Binary room-temperature ionic liquid (RTIL) samples including a lithium salt were prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with LiBF4. The ionic conductivity, viscosity, thermal properties, and ion self-diffusion coefficients in [emim][BF4] and the binary [Li][emim][BF4] at six concentrations of LiBF4 ranging from 0.25 to 1.50 M were measured at various temperatures. The self-diffusion coefficients of the individual components, [emim], BF4, and Li, were measured by using 1H, 19F, and 7Li pulsed gradient spin−echo NMR, respectively. Since the Walden product holds similar to typical solution electrolytes, the ion conduction mechanism is interpreted using a flux basis electrolyte theory. The ions form associated structures and diffuse under the influence of the counterions in the binary IL systems. An attempt to correlate the ion diffusion with the ionic conduction was made in the framework of the Nernst−Einstein relationship. The Li net transference number and the apparent ion activity are also discussed.

The lithium ion binary room-temperature molten salt ͑i.e., ionic liquid͒, LiEMIBF 4 was prepared by mixing 1-ethyl-3methylimidazolium tetrafluoroborate (EMIBF 4 ) with LiBF 4 . The ionic conductivity of LiEMIBF 4 was 7.4 mS cm Ϫ1 at 20°C and lower than that of EMIBF 4 . A solidified LiEMIBF 4 , named GLiEMIBF 4 , was prepared by in situ polymerization of poly͑ethyl-eneglycol͒ diacrylate with LiEMIBF 4 . The ionic conductivity of the homogeneous transparent membrane obtained was smaller than that of LiEMIBF 4 . The thermal decomposition temperatures of these ''ionic media'' measured by thermogravimetrydifferential thermal analysis showed that LiEMIBF 4 and GLiEMIBF 4 have high thermal stability around 300°C. The cathodic limit of EMIBF 4 was ca. 1.1 V vs. Li/Li ϩ measured by linear sweep voltammetry. To test the possibility of use of these ionic media for lithium-ion batteries, demonstration cells of Li͓Li 1/3 Ti 5/3 ͔O 4 /LiEMIBF 4 or GLiEMIBF 4 /LiCoO 2 were assembled. The capacity retention after 50 cycles was 93.8% of the initial capacity in the LiEMIBF 4 cell. Discharge potential profile of the GLiEMIBF 4 cell showed decline probably due to the concentration polarization in the gelled electrolyte. Liquid and gelled electrolytes composed of ''lithium ion coexisting room-temperature molten salt'' are shown to function as nonflammable electrolytes in the lithium-ion batteries.

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