Molecular dynamics simulation of polymer electrolytes based on poly(ethylene oxide) and ionic liquids. I. Structural properties

Molecular dynamics (MD) simulations have been performed for prototype models of polymer electrolytes in which the salt is an ionic liquid based on 1-alkyl-3-methylimidazolium cations and the polymer is poly(ethylene oxide), PEO. The MD simulations were performed by combining the previously proposed models for pure ionic liquids and polymer electrolytes containing simple inorganic ions. A systematic investigation of ionic liquid concentration, temperature, and the 1-alkyl- chain length, [1,3-dimethylimidazolium]PF6, and [1-butyl-3-methylimidazolium]PF6, effects on resulting equilibrium structure is provided. It is shown that the ionic liquid is dispersed in the polymeric matrix, but ionic pairs remain in the polymer electrolyte. Imidazolium cations are coordinated by both the anions and the oxygen atoms of PEO chains. Probability density maps of occurrences of nearest neighbors around imidazolium cations give a detailed physical picture of the environment experienced by cations. Conformational changes on PEO chains upon addition of the ionic liquid are identified. The equilibrium structure of simulated systems is also analyzed in reciprocal space by using the static structure factor, S(k). Calculated S(k) display a low wave-vector peak, indicating that spatial correlation in an extended-range order prevail in the ionic liquid polymer electrolytes. Long-range correlations are assigned to nonuniform distribution of ionic species within the simulation box.     

Molecular dynamics simulation of the polymer electrolyte poly(ethylene oxide)/LiClO(4). II. Dynamical properties

The dynamical properties of the polymer electrolyte poly(ethylene oxide) (PEO)LiClO(4) have been investigated by molecular dynamics simulations. The effect of changing salt concentration and temperature was evaluated on several time correlation functions. Ionic displacements projected on different directions reveal anisotropy in short-time (rattling) and long-time (diffusive) dynamics of Li(+) cations. It is shown that ionic mobility is coupled to the segmental motion of the polymeric chain. Structural relaxation is probed by the intermediate scattering function F(k,t) at several wave vectors. Good agreement was found between calculated and experimental F(k,t) for pure PEO. A remarkable slowing down of polymer relaxation is observed upon addition of the salt. The ionic conductivity estimated by the Nernst-Einstein equation is approximately ten times higher than the actual conductivity calculated by the time correlation function of charge current.     

Composite electrolytes based on poly(ethylene oxide) and binary ionic liquids for dye-sensitized solar cells

The sunlight is the largest single available source of clean and renewable energy to ensure human society’s sustainable development. Owing to their low production cost and high energy conversion efficiency, dye-sensitized solar cells (DSSCs) have been regarded as good alternatives to conventional photovoltaic devices. Herein, a series of composite electrolytes based on poly(ethylene oxide) (PEO) and the binary ionic liquids 1-propyl-3-methy-imidazolium iodide ([PMIm]I) and 1-ethyl-3-methylimidazolium thiocyanate ([EMIm][SCN]) were prepared and then applied to fabricate six DSSCs. The composite electrolytes were characterized by fourier transform infrared spectroscopy (FTIS), X-ray diffraction (XRD), and electrochemical impedance spectra (EIS). It was shown that the addition of binary ionic liquids would reduce the degree of crystallinity of PEO, thus improving the ionic conductivities of the electrolytes by about 2 orders of magnitude. Investigation on the photovoltaic performances of these DSSCs showed that the fill factor (FF) could reach up to 0.67 and energy conversion efficiency (η) could reach up to 4.04% under AM 1.5 full sunlight (100 mW/cm2).     

Vibrational spectroscopic study of ionic association in poly(ethylene oxide)-NH4SCN polymer electrolytes

The polymer-ammonium complexes are an important class of proton conducting polymer electrolytes. In this work, poly(ethylene oxide) (PEO)-NH(4)SCN electrolytes were prepared over a large range of the salt content, and their FT-IR spectra were measured at room temperature. Based on the assignments of each band in the spectral envelope of SCN(-1), their relative intensities are determined by the use of FT-IR technique. Following the experimental results and spectral analyses, this paper reports the interactions, the various ionic associations, the changes of the ionic association with NH(4)SCN content, and the characteristics of structure in PEO-NH(4)SCN electrolytes. It is shown that the hydrogen bonds of PEO and NH(4)SCN exert the great effect to the ionic association, the interactions of PEO with NH(4)SCN, and PEO crystallinity, in particular, under the condition of high NH(4)SCN content. In addition, the differences of ionic association among PEO-NaSCN, PEO-KSCN and NH(4)SCN electrolytes are also compared in this paper.     

