Transport and Electrochemical Properties of Three Quaternary Ammonium Ionic Liquids and Lithium Salts Doping Effects Studied by NMR Spectroscopy

Ionic liquids (IL) composed of a quaternary ammonium cation having an ether chain, N,N-diethyl-N-methyl-N-(2-methoxyethyl)­ammonium (DEME) are electrochemically stable and used in electric double layer capacitors and also one of important candidates to use lithium secondary batteries. In this study, three DEME-based ILs with anions BF₄, CF₃BF₃, and N­(SO₂CF₃)₂ were studied by measuring temperature dependences of ionic conductivity σ, viscosity η and density ρ. Also ion diffusion coefficients D_anion and D_cation were obtained by NMR spectroscopy in the wide temperature range. Using the classical Stokes–Einstein (SE) and Nernst–Einstein (NE) equations, the relationships between η and D, and σ and D were evaluated. The lithium salt doping effects were studied by ⁷Li NMR spectroscopy. The lithium ion diffusion was slower than other ion diffusion at every temperature. Arrhenius-type plots of ⁷Li T₁ showed minima in the three doped samples. Then one-jump distance of lithium ion was estimated in the temperature range between 273 and 373 K.