Rechargeable lithium-polymer batteries offer several advantages over their liquid and gel electrolyte counterparts, particularly in terms of safety. Typical lithium ion batteries contain volatile and flammable solvents in the conducting medium. A short circuit can cause localized heating that can create a fire. This is a particular concern for batteries that are used for human-rated space applications. One way to avoid this problem is to replace the liquid with a solid polymer that can conduct lithium ions. The polymer used most often is poly(ethylene oxide) (PEO). The use of PEO would also be advantageous because lithium metal anodes could be used instead of the normally used graphite intercalation anodes, increasing the potential power density. Unfortunately, PEO and its derivatives have yet to reach a lithium ion conductivity sufficient for practical use. Polymer electrolytes appear to have reached an upper conductivity limit1 of 104 S/cm, but at least 103 S/cm is needed. Furthermore, PEO has poor mechanical properties at higher operating temperatures.
The NASA Glenn Research Center had previously synthesized a series of hyperbranched polymer electrolytes that contain PEO-based oligomers that were connected via a triazine linkage. The polymers were crosslinked via a sol-gel technique to improve their dimensional stability. When doped with lithium salts, these polymers were strong, flexible, thermally and mechanically stable up to high temperatures, and completely amorphous over the required temperature range. These polymers have room-temperature conductivities up to 4×105 S/cm, which is still too low for practical use. Recently, researchers from Glenn and the Ohio Aerospace Institute (OAI) greatly increased the conductivity by adding room-temperature ionic liquids (ILs). ILs are made from an asymmetric organic cation and a bulky anion with a highly delocalized charge to minimize packing. ILs are both nonvolatile and nonflammable, thereby maintaining the safety advantages of the solid polymer electrolyte while adding a conductive liquid component.
Chemical structure of the polymer, IL, and lithium salt that make the polymer electrolyte.
The addition of only 30 wt% IL increased the conductivity by over an order of magnitude. Conductivity further increased as more IL was added. The hyperbranched polymer films are now capable of holding over 150 wt% IL, with the highest conductivity being 8.8×104 S/cm at ambient temperatures. Furthermore, these films maintain their mechanical properties even at such high IL loadings.
Room-temperature conductivity of several polymer electrolytes as a function of IL content.
Find out more about the research of Glenns Polymers Branch:
http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/polymers/
Last updated: November 13, 2007
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
