Solid polymer electrolytes V: microstructure and ionic conductivity of epoxide-crosslinked polyether networks doped with LiClO4

A crosslinked polyether network was prepared from poly(ethylene glycol) diglycidyl ether (PEGDE) cured with poly(propylene oxide) polyamine. Significant interactions between ions and polymer host have been observed for the crosslinked polyether network in the presence of LiClO₄ by means of FT-IR, DSC, TGA, and ⁷Li MAS solid-state NMR. Thermal stability and ionic conductivity of these complexes were also investigated by TGA and AC impedance measurements. The results of FT-IR, DSC, TGA and ⁷Li MAS solid-state NMR measurements indicate the formation of different types of complexes through the interaction of ions with different coordination sites of polymer electrolyte networks. The dependence of ionic conductivity was investigated as a function of temperature, LiClO₄ concentration and the molecular weight of polyether curing agents. It is observed that the behavior of ion transport follows the empirical Vogel-Tamman-Fulcher (VTF) type relationship for all the samples, implying the diffusion of charge carrier is assisted by the segmental motions of polymer chains. Moreover, the conductivity is also correlated with the interactions between ions and polymer host, and the maximum ionic conductivity occurs at the LiClO₄ concentration of [O]/[Li⁺]=15.     

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.     

Effect of modified montmorillonites on the ionic conductivity of (PEO)16LiClO4 electrolytes

Three kinds of modified montmorillonites were prepared by ion exchange method, and added into (PEO)₁₆LiClO₄ matrix to study the effect on the ionic conductivity of (PEO)₁₆LiClO₄ electrolytes. The structure of the modified montmorillonites and polymer composites were characterized by wide-angle X-ray diffraction. HP 4192A was used to measure the ionic conductivity of the polymer electrolytes. The results show that the addition of optimum content of 250-Li-mont enhances the ionic conductivity of the PEO based electrolyte by nearly 30 times more than the plain system and that is much higher than the other two modified montmorillonites. The difference of enhancement in conductivity caused by adding these three montmorillonites can be attributed to the difference in structure of the samples as characterized by wide-angle X-ray diffraction.     

Thermal, electrical and mechanical properties of plasticized polymer electrolytes based on PEO/P(VDF-HFP) blends

The polymer electrolytes composed of a blend of poly(ethylene oxide) (PEO) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as a host polymer, mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a plasticizer, and LiClO₄ as a salt were prepared by a solution casting technique. SEM micrographs show that P(VDF-HFP) is very compatible with PEO. The ionic conductivity of the electrolytes increases with increasing plasticizer content, while the mechanical properties become obviously worse. By addition of a certain content of PEO in P(VDF-HFP) matrix, a good compromise between high ionic conductivity and mechanical stability can be obtained.     

Examination of the fundamental relation between ionic transport and segmental relaxation in polymer electrolytes

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes is strongly limited by their poor ionic conductivity. The classical theory predicts that the ionic transport is dictated by the segmental motion of the polymer matrix. As a result, the low mobility of polymer segments is often regarded as the limiting factor for development of polymers with sufficiently high ionic conductivity. Here, we show that the ionic conductivity in many polymers can be strongly decoupled from their segmental dynamics, in terms of both temperature dependence and relative transport rate. Based on this principle, we developed several polymers with “superionic” conductivity. The observed fast ion transport suggests a fundamental difference between the ionic transport mechanisms in polymers and small molecules and provides a new paradigm for design of highly conductive polymer electrolytes.     

