Hans Werner Kammer

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.

Polymer electrolytes—relaxation and transport properties

Relaxations in polymer electrolytes were studied in poly(ethylene oxide) and epoxidized natural rubbers both filled with lithium perchlorate. Impedance relaxation was investigated over a wide range of salt concentration at room temperature. Imaginary part of impedance as a function of frequency exhibits generally one maximum and one minimum. These two extreme values rule properties in the DC limit. They can be related to transport properties and degree of dissociation. It turns out that DC conductivity is dominantly ruled by transport coefficient.

Compatibility and conductivity of LiClO4 free and doped polyacrylate – poly(ethylene oxide) blends

Abstract Thermal behaviour of polyacrylate (PAc)/poly(ethylene oxide) (PEO) and PAc/PEO doped with LiClO4 were investigated by differential scanning calorimetry. The constituents, PAc and PEO, show immiscibility after glass transition temperature T g analysis. The addition of LiClO4 increases the T gs of neat PAc, PEO and their blends. The conductivities at 30°C of PAc, PAc/PEO 40 : 60 doped with 15 wt-% LiClO4 and PEO are 8·3 × 10–10, 1·2 × 10–6 and 7·9 × 10–6 S cm–1 respectively. Reasonably high conductivity of the PAc/PEO/LiClO4 blends makes it a potential polymer electrolyte.

Ionic transport and glass transition temperature of polyether–salt complexes: dependence on molecular mass of polymer

Abstract Poly(ethylene oxide) (PEO) of different molecular masses (Mη = 6×105, 1×106 and 4×106 g mol−1) and lithium perchlorate complexes were prepared by solution casting method. The salt concentrations of the samples were varied between 2 and 23 wt-%. Room temperature (30°C) conductivity, glass transition temperature and Fourier transform infrared spectroscopy (FTIR) for the samples are reported. The conductivities acquired at 13 wt-% salt concentration for PEO with Mη = 6×105, 1×106 and 4×106 g mol−1 are 9×10−6, 1×10−4 and 7×10−4 S cm−1 respectively. This indicates that higher molecular mass PEO samples exhibit higher conductivity. An interesting trend is found for variation in glass transition temperature Tg with variation in concentration of complexing salt. It shows a linear variation until ∼13 wt-% and then slowly levels off. The slope of the linear region is found to be dependent on the polymer molecular masses. The rise in Tg for each salt concentration increases linearly with ascending molecular mass of PEO. This suggests that PEO with higher molecular mass exhibits greater extent of complexation with LiClO4 as supported by FTIR studies.

Melting behaviour, morphology and conductivity of solid solutions of PEO/PAc blends and LiClO4

Abstract The melting behaviours of poly(ethylene oxide)/polyacrylate (PEO/PAc) and PEO/PAc doped with LiClO4 were investigated by differential scanning calorimetry. The equilibrium melting temperature and the crystallinity of PEO show no significant variation with the addition of PAc but are suppressed by the incorporation of LiClO4. The deformed structure of PEO spherulite resulting from disrupted crystallisation is observed in salt doped PEO and its blends. Conductivity as a function of salt concentration is studied using the power law. Lower ion mobility and correlation of ions for PAc corroborate with its lower conductivity as compared to PEO. Despite a reasonable amount of LiClO4 being solvated by PAc, the conductivity of the solid solutions of PEO/PAc blends is primarily governed by PEO.

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, cS, of relaxing species provides a suitable variable for discussing electrochemical behavior of the electrolytes. Quantity cS 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 (cS)1/2 dependence as well known from strong electrolytes, due to the low effective concentration cS.

Selective localization of lithium perchlorate in immiscible blends of poly(ethylene oxide) and epoxidized natural rubber

Immiscible blends of poly(ethylene oxide) (PEO) and epoxidized natural rubber (ENR) comprising of lithium perchlorate (LiClO4) were prepared by solution casting method. Blends of PEO and ENR are found to be thermodynamically immiscible via thermal analysis and the result has been reported previously. It is also noted in the previous study that LiClO4 dissolves preferentially in PEO than in ENR. Fourier-transform infrared (FTIR) spectroscopic studies on the blends of PEO/ENR and PEO/ENR/LiClO4 at all compositions in this work aim at investigating the ion-dipole interactions between the Li+ ions and the polymer components of the PEO/ENR/LiClO4 blends from a different approach so as to verify the selective localization of the salt in the different phases of the blends. FTIR results show that Li+ ions coordinate individually to the neat polymers of the PEO/ENR/LiClO4 blends at the ether oxygen of PEO and the oxirane group of ENR. Spectroscopic studies reveal again the preferential solubility of the salt in PEO.

The influence of the amorphous polymer on conductivity, morphologies and thermal properties of polyether-based blends with addition of inorganic salt

Thermodynamic control of the dispersion of lithium (Li) salt in different phases of semicrystalline/amorphous polymer blends is elucidated in this study. Solid polymer electrolytes of poly(ethylene oxide) (PEO), epoxidized natural rubber (ENR), random copolymer of poly(acrylate) (PAc) and as well as polymer blends of PEO with ENR and PAc doped with various concentrations of Li salt were studied. The salt concentrations (CLi) of solid polymer electrolytes vary between CLi = 0.02 and 0.15. The influence of the ENR or PAc on the properties of PEO after addition of Li salt is discussed. Blends of PEO/ENR and PEO/PAc are immiscible by elucidation of the glass transition temperature (Tg) as well as the morphological analyses. PEO, ENR and PAc possess oxygen in their respective chemical structures, which may be able to coordinate with the Li salt added. Non uniformity of Li salt concentration in different phases of the blends is highlighted for both systems. The conductivity of PEO/ENR and PEO/PAc blends doped w...

Characterization of polymer electrolytes by dielectric response using electrochemical impedance spectroscopy

Abstract Authors present a phenomenological view on dielectric relaxation in polymer electrolytes, which is monitored by electrochemical impedance spectroscopy. Molecular interaction of polymer chains with salt molecules (or dipole-dipole interaction between segments and salt molecules) leads to dipolar molecular entities. Frequency-dependant impedance spectra are the key quantities of the interest for determination of electric properties of materials and their interfaces with conducting electrodes. Salt concentration serves as parameter. Bulk and interfacial properties of the samples are discussed in terms of impedance (Z*) and modulus (M*) spectra. We focus on two different classes of systems, i.e. high molar mass of poly(ethylene oxide) (PEO)+lithium perchlorate (LiClO4) (i.e. the inorganic salt) and epoxidized natural rubber (ENR-25) with 25 mol% of epoxide content+LiClO4. Impedance spectra with salt content as parameter tell us that we have interaction between dipolar entities leading to dispersion of relaxation times. However, as scaling relations show, dispersion of relaxation times does not depend on salt content in the PEO system. The relaxation peak for the imaginary part of electric modulus (M″) provides information on long-range motion of dipoles. Summarizing the results from imaginary part of impedance spectrum (Z″), tan δ (imaginary/real of permittivities) and M″ for the two systems under the discussion, PEO behaves like a mixture of chains with dipoles. There are interactions between the dipoles, but they are relaxing individually. Therefore, we see PEO-salt system as a polymer electrolyte where only a tiny fraction of added salt molecules becomes electrically active in promoting conductance. However, ENR-25-salt system behaves just as a macroscopic dipole and it can not display electrode polarization or electric relaxation because there is no mobility of individual dipoles. Hence, ENR-25-salt does not form a polymer electrolyte in the classic sense.