ARTICLES Temperature-Dependent Raman Spectroscopy of Lithium Triflate-PEO Complexes: Phase Equilibrium and Component Interactions Fannie Alloin,* Gurusamy Hirankumar, and Thierry Pagnier LEPMI-Laboratoire d’Electrochimie et de Physico-chimie des Mate ´riaux et des Interfaces (Grenoble-INP, UJF, CNRS), 1130 Rue de la Piscine, BP 75 F-38402 Saint Martin d’He `res, France ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: NoVember 9, 2009 Poly(ethylene oxide) and complexes of lithium trifluorosulfonate-poly(ethylene oxide) (LiTf-PEO) with 4 e O/Li e 18 have been studied by Raman spectroscopy from room temperature up to 160 °C. The Raman spectrum of the (PEO) 3 LiTf defined compound has been deduced from the experimental data. Subtraction of the Raman spectrum due to (PEO) 3 LiTf in each sample allowed us to determine for the first time the composition and the Li + -Tf - and Li + -PEO interactions in the part of the polymer not crystallized as (PEO) 3 LiTf. It is shown that the local interactions between cation and anion or between cation and PEO chain persist even in the melted state, up to near the liquidus temperature. In particular, the Li + -PEO interactions decrease significantly just below the liquidus temperature with a simultaneous strong increase in the ion pair concentration. Introduction Solid polymer electrolytes have been extensively studied due to their ionic conduction properties and their potential application in lithium batteries, solar cells, and electrochromic systems. 1 Lithium salt -poly(ethylene oxide) (PEO) complexes have attracted much attention, in spite of the need to heat them near 60 °C to get a sufficient ionic conductivity. Ionic conduction occurs in the amorphous parts of the polymer, and it depends on the salt concentration and dissociation. Vibrational spectroscopies, 2,3 IR and Raman, have proved to be efficient tools to determine these characteristics, especially for lithium triflate (LiCF 3 SO 3 , hereafter denoted LiTf)-PEO complexes. Internal modes of the triflate anion (Tf - ) CF 3 SO 3 - have been particularly studied with a special attention to the SO 3 symmetric stretching mode near 1050 cm -1 and the CF 3 symmetric deformation 4 near 760 cm -1 since these two modes show bands depending on the Li + -Tf - association. Another region of interest is located near 860 cm -1 , where a band appears in the Li salt-PEO complexes and which is associated with ether oxygen breathing about a Li + ion solvated by the macromolecular chain 5 or with CH 2 rocking influenced by the Li + solvation. 6 It has to be noticed that most studies were conducted at room temperature, while PEO-based electrolytes have good electrical properties above the PEO melting temperature of 60 °C. One noticeable exception is the IR study by Frech et al. 7,8 performed at high temperature, who concluded an amorphous phase existed in addition to the pure PEO and compound PEO 3 LiTf was defined with a O/Li ratio of 3, as expected from the phase diagram. 9 Our objective in this study was to provide a more systematic view of the structure of PEO-LiTf complexes as a function of temperature and composition, as it can be observed by Raman spectroscopy. For this purpose, we have determined the Raman spectrum of PEO and of PEO-LiTf complexes with O/Li ranging from 4 to 18 between room temperature and 160 °C. We have used as excitation light a near-infrared source, thus minimizing the problems associated with fluorescence. We have determined the Raman spectrum of the (PEO) 3 LiTf defined compound and used it to determine the Raman spectrum of the fraction of the complex not belonging to this compound, with a particular view to the interactions of Li + with Tf - and with the polymeric chain. Experimental Section LiCF 3 SO 3 was purchased from Acros, dried in vacuum at 120 °C, and stored in a glovebox in argon atmosphere with less than 3 ppm O 2 and H 2 O. High molecular weight PEO (M w ) 5 × 10 6 g mol -1 ) was purchased from Aldrich and used as- received. Mixtures were prepared in a glovebox by dissolving the appropriate amounts of PEO and LiTf in acetonitrile. The resulting solution was stirred overnight and cast on a Teflon plate. Solvent evaporation was carried out in a glovebox for 12 h at room temperature. Films were then dried in vacuum at 80 °C for 48 h and stored in a glovebox in argon. For Raman measurements, films were placed between two glass plates sealed with Torr-seal glue. * To whom correspondence should be addressed. E-mail: Fannie.alloin@ lepmi.grenoble-inp.fr. Fax: 33 (0)4 76 82 65 77. Tel.: 33 (0)4 76 82 65 61. Figure 1. Raman spectrum of (PEO) 11 LiTf at 30 °C as acquired (bottom) and after fluorescent background and borosilicate glass spectra removal (top). J. Phys. Chem. B 2009, 113, 16465–16471 16465 10.1021/jp904356a  2009 American Chemical Society Published on Web 12/08/2009