Poly(ethylene oxide) LiN(SO[sub 2]CF[sub 2]CF[sub 3])[sub 2] Polymer Electrolytes

Abstract

The present report describes a Raman spectroscopy investigation of polymer electrolytes based on poly͑ethylene oxide͒ ͑PEO͒-lithium bis͑perfluoroethylsulfonyl͒imide ͑LiBETI͒, LiN(SO 2 CF 2 CF 3) 2 . A wide range of ether oxygen/salt, EO/LiBETI, molar ratios extending from 2 to 50 was considered. The crystalline salt was also investigated and a preliminary assignment of the Raman peaks of the salt, pure and dissolved in PEO, is made. The analysis of the Raman spectra supports the suggested formation of a solvent-separated P(EO) 6(or 7) -LiBETI crystalline solvate. The highest conductivities of polymer electrolytes reported in previous works were for those electrolytes having compositions close to this solvate stoichiometry. The appearance of broad vibrational bands associated with amorphous PEO, even in very salt-diluted samples, confirms the ability of the BETI Ϫ anion to strongly affect the polymer morphology. The stringent demand for environmentally compatible electric and hybrid vehicles has motivated the research and development of electrochemical power sources characterized by high energy and/or high power densities, good cyclability, reliability, and safety. 1-4 Re-chargeable lithium metal anode batteries are one of the most prom-ising answers to these demands because of their theoretically high gravimetric energy and power densities. 3,4 Nevertheless, the reactiv-ity of lithium metal anodes with liquid electrolytes and the poor conductivity of dry polymer electrolytes combined with the high cost of both systems are preventing the introduction of lithium-based batteries in pure and hybrid electric vehicle ͑EV͒ applications. However, while the reactivity of lithium toward liquid electrolytes is presently considered problematic especially in terms of safety, in-creasing the conductivity of dry polymer electrolytes, and reducing the cost of manufacturing of devices; incorporating such electrolytes is still an open issue. Solvent-free, poly͑ethylene oxide͒ ͓P(EO)͔ n LiX complexes ap-pear to be one of the most suitable electrolytes for the development of reliable lithium polymer batteries ͑LPBs͒. 5,6 These complexes are formed by dissolving a lithium salt, LiX, in a PEO polymer matrix 7