Gel electrolytes based on crosslinked tetraethylene glycol diacrylate/poly(ethylenimine) systems

Gel electrolytes were prepared by crosslinking low molecular weight poly(ethylenimine) (PEI) with tetraethylene glycol diacrylate (TEG) in the presence of 2-methoxyethyl ether (diglyme) and lithium triflate (LiTf). Impedance and infrared (IR) spectroscopies were used as complimentary tools for studying the mode of ion conduction in these gel electrolytes. Ionic conductivity measurements for all samples tested exhibited significant LiTf and diglyme composition dependency. The maximum ionic conductivity at 20 °C was 2×10⁻⁴ S/cm with moderate LiTf and high diglyme compositions. The calculated molal concentration of non-ionically bound ‘free’ triflate ion was found to vary directly with ionic conductivity with the highest molality ‘free’ triflate samples yielding the highest ionic conductivity. Lithium ion interactions with the triflate ion, diglyme and the crosslinked polymer matrix were observed with IR spectroscopy. A lower frequency shoulder on the v_s(CO) vibrational mode increases in intensity as LiTf composition is increased. Curve fitting and molar calculations suggest that over 85% of the total lithium ions available are coordinated to the TEG carbonyl at dilute LiTf compositions.     

Evaluation of the role of an ionic liquid as organophilization agent into montmorillonite for NBR rubber nanocomposite production

In this work we have shown the preparation of montmorillonite (Na + Mt) co-intercalated with a quaternary ammonium salt (hexadecyltrimethylammonium — HDTMA) and an ionic liquid (IL) (1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide — OMImTf₂N) using a mechanochemical process. Changes in the structure of the organophilized montmorillonite were characterized using X-ray diffraction (XRD), thermal gravimetry (TG), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The presence of both cations (HDTMA and OMIm) into the clay mineral layers was confirmed by FTIR. XRD studies support that the co-intercalation process of the IL cation is more easily established in phases in which the interlayer space is not totally packed with HDTMA and an additional order in ionic liquid phase was observed when the IL amount exceeds 15 mass%. The addition of IL induced a thermal stability gain of 90 °C in comparison with the ammonium treated montmorillonite. The incorporation of IL in the ammonium based organoclays also displayed their better compatibility with the elastomeric matrix (NBR). Morphology studies by SEM of cryofractured surfaces evidence an improved interface interaction between the clay mineral and elastomer, indicating that ionic liquid has a direct and pronounced effect on the final clay polymer nanocomposites. This result can be assigned to the covered clay mineral particles mainly for higher IL content.     

Morphological,infrared, and ionic conductivity studies of poly(ethylene oxide)–49% poly(methyl methacrylate) grafted natural rubber–lithium perchlorate salt based solid polymer electrolytes

The potential of poly(ethylene oxide) (PEO) and 49% poly(methyl methacrylate) grafted natural rubber (MG49) as a polymer host in solid polymer electrolytes (SPE) was explored for electrochemical applications. PEO–MG49 SPEs with various weight percentages of lithium perchlorate salt (LiClO₄) was prepared with the solution casting technique. Characterization by scanning electron microscopy, Fourier transform infrared spectroscopy, and impedance spectroscopy was done to investigate the effect of LiClO₄ on the morphological properties, chemical interaction, and ionic conductivity behavior of PEO–MG49. Scanning electron microscopy analysis showed that the surface morphology of the sample underwent a change from rough to smooth with the addition of lithium salts. Infrared analysis showed that the interaction occurred in the polymer host between the oxygen atom from the ether group (C? O? C) and the Li⁺ cation from doping salts. The ionic conductivity value increased with the addition of salts because of the increase in charge carrier up to the optimum value. The highest ionic conductivity obtained was 8.0 × 10^?6 S/cm at 15 wt % LiClO₄. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Hydrogen bonding effect on the poly(ethylene oxide), phenolic resin,and lithium perchlorate–based solid‐state electrolyte

The interaction behavior of solid‐state polymer electrolytes composed of poly(ethylene oxide) (PEO)/novolac‐type phenolic resin and lithium perchlorate (LiClO₄) was investigated in detail by DSC, FTIR, ac impedance, DEA, solid‐state NMR, and TGA. The hydrogen bonding between the hydroxyl group of phenolic and ether oxygen of the PEO results in higher basicity of the PEO. The higher basicity of the ether group can dissolve the lithium salts more easily and results in a greater fraction of “free” anions and thus higher ionic conductivity. DEA results demonstrated that addition of the phenolic increases the dielectric constant because of the partially negative charge on the ether group induced by the hydrogen bonding interaction between ether oxygen and the hydroxyl group. The study showed that the blend of PEO(100)/LiClO₄(25)/phenolic(15) possesses the highest ionic conductivity (1.5 × 10^?5 S cm^?1) with dimensional stability. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1207–1216, 2004     

