A computer simulation study of ionic conductivity in polymer electrolytes

In this paper we present a computer simulation study of ionic conductivity in solid polymeric electrolytes. The multiphase nature of the material is taken into account. The polymer is represented by a regular lattice whose sites represent either crystalline or amorphous regions with the charge carrier performing a random walk. Different waiting times are assigned to sites corresponding to the different phases. A random walk (RW) is used to calculate the conductivity through the Nernst-Einstein relation. Our walk algorithm takes into account the reorganization of the different phases over time scales comparable to time scales for the conduction process. This is a characteristic feature of the polymer network. The qualitative nature of the variation of conductivity with salt concentration agrees with the experimental values for PEO-NH₄I and PEO-NH₄SCN. The average jump distance estimated from our work is consistent with the reported bond lengths for such polymers.     

Oxygen self-diffusion and ionic conductivity of oxide solid electrolytes

The self-diffusion coefficient for a stochastically nonuniform thermodynamic system is represented as the mean value of the transition rates. A model for the ionic transport in solid oxide electrolytes is proposed. The existence of percolation cluster of the doping cations is taken into account in the model. The maximum of the concentration dependence of the ionic conductivity is explained by the blocking effect and random distribution of traps. The problem of inconsistency of theoretical and experimental values for the pre-exponential factor is discussed and an approach is proposed to overcome this disagreement.     

Pipe-sphere model for enhancement of ionic conductivity in composite solid electrolytes

It is well-known that the ionic conductivity of a superionic conductor when dispersed with an insulator shows a remarkable enhancement. In this work we suggest that the contribution coming from grain-boundaries and dislocations is primarily responsible for this phenomenon in a number of cases. We propose a simple theoretical model for such composites and with the aid of the Effective Medium Theory (EMT) under self-consistent scheme we estimate the effective conductivity as a function of insulator volume fraction and particle size for four composites, namely CaF₂-Al₂O₃, CuCl-Al₂O₃, Sr(NO₃)₂-Al₂O₃ and SrCl₂-Al₂O₃. This model is applicable to composites where enhancement is observed for a very low insulator volume fraction and other prevalent models are inadequate. The results exhibit a good qualitative fit to the experimental data and all characterisitic experimental observations.     

Factors affecting ionic conductivity in the lithium conducting glassy solid electrolytes

The electrical conductivity results of lithium borosilicate glasses with addition of Li₂SO₄ and LiCl have been critically analyzed. In general, it is observed that the factors viz. lithium fraction, f_Li and the number of non-bridging oxygens (NBOs) govern the ionic conductivity in the lithium conducting glasses. For the same f_Li, the presence of mixed formers in the glass gives higher conductivity compared to that of the glass with only one former. Thus the competitive network of glass in mixed former systems provides higher mobilities for lithium ions and hence high ionic conductivity. The addition of Li₂SO₄ and LiCl in the lithium borosilicate glasses gave enhancement in the conductivity. However, the mechanism of enhancement in conductivity is different in the two glass systems. The comparison of the result of binary, ternary and quaternary glass systems suggests that in general, the decrease in activation energy, increase in f_Li and increase in NBOs gives rise to enhancement in conductivity. For the same value of f_Li the higher conductivity is exhibited by glasses with lower value of K (K=SiO₂/B₂O₃). Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Influence of plasticizer on ionic conductivity of PVC-PBMA polymer electrolytes

Plasticized polymer electrolytes comprising of ethylene carbonate as the plasticizing agent in poly (vinyl chloride) [PVC]–poly (butyl methacrylate) [PBMA] blended polymer electrolytes were prepared by solution casting technique. Complex formation, structural elucidation, conductivity, dielectric parameters (?′, ?″, M′, and M″), thermal stability, and surface morphology are brought out from FTIR, XRD, ac impedance analysis, dielectric studies, thermogravimetry/differential thermal analysis, and scanning electron microscopic studies, respectively. Polymer electrolytes are found to exhibit higher ionic conductivity at higher concentration of plasticizer at the cost of their mechanical stability. Conductivity of 1.879 × 10^?4 S cm^?1 is exhibited by the polymer electrolyte consisting of 69% of plasticizer with appreciable thermal stability up to 523 K. Temperature and frequency dependence of conductivity is found to follow Vogel Tammann Fulcher relation and Jonscher power law, respectively. Real and imaginary parts of dielectric constants are found to decrease with increase in frequency which could be due to the electrode polarization effect.     

