Effect of a plasticizer on a solid polymer electrolyte

Adding a polyethylene glycol plasticizer did not significantly improve the ionic conductivity of two polyethylene oxide-metal salt electrolytes with nanofillers.Polymer electrolytes have been the focus of much fundamental and applied research for many years. The discovery of ionic conduction in…

Adding a polyethylene glycol plasticizer did not significantly improve the ionic conductivity of two polyethylene oxide-metal salt electrolytes with nanofillers.Polymer electrolytes have been the focus of much fundamental and applied research for many years. The discovery of ionic conduction in polyethylene oxide (PEO)-alkali metal salt solutions opened the door to the use of polymer electrolytes in devices such as batteries and electrochromic (electronically tintable) windows.1 Since then, a significant amount of work on various such polymer electrolytes, e.g., polymer-in-salt and salt-in-polymer complexation systems, has been reported in the literature. Though many researchers have studied silver ion2–9 conducting polymer electrolytes, there are very few reports on PEO and silver triflate (AgCF3SO3) salt systems. Yet evidence suggests that such complexes have favorable characteristics that make them well-suited for certain applications in electrochemical devices, gas sensors, and membrane systems.10Adding AgCF3SO3 salt to the PEO host polymer yields an increase in conductivity, owing to the dissociation of AgCF3SO3 salt, which produces more ions in the material, thus enhancing its ability to carry a current.11 However, a major limitation of these polymer salt complexes is their low ionic conductivity at ambient temperature. To overcome this, nanosized fillers such as aluminum oxide (Al2O3), silicon dioxide (SiO2), tellurium dioxide (TeO2), zirconium dioxide (ZrO2), and cerium dioxide (CeO2) are added to polymer-salt systems.12, 13 This method of incorporating fillers achieves improved mechanical, thermal, and ionic conductivity, as well as stabilizing the amorphous phase at lower temperature.14, 15 In previous work, we found that adding an SiO2 nanofiller to a PEO:AgCF3SO3 polymer salt complex enhances the ionic conductivity of the system.16, 17 The nanofiller particles provide almost continuous crystalline domain channels, leading to an increase in free volume, and thus allowing ion migration to take place easily. However, once the optimum conductivity has been achieved, any further additions of nanofiller begin to segregate due to non-complexation in the polymer salt matrix, and conductivity drops.Figure 1.Logarithm of ionic conductivity (σ) vs. reciprocal temperature (T) of the PEO:AgCF3SO3:SiO2solid electrolyte system with (black) and without (red) the polyethylene glycol (PEG) plasticizer. PEO: Polyethylene oxide host polymer. AgCF3SO3: Conducting silver triflate salt. SiO2: Silicon dioxide nanofiller. Figure 2.X-ray diffraction patterns of intensity vs. detector position angle (2θ) for the PEO:AgCF3SO3:SiO2nanocomposite polymer electrolyte system with and without the PEG plasticizer. A.U.: Arbitrary units.In an attempt to further optimize ionic conductivity, we have therefore investigated another attractive approach, which involves adding a low molecular weight plasticizer to a nanocomposite polymer electrolyte system. Though there are many plasticizers available, including polyethylene glycol (PEG), ethylene carbonate, propylene carbonate, and dimethylformamide,18 only an appropriate choice of plasticizer enhances ionic conductivity. In particular, we have found that the dielectric value of the plasticizer is a key factor in achieving this.19To further pursue this line of work, we investigated adding a PEG plasticizer to a PEO:AgCF3SO3:SiO2 nanocomposite polymer electrolyte system.20 Though addition of a plasticizer does not supply ions to the electrolyte system, it helps dissociate more of the salt into ions and generates separate conducting pathways for the migration of free ions, all of which should produce an increase in conductivity. However, we found that adding PEG to the PEO:AgCF3SO3:SiO2 electrolyte system did not yield the expected increase in conductivity (see Figure 1). This may be due to the similarity between the chemical structure of