Characterization of solid polymer electrolytes based on poly(trimethylenecarbonate) and lithium tetrafluoroborate

The results of an investigation of a polymer electrolyte system based on the poly(trimethylene carbonate) host matrix, designated as p(TMC), with lithium tetrafluoroborate guest salt are described in this presentation. Electrolytes with lithium salt compositions with n between 3 and 80 (where n represents the number of (OCOCH₂CH₂CH₂O) units per lithium ion) were prepared by co-dissolution of salt and polymer in anhydrous tetrahydrofuran. The homogeneous solutions obtained by this procedure were evaporated, within a preparative glovebox and under a dry argon atmosphere, to form thin films of electrolyte.The solvent-free electrolyte films produced were obtained as very flexible, transparent, completely amorphous films and were characterized by measurements of total ionic conductivity, cyclic voltammetry, differential scanning calorimetry and thermogravimetry.     

New solid polymer electrolytes based on PEO/PAN hybrids

Hybrid polymer dry electrolytes comprised of poly(ethylene oxide) (PEO), polyacrylonitrile (PAN), and LiClO₄ were investigated. The impedance spectroscopy showed that the effect of PAN on the ion conductivity of PEO‐based electrolytes depends on the concentration of lithium salt. When the mole ratio of lithium to oxygen is 0.062 (15%LiClO₄‐PEO), adding PAN will increase the ionic conductivity. Differential scanning calorimetry, NMR, and IR data suggested that the enhanced conductivity was due to both the decreasing of the PEO crystallinity and increasing of the degree of ionization of lithium salt. There was obviously no interaction between PAN and lithium ions, and PAN acts as a reinforcing filler, and hence contributes to the mechanical strength besides reducing the crystallinity of the polymer electrolytes. When the LiClO₄‐PEO‐PAN hybrid polymer electrolyte was heated at 200°C under N₂, PAN crosslinked partially, which further decreased the crystallinity of PEO and increased the ionic conductivity, and at the same time prevented the recrystallization of PEO upon sitting at ambient environment. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1530–1540, 2006     

Ionic properties of non-aqueous liquid and PVDF-based gel electrolytes containing a cesium thiolate/disulfide redox couple

Liquid electrolytes containing a cesium thiolate/disulfide redox couple, prepared from 5-mercapto-1-methyltetrazole cesium salt (CsT) and di-5-(1-methyltetrazole)disulfide (T₂) dissolved in several aprotic solvents and solvent mixtures, have been studied using various techniques. FTIR spectroscopy reveals that relatively strong interactions occur between the reduced species T⁻ and DMSO or DMF while Cs⁺ ions are very weakly coordinated to the SO or CO bond. It is shown that the electrolyte consisting of 1.55 mol kg⁻¹ CsT in the solvent mixture DMSO/DMF (40/60%) exhibits the highest conductivity (1.1×10⁻² and 2.3×10⁻² S cm⁻¹ at 23 and 80 °C, respectively), and that the presence of the oxidized species T₂ does not affect significantly its electrical properties up to a CsT:T₂ molar ratio of 5:1. Conductivity values as a function of salt concentration are discussed in terms of the effective number of charge carriers, taking into account the level of ionic association, and of the ionic mobility. Optically transparent gel electrolytes have been prepared by incorporation of the optimal liquid electrolyte into various amounts of poly(vinylidene fluoride) (PVDF). It is shown that ionic mobility is not much affected by the polymer concentration, suggesting that migration of ions occurs mainly through the solvent mixture surrounded by the PVDF matrix.     

The effect of EPIDA units on the conductivity of poly(ethylene glycol)-4,4′-diphenylmethane diisocyanate-EPIDA polyurethane electrolytes

Novel thermoplastic polyurethanes with chelating groups were synthesized from 4,4′-diphenylmethane diisocyanate (MDI), poly(ethylene glycol) (PEG), and EP-IDA. Differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FT-IR), and impedance spectroscopy (IS) were used to monitor changes in the morphology of these polyurethanes with the concentration of lithium perchlorate (LiClO₄) dopants. Adding the salt significantly changes the FTIR spectrum of the polyurethane, indicating an interaction between the lithium cation within the urethane group and the chelating group. The soft segment T_g increases with LiClO₄ concentration, as determined by DSC, indicating that solubility of the lithium cation in the host polyurethane increases with the chelating groups. IS shows that the bulk conductivity reaches a maximum as the salt concentration is increased. One of the investigated polyurethane electrolytes has an ionic conductivity as high as ∼10⁻⁶ S cm⁻¹ at room temperature.     

