Liquid and Polymer Gel Electrolytes for Lithium Batteries Composed of Room-Temperature Molten Salt Doped by Lithium Salt

Abstract

The lithium ion binary room-temperature molten salt (i.e., ionic liquid), LiEMIBF4LiEMIBF4 was prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4)(EMIBF4) with LiBF4.LiBF4. The ionic conductivity of LiEMIBF4LiEMIBF4 was 7.4 mS cm⁻¹ at 20°C and lower than that of EMIBF4.EMIBF4. A solidified LiEMIBF4,LiEMIBF4, named GLiEMIBF4,GLiEMIBF4, was prepared by in situ polymerization of poly(ethyleneglycol) diacrylate with LiEMIBF4.LiEMIBF4. The ionic conductivity of the homogeneous transparent membrane obtained was smaller than that of LiEMIBF4.LiEMIBF4. The thermal decomposition temperatures of these “ionic media” measured by thermogravimetry-differential thermal analysis showed that LiEMIBF4LiEMIBF4 and GLiEMIBF4GLiEMIBF4 have high thermal stability around 300°C. The cathodic limit of EMIBF4EMIBF4 was ca. 1.1 V vs. Li/Li+Li/Li+ measured by linear sweep voltammetry. To test the possibility of use of these ionic media for lithium-ion batteries, demonstration cells of Li[Li1/3Ti5/3]O4/LiEMIBF4Li[Li1/3Ti5/3]O4/LiEMIBF4 or GLiEMIBF4/LiCoO2GLiEMIBF4/LiCoO2 were assembled. The capacity retention after 50 cycles was 93.8% of the initial capacity in the LiEMIBF4LiEMIBF4 cell. Discharge potential profile of the GLiEMIBF4GLiEMIBF4 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. © 2003 The Electrochemical Society. All rights reserved.