Structural studies and ionic conductivity of lithium iodide-lithium tungstate solid electrolytes

Solid mixtures of calcined lithium iodide - lithium tungstate (LiI -Li₂WO₄) have been found to be potential solid electrolytes for practical applications with high conductivities of about 10⁻³ S·cm⁻¹ at room temperature. The highest ionic conductivity was recorded for the sample containing 20 wt.-% of lithium iodide. The ionic conductivity was related to the structure of the material using X-ray diffraction (XRD) and infrared techniques (FTIR). These experiments confirm the evidence of interaction between LiI and Li₂WO₄. FTIR spectroscopy revealed the existence of a band at 1505 cm⁻¹ which is formed as a result of this LiI -Li₂WO₄ interaction. The new phase acts as a conducting pathway for the ions to migrate through the material. Lithium ionic conduction was confirmed by measuring the transference number by Wagner's polarization technique. The ionic transference number of this solid electrolyte was found to be 1 within the limits of error.     

Development of ceria-doped zirconia electrolytes with high toughness and ionic conductivity

Development of Y₂O₃ stabilized ZrO₂ electrolytes for solid oxide fuel cell with better mechanical properties was attempted. 3 mol% Y₂O₃ stabilized ZrO₂ doped with 3–15 mol% CeO₂ was investigated in the present work. The results reveal that the toughness and the bending strength of 3–6 mol% CeO₂ doped 3 mol% Y₂O₃–ZrO₂ are much higher than that of 8 mol% Y₂O₃–ZrO₂. The best ionic conductivity observed in 6 mol% ceria-doped 3 mol% Y₂O₃–ZrO₂ electrolyte is better than that of 8 mol% Y₂O₃–ZrO₂ at 800 °C, which indicates the possibility of developing ZrO₂-based electrolyte with enhanced toughness.     

Effect of branching in base polymer on ionic conductivity in hyperbranched polymer electrolytes

Novel hyperbranched polymer, poly[bis(diethylene glycol)benzoate] capped with a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group (poly-Bz1a), was prepared, and its polymer electrolyte with LiN(CF₃SO₂)₂, poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte, was all evaluated in thermal properties, ionic conductivity, and electrochemical stability window. The poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte exhibited higher ionic conductivity compared with a polymer electrolyte based on poly[bis(diethylene glycol)benzoate] capped with an acetyl group (poly-Ac1a), and the ionic conductivity of poly-Bz1a/LiN(CF₃SO₂)₂ electrolyte was to be 7×10⁻⁴ S cm⁻¹ at 80 °C and 1×10⁻⁶ S cm⁻¹ at 30 °C, respectively. The existence of a 3,5-bis[(3′,6′,9′-trioxodecyl)oxy]benzoyl group as a branching unit present at ends in the base polymer improved significantly ionic conductivity of the hyperbranched polymer electrolytes. The polymer electrolyte exhibited the electrochemical stability window of 4.2 V at 70 °C and was stable until 300 °C.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

Ionic conductivity,ionic transport and electrochemical characterizations of plastic crystal polymer electrolytes