The environment of lithium ions and conductivity of comb-like polymer electrolyte with a chelating functional group

The behavior of lithium ions in a comb-like polymer electrolyte with a chelating functional group have been characterized by differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), Fourier transform infrared (FTIR) spectroscopy, ac impedance and ⁷Li solid-state NMR measurements. The comb-like copolymer is synthesized by poly(ethylene glycol-methyl ether methacrylate) (PEGMEM) and (2-methylacrylic acid 3-(bis-carboxymethylamino) -2-hydroxy-propyl ester) (GMA-IDA). FTIR and ⁷Li solid-state NMR spectra demonstrate the interactions of Li⁺ ions with both the ether oxygen of the PEGMEM and the nitrogen atom of the GMA-IDA segments. Moreover, ⁷Li solid-state NMR shows that the lithium ions are preferentially coordinated to the GMA-IDA segment. The T_g increases for the copolymers doped with LiClO₄. These results indicate the interactions of Li⁺ with both PEGMEM and GMA-IDA segments form transient cross-links. The Vogel-Tamman-Fulcher (VTF)-like behavior of conductivity implies the coupling of the charge carriers with the segmental motion of the polymer chains. The dependence of the maximum conductivity on the composition of the copolymers and the doping lithium ion concentration was determined. The GMA-IDA unit in the copolymer improves the dissociation of the lithium salt, the mechanical strength and the conductivity.     

Ionic conductivity of polymer gels deriving from alkali metal ionic liquids and negatively charged polyelectrolytes

We have prepared polymer gel electrolytes with alkali metal ionic liquids (AMILs) that inherently contain alkali metal ions. The AMIL consisted of sulfate anion, imidazolium cation, and alkali metal cation. AMILs were mixed directly with poly(3-sulfopropyl acrylate) lithium salt or poly(2-acrylamido-2-methylpropanesulfonic acid) lithium salt to form polymer gels. The ionic conductivity of these gels decreased with increasing polymer fraction, as in general ionic liquid/polymer mixed systems. At low polymer concentrations, these gels displayed excellent ionic conductivity of 10⁻⁴ to 10⁻³ S cm⁻¹ at room temperature. Gelation was found to cause little change in the diffusion coefficient of the ionic liquid, as measured by pulse-field-gradient NMR. These data strongly suggest that the lithium cation migrates in successive pathways provided by the ionic liquids.     

Determination of lithium transference number in PEO-DME-LiClO4 modified with alumina powders of various surface acidity

The effect of alumina additives bearing various surface groups on conductivity and lithium cation transference numbers in poly(ethylene oxide) dimethyl ether (PEO-DME)-LiClO₄ electrolytes is examined. It is demonstrated that an increase in the conductivity and lithium transference number in composite electrolytes compared to pure PEO-DME-LiClO₄ electrolyte is observed in the limited salt concentration range. Both quantities seem to depend mostly on ionic species mobility. Also, their salt concentration dependence resembles that of viscosity of electrolytes studied. The conduction mechanism is discussed on the basis of conductivity, transference numbers and ionic association studies.     

Spectroscopic and electrochemical studies on block‐polymer/PMMA blend based composite polymer electrolytes

Poly(methylmethacrylate)(PMMA)/oxymethylene‐linked polyoxyethylene multi‐block polymer(Om‐POE_n, where n represents number of unit  CH₂CH₂O ) blend based composite electrolyte films containing different lithium salt concentration and nanofillers' content are prepared using solvent evaporation technique. The interaction of polymer–salt complex has been confirmed using FTIR spectral studies. The figuration of CPE was studied by XRD. Ionic conductivity and thermal behavior of the CPEs were studied with various salt concentrations, temperature, and nanofillers' content. The surface structure of the CPE is also investigated using scanning electron microscopy. The high room temperature ionic conductivity, transmittivity in the visible region, and thermal stability make these CPEs potential candidates as solid‐like electrolytes for electrochemical devices. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Chemical–physical properties of bis(perfluoroalkylsulfonyl)imide-based ionic liquids

The chemical–physical properties of hydrophobic ionic liquids, tailored as electrolyte components for supercapacitors and lithium batteries, were investigated and compared. The ionic liquid samples are based on bis(perfluroalkylsulfonyl)imide anions coupled with different types of cation. The effect of the cation as well as the main cationic aliphatic side group, in combination with different anions, on the chemical–physical properties of the investigated ionic liquid samples was evaluated and discussed. The activation energy for the conduction mechanism and the glass transition temperature were obtained by fitting the conductivity vs. temperature data.     

