Nano-sponge ionic liquid-polymer composite electrolytes for solid-state lithium power sources

Solid polymer gel electrolytes composed of 75 wt.% of the ionic liquid, 1-n-butyl-2,3-dimethylimidazolium bis-trifluoromethanesulfonylimide with 1.0 M lithium bis-trifluoromethanesulfonylimide and 25 wt.% (polyvinylidenedifluoro-hedafluoropropene) are characterized as the electrolyte/separator in solid-state lithium batteries. The ionic conductivity of these gels ranges from 1.5 to 2.0 mS cm(-1), which is several orders of magnitude more conductive than any of the more commonly used solid polymers, and comparable to the best solid gel electrolytes currently used in industry. TGA indicates that these polymer gel electrolytes are thermally stable to over 280 degrees C, and do not begin to thermally decompose until over 300 degrees C; exhibiting a significant advancement in the safety of lithium batteries. Atomic force microscopy images of these solid thin films indicate that these polymer gel electrolytes have the structure of nano-sponges, with a sub-micron pore size. For these thin film batteries, 150 charge-discharge cycles are run for LixCoO2 where x is cycled between 0.95 down to 0.55. Minimal internal resistance effects are observed over the charging cycles, indicating the high ionic conductivity of the ionic liquid solid polymer gel electrolyte. The overall cell efficiency is approximately 98%, and no significant loss in battery efficiency is observed over the 150 cycles. (C) 2009 Elsevier B.V. All rights reserved.