In this work, the plastic crystal polymer electrolytes (PCPEs), composed of polyacrylonitrile (PAN), succinonitrile (SN) and lithium bis(trifluoromethane)sulfonimide (LiTFSI) were prepared. The concentrations of lithium salt were varied by weight percentage from 10 wt% to 50 wt%. The ionic conductivity of the PCPE films increases with the increase of lithium salt, where the highest value recorded is in the order of ~10^?2 S cm^?1. The temperature-dependence conductivity analysis shows that the PCPE films exhibit Arrhenius behaviour when subjected to the temperature range from 303 K to 343 K. The decrease in crystallinity was confirmed by X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) analyses. The cationic transport number also increases with the increase of salt which corresponds well to their conductivity values. It is found that the films are electrochemically stable up to ~3.6 V as revealed by the linear sweep voltammetry (LSV) analysis. The cyclic voltammetry (CV) plots of the films shown no substantial change in the redox peaks which mean that the charge transfer reaction is reversible.     

Preparation and ionic conductivity of composite polymer electrolytes based on hyperbranched star polymer

Hyperbranched star polymer HBPS-(PPEGMA) _x was synthesized by atom transfer radical polymerization (ATRP) using hyperbranched polystyrene (HBPS) as macroinitiator and poly(ethylene glycol) methyl ether methacrylate (PEGMA) as monomer. The structure of the prepared hyperbranched star polymer was characterized by ¹H NMR, ATR-FTIR, and GPC. Polymer electrolytes based on HBPS-(PPEGMA) _x , lithium salt, and/or nano-TiO₂ were prepared. The influences of lithium salt concentration and type, nano-TiO₂ content, and size on ionic conductivity of the obtained polymer electrolytes were investigated. The results showed that the low crystallinity of the prepared polymer electrolyte was caused by the interaction between lithium salt and polymer. The addition of TiO₂ into HBPS-(PPEGMA) _x /LiTFSI improved the ionic conductivity at low temperature. The prepared composite polymer electrolyte showed the highest ionic conductivity of 9?×?10^?5 S cm^?1 at 30 °C when the content of TiO₂ was 15 wt% and the size of TiO₂ was 20 nm.     

Effect of nano-size fumed silica on ionic conductivity of PVdF-HFP-based plasticized nano-composite polymer electrolytes

Nano-composite polymer electrolytes containing poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), ammonium tetrafluoroborate (NH₄BF₄), and nano-size fumed silica (SiO₂) have been prepared and characterized by complex impedance spectroscopy. Ionic conductivity of polymer has been found to increase with the addition of NH₄BF₄, and a maximum conductivity of 3.62 × 10^?6 S/cm has been obtained at 30 wt% NH₄BF₄. The formation of ion aggregates at high concentration of salt has been explained by Bjerrum’s law and mass action considerations. The conductivity of polymer electrolytes has been increased by three orders of magnitude (10^?6 to 10^?3 S/cm) with the addition of plasticizer, and a maximum conductivity of 1.10 × 10^?3 S/cm has been observed at 80 wt% DMA. An increase in conductivity with the addition of nano-size fumed silica is attributed due to the formation of space-charge layers. A maximum conductivity of 7.20 × 10^?3 S/cm has been observed for plasticized nano-composite polymer electrolytes at 3 wt% SiO₂. X-ray diffraction analysis of polymer electrolyte system was also carried out. A small change in conductivity of nano-composite polymer electrolytes observed over the 30–130 °C temperature range and for a period of 30 days is also desirable for their use in various applications.     

Anomalous ionic transport in glassy electrolytes

Summary We discuss the dynamic structure model recently introduced by Bunde, Maass and Ingram to account for the anomalies of ionic transport in glassy ionic conductors. The model is based on the experimental evidence that ions in glass maintain their distinct local environments. Key features include a site memory effect that introduces vacancies appropriate to each kind of mobile ion, and a mismatch energy that emerges whenever an ion attempts to enter a different kind of site, the combination of both leads to the formation of fluctuating percolation pathways. The connectivity of these pathways determines the ion mobility in the glassy network. The exploration of this model by numerical methods leads i) to a power law relationship between ionic conductivity and cation content (now confirmed in the literature) and ii) to the elucidation of many facets of the mixed alkali effect. It is suggested that the model could form the basis for a comprehensive theory of vitreous electrolytes. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The effect of point defects on the ionic conductivity of RbAg4I5 solid electrolytes