Molecular dynamics simulation of interactions between a sodium dodecyl sulfate micelle and a poly(ethylene oxide) polymer

We have performed atomistic molecular dynamics simulations of an anionic sodium dodecyl sulfate (SDS) micelle and a nonionic poly(ethylene oxide) (PEO) polymer in aqueous solution. The micelle consisted of 60 surfactant molecules, and the polymer chain lengths varied from 20 to 40 monomers. The force field parameters for PEO were adjusted by using 1,2-dimethoxymethane (DME) as a model compound and matching its hydration enthalpy and conformational behavior to experiment. Excellent agreement with previous experimental and simulation work was obtained through these modifications. The simulated scaling behavior of the PEO radius of gyration was also in close agreement with experimental results. The SDS-PEO simulations show that the polymer resides on the micelle surface and at the hydrocarbon-water interface, leading to a selective reduction in the hydrophobic contribution to the solvent-accessible surface area of the micelle. The association is mainly driven by hydrophobic interactions between the polymer and surfactant tails, while the interaction between the polymer and sulfate headgroups on the micelle surface is weak. The 40-monomer chain is mostly wrapped around the micelle, and nearly 90% of the monomers are adsorbed at low PEO concentration. Simulations were also performed with multiple 20-monomer chains, and gradual addition of polymer indicates that about 120 monomers are required to saturate the micelle surface. The stoichiometry of the resulting complex is in close agreement with experimental results, and the commonly accepted "beaded necklace" structure of the SDS-PEO complex is recovered by our simulations.     

Single-ion and salt conductor polymer electrolytes based on poly(4-vinylpyridine) quaternized with poly(ethylene oxide) side chains

A new type of single-ion conductor with fixed cation was synthesized by spontaneous anionic polymerization of 4-vinylpyridine in the presence of short polyethylene oxide ( PEO ) chains as alkylating agents. These comblike polymers have low T_gs and are amorphous with the shorter PEO s. Their conductivities are unaffected by the nature of the anion ( Br ⁻, ClO ₄⁻, and tosylate) and are controlled by the free volume and the mobility of the pendant cation. By comparison of the results at constant free volume, it is shown that the charge density decreases with the increasing length of pendant PEO demonstrating that PEO acts only as a plasticizing agent. Best conductivity results (σ = 10⁻⁵ S cm⁻¹ at 60°C) are obtained with PEO side chains of molecular weight 350. With this sample, the conductivity in the presence of various amounts of added salt (LiTFSI) was studied. A best value of 10⁻⁴ S cm⁻¹ at 60°C is obtained with a molar ratio EO/Li of 10. It is shown that, over the range of examined concentrations (0.2–1.3 mol Li kg⁻¹), the reduced conductivity σ_r/c increases linearly with increasing salt concentration showing that the ion mobility increases continuously. Such behavior is quite unusual since in this concentration range a maximum is generally observed with PEO systems. To interpret this result and by analogy with the behavior of this type of polymer in solution, it is proposed that the conformation of these polymers in the solid state is segregated with the P4VP skeleton more or less confined inside the dense coils surrounded by the PEO side chains. Under the influence of the increasing salt concentration, this microphase separation vanishes progressively: The LiTFSI salt exchanges with the tosylate anions and acts as a miscibility improver agent. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2719–2728, 1997     

Highly conducting solid electrolytes based on poly (ethylene oxide-co-propylene oxide)

The conducting properties of solid electrolytes comprising random poly(ethylene oxide-co-propylene oxide) (of 84 : 16 monomer units mole ratio) and lithium, sodium, potassium, cesium, and rubidium salts have been studied. The systems containing some lithium or sodium salts achieved conductivity levels as high as 10^?5–10^?4 S/cm at ambient temperature and greater than 10^?3 S/cm at 100°C. However, the systems with rubidium and cesium salts exhibit conductivities a few orders of magnitude smaller. DSC studies show that the electrolytes studied are characterized by a high content of an amorphous phase (95–100%). It is suggested that the copolymer exhibits lower complexing abilities than that of poly(ethylene oxide), which results in a higher flexibility of electrolytes containing small cations and poor dissociation of the salts having large cations. © 1995 John Wiley & Sons, Inc.     

