About impedance spectra and dynamics of charge carriers in neat poly(ethylene oxide) structures

Impedance spectra of linear and cross-linked poly(ethylene oxide) (PEO) are analyzed in a wide range of temperature. Dielectric responses differ at low and high temperature due to tendency of PEO to crystallization below melting temperature. Extent of crystallization depends on cross-linking density. The network in PEO with high cross-linking density is rigid and morphology transition shifts to very low temperature. Debye-like relaxation appears at low temperature similar as in ionic liquids. Onset of polarization relaxation shifts to higher temperature with increasing mesh size that is coupling of electric and structural relaxation appears. This is also nicely reflected by scaled conductivity. It demonstrates that the structure in cross-linked systems is a superposition of chemical and physical networks. They relax separately at low temperature and frequency under condition of sufficient rigidity.     

Impedance spectra of polymer electrolytes

The authors present a phenomenological view on dielectric relaxation in polymer electrolytes. Polymer electrolytes are seen as molecular mixtures of an organic polymer and an inorganic salt. The following is based on systems with high molar mass poly(ethylene oxide) (PEO) and epoxidized natural rubber with 25 mol% of epoxide content (ENR-25) filled with lithium perchlorate (LiClO₄). Dielectric properties of these systems have been studied as a function of salt content at room temperature. Additionally, properties of neat low molar mass PEO were studied as function of temperature. Relaxation-coined dielectric behavior rules the system with PEO in the frequency that ranged up to 10⁶ Hz. Imaginary parts of impedance, tangent loss, and electric modulus spectra show distribution of relaxation times. Comparison of tangent loss (tan δ) spectra and imaginary part of electric modulus (M″) spectra reveals that localized motion dominates long-range motion of dipoles in the low-frequency range. However, discrepancy between them decreases with growing salt content. Scaling of tan δ spectra demonstrates that distribution of relaxation times does not depend on salt content in the range of low frequencies. The ENR-25 system exhibits solely relaxation like a macroscopic dipole. In conclusion, the system with PEO is characterized by individual relaxation of well-interacting dipoles, whereas the system based on ENR-25 is coined by immobilized dipoles that lead in the state of high-salt content to the relaxation behavior of a macroscopic dipole.     

On dielectrics of polymer electrolytes studied by impedance spectroscopy

We present a phenomenological view on dielectric relaxation in polymer electrolytes in the frequency range where conductivity is independent of frequency. Polymer electrolytes are seen as molecular mixtures of an organic polymer and an inorganic salt. The discussion applies also to ionic liquids. The following is based on systems with poly(ethylene oxide) (PEO) comprising the lithium perchlorate salt (LiClO₄) and also pure low-molecular PEO. In those systems, dipole-dipole interactions form an association/dissociation equilibrium which rules properties of the system in the low-frequency region. It turns out that effective concentration, c _S, of relaxing species provides a suitable variable for discussing electrochemical behavior of the electrolytes. Quantity c _S is proportional to the ratio of DC conductivity and mobility. Polymer salt mixtures form weak electrolytes. However, diffusion coefficient and corresponding molar conductivity display the typical (c _S)^1/2 dependence as well known from strong electrolytes, due to the low effective concentration c _S.     

Conductivity and dielectric relaxation of Li salt in poly(ethylene oxide) and epoxidized natural rubber polymer electrolytes

Two types of polymer electrolytes were studied: poly(ethylene oxide) (PEO) and epoxidized natural rubber (ENR) both filled with lithium perchlorate. Universal dielectric behavior and impedance relaxation were investigated at room temperature over a wide range of salt concentration. Complex impedance plots exhibit one semicircle in some cases (PEO polymer electrolytes) with an extended spike at low frequencies. This implies a double layer capacity strongly influences conductivity at low frequencies. In the ENR–salt system, semicircles can be obtained only at very high concentrations. This points towards stable resistor dominated networks only develop at very high salt concentrations for this system. Centers of the semicircles lie below real axis indicating non-Debye dielectric relaxation. The relaxation peak broadens and shifts to higher frequencies with increasing salt content. It indicates that the relaxation time of polarization relaxations decreases with ascending salt content. Relaxations occur at extremely low salt concentrations in PEO and only at very high salt concentrations in ENR. Hence, conductivity of ENR–salt is one to two orders of magnitude lower as for PEO–salt.     

