Anion Effects on Solid Polymer Electrolytes Containing Sulfur Based Ionic Liquid for Lithium Batteries

Solid polymer electrolytes for lithium batteries possess increased safety over traditional carbonate electrolytes, but have not been shown effective at temperatures close to ambient. The inclusion of triethyl sulfonium based ionic liquids (IL) into poly(ethylene oxide) (PEO) homopolymers has demonstrated the ability to reach the necessary figures of merit for use in a lithium battery below physiological temperature. The effect of the anion for the IL and the lithium salt was measured for bis(perfluoroethylsulfonyl) imide, bis(oxalato) borate, perchlorate, hexafluorophosphate (PF6) and bis(trifluorosulfonyl) imide (TFSI), and all showed ionic conductivity on the order of 1 mS/cm at physiological temperature. TLi+ ranged from 0.19-0.31 for the systems and some anions demonstrated stability exceeding 5 V vs. Li/Li+. The ability to reversibly plate and strip lithium was also measured and was largely influenced by the resistance of the electrolyte-electrode interface. This research demonstrated that TFSI is the best choice of anions for the solid polymer electrolyte system, while PF6 is the worst. The other anions demonstrated a range of properties that depending on the needed properties could potentially be favored relative to TFSI. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.089205jes] All rights reserved.