Abstract

The effect has been studied of additive coloring on the magnitude of ion conductivity of RbAg₄I₅ crystals. It has been found that slight changes of silver stoichiometry of 10^?3 at.% can lead to considerable variations of the ionic conductivity Δ[sgrave]_i/[sgrave]_i ? 0.1. The dependence has been observed of the magnitude of ion conductivity on the ratio between the integral intensities of the main bands in the photoluminescence spectrum of the γ-phase of RbAg₄I₅ which associated with the luminescence centres containing vacancies and interstitials of silver cations.     

Study on the ionic conductivity of polyether network polymer electrolytes: effect of the preparation method

In preparing the network polymer electrolytes, two different methods were taken in the incorporation of salt into the polymer network. In one method, the network polyether was dipped into the lithium salt solution with or without plasticizer, and in the other method the network formation was proceeded in the presence of the lithium salt in the reaction medium with or without plasticizer. We designated the former as the network polymer electrolyte (NPE), the latter as the salt added network polycondensate electrolyte (SNPE). For the NPEs, the ionic conductivities increased with decreasing the cross-linking degree, which was mainly attributed to the increase of the free ion fraction with the decrease in the cross-linking degree that was evidenced by ⁷Li NMR relaxation studies. For the NPEs plasticized with MPEG₇, the conductivities increased with increase of the MPEG₇ content, which was largely due to the increase of the mobility of the free ions. The ionic conductivity of the SNPE was higher than that of the NPE, which resulted from the lower microviscosity of the SNPE due to the larger amount of the contained linear polyether and the lower cross-linking degree.     

Frequency-dependent conductivity in ionic glasses: A possible model

The frequency-dependent (ac) conductivity behaviour of ionically conducting glasses is discussed. A critique is given of the treatment of Almond and West for this behaviour. Two models are proposed instead to account for the observed ac loss data. One involves parallel relaxation processes and is a modification of a model originally developed for the case of large-polaron transport in amorphous semiconductors. The other is based on series relaxation, and is based on a model originally proposed by Glarum for the case of dielectric relaxation in molecular liquids. In the development given here, this model is modified so as to describe ionic transport, and a new microscopic transport mechanism is proposed—“diffusion-controlled relaxation”—which satisfies the requirements of the Glarum theory and which naturally incorporates cooperative ionic motions. For this reason, it is intuitively more appealing.     

Entropy and ionic conductivity

It is known that the ionic conductivity can be obtained by using the diffusion constant and the Einstein relation. We derive it here by extracting it from the steady electric current which we calculate in three ways, using statistics analysis, an entropy method, and an entropy production approach.     

Effect of sintering aid MoO3 on the microstructure and ionic conductivity of ceria- and lanthanum gallate-based electrolytes

The effect of a small addition of MoO₃ on the microstructure and ionic conductivity of Nd_0.2Ce_0.8O_1.9 (NDC), La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM) and Nd_0.2Ce_0.8O_1.9-La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (NDC-LSGM) has been investigated. The microstructure and electrical properties of the samples were characterized by X-ray diffraction, field-emission scanning electron microscopy and electrochemical impedance spectroscopy. The results show that MoO₃ doping can obviously increase the densification and grain sizes, and decrease the grain and grain boundary resistances of the NDC, LSGM and NDC-LSGM electrolytes. It expands the oxygen ion channels and reduces the total conductance activation energy of the system. The total conductivities of MoO₃-doped NDC and NDC-LSGM samples are 1.56 and 2.10 times higher than that of the undoped NDC system at 450°C. The total conductivity of LSGM-Mo is 1.46 times higher than that of LSGM at 450°C. These finding suggest that MoO₃ is considered to be an effective sintering aid that optimizes the electrical properties of NDC, LSGM and NDC-LSGM electrolytes.