Ionic liquids consist of only ionic components, having high ionic conductivity suitable for a liquid electrolyte. An electrolyte for a secondary battery requires not only high ionic conductivity but also non-volatility, heat-resistivity, non-inflammability, and non-corrosiveness. Ionic liquids cover these conditions. The cationic component of ionic liquid involves alkyl-substituted imidazolium, pyrrolidinium, piperidinium, ammonium, phosphonium, sulfonium and the anionic component involves halide, BF4, PF6, thiocyanate, and di(sulfonyl)imide. Chemical modifications of the cation and anion control melting point, viscosity and ionic conductivity. Hamaguchi et al. observed magnetic ionic liquids by introducing FeCl4 to the anion part to form 1-butyl-3-methylimidazolium tetrachloroferrate, (Bmim)FeCl4.1,2) A magnetic fluid is a liquid with magnetic properties. However, a conventional magnetic fluid contains volatile solvents. It causes a change of viscosity and phase separation by cohesion/precipitation. On the other hand, the magnetic ionic liquid is a highly stable and non-volatile liquid. Moreover, this magnetic ionic liquid responds to a magnetic field by a permanent magnet, because it shows large magnetic susceptibility at room temperature.