Current lithium-ion batteries (LiB) have limited usefulness in high temperature applications due to the flammability of the organic electrolyte. Alternatively, solid electrolytes can mitigate safety issues and enable high temperature applications. Here, a new solid-state lithium ion battery design is investigated using LiSICON as the electrolyte. To reduce the interfacial resistance between the solid electrolyte and graphite electrode, an ionic liquid was used as the ionic conductor. N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR(13)(+)FSI(-)) was used as the ionic liquid due to its high efficiency for lithiation/delithiaion. The cell capacity and efficiency was measured at temperatures from 25 degrees C to 120 degrees C. The lithium salt concentration in the ionic liquid was found to change the electrolyte reduction potential. A stable solid electrolyte interface (SEI) was achieved by adjusting the lithium salt concentration in the wetting agent. Impedance spectroscopy was performed to study the cell behavior with different Li salt concentrations at elevated temperature. A carbon anode in a LiSICON cell with 0.75 M LiTFSI/PYR(13)(+)FSI(-) had a capacity of 325 mAh g(-1) and coulombic efficiency of 99.8% during cycling at 80 degrees C. (C) 2011 Elsevier B.V. All rights reserved.