Innovative Ionic Liquids: Electrolytes for Ion Power Sources

摘要

As technologies evolve from fixed-site equipment to warfighter- portable devices, the need for safe, compact, rechargeable power sources continues to grow. In spite of the emphasis on high energy density lithium ion (Li-ion) power sources, the fire and/or explosive hazards of these severely limit their utility to the warfighter. Logistical concerns further compound these difficulties, and are reflected in numerous federal and international regulations that limit how, and how many, lithium ion batteries can be shipped together. Thus, a twofold problem exists: increasing the energy density of batteries, while at the same time improving the safety of these devices to enhance their utility to the U.S. Navy and the Joint Services. The key component responsible for the hazards associated with lithium ion batteries is the electrolyte. Typically, these electrolytes, even in gel form, are more flammable than jet fuels. Therefore, enhancing the safety of these systems is dependent on being able to replace these common electrolytes with a safer system. An ideal replacement choice is ionic liquids (ILs). ILs are organic- based molten salts that are liquid at ambient temperatures. In general, the composition of these ILs incorporates a thermally and electrochemically stable, positively charged ring system, and an anion, such as a perfluorinated imide. It is this thermal and electrochemical stability and the stability of the organic components that have made ILs attractive alternatives to the more common organic solvents. As salts, the ILs are highly ionically conductive, have nearly no vapor pressure, and as such are non-volatile. Thermal analysis indicates that not only are they stable to over 350 deg C, but also they do not explosively ignite as most other common organic solvents do, but instead thermally degrade by charring.