Study of Crown Ether - Lithium Salt Complexes for Solid-State Electrolyte of Li Ion Battery

Abstract

Electrolyte plays a pivotal role in Li-ion battery (LIB). Generally, it has been considered as a liquid phase material that allows for reasonable dissolution ability of inorganic salt and ion transportability across the electrodes. Since conventional liquid electrolytes faced limitations to improve battery performance, the solid-state electrolyte has raised rapidly to replace the liquid electrolyte. Solid polymer electrolyte (SPE) is one of the most promising solid-state electrolytes, it has attracted attention due to its strength as an electrolyte such as high accessibility, tolerance against volume change of electrodes, and reasonable chemical/thermal stability. Especially poly(ethylene oxide) (PEO) or PEO-based electrolytes have been widely developed, because of their outstanding Li salt dissolution ability. PEO forms a unique helical complex structure with Li salt, the complex is highly stable. It allows PEO to dissolve Li salt much easier than other polyethers, such as poly(propylene oxide) or poly(oxymethylene). However, the high stability of the complex could disturb fast ionic conduction in electrolyte medium, it is the reason why PEO electrolyte has poor ionic conductivity at ambient temperature (10-7 to 10-6S/cm at room temperature). Crown ethers, the macrocycles composed of ethylene oxide, also attract attention for SPEs. Even though the monomer is the same (ethylene oxide) for both PEO and Crown ethers, the properties of each complex are different. Crown ether electrolyte shows higher ionic conductivity than PEO electrolyte at ambient temperature, it is assumed that the crown ether electrolyte is an amorphous phase while PEO electrolyte has high crystallinity below the melting point (60~80%). Crown ether is also promising for Li metal batteries. Crown ether is the hosting material that has a preorganized acceptor structure (also well-known as macrocyclic effect), therefore it can adjust kinetics of reduction on the surface of Li metal anode. Even though promising abilities of crown ether electrolytes have been reported, their ion conduction mechanisms are yet understood well. Therefore, it is worth studying about complex structure and ion conduction mechanism of crown ether electrolyte for a better understanding of SPE, and further, it can be a milestone to design new SPEs. To understand the complex structure and ion conduction mechanism in crown ether electrolytes, we performed both experimental and theoretical analysis. We synthesized the complexes of different sizes of crown ethers (12-crown-4, 15-crown-5, and 18-crown-6) and Li salts (LiTFSI, LiClO4, LiBF4, and LiI). The complexes are characterized by Fourier transformed infrared (FTIR), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), and AC impedance analysis. DFT calculation and molecular dynamics for each complex were performed to obtain an atomistic description of the complexes.