Dielectric spectroscopy and viscosity studies of aqueous poly(ethylene oxide) and poly(vinyl pyrrolidone) blend

The complex dielectric function, electric modulus, impedance and ac electrical conductivity behaviour of aqueous solutions of 5 wt% poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP) and their different volume percent blends were investigated in the frequency range 20 Hz to 1 MHz at 15, 30 and 45 °C. It is found that the real part of dielectric function of these blends at 1 MHz decreases with the increase of PEO concentration and their dc electrical conductivity has strong correlation with the electrode polarization relaxation time. The static permittivity, ionic conductivity, electrode polarization relaxation time and apparent viscosity have linear behaviour with temperature variation at fixed volume concentration of the aqueous polymers blend. The viscosity of these aqueous polymeric blends increases with the increase of PEO concentration. The behaviour of hydrogen bond interactions between the polar segments of PEO and PVP were explored from the comparative change in dielectric parameters and viscosity of the two phase aqueous polymeric systems.     

Dielectric spectroscopy and confirmation of ion conduction mechanism in direct melt compounded hot-press polymer nanocomposite electrolytes

Solid-type polymer nanocomposite electrolyte (PNCE) comprising poly(ethylene oxide) (PEO), lithium perchlorate (LiClO₄) and montmorillonite (MMT) nano-platelets were synthesized by direct melt compounded hot-press technique at 70 °C under 3 tons of pressure. The spectra of complex dielectric function, electric modulus and alternating current (ac) electrical conductivity, and complex impedance plane plots of these materials were investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The variation of electrode polarization and ionic conduction relaxation times with MMT concentration up to 20 wt.% confirms their strong correlation with direct current ionic conductivity. The predominance of exfoliated MMT structures in PEO matrix and their effect on cation conduction mechanism and ion pairing were discussed by considering a supramolecular transient cross-linked structure. The normalized ac conductivity as a function of scaled frequency of these PNCE materials obey the universal time–concentration superposition behaviour alike the disordered solid ionic conductors.     

Broad band dielectric behaviour of plasticized PEO-based solid polymer electrolytes

The conductivity and dielectric response of poly(ethylene oxide) (PEO) based plasticized polymer electrolyte systems were studied in the broad frequency range from 5 Hz to 1.8 GHz and in the temperature range from 248 K to 353 K. Propylene carbonate (PC) and ethylene carbonate (EC) were used as conventional plasticizers while poly(perfluorinated ethylene methylene oxide) (M03) was used as a new type of plasticizer. PEO-LiN(CF₃SO₂)₂ plasticized with M03 shows high enough conductivity values to be used as electrolyte in rechargeable lithium polymer batteries. At high frequency a dielectric relaxation is observed for pure PEO as well as for the salt containing systems in the GHz region that is assumed to be due to segmental motion of the polymer chains. In the salt containing systems, this relaxation is shifted to lower frequencies relative to that of pure PEO, this is attributed to transient cross-linking. However, at lower frequencies another dielectric response peak was detected in all samples containing salts. The effect of the plasticizer on this relaxation is complex. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Dielectric permittivity and electric modulus of polyethylene oxide (PEO)-LiClO4 composite electrolytes

Dielectric permittivity and conductivity relaxation in polyethylene oxide (PEO)-LiClO₄ salt polymer electrolytes have been investigated for different lithium ion concentrations. We have observed that imaginary modulus spectra exhibit asymmetric maxima with peak-width much broader than that of the Debye peak and are skewed toward the high frequency sides of the maxima. The charge carriers for the electrolyte having higher lithium salt concentration relax much faster than that for other electrolytes and produces higher conductivity. The modulus data have been fitted using non-exponential Kohlrausch-Williams-Watts (KWW) function φ(t). We have observed that the value of the non-exponential parameter (β) is fairly low and nearly constant for different salt concentrations. The low value of β suggests a wide distribution of non-exponential relaxation times. Using the scaling of modulus data we have observed that the relaxation dynamics of charge carriers in these PEO-Li salt based electrolytes is independent of temperature and salt concentration.     