Effects of electrolytes on adsorbed polymer layers: poly(ethylene oxide)-silica system

The effects of various electrolytes on the adsorption of poly(ethylene oxide) onto silica have been studied. The salts were the chlorides of Na+, Mg2+, Ca2+, and La3+. The methods used were adsorption isotherms, found using a depletion method with phosphomolibdic acid, photon correlation spectroscopy, and solvent relaxation NMR. All the salts increased the particle-polymer affinity and adsorbed amount according to the adsorption isotherms, and a linear relationship was found between the initial slope of the isotherms and the ionic strength of the solution. Final adsorbed amounts were approximately 0.4-0.5 mg m(-2). The polymer layer thicknesses as found by PCS were of the same order as the radius of gyration of the polymer and increased with both the concentration and the valency of the salt due to increased adsorption. Solvent relaxation NMR showed that NaCl is too weak to have a noticeable effect on the polymer train layer, but the divalent salts clearly did increase both the strength of solvent binding close to the silica surface and the amount of PEO required to reach the maximum train density.     

Molecular dynamics simulations of a poly(ethylene oxide) surface

Potentials developed earlier for crystalline and amorphous bulk PEO systems have been used for the MD simulation of a PEO surface model. The surface comprises the outer region of a 122 Å-thick sheet of PEO in which the PEO, -(CH₂-CH₂-O)_n- chains run obliquely across the cell, and are terminated by C₂H₅ ethyl groups. The atoms on one side of the sheet are tethered to facilitate a satisfactory Ewald summation. The sheet expands from its ‘crystalline’ width of 122 Å to 128 Å in the simulated model. Simulations were performed at three temperatures: 300 K, 400 K and 500 K. Different behaviour in the surface layer was found compared to that in the bulk. The structural and dynamical properties of the surface were analyzed at each temperature.     

Solid polymer electrolytes I,preparation, characterization,and ionic conductivity of gelled polymer electrolytes based on novel crosslinked siloxane/poly(ethylene glycol) polymers

A series of crosslinked siloxane/poly(ethylene glycol) (Si–PEG) copolymers were synthesized from the reactive methoxy‐functional silicone resin (Si resin) and PEGs with different molecular weights via two kinds of crosslinking reactions during an in situ curing stage. One of the crosslinking reactions is the self‐condensation between two methoxy groups in the Si resin, and another one is an alkoxy‐exchange reaction between the methoxy group in the Si resin and the OH group in PEG. The synthesized crosslinked copolymers were characterized by Fourier transform infrared spectroscopy, DSC, and ¹³C NMR. The crosslinked copolymers were stable in a moisture‐free environment, but the Si? O? C linkages were hydrolyzed in humid conditions. The gel‐like solid polymer electrolytes (SPEs) were prepared by impregnating these crosslinked Si–PEG copolymers in a propylene carbonate (LiClO₄/PC) solution. The highest conductivity reached 2.4 × 10^?4 S cm^?1 at 25 °C and increased to 8.7 × 10^?4 S cm^?1 at 85 °C. The conductivities of these gel‐type SPEs were affected by the content of LiClO₄/PC, the molecular weights of PEGs, and the weight fraction of the Si resin. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2051–2059, 2004     

Novel zinc ion conducting polymer gel electrolytes based on ionic liquids

We report novel zinc ion conducting polymer gel electrolytes (PGEs) based on non-volatile room temperature ionic liquids. The PGEs consist of an ionic liquid, with a zinc salt dissolved in it, blended with a polymer matrix, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The resultant electrolyte membranes are freestanding, translucent, flexible and elastic, with excellent mechanical integrity and strength. They possess exceptional thermal stability, exhibit essentially no weight loss under dynamic vacuum or upon heating to 200 °C, and remain the same gel phase in wide temperature ranges, with ionic conductivities on the order of 10⁻³ S/cm at room temperature, 10⁻⁴ S/cm at −20 °C and 4–5 × 10⁻³ S/cm at 80 °C. Electrochemical tests show that zinc ions are mobile in the membranes and zinc metal is capable of dissolution into and deposition from the membranes. The membranes also exhibit wide electrochemical stability windows. The results of this study demonstrate the promise of developing PGEs based on ionic liquids for potential application in next-generation non-aqueous zinc battery systems.     

