Journal of Power Sources 189 (2009) 1174–1178 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Short communication Are ionic liquids based on pyrrolidinium imide able to wet separators and electrodes used for Li-ion batteries? Claudia Simona Stefan, Daniel Lemordant, Bénédicte Claude-Montigny, David Violleau ∗ Laboratoire de Physico-Chimie des Matériaux et des Biomolécules (LPCMB, EA 4244), Equipe Chimie-Physique des Interfaces et des Milieux Electrolytiques (CIME), Université F. Rabelais, Faculté des Sciences et Techniques, Parc de Grandmont, 37200 Tours, France article info Article history: Received 5 September 2008 Received in revised form 18 December 2008 Accepted 20 December 2008 Available online 3 January 2009 Keywords: Li-ion battery Alkyl-pyrrolidinium imide Room temperature ionic liquids Separators Contact angle Surface free energy abstract Surface free energy and contact angle measurements were conducted with a series of room temperature ionic liquids (RTILs) based on N,N ′ -alkyl-pyrrolidinium imide. Wetting characteristics of various separa- tors (Celgard ® and Separion ® ) and electrodes (LiCoO 2 , Li 4 Ti 5 O 12 and graphite), commonly used in Li-ion batteries, were performed. Initially, the free surface energies were determined for both smooth polymeric materials, constituent of the separators, and pyrrolidinium RTILs. Experimental results and calculations show that (i) N-methyl-N-pentyl pyrrolidinium imide is the most wetting RTIL whatever the separator used, and that (ii) the separator wettability is one of the most important factor to take into account in electrochemical devices. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Many factors affect the performances of Li-ion batteries. One of the key factors for improving the cycling ability and the power of Li-ion batteries is the wettability of both electrodes and sep- arators, especially when the temperature decreases. It has been previously shown that room temperature ionic liquids (RTILs) have a real potential for use in Li-ion batteries [1–5], but their use as sin- gle electrolyte, when mixed to a lithium salt, is held up because of their high viscosity at room temperature. At the opposite to organic solvents which are usually very volatile and highly flammable, RTILs exhibit low vapour pressure and are mostly non-flammable. More- over, these compounds respect the environment, thanks to several physico-chemical properties conjugated with their chemical struc- ture. Constituted only by organic cations and inorganic (seldom organic) anions, these room temperature molten salts present many advantages apart their non-flammability and negligible vapour pressure such as high chemical and thermal stability, a strong resis- tance toward both oxidation and reduction and correlatively a large electrochemical window. Since a decade, researchers have focused their interest on RTIL thermo-physical properties for electrochemical applications ∗ Corresponding author at: LPCMB (EA 4244) équipe CIME, Université Franc ¸ ois Rabelais, UFR des Sciences et Techniques, Parc de Grandmont, 37200 Tours, France. Tel.: +33 2 47 36 69 13; fax: +33 2 47 36 70 73. E-mail address: david.violleau@univ-tours.fr (D. Violleau). in batteries (melting point, crystallisation temperature, thermal decomposition temperature, etc.) [1–8], but at our knowledge, no data are available concerning RTIL wetting properties. The only published results concerning wettability are dealing with con- ventional Li battery electrolytes based on organic compounds [9–10]. Characterization and prediction of wetting phenomenon by contact angle (CA) measurements and surface free energy (SFE) calculations are powerful analysis tools widely used for many appli- cations [11], even if problems are encountered with the application of these methods and particularly the choice of an appropriate set of liquids and the ill-conditioning system of mathematical equa- tions [12–14]. The thermodynamics of the sessile drop were first described by Young [15], which establishes the relation between the surface free energies of a liquid, a solid and a gas and the CA formed at the interface of the three phases. If the surface free energy of a liquid is easily obtained by surface free energy mea- surement, this is not the case of solids. For this reason, many methods were developed for the determination of the SFE of solids. The most often applied are those of Zisman [16], Owens–Wendt (OW) [17] and the most recent of van Oss–Chaudhury–Good (vOCG) [18–20]. The aim of this work is (i) to determine the surface free energy of N-alkyl-N-alkyl ′ -pyrrolidinium bis(trifluoromethane- sulfonyl)imide RTILs, and (ii) to determine the wettability of com- mercial separators and electrodes commonly used in the field of Li-ion batteries in the presence of these RTILs using the OW and vOCG methods. 0378-7753/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2008.12.114