PEG and PEO, and also to the very small difference between their values of dielectric constant. The similarity in chemical structure between PEO and PEG gives rise to a chemical interaction—(CH2CH2)n + HO (H2CH2OH) → HO (CH2CH2O)n+1 H—which promotes the organization of segmental units to form a complex rigid structure, whose presence was confirmed by x-ray diffraction (XRD): see Figure 2. Fourier transform IR spectra20 of the system showed that peak intensities increased when PEG was added to the nanocomposite polymer electrolyte, indicating a reduction in the amorphous phase of the system. Similar inferences were also drawn from XRD. Differential scanning calorimetry data revealed a shift in the glass transition temperature (Tg), which tended to decrease with addition of the plasticizer. This may be a result of linkage between PEO and PEG chains, which reduces the segmental mobility of the chains and increases the crystalline content in the system. Scanning electron microscopy (SEM) showed that the nanocomposite polymer electrolyte system without a PEG plasticizer appears more smooth and homogeneous than its plasticized counterpart.20 The chemical bonds and linkages in the polymer complex are affected by the addition of a plasticizer, and two factors that play an important part in enhancing ionic conductivity are the dissociation constant and the dielectric constant of the plasticizer. The dielectric value of the plasticizer must be higher than that of the PEO host polymer to achieve an enhancement in conductivity.19 The small difference between the dielectric constants of PEO and PEG could thus be one reason why we did not find any significant increase in conductivity.Figure 3.Logarithm of ionic conductivity (σ) vs. reciprocal temperature (T) of the PEO:LiCF3SO3:Al2O3nanocomposite polymer electrolyte system with (black) and without (red) the PEG plasticizer. LiCF3SO3: Lithium trifluoromethanesulfonate conducting salt. Al2O3: Aluminum oxide nanofiller.The above outcomes prompted us to analyze a different PEO-based system, in which we replaced the AgCF3SO3 conducting salt with lithium trifluoromethanesulfonate (LiCF3SO3), and the SiO2 nanofiller with Al2O3. As before, we then prepared two versions of this PEO:LiCF3SO3:Al2O3 system, one with and one without a PEG plasticizer. On comparing the conductivity of these two systems, we found that at higher temperatures the plasticized system actually had a significantly lower conductivity than the system without PEG (see Figure 3). The shift in the value of Tg in the plasticized system was again toward a lower temperature (see Figure 4), and SEM images of the samples confirmed that addition of the PEG made the surface of the film rougher (see Figure 5).Figure 4.Differential scanning calorimetry thermograms of the PEO:LiCF3SO3:Al2O3nanocomposite polymer electrolyte system with and without the PEG plasticizer.Figure 5.Scanning electron micrographs of the PEO:LiCF3SO3:Al2O3solid electrolyte system with (a) and without (b) the PEG plasticizer.In summary, we investigated the effect of adding a PEG plasticizer to two different PEO-based solid electrolyte systems, and found that (unlike what happens with other plasticizers having higher dielectric values) this failed to improve the conductivity of the system. Moving forward, we plan to further understand other parameters related to the conduction and relaxation phenomena in these polymer electrolytes through dielectric, modulus, and other relaxation investigations.AuthorsNirali GondaliyaDepartment of Engineering Physics Shri Sad Vidhya Mandal Institute of TechnologyDinesh K. KanchanPhysics Department, Faculty of Science MSUDinesh Kanchan has a PhD from the Indian Institute of Technology Delhi. He is a professor in the Physics Department and director of the Research and Consultancy cell at MSU. His field of expertise is solid-state ionics, which includes glassy electrolytes, polymer nanocomposite solid electrolytes, polymer blends, and polymer blend nanocomposites.Poonam SharmaPhysics Department, Faculty of Science MSUReferencesM. Armand, The history of polymer electrolytes, Solid State Ionics 69 (3–4), pp. 309-319, 1994. S. Sunderrajan, B. D. Freeman, C. K. Hall and I. 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