Study of ionic conductivity and microstructure of a cross-linked polyurethane acrylate electrolyte

A cross-linked polyurethane acrylate (PUA) was synthesized by end capping 4,4′-methylene bis(cyclohexyl isocyanate), H₁₂MDI/poly-(ethylene glycol), PEG based prepolymer with hydroxy ethyl acrylate (HEA). Significant interactions of the Li⁺ ions with the soft and hard segments of the host polymer have been observed for the PUA complexed with lithium perchlorate (LiClO₄) by means of differential scanning calorimetry (DSC), Fourier transform infra-red (FTIR) spectroscopy, ⁷Li magic angle spinning (MAS) NMR measurements and thermogravimetric analysis (TGA). The ⁷Li MAS NMR investigation of the PUA indicates the presence of at least three distinct Li⁺ sites at lower temperature, which merge to a single one at higher temperature in similar line with uncross-linked polyurethane. The results of TGA, DSC and FTIR spectroscopy support the formation of different types of complexes by the interaction of the Li⁺ ions with different coordination sites of PUA. No detectable interactions could be observed between Li⁺ ions and groups in HEA. The DSC data indicates the formation of transient cross-links with the ether oxygens of the soft segment and mixing of soft and hard phases induced by the Li⁺ ions. In addition, a Vogel-Tamman-Fulcher (VTF) like temperature dependence of ionic conductivity implies coupling of the ion movement with the segmental motion of the polymer chains in the cross-linked environment. Predominant formation of contact ion pairs of LiClO₄ has been consistently observed through AC conductivity, DSC and NMR spectroscopic results. Swelling measurements of PUA with plasticizers reveal the improved dimensional stability for these cross-linked PUA in comparison with uncross-linked polyurethane.     

Complicated phase behavior and ionic conductivities of PVP-co-PMMA-based polymer electrolytes

We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly(vinylpyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) and lithium perchlorate (LiClO₄) as well as these systems' phase behavior and ionic conductivities. The presence of MMA moieties in the PVP-co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature (T_g). In the binary LiClO₄/PVP blends, the presence of a small amount of LiClO₄ reduces the strong dipole-dipole interactions within PVP and leads to a lower T_g. Further addition of LiClO₄ increases T_g as a result of ion-dipole interactions between LiClO₄ and PVP. In LiClO₄/PVP-co-PMMA blend systems, for which the three individual systems—the PVP-co-PMMA copolymer and the LiClO₄/PVP and LiClO₄/PMMA blends—are miscible at all compositional ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO₄, PVP and PMMA units. The PMMA-rich component in the PVP-co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO₄ content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO₄/VP57 blend (i.e., 57 mol% VP units in the PVP-co-PMMA copolymer).     

Conductivity relaxation in the PEO-salt polymer electrolytes

The ionic conductivity of polyethylene oxide (PEO)-based electrolytes is complicated due to the coexistence of crystalline and amorphous phase below melting point of PEO complexes. The two-phase characteristics are greatly dependent upon thermal history, exhibiting variety of spherulitic morphology and crystallinity. Further complicacy comes from slow crystallization kinetics of the spherulites. We found that the ionic conductivity of PEO_nLiClO₄ polymer electrolytes under isothermal conditions, after quenching from high-temperature phase, drops significantly for roughly first 10 h and then decreases very slowly thereafter. The conductivity relaxation observed can be assigned to be a consequence of the slow recrystallization kinetics of PEO. It corresponds to a gradual, slow secondary crystallization of PEO and PEO-salt complexes corresponding to thickening of spherulitic aggregates, possibly through a development of subsidiary lamellae which fill in the space between the dominant lamellae crystals. Hence, large inconsistencies in the conductivity values reported in many papers, varying more than three orders of magnitude, are rather obvious, originated from non-equilibrium nature and slow recrystallization kinetics of semicrystalline state.     

Highly conductive ionic liquid based ternary polymer electrolytes obtained by in situ photopolymerisation

New ternary polymer electrolytes based on a commercial cross-linking resin dianol diacrylate (DDA), tetramethylene sulphone (TMS) as a compatibilizing solvent and ionic liquids (ILs) were prepared by in situ photopolymerisation. The electrolytes containing two polymer-to-TMS-to-IL ratios, 20:30:50 and 20:20:60 by weight, respectively and six various ILs were investigated. The obtained materials were flexible, self-standing with good mechanical properties and a long-term stability. They showed high ionic conductivities, in the range of ca. 7 × 10⁻³ to 3 × 10⁻² S cm⁻¹ at 25 °C. A very important result is that the conductivities of all the prepared polymer electrolytes exceeded the conductivities of the corresponding neat ILs by 2–3.5 times. The temperature dependence of the ionic conductivity correlates with VTF equation. All the materials prepared were characterized by a broad electrochemical stability window (3.3–3.7 V).     

