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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