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J. Phys. Chem. B 2001, 105, 9016-9021
The Influence of Inert Oxide Fillers on Poly(ethylene oxide) and Amorphous Poly(ethylene oxide) Based Polymer Electrolytes† Patrik Johansson,‡ Mark A. Ratner, and Duward F. Shriver* Chemistry Department and Materials Research Center, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 ReceiVed: March 7, 2001; In Final Form: May 22, 2001
An FT-Raman study was performed on lithium salt/polymer/nanosized inert oxide filler (SiO2, Al2O3) systems. Molecular spectroscopy suggests that there is little effect of the filler particles on the “free” anion/ion-pair/ higher aggregate equilibria. Raman line widths indicate enhanced anion disorder when a small amount of filler is introduced. Similarly, DSC measurement indicates a reduction of crystallinity. By contrast, fully amorphous polymer samples display broader Raman line widths and higher glass transition temperatures as the filler concentration is increased, with concomitant lower ion conductivity. The influence of the fillers differs in proportion to the extent of crystallinity of the polymer-salt complex, but within our experimental range the particle size or composition of the filler has little influence on ionic conductivity.
Introduction Neat polymer electrolytes, consisting of salts in polar polymers, were prepared by Wright1 and collaborators, and their use as components in advanced batteries was suggested by Armand.2 Subsequent work has shown that these materials exhibit ionic conductivity above the glass transition temperature Tg, but the polymer host3 imparts some rigidity. Although the major interest of these materials has been in energy storage,4 they have also been used as hosts for electrochemical studies and redox chemistry5,6 and in other electrochemical systems such as light-emitting diodes7 and smart windows. Numerous reports have been published on solid polymer electrolytes containing nanosized particles.8-27 The first report on the use of inert filler particles in lithium salt/PEO, poly(ethylene oxide), polymer electrolytes was published by Weston and Steele in 1982.8 Their objective was to enhance the mechanical properties of the materials. More recently, the focus has centered mainly on the possibility of increased ion conductivity for materials containing small particles of oxides such as Al2O3,9-15 TiO2,9,14 γ-LiAlO2,16-18 and SiO2,14,19-23 and other filler compounds.10,12,24-26 The introduction of high surface area silica into polar polymers stabilizes PEO/ lithium salt electrolytes toward lithium electrodes, and improves the mechanical properties of the electrolyte.20-23 Most studies in this area focus on traditional long chain, high molecular weight, PEO/ lithium salt mixtures with added nanosized fillers. Properties such as ionic conductivity,9,15,16-19,25 stability against lithium anodes, and thermal properties have been investigated.8,9,10,11,16,19,25-27 Efforts have been made to determine the origin of the enhanced ion conductivity, and the general consensus is that the dominant conductivity enhancement originates from a reduction of the amount of polymer crystallinity. Little work is available on the action of fillers at the molecular level. †
Part of the special issue “Royce W. Murray Festschrift”. * Corresponding author. E-mail:
[email protected]. ‡ Current address: Experimental Physics, Chalmers University of Technology, Gothenburg, Sweden.
There is support for the transport of cations on particle surfaces.28-30 For example, Phipps and Whitmore measured surface transport on fused silica plates embedded in LiI, and they found ion conductivity values parallel to the surface as high as 2 S cm-1 at room temperature.31 This study opens the interesting possibility of surface-assisted fast ion transport in composite materials. Few studies have been performed on these composite materials to evaluate the local environment of the charge carriers with respect to “free” ion/ion-pair/higher aggregate equilibria. Zax and co-workers performed an NMR line-width study on different cations mobilities of metal iodides (M ) Li, Na, Rb, and Cs) in PEO/montmorillonite systems, and a change in the cation dynamics was found, but the specific molecular level interactions were not elucidated.26 This system is different from the present work in the sense that the PEO chains, with cations, are intercalated and monitored within the ∼1 nm monmorillonite galleries, whereas the present system consists of filler dispersed in a polymer matrix, which may strongly affect the polymer (and cation) dynamics. In a previous NMR study on LiClO4/ PEO/γ-LiAlO2, it was concluded that the Li mobility is not changed by the presence of the lithium aluminate filler.18 Vibrational spectroscopy has been used extensively to probe the local environments of the charge carriers.32-39 To our knowledge, the first vibrational spectroscopic studies of fillers in ternary systems utilized FT-IR to probe MClO4/amorphous PEO (aPEO) or PEO/R-Al2O3 systems, where M is Na+ or Li+.10 It was concluded that the sodium ions form ion pairs or higher aggregates as small amounts of fillers are added, as indicated by the C-O-C modes of the polymer backbone around 1116 cm-1. Also, the intensity of the symmetric stretching of the ClO4- anion at ∼938 cm-1 was observed to increase, and this was ascribed to ion-pair formation. However, the opposite effect occurs when lithium is the cation: an increase in “free” anion concentration is observed with increasing filler concentration, and a C-O-C band appears at ∼1090-1095 cm-1, which was attributed to lithium cation coordination of the ether oxygens. The two studies, however, did not use the same anion mode as the probe: the 620-635 cm-1 region was used in the latter
10.1021/jp010868r CCC: $20.00 © 2001 American Chemical Society Published on Web 06/27/2001
Influence of Oxide Fillers on PEO-Based Electrolytes study, and therefore a straightforward comparison is difficult. These two, somewhat contradictory studies, along with Raman studies by Croce et al.16 and by Best et al.,40 are the only attempts to use vibrational spectroscopy to probe local interactions in systems like these. The present work combines Raman spectroscopy with DSC measurements on several different systems to provide information on local interactions. Our emphasis is on the vibrational modes of the triflate, Tf, (CF3SO3-), and the TFSI, ([(CF3SO2)2N]-) anions. IR and Raman spectroscopy are very useful to resolve issues such as “free” ion/ion-pair equilibria for pure salt/polymer systems.32-39 Furthermore, electrolytes using the TFSI anion have among the highest ion conductivities, without the severe safety risks associated with the ClO4- anion, which also promotes high conductivity. The present work focuses on the concentration range where most other experiments are made and where “free” anions and ion pairs are the dominant species. For TFSI we expect no ion pairs for n > 6. For Tf we limit the system to n ) 40 due to the early occurrence of higher aggregates. Since particle size is proposed to be a crucial parameter,11 four different sizes of silica particles were employed. The components of the systems studied here are LiTFSI, LiTf (LiSO3CF3), PEO, amorphous PEO (aPEO), γ-Al2O3, and SiO2. Experimental Section All chemicals were carefully dried prior to use, and handled under a dry nitrogen atmosphere or vacuum. PEO (Mw ) 600 000, Aldrich) was dried at ∼50 °C for 24 h at 10-3 Torr, and thereafter on a high-vacuum line (