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO<Subscript>4</Subscript>–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO₄ monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO₄ monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.     

Electrical conduction and polarization in PbWO<Subscript>4</Subscript> crystals

The regularities of ion-electron processes in an undoped PbWO₄ single crystal upon transition to a quasi-equilibrium state in an external dc electric field with a linear variation in the temperature in the range 290–600 K are investigated using different methods. The total conductivity, thermally stimulated polarization current, and thermally stimulated depolarization current are measured. It is assumed that the temperature dependence of the conductivity can be described within the theory of small-radius polarons. The thermally stimulated polarization (depolarization) currents are interpreted in terms of the space-charge (peaks of the current in the range 400–550 K) and dipole (peaks of the current in the range 290–370 K) mechanisms of generation of a polarization charge in the sample. The inference is drawn that the dominant contribution to the dipole polarization is made by dipolons, namely, doubly charged (cation-anion) vacancy pairs coupled through electrostatic interaction. The basic parameters of relaxation phenomena and charge transfer are calculated.     

Alternating current characterization of nano-Pt(Ⅱ)octaethylporphyrin(PtOEP) thin film as a new organic semiconductor

Alternating current(AC) conductivity and dielectric properties of thermally evaporated Au/Pt OEP/Au thin films are investigated each as a function of temperature(303 K–473 K) and frequency(50 Hz–5 MHz).The frequency dependence of AC conductivity follows the Jonscher universal dynamic law.The AC-activation energies are determined at different frequencies.It is found that the correlated barrier hopping(CBH) model is the dominant conduction mechanism.The variation of the frequency exponent s with temperature is analyzed in terms of the CBH model.Coulombic barrier height Wm,hopping distance Rω,and the density of localized states N(EF) are valued at different frequencies.Dielectric constant ε_1(ω,T) and dielectric loss ε_2(ω,T) are discussed in terms of the dielectric polarization process.The dielectric modulus shows the non-Debye relaxation in the material.The extracted relaxation time by using the imaginary part of modulus(M')is found to follow the Arrhenius law.     

Impedance and dielectric studies of nanocomposite polymer electrolyte systems using MMT and ferroelectric fillers

Impedance and dielectric properties of nanocomposite polymer electrolyte systems modified with nano size MMT and ferroelectric fillers have been investigated for varying lithium to oxygen ratios. The changes in the structural properties of the electrolyte samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetric (DSC) technique. The ion transport number estimated by DC polarization technique is found to be between 0.86 and 0.95. The bulk conductivities of nanocomposite polymer electrolyte films were studied using impedance spectroscopic technique. The impedance plot shows high frequency semicircle, due to the bulk effect of sample and maximum ionic conductivity of 2.15 × 10⁻⁴ Scm⁻¹ was observed for (PEO)₄LiCBSM at 323 K with lithium to oxygen ratio 1: 4. The complex impedance data was used to evaluate ionic conductivity and dielectric relaxation process, to understand the ion transport mechanism in these systems.

     

Effects of plasticizer and nanofiller on the dielectric dispersion and relaxation behaviour of polymer blend based solid polymer electrolytes