Synthesis and properties of all solid polymer electrolytes based on poly(vinyl acetate‐co‐acetyl ethylene oxide acrylate)

Poly(acetyl ethylene oxide acrylate‐co‐vinyl acetate) (P(AEOA‐VAc)) was synthesized and used as a host for lithium perchlorate to prepare an all solid polymer electrolyte. Introduction of carbonyl groups into the copolymer increased ionic conductivity. All solid polymer electrolytes based on P(AEOA‐VAc) at 14.3 wt% VAc with 12wt% LiClO₄ showed conductivity as high as 1.2 × 10^?4 S cm^?1 at room temperature. The temperature dependence of the ionic conductivity followed the VTF behavior, indicating that the ion transport was related to segmental movement of the polymer. FTIR was used to investigate the effect of the carbonyl group on ionic conductivity. The interaction between the lithium salt and carbonyl groups accelerated the dissociation of the lithium salt and thus resulted in a maximum ionic conductivity at a salt concentration higher than pure PAEO‐salts system. Copyright © 2010 John Wiley & Sons, Ltd.     

Amphiphilic polymer gel electrolytes. I. preparation of gels based on poly(ethylene oxide) graft copolymers containing different ionophobic groups

Amphiphilic graft copolymers were prepared via the radical copolymerization of poly(ethylene oxide) (PEO) macromonomers with fluorocarbon or hydrocarbon acrylates in toluene with 2,2′‐azobisisobutyronitrile (AIBN) as an initiator. ¹H NMR spectroscopy confirmed that the composition of the graft copolymers corresponded well to the monomer feed. For gel electrolytes prepared from the amphiphilic copolymers, the nature of the ionophobic parts of the amphiphilic graft copolymers had a great influence on the ion conductivity. Gel electrolytes based on graft copolymers containing fluorocarbon side chains showed significantly higher ion conductivity than electrolytes based on graft copolymers containing hydrocarbon groups. The ambient‐temperature ion conductivity was about 2.6 mS/cm at 20 °C for a gel electrolyte based on an amphiphilic graft copolymer consisting of an acrylate backbone carrying PEO and fluorocarbon side chains. Corresponding gels based on graft copolymers with PEO side chains and hydrocarbon groups showed an ambient‐temperature ion conductivity of about 1.2 mS/cm. The gel electrolytes contained 30 wt % copolymer and 70 wt % 1 M LiPF₆ in an ethylene carbonate/γ‐butyrolactone (2/1 w/w) mixture. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2223–2232, 2001     

Morphology of ultrathin polymer films based on poly(ethylene oxide) blends

The structure of ultrathin (15–200 nm) films of two types prepared from polymer blends based on PEO (the crystallizable component), namely, PEO-poly(arylene sulfone oxide) (the amorphous component) and PEO-PB (the amorphous component), has been studied by atomic force microscopy. The content of PEO in both blends is 76 wt %. Ultrathin blend films have been applied on a Si substrate via substrate dipping into dilute solutions of polymer blends in chloroform at room temperature. The rate of the substrate lift has been varied from 0.1 to 1 mm/min. The amorphous-amorphous separation takes place during formation of ultrathin films of the above blends in the course of the substrate lift at the stage of gelation. The crystallization of PEO and dewetting in the resulting two-phase blend gels depend on the rate of the substrate lift and the rigidity of macromolecules of the amorphous component. Moreover, the predominant interaction of the substrate with one of the components plays a significant role in structure formation of ultrathin films of both polymer blends.     

