Recently, lithium-ion batteries (LIBs) have been applied to large-scale devices such as electric vehicles and energy storage systems (ESSs), requiring high energy density and long cycle life. The use of organic electrolytes in lithium ion batteries in such large-scale devices has some safety issues. Organic electrolytes have a high ionic conductivity, but there are many risks of leakage, explosion and ignition due to the decomposition reaction of the electrolyte. As a promising solution to this issues, the use of a solid electrolyte have been proposed. In contrast to organic-based liquid electrolytes, all-solid-state lithium batteries (ASLBs) with a solid electrolyte are considered safe because of their non-flammability. These solid electrolytes have the advantages of high stability and high energy density, but they have a disadvantage of very low ionic conductivity at room temperature.

In previous studies, PEO-based membrane is available in a free form for all solid state battery designs, but it has very low ionic conductivity (10^-6-10^-8 S/cm) at room temperature because of their very low ion mobility below their melting point (~ 60°C). The PEO-based membrane with inorganic filler increases ion diffusion or mobility by stabilizing the highly conductive amorphous phase.

In our previous research, the composite membrane was prepared by incorporating tetragonal Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte in a polyethylene oxide (PEO) matrix. LLZO was used as an inorganic filler because of its chemical stability (LLZO does not react with Li metal), high ionic conductivity of ~10⁴ S/cm and wide potential window (~5V vs Li/Li⁺). Therefore, we made a composite membrane by incorporating the ceramic inorganic filler LLZO and improved the ion conductivity such as 4.4 x 10^-4 at 55°C of the membrane. However, this membrane still has a low ionic conductivity to be applied to real battery systems because their interface resistance between the electrode and solid electrolyte is large. Mostly, the main internal resistance in ASLBs is the internal resistance between the electrode and the solid electrolyte, not the bulk resistance of the solid electrolyte.

In order to enhance further the ionic conductivity of composite membranes, we made hybrid composite membranes by adding small amounts of solvated ionic liquids (SILs). The representative of the SILs is an equimolar mixtures of Li[TFSA] and tetraglyme (abbreviated as [Li₁(G4)₁][TFSA]), where stable complex cation [Li₁(glyme)₁]⁺ is formed and it serves as an analog of the weakly-coordinating cation of the aprotic ILs. [Li₁(G4)₁][TFSA] showed faster Li⁺ ion transport than the binary systems of aprotic ILs and Li salts. Also, SILs has low volatility, high ionic conductivities and superior electrochemical stability. Because of these properties, we expected that the ionic conductivity would be increased while maintaining the high thermal and electrochemical stability of the composite membrane.

In this study, we propose the novel concept of a free-standing composite membrane with a new family of SILs which has a high ionic conductivity of 1.8 x 10^-3 at 55°C. In addition, it had an excellent thermal stability at the temperature. Then we discuss whether the ion conductivity of the composite membrane is improved by adding a small amount of SILs with LLZO based solid electrolyte because of synergetic effects. Finally, the effect of SILs with LLZO based solid electrolyte on the battery performance of the ASLBs cells was investigated.