Journal of Power Sources 195 (2010) 6130–6137 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour UV cross-linked, lithium-conducting ternary polymer electrolytes containing ionic liquids G.T. Kim a , G.B. Appetecchi a,∗∗ , M. Carewska a , M. Joost b , A. Balducci b , M. Winter b , S. Passerini a,b,∗ a ENEA, IDROCOMB, Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy b Institute of Physical Chemistry, University of Muenster, Corrensstr. 28/30, D 48149 Muenster, Germany article info Article history: Received 24 September 2009 Received in revised form 25 October 2009 Accepted 29 October 2009 Available online 10 November 2009 Keywords: Polymer electrolyte Lithium batteries Ionic liquid PEO LiTFSI UV cross-linking abstract In this manuscript is reported an attempt to prepare high ionic conductivity lithium polymer elec- trolytes by UV cross-linking the poly(ethyleneoxide) (briefly called PEO) polymer matrix in presence of the plasticizing lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and an ionic liquid of the pyrrolidinium family (N-alkyl-N-methylpyrrolidinium TFSI) having a common anion with the lithium salt. It is demonstrated that polymer electrolytes with room temperature ionic conductivities of nearly 10 -3 S cm -1 could be obtained as a result of the reduced crystallinity of the ternary electrolytes. The results clearly indicate that the cross-linked ternary electrolyte shows superior mechanical proper- ties with respect to the non-cross-linked electrolytes and higher conductivities with respect to polymer electrolytes containing none or less ionic liquid. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Rechargeable lithium batteries outpaced all other battery sys- tems in the consumer portable electronic and telecommunications markets within the last few years. However, to power the hybrid and/or pure electric vehicles of tomorrow, lithium batteries have to provide even higher energy and/or higher power densities, better cyclability, reliability, and, overall, safety. Lithium metal batteries with their theoretically high gravimetric energy and power densi- ties are a desired solution. However, the liquid organic electrolytes that have warranted the great success to present lithium-ion bat- teries, appears to be not useful because of their high reactivity with Li-metal that results in poor performance and uneven (dendritic) anode deposition. This latter phenomenon raises serious safety issues considering that the conventional electrolytes for Li-ion bat- teries are mostly composed of volatile flammable solvents, which could easily cause fire or explosion of the battery upon dendritic short-circuit. Actually, the safety issue caused by the electrolytes’ fire and explosion hazards is indeed present in large Li-ion bat- teries and, so far, it has prevented the wide deployment of Li-ion batteries in hybrid and electric vehicles. The thermal runaway of a ∗ Corresponding author. Tel.: +49 251 83 36026; fax: +49 251 8336032. ∗∗ Corresponding author. Tel.: +39 06 3048 4985; fax: +39 06 3048 6357. E-mail addresses: gianni.appetecchi@enea.it (G.B. Appetecchi), stefano.passerini@uni-muenster.de (S. Passerini). Li-ion battery is a catastrophic, uncontrollable event that occasion- ally happens even in long established applications such as laptop PC and cell phones [1]. Lithium metal, polymer electrolyte batteries (LMPBs) have been proposed since early 1980s as the solution to the safety issues. So far, however, LMPBs are substantially limited to high-temperature operations since solvent-free polymer electrolytes, such as those with dissolved lithium salt in poly(ether/glycols), are characterized by a relative low ionic conductivity at room temperature. Most of the work on polymer electrolytes, so far, has been focus- ing on poly(ethylene oxide) (PEO). PEO is an inert polymer and its application as electrolyte host has been intensively studied for almost four decades now [2]. The fact that PEO builds complexes with Li salts and displays both thermal and interfacial stabilities [3] makes it a promising candidate as polymer electrolyte host. However, at room temperature the ionic conductivity of Li salts dissolved in PEO is limited because the highly symmetrical repeat- ing ethylene oxide units tend to crystallize. Amorphous areas are necessary for sufficient ionic conductivity [4,5]. Li salts with large anions free the Li + from the strong EO coordination. An ionic liquid as additional salt, providing the same anion seems to promote this process and the voluminous cations create “free-volume” [6,7] for diffusion. Recently, the combination of a polymer and an ion con- ducting, electrochemically stable ionic liquid has been explored as a polymer-based electrolyte in lithium ion batteries [8]. A broad and stable amorphous region of PEO was created. Ionic liquids are per- fectly suitable for battery applications, because they present high 0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2009.10.079