The study on the conductivity and morphology of polyurethaneurea electrolytes based on poly(ethylene glycol) 2000 and LiClO4

Polyurethaneureas (PUU), which were synthesized from 4,4′-diphenylmethane diisocyanate (MDI), poly(ethylene glycol) (PEG, MW=2000), and 3,5-diaminobenzoic acid, were used as the matrix of the polyelectrolytes in this study. Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR), and ⁷Li magic angle spinning (MAS) solid-state NMR were used to monitor changes in the morphology of PUU electrolytes corresponding to the concentration of lithium perchlorate (LiClO₄) dopants. The results of DSC and FT-IR indicate the different polymer complexes formed by the interaction of the Li⁺ ions with the different coordination sites of PUU. The ⁷Li MAS solid-state NMR investigation of the PUU electrolytes points out that two different Li⁺ environments exist at lower temperature. The results of DSC and the ⁷Li MAS solid-state NMR show that Li⁺ ions are preferentially coordinated to the ether oxygen of the PEG soft-segment when the salt concentration is below 0.1 mmol LiClO₄(gPUU)⁻¹. Impedance spectroscopy measurements show that the conductivity behavior followed the Arrhenius equation and was influenced by the hard-segment T_g. One of the PUU electrolytes under the investigation has an ionic conductivity as high as 3.0×10⁻⁵ S cm⁻¹ at 30 °C.     

The effects of anion structure of lithium salts on the properties of in-situ polymerized thermoplastic polyurethane electrolytes

Thermoplastic polyurethane (TPU) electrolytes with lithium salts were prepared by an in-situ polymerization method. Three different lithium salts were used to study the effects of the anion structure on the properties of polyurethane electrolytes: LiCl, LiClO₄, LiN(SO₂CF₃)₂ (LiTFSI). The effects of the anion structure on monomer (PTMG) prior to polymerization and on the properties of TPU electrolytes post polymerization were investigated. The anion structure of lithium salt has a significant influence on the ionic conductivity, thermal stability and tensile property of TPU electrolytes. The TPU electrolytes with LiTFSI demonstrated a high ionic conductivity up to 10⁻⁵ S/cm at 300 K. The ionic conductivity of polyurethane electrolytes with lithium salts is in the order: LiCl < LiClO₄ < LiTFSI. It was found that the lithium salts with larger anions were easily dissociated in TPU and had stronger interaction with TPU, which provided more charge carriers and gave higher ionic conductivity.     

Ion-pairing effects and ion-solvent-polymer interactions in LiN(CF3SO2)2-PC-PMMA electrolytes: a FTIR study

FTIR spectroscopic investigations coupled with ionic conductivity and viscosity measurements on lithium imide (LiN(CF₃SO₂)₂)-propylene carbonate (PC)-poly(methyl methacrylate) (PMMA) based liquid and gel electrolytes over a wide range of salt (0.025-3 M) and polymer (5-25 wt.%) concentration range furnish a novel insight into the ion-ion and ion-solvent-polymer interactions. Vibrational spectral data for LiN(CF₃SO₂)₂-PC electrolytes reveal that the solvation of lithium ions manifests from Li⁺OC and Li⁺O (ring oxygens) interactions as the ν_s(CO), the ring breathing and the δ(CH) modes of the pentagonal solvent ring are strongly perturbed for all salt concentrations. The split of the ν(SO₂) mode (that appears at 1355 cm⁻¹ for the “free imide ion”) into two components at 1337 and 1359 cm⁻¹ confirms the existence of contact ion-pairs possessing two different stable optimized geometries wherein the Li⁺ ion coordinates in a bidentate fashion in liquid and gel electrolytes of 3 M LiN(CF₃SO₂)₂-PC strength. Perturbations observed for the ν_a(SNS) and ν_s(SNS) modes of the imide ion and the symmetric ring deformation mode of PC confirms the presence of ion-pairs in both 2 and 3 M electrolytes. Incorporation of even upto 25 wt.% of PMMA in a solution of LiN(CF₃SO₂)₂-PC of 3 M strength results in an insignificant conductivity decline (as σ₂₅>10⁻³ S cm⁻¹) which is simultaneously accompanied by a massive increase in its macroscopic viscosity (as η₂₅>10⁸ cSt). Gels containing 25 wt.% of PMMA exhibit a complex pattern of Li⁺-PMMA interactions through the carbonyl oxygen of its ester group which is evidenced from the perturbations observed for the ν_s(CO) mode of PMMA. Ionic conductivity decline that occurs at salt concentrations ≥1.25 M LiN(CF₃SO₂)₂-PC in both liquid and gel electrolytes, is therefore attributable to (i) ion-pairing phenomenon and (ii) an enhancement in the solution viscosity due to a high salt proportion.     

