Low-viscosity quaternary ammonium-based ionic liquid electrolytes for lithium air batteries

Ionic liquids (ILs) have attracted significant attention as candidates for Li-air battery electrolyte solvents owing to their novel properties, such as non-volatility and low reactivity with lithium metal anodes. However, the main drawback of IL-based electrolytes is their high viscosity, which results in poor battery capacity and conductivity. In this study, we synthesized low-viscosity ILs with reduced molecular weights using the side-chain shortening method. In particular, the IL: trimethyl ethoxymethyl ammonium bis(fluoromethylsulfonyl)imide (N111(1O2) FSI), has a lower viscosity (111.4 cP) than that of the electrochemically stable reference material, diethylmethyl (2-methoxyethyl) ammonium bis(fluoromethylsulfonyl)imide (DEME FSI; viscosity of 131.1cP). The use of the optimal IL, N111(1O2) FSI, resulted in an initial capacity 3.75 times higher than that obtained using DEME FSI in the cell test. The chemical stability of N111(1O2) FSI with lithium metal was confirmed by microstructure analysis of the lithium surface. The physicochemical properties, such as viscosity, ionic conductivity, melting point, and electrochemical properties, were thoroughly characterized.