Solid polymer electrolytes consisted of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) blend (50:50 wt/wt%) with lithium triflate (LiCF₃SO₃) as a dopant ionic salt at stoichiometric ratio [EO + (CO)]:Li⁺ = 9:1, poly(ethylene glycol) (PEG) as plasticizer (10 wt%) and montmorillonite (MMT) clay as nanofiller (3 wt%) have been prepared by solution cast followed by melt–pressing method. The X–ray diffraction study infers that the (PEO–PMMA)–LiCF₃SO₃ electrolyte is predominantly amorphous, but (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG electrolyte has some PEO crystalline cluster, whereas (PEO–PMMA)–LiCF₃SO₃–10 wt% PEG–3 wt% MMT electrolyte is an amorphous with intercalated and exfoliated MMT structures. The complex dielectric function, ac electrical conductivity, electric modulus and impedance spectra of these electrolytes have been investigated over the frequency range 20 Hz to 1 MHz. These spectra have been analysed in terms of the contribution of electrode polarization phenomenon in the low frequency region and the dynamics of cations coordinated polymer chain segments in the high frequency region, and also their variation on the addition of PEG and MMT in the electrolytes. The temperature dependent dc ionic conductivity, dielectric relaxation time and dielectric strength of the plasticized nanocomposite electrolyte obey the Arrhenius behaviour. The mechanism of ions transportation and the dependence of ionic conductivity on the segmental motion of polymer chain, dielectric strength, and amorphicity of these electrolytes have been explored. The room temperature ionic conductivity values of the electrolytes are found ∼10⁻⁵ S cm⁻¹, confirming their use in preparation of all-solid-state ion conducting devices.     

Dielectric Relaxations and Structures of Nanoclay in Solution Cast Poly(Ethylene Oxide)– Montmorillonite Clay Nanocomposites

The relative complex dielectric function, electric modulus and alternating current electrical conductivity spectra and complex impedance plane plots of aqueous solution cast poly(ethylene oxide)–montmorillonite clay (PEO–MMT) nanocomposite films were investigated over the frequency range 20 Hz to 1 MHz at ambient temperature. The intercalated and exfoliated structures of nanoclay dispersed in PEO matrix were recognized by the significant change in real part of dielectric function with clay concentration in the range 0%–20 wt%. The relaxation times corresponding to PEO chain segmental motion and ionic conduction relaxation processes were used to explore the interactions compatibility between PEO molecules and the dispersed MMT clay nanoplatelets and their effect on PEO chain dynamics. Real part of conductivity spectra of these nanocomposites over five decades of frequency has nonlinear behavior, which is influenced by the MMT clay concentration. The complex impedance plane plots confirm the bulk properties of these nanocomposites over the experimental frequency range.     

Dielectric relaxation and ionic conductivity studies of LiCaPO<Subscript>4</Subscript>

The AC conductivity of the LiCaPO₄ compound has been measured in the temperature range 634–755 K and the frequency range 300 Hz–5 MHz. The impedance data were fitted to an equivalent circuit consisting of series combination of grains, grains boundary, and electrode elements. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The modulus plots are characterized by the presence of two relaxation peaks thermally activated. The activation energies obtained from the analysis of M″ (0.90 eV) and conductivity data (0.94 eV) are very close, revealing an ionic hopping mechanism.     

Effect of different anions of lithium salt and MMT nanofiller on ion conduction in melt-compounded PEO–LiX–MMT electrolytes

Polymer nanocomposite electrolyte (PNCE) films composed of poly(ethylene oxide) (PEO), lithium salt ( \textLiX;  \textX = ClO₄ ^- ,  BF₄ ^- ,  CF₃SO₃ ^- {\text{LiX}};\;{\text{X}} = ClO_4^{ - },\;BF_4^{ - },\;C{F_3}SO_3^{ - } ) and montmorillonite (MMT) clay as nanofiller were prepared by melt-compounded hot-pressed technique at 70 °C under 3 tons of pressure. The ionic conductivity and relaxation behaviour of the films were investigated by dielectric relaxation spectroscopy in the frequency range of 20 Hz to 1 MHz at ambient temperature. The results revealed that the ionic conductivity of the PNCE films having 20:1 stoichiometric ratio of ethylene oxide monomer units to the lithium cation are governed by the size of different anions and the dissociation constant of salt, and also MMT concentration. It was found that PEO–LiBF₄ film has comparative high dc ionic conductivity, whereas both the LiBF₄ and LiClO₄ containing PNCE films exhibit anomalous conductivity behaviour with varying MMT concentration. The PEO–LiCF₃SO₃ film has two orders of magnitude low value of dc ionic conductivity as compared to that of the other salts electrolyte films, but its conductivity enhances by one order of magnitude when 2 wt.% MMT is added as filler. A correlation between the values of ionic conductivity, conductivity relaxation time and the real part of permittivity at 1 MHz were found and the same was discussed in relation to the transient ion-dipolar type cross-linked structural behaviour of the polymeric nanocomposite electrolytes.     