Morphology and ionic conductivity of thermoplastic polyurethane electrolytes

Thermoplastic polyurethane (TPU) with a mixture of soft segments [poly(ethylene glycol) (PEG) and poly(tetramethylene glycol) (PTMG)], denoted TPU‐M, was prepared as an ion‐conducting polymer electrolyte. TPUs with PEG and PTMG as soft segments were also synthesized individually as polymer electrolytes. The changes in the morphology and ion conductivity of the phase‐segregated TPU‐based polymer electrolytes as a function of the lithium perchlorate concentration were determined with differential scanning calorimetry, Fourier transform infrared spectroscopy, alternating‐current impedance, and linear sweep voltammetry measurements. Both solid and gelatinous polymer electrolytes were characterized in this study. The effect of temperature on conductivity was studied. The conductivity changes revealed the combined influence of PTMG and PEG units in TPU‐M. The swelling characteristics in a liquid electrolyte and the dimensional stability were evaluated for the three TPUs. Because of its dimensional stability and ionic conductivity, the TPU system containing both PEG and PTMG as soft segments was found to be more suitable for electrolyte applications. A room‐temperature conductivity of approximately 1 × 10^?4 was found for TPU‐M containing 50 wt % liquid electrolyte. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1154–1167, 2004     

Agar-based films for application as polymer electrolytes

New types of polymer electrolytes based on agar have been prepared and characterized by impedance spectroscopy, X-ray diffraction measurements, UV-vis spectroscopy and scanning electronic microscopy (SEM). The best ionic conductivity has been obtained for the samples containing a concentration of 50 wt.% of acetic acid. As a function of the temperature the ionic conductivity exhibits an Arrhenius behavior increasing from 1.1 × 10⁻⁴ S/cm at room temperature to 9.6 × 10⁻⁴ S/cm at 80 °C. All the samples showed more than 70% of transparency in the visible region of the electromagnetic spectrum, a very homogeneous surface and a predominantly amorphous structure. All these characteristics imply that these polymer electrolytes can be applied in electrochromic devices.     

Solid polymer electrolytes VIII: effect of LiClO4-doped level on the microstructure and ionic conductivity of a chemical-covalently polyether-siloxane hybrid electrolyte

A new hybrid polymer electrolyte system based on chemical-covalently polyether and siloxane phases is designed and prepared in the presence of lithium perchlorate (LiClO₄) which acted as both ionic source and the epoxide ring-opening catalyst. The effect of salt-doped level on the microstructure and ionic conductivity of these composite electrolytes were investigated by means of Fourier transform infra-red spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis, a.c. impedance and multinuclear solid-state nuclear magnetic resonance measurements. DSC results indicate that the formation of transient cross-links between Li⁺ ions and the ether oxygens on complexation with LiClO₄ results in an increase in polyether segment T_g. However, the polyether segment T_g decreases at the highest salt concentration (5.0 mmol LiClO₄/g PEGDE), ascribing to the plasticizing effect. The behavior of ion transport is coupled with the segmental motions of polymer chains and also correlated with the interactions between ions and polymer host.     

