Concentration effect of BMIMTf on P(VdF-HFP)/MgTf-based solid polymer electrolyte system
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Soon-Chien Lu Centre for Surface Chemistry and Catalysis, Faculty of Bioengineering Science, Katholieke Universiteit Leuven, 3001 Leuven, Belgium (Received 5 December 2011; accepted 8 February 2012)
Solid polymer electrolytes (SPEs) with poly(vinylidene fluoride-hexafluoropropylene) [P(VdF-HFP)] as polymer host, doped with magnesium trifluoromethanesulfonate (MgTf) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIMTf) have been synthesized via solution casting method. This P(VdF-HFP)/MgTf/BMIMTf-based SPE achieves ;3 103 and ;7 103 Scm1 at 30 and 80 °C, respectively, with 75 part by weight (pbw) of BMIMTf. At the same time, they are also examined by means of frequency-dependent conductivity, dielectric permittivity, and dielectric modulus studies. Scanning electron microscopy reveals drastic morphological changes on SPE with small amount of BMIMTf. Even though it gradually changes back to its undoped state with higher concentration, it appears to be swollen. Examination on relationship between ionic conductivity and crystallinity by differential scanning calorimetry technique shows inconsistency at concentration higher than 75 pbw. This observation is related to greater ion–ion interaction due to excessive BMIMTf. Photoluminescence is also used to detect structural alterations in the local environment of SPE. I. INTRODUCTION
Polymer electrolytes are polymer-based systems that show ionic conductivity, which allows their use in solidstate devices such as batteries and capacitors.1 However, conventional organic sol–gel electrolyte used in these devices is expected to be replaced by solid polymer electrolyte (SPE) in the near future.2 SPE offers several advantages to battery technology: low manufacturing cost and improved performance, economy of volume and mass, structural and chemical stability, low toxicity, and dual function as an electrode separator.3 However, its lower ionic conductivity is the main hindrance to be extensively commercialized. Incorporation of room temperature ionic liquids is one of the many ways to overcome this difficulty. Ionic liquids are molten organic salts at room temperature. They are nonvolatile, highly ionic, and display wide electrochemical stability window, and most of them are transparent due to their nature.4 Poly(vinylidenefluoride-hexafluoropropylene) [P(VdFHFP)] has been studied extensively for application in polymer electrolytes as it shows low crystallinity compared to PVdF.5 It has been used as quasi-solid-state materials in dye-sensitized solar cells6 and shown to have good compatibility with ionic liquid.7 To resolve the
problem of low compatibility with polymer and safety problem faced by lithium-conducting SPEs,8 it is essential to develop other alternatives to the lithium-based batteries. The search for magnesium containing SPE can be interesting not only for understating the multivalent cationic conductivity mechanism in the polymer but also due to their lower cost, ease of handling, and fabrication as thin film membranes,9 as compared to its lithium count
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