes are often a critical component in ensuring LIB safety because they generally contain flammable organic solvents. To this end, various electrolyte materials have been reported, including ionic liquids (ILs) and inorganic solid electrolytes. ILs have been utilized in different ways: i) as an electrolyte itself (IL−Li salt system), ii) as a majority component in electrolytes, or iii) as an additive for electrolytes. Recently, ILs are also used iv) as an additive for solid electrolytes in order to fill grain boundary of electrolyte particles.

In our laboratory, we have synthesized various classes of ILs taking their use into consideration. For i) and iv), the most important feature of ILs is their high ionic conductivity over a wide temperature range. In contrast to this, in the case of ii) and iii), in which ILs are combined with other liquid electrolytes, functionalities, such as suppression of flammability and formation of stable interphase with electrodes, become more important rather than the conductivity. For the former purposes i) and iv), bis(fluorosulfonyl)imide-based ILs (FSI ILs) were synthesized and mixed with LiFSI to form Li⁺ conductive electrolytes. The FSI IL−LiFSI electrolytes were also used as a coating material for the solid electrolyte, specifically Al-doped Li₇La₃Zr₂O₁₂ (LLZO).

The FSI ILs exhibited ionic conductivity above 10⁻³ S cm⁻¹ at room temperature both with and without the presence of LiFSI. In a linear sweep voltammetry, oxidation currents were observed from 4.5 V vs. Li/Li⁺. Taking this electrochemical stability of FSI IL−LiFSI electrolytes, they were tested in Li-metal cells with LiFePO₄ cathode. The cells exhibited high specific capacities (>130 mAh g⁻¹) together with flat voltage profiles at C/10. Then, a small amount of FSI IL−LiFSI was mixed with LLZO to form composites. When FSI IL−LiFSI is absent, a LLZO pellet, pressed with 1 ton, without any high-temperature sintering process, showed room temperature ionic conductivity of 1.1× 10⁻⁹ S cm⁻¹ due to a large grain boundary resistance. By making the composite, however, the ionic conductivity was improved to 10⁻³ S cm⁻¹. Employing the composite electrolytes, quasi-solid-state Li-metal batteries having LiFePO₄ were assembled. The cells exhibited a capacity above 100 mAh g⁻¹ throughout the course of charge–discharge cycle at C/20. This demonstrates that FSI ILs are the effective additive for LLZO that can guarantee the ion conduction enough for battery cycling.