Novel polymer electrolyte composed of poly(ethylene oxide), lithium triflate,and benzimidazole

This work has demonstrated that the incorporation of benzimidazole derivatives, 2,2′‐p‐phenylene‐bisindole (PPBI), enhances the ionic conductivity of a poly(ethylene oxide) (PEO)–based electrolyte by 20 times more than the plain system. Specific interactions among amino group, ethyl oxide, and lithium cation were investigated using differential scanning calorimetry (DSC), FTIR, and alternating current impedance. The DSC characterization confirms that the initial addition of PPBI is able to enhance the PEO crystallinity attributed to the interaction between the negative charge from the amino group and the lithium cation. Three types of complexes are present: complex I is present in the PEO phase, complex II resides at interphase, and complex III is located within the PPBI domain. Complex II plays the key role in stabilizing these two microstructure phases. FTIR spectra confirm that because of the presence of PPBI one is able to dissolve lithium salts more easily than in the plain electrolyte system and thus increase the fraction of free ions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 719–725, 2004     

Effects of ultrasonic assisted processing and clay nanofiller on dielectric properties and lithium ion transport mechanism of poly(methyl methacrylate) based plasticized polymer electrolytes

Lithium ion conducting solid polymer electrolyte (SPE) films consisted of poly(methyl methacrylate) (PMMA) matrix with lithium perchlorate as a dopant ionic salt, poly(ethylene glycol) as plasticizer and montmorillonite clay as inorganic nanofiller have been prepared by classical solution casting and high intensity ultrasonic assisted solution casting methods. The X‐ray diffraction study confirmed the amorphous structure of all these PMMA‐based solid electrolytes and the clay nanosheets existed in exfoliated form in their amorphous phase. Dielectric relaxation spectroscopy had been employed for the investigation of complex dielectric function, ac electrical conductivity, electric modulus, and impedance spectra of these electrolytes over the frequency range from 20 Hz to 1 MHz. It was observed that the dielectric properties and ionic conductivity of the electrolytes strongly depended on the sample preparation methods, and also had changes with addition of the clay nanofiller. Temperature‐dependent dielectric study of the electrolyte films confirmed that their dc ionic conductivity and conductivity relaxation time values obeyed the Arrhenius behavior. This study also revealed that the lithium ion transportation in the ion–dipolar complexes of these electrolytes occurred through hopping mechanism and it was correlated with the conductivity relaxation time. Preparation of these electrolyte films through ultrasonic assisted solution casting method increased the ionic conductivity by more than one order of magnitude in comparison to that of the classical solution casting method, which revealed that the former was a novel method for the preparation of these SPEs of relatively enhanced ionic conductivity. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42188.     

Effect of cation structure on the electrochemical and thermal properties of ion conductive polymers obtained from polymerizable ionic liquids

Several onium cations having vinyl group formed ionic liquids after coupling with bis(trifluoromethanesulfonyl)imide. These monomers were polymerized, and the relation between onium cation structure and properties of thus polymerized ionic liquids was investigated. The polymerized ionic liquid having ethylimiadzolium cation unit showed the highest ionic conductivity of around 10⁻⁴ S cm⁻¹ at 30 °C among the obtained polymers reflecting the lowest glass transition temperature of −59 °C. These polymers were thermally stable and their decomposition temperatures were about 350 °C. The ionic conductivity of the polymerized ionic liquids decreased by both the addition of lithium bis(trifluoromethanesulfonyl)imide and the polymerization in the presence of cross-linker. However, the polymerized ionic liquid having 1-methylpiperidinium cation structure showed good lithium ion transference number of 0.43 at room temperature.     

Ionic conduction in polyether-based lithium arylfluorosulfonimide ionic melt electrolytes

We report synthesis, characterization and ion transport in polyether-based ionic melt electrolytes consisting of Li salts of low-basicity anions covalently attached to polyether oligomers. Purity of the materials was investigated by HPLC analysis and electrospray ionization mass spectrometry. The highest ionic conductivity of 7.1 × 10⁻⁶ S/cm at 30 °C was obtained for the sample consisting of a lithium salt of an arylfluorosulfonimide anion attached to a polyether oligomer with an ethyleneoxide (EO) to lithium ratio of 12. The conductivity order of various ionic melts having different polyether chain lengths suggests that at higher EO:Li ratios the conductivity of the electrolytes at room temperature is determined in part by the amount of crystallization of the polyether portion of the ionic melt.