Synthesis, structure characterization and ionic conductivity of star-branched organic-inorganic hybrid electrolytes based on cyanuric chloride, diamine-capped poly(oxyalkylene) and alkoxysilane

A synthesis route for preparing highly conductive solid organic-inorganic hybrid electrolytes has been developed by using cyanuric chloride as the coupling core to react with diamino-terminated poly(oxyalkylene) triblock copolymers, followed by cross-linking with an epoxy alkoxysilane 3-glycidyloxypropyl trimethoxysilane via a sol-gel process. The present hybrid electrolyte with a [O]/[Li] ratio of 32 was found to be the most conductive, reaching a maximum lithium ion conductivity of 6.8 × 10⁻⁵ Scm⁻¹ at 30 °C. The Li-ion mobility was determined from ⁷Li static NMR line width measurements and correlated with their ionic conductivities. The onset of ⁷Li line narrowing was closely related to the T_g of the hybrid electrolytes as measured by DSC experiments. Thus, the motions of the lithium cations are strongly coupled with the segmental motion of the polymer chains, which is in line with the Vogel-Tamman-Fulcher behavior as observed in ionic conductivity.     

Synthesis and electrochemical characterization of PEO-based polymer electrolytes with room temperature ionic liquids

New gel polymer electrolytes containing 1-butyl-4-methylpyridinium bis(trifluoromethanesulfonyl)imide (BMPyTFSI) ionic liquid are prepared by solution casting method. Thermal and electrochemical properties have been determined for these gel polymer electrolytes. The addition of BMPyTFSI to the P(EO)₂₀LiTFSI electrolyte results in an increase of the ionic conductivity, and at high BMPyTFSI concentration (BMPy⁺/Li⁺ = 1.0), the ionic conductivity reaches the value of 6.9 × 10⁻⁴ S/cm at 40 °C. The lithium ion transference numbers obtained from polarization measurements at 40 °C were found to decrease as the amount of BMPyTFSI increased. However, the lithium ionic conductivity increased with the content of BMPyTFSI. The electrochemical stability and interfacial stability for these gel polymer electrolytes were significantly improved due to the incorporation of BMPyTFSI.     

Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li_6.5La₃Zr_1.5(Ta/Nb)_0.5O₁₂, typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm^?1. However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li_6.5La₃Zr_1.5Ta_(0.5?x)Nb_xO₁₂, where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li_6.5La₃Zr_1.5Ta_0.1Nb_0.4O₁₂ (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10^?3 S cm^?1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.     

Application of electrochemical impedance spectroscopy to the study of ionic transport in polymer-based electrolytes

Lithium-ion batteries should benefit from new polymer-based electrolytes to answer both safety and performance issues for use in transportation applications. Rubber-like gel electrolytes are prepared by gelling of a ionic liquid based solution by a cross-linked epoxide-amine, as a simple and effective means to obtain safe electrolyte materials that exhibit minimal flammability and toxicity while possibly preserving high mechanical and ionic transport properties over a wide temperature window. The synthesis of gel polymer electrolytes based on epoxy-amine thermosetting polymers, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI TFSI) and the lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is presented. Self-standing, transparent, optically homogeneous films are obtained after curing. Two gel polymer electrolytes differing by the nature of the epoxide precursor (aromatic versus aliphatic) are characterized in terms of thermo-mechanical properties with respect to their neat epoxy-amine counterparts. Electrochemical impedance spectroscopy (EIS) is used to investigate the electrical properties and measure the ionic conductivities over the temperature range of interest. The choice of a more flexible moiety in the thermoset precursors leads to an enhanced ionic conductivity. An electric model is presented to predict the gel conductivity on the basis of the conductivity values of neat compounds and to discuss the gel morphology at the molecular scale.     

Gel electrolytes with ionic liquid plasticiser for electrochromic devices

The comparative performance of conducting polymer electrochromic devices (ECDs) utilising gel polymer electrolytes (GPEs) plasticised with ethylene carbonate/propylene carbonate or (N-butyl-3-methylpyridinium trifluoromethanesulphonylimide (P₁₄TFSI) has been made. Lithium perchlorate and lithium trifluoromethanesulphonylimide salts were used in the GPEs to provide enhanced ionic conductivity and inhibit phase separation of the polyethyleneoxide (PEO) and plasticiser. ECDs were assembled from cathodically colouring, polyethylenedioxythiophene (PEDOT), and anodically colouring, polypyrrole (PPy), conducting polymer electrochromes deposited by vapour deposition. The photopic contrast switching over the visible light spectrum, switching speeds and device stability of the ECDs were obtained. These studies demonstrate that the ionic liquid (IL) plasticised GPEs are a suitable replacement for pure IL based devices and volatile organic solvent plasticisers based upon ethylene carbonate/propylene carbonate mixtures.     