Glycolysis of poly(ethylene terephthalate) catalyzed by ionic liquids

Poly(ethylene terephthalate) (PET) from an industrial manufacturer was depolymerized by ethylene glycol in the presence of a novel catalyst: ionic liquids. It was found that the purification process of the products in the glycolysis catalyzed by ionic liquids was simpler than that catalyzed by traditional compounds, such as metal acetate. Qualitative analysis showed that the main product in the glycolysis process was the bis(hydroxyethyl) terephthalate (BHET) monomer. Thermal analysis of the glycolysis products was carried out by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The influences of experimental parameters, such as the amount of catalyst, glycolysis time, reaction temperature, and water content in the catalyst on the conversion of PET, selectivity of BHET, and distribution of the products were investigated. Results show that reaction temperature is a critical factor in this process. In addition, a detailed reaction mechanism of the glycolysis of PET was proposed.     

Molecular dynamics simulation of dual amino-functionalized imidazolium-based ionic liquids

In this work, the all-atom (AA) force fields were set up for three kinds of dual amino-functionalized imidazolium-based ionic liquids (ILs), composed by cations with different alkyl chain length and amino acid anion [Gly]⁻. The force field was based on our previous work and the default parameters were developed in this study. Molecular dynamics simulations were performed. Validation was carried out by comparing simulation densities with experimental data, and good agreement was obtained. Molar volume and heat capacity at constant pressure were predicted. Mean square displacements for these ILs were computed and these ILs were proved to move very slowly. It may be caused by hydrogen-bonded network between ions and the terminal azyl. To depict the microscopic structures of the ILs, many types of radial distribution functions were investigated. It is interesting to find that not only the cation and anion, but also the anions themselves will form hydrogen bonds.     

Recent advances in solid-state polymer electrolytes and innovative ionic liquids based polymer electrolyte systems

Solid polymer electrolytes are a promising alternative to widely used liquid carbonate electrolytes to deliver next-generation lithium-ion batteries with improved safety. However, the limited ionic conductivity and high interfacial resistance with electrodes limit their widespread use. This review aims to give an overview of the recent research on performance aspects and strategies of solid polymer electrolytes, including ionic conductivity, lithium transference number, design flexibility, scale-up, and integration of ionic liquids with a focus on safety.     

Dielectric relaxation and ionic transport in poly(ethylene carbonate)‐based electrolytes

To study the ion‐conductive and dielectric properties of polymer electrolytes based on poly(ethylene carbonate) (PEC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the complex permittivity and conductivity were measured using broadband dielectric spectroscopy. The temperature dependence of the relaxation frequency and ionic conductivity for PEC‐LiTFSI electrolytes (1 – 200 mol%) indicates that the segmental motion of PEC chains decreases with the addition of just 1 mol% of Li salt and increases with increasing concentration above 10 mol%. According to the Walden rule for PEC‐based electrolytes, the value of deviation from the reference line increased, and the fragility and decoupling exponents decreased with increasing salt concentration. These results indicate that there are large numbers of ion pairs and aggregated ions, which imply low ionicity and reduced fragility in highly concentrated PEC‐based electrolytes. Copyright © 2016 John Wiley & Sons, Ltd.     

Molecular dynamics simulation study of the structure of poly(ethylene oxide) brushes on nonpolar surfaces in aqueous solution

The structure of poly(ethylene oxide) (PEO, M(w) = 526) brushes of various grafting density (sigma) on nonpolar graphite and hydrophobic (oily) surfaces in aqueous solution has been studied using atomistic molecular dynamics simulations. Additionally, the influence of PEO-surface interactions on the brush structure was investigated by systematically reducing the strength of the (dispersion) attraction between PEO and the surfaces. PEO chains were found to adsorb strongly to the graphite surface due primarily to the relative strength of dispersion interactions between PEO and the atomically dense graphite compared to those between water and graphite. For the oily surface, PEO-surface and water-surface dispersion interactions are much weaker, greatly reducing the energetic driving force for PEO adsorption. This reduction is mediated to some extent by a hydrophobic driving force for PEO adsorption on the oily surface. Reduction in the strength of PEO-surface attraction results in reduced adsorption of PEO for both surfaces, with the effect being much greater for the graphite surface where the strong PEO-surface dispersion interactions dominate. At high grafting density (sigma approximately 1/R(g)(2)), the PEO density profiles exhibited classical brush behavior and were largely independent of the strength of the PEO-surface interaction. With decreasing grafting density (sigma < 1/R(g)(2)), coverage of the surface by PEO requires an increasingly large fraction of PEO segments resulting in a strong dependence of the PEO density profile on the nature of the PEO-surface interaction.