Dielectric relaxation in poly(ethylene oxide) complexed with alkali metal perchlorates

Audio frequency complex admittance measurements over the temperature range 5.5–350 K have been performed on poly(ethylene oxide) (PEO) complexed with lithium and sodium perchlorates. No evidence of an α_a relaxation was seen in any sample. In the Li-complexed material, the γ relaxation occurs at about 174 K and 1000 Hz which is the same as for pure PEO. In the Na-complexed materials, there appear to be more than one peak in this region, and the relative peak heights are concentration dependent. The result is that at the 8:1 doping level, there is a broad, skewed relaxation whose peak occurs at about 203 K and 1000 Hz, while at 4.5:1 NaClO₄ the relaxation appears much closer to a normal peak with a maximum at 185 K for the 1000 Hz data. Combined with previous results for thiocyanate complexed PEO, these results show that distortions in the polymer chain depend on both the cation and anion. Next, two new relaxations are observed at low temperatures. The first, occuring at 60–100 K is very weak and is common to all PEO samples. The second occuring at very low temperatures, is only found for perchlorate complexed PEO and is attributed to the localized motion of perchlorate-induced defects on the exterior of the chains. The high temperature electrical conductivity for PEO₈:NaClO₄ is found to be larger than PEO_4.5:NaClO₄ in agreement with the trend observed by other workers for PEO complexed with different alkali metal salts.     

Structural, vibrational, thermal, and conductivity studies on proton-conducting polymer electrolyte based on poly (N-vinylpyrrolidone)

The proton-conducting polymer electrolytes based on poly (N-vinylpyrrolidone) (PVP), doped with ammonium chloride (NH₄Cl) in different molar ratios, have been prepared by solution-casting technique using distilled water as solvent. The increase in amorphous nature of the polymer electrolytes has been confirmed by XRD analysis. The FTIR analysis confirms the complex formation of the polymer with the salt. A shift in glass transition temperature (T _g) of the PVP/NH₄Cl electrolytes has been observed from the DSC thermograms which indicates the interaction between the polymer and the salt. From the AC impedance spectroscopic analysis, the ionic conductivity of 15?mol% NH₄Cl-doped PVP polymer complex has been found to be maximum of the order of 2.51?×?10^?5?Scm^?1 at room temperature. The dependence of T _g and conductivity upon salt concentration has been discussed. The linear variation of the proton conductivity of the polymer electrolytes with increasing temperature suggests the Arrhenius type thermally activated process. The activation energy calculated from the Arrhenius plot for all compositions of PVP doped with NH₄Cl has been found to vary from 0.49 to 0.92?eV. The dielectric loss curves for the sample 85?mol% PVP:15?mol% NH₄Cl reveal the low-frequency ?? relaxation peak pronounced at high temperature, and it may be caused by side group dipoles. The relaxation parameters of the electrolytes have been obtained by the study of Tan?? as a function of frequency.     

Physical meaning of conductivity spectra for ZnO ceramics

With the help of broadband dielectric spectroscopy in a wide temperature and frequency range, the conductivity spectra of ZnO polycrystalline ceramics are measured and the direct-current-like (DC-like) conductivity and relaxation polarization conductivity are observed successively along the frequency axis. According to the classical Debye theory and Cole-Cole equation, the physical meanings of the two conductivities are discussed. It is found that the DC-like conductivity corresponds to electron transportation over the Schottky barrier at the grainboundary. The relaxation polarization conductivity corresponds to electronic trap relaxation of intrinsic point defects (zinc interstitial and oxygen vacancy). When in the high frequency region, the relaxation conductivity obeys the universal law with the index n equal to the index α in the Cole-Cole equation as an indictor of disorder degree.