First international symposium on polymer electrolytes

We review the effects of temperature on conductance and viscosity in the liquid state of vitreous ionic conductors and show how differences may be best understood by comparison of relaxation times for electrical and mechanical stresses acting on liquid or glassy states of the material. This leads to the definition of a conductivity/viscosity mode decoupling index, useful as a figure of merit for the solid electrolyte. In applying the same approach to polymer electrolyte systems a problem is encountered due to the molecular weight dependence of the viscosity. This is resolved by deriving a ‘monomer’ shear relaxation time for the polymer electrolyte solution and showing that this quantity corresponds closely with the ‘local’ mechanical relaxation time obtained from light scattering studies (which is a molecular weight independent quantity for pure polypropylene oxide and other polymers of low T_g). Comparison of the electrical relaxation times of the polymer solution with the ‘local’ (or ‘matrix’) relaxation times then shows that the relationship found for superionic glass-forming systems is inverted in the case of polymer electrolytes. The latter have fractional decoupling indexes which may be interpreted in terms of serial coupling phenomena; i.e. ions must first decouple from their partner ions (in ion pairs), or from their intramolecular solvation states, before they can contribute to conductivity relaxation which is itself coupled to the local polymer matrix relaxation. An extreme case is illustrated using the weak electrolyte, lithium acetate, for which the decoupling index is ～6 × 10^?5.     

Increasing ionic conductivity and mechanical strength of a plastic electrolyte by inclusion of a polymer

In this contribution we present a soft matter solid electrolyte which was obtained by inclusion of a polymer (polyacrylonitrile, PAN) in LiClO₄/LiTFSI-succinonitrile (SN), a semi-solid organic plastic electrolyte. Addition of the polymer resulted in considerable enhancement in ionic conductivity as well as mechanical strength of LiX-SN (X = ClO₄, TFSI) plastic electrolyte. Ionic conductivity of 92.5%-[1 M LiClO₄-SN]:7.5%-PAN (PAN amount as per SN weight) composite at 25 °C recorded a remarkably high value of 7 × 10⁻³ Ω⁻¹ cm⁻¹, higher by few tens of order in magnitude compared to 1 M LiClO₄-SN. Composite conductivity at sub-ambient temperature is also quite high. At −20 °C, the ionic conductivity of (100 − x)%-[1 M LiClO₄-SN]:x%-PAN composites are in the range 3 × 10⁻⁵-4.5 × 10⁻⁴ Ω⁻¹ cm⁻¹, approximately one to two orders of magnitude higher with respect to 1 M LiClO₄-SN electrolyte conductivity. Addition of PAN resulted in an increase of the Young's modulus (Y) from Y → 0 for LiClO₄-SN to a maximum of 0.4 MPa for the composites. Microstructural studies based on X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy suggest that enhancement in composite ionic conductivity is a combined effect of decrease in crystallinity and enhanced trans conformer concentration.     

Preparation of thermally stable polymer electrolytes from imidazolium-type ionic liquid derivatives

Thermally stable polymer electrolytes based on ionic liquids were prepared and analyzed. Mono-functional ionic liquid monomers, ionic liquid cross-linkers, and ethylimidazolium-type ionic liquid salts were mixed and polymerized. The ionic liquid-type cross-linkers were effective to prepare thermally stable polymer films. In particular, the copolymerization of cross-linker and ethylimidazolium-type ionic liquid monomers were used to make polymer electrolytes with high ionic conductivities. The copolymerization in ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide gave a transparent film showing no thermal degradation up to 400 °C.