Polymer electrolytes amorphous

A method of characterising transport mechanisms in solid ionic conductors has been proposed which involves a comparison of a structural relaxation time, t, and a conductivity relaxation time, t . This differentiates between the amorphous glass electrolyte and the amorphous polymer electrolyte, the latter being a very poor conductor below the 7. A decoupling index has been defined where... 

Thermal and Conducting Properties of Polymer-LiC104 Mixtures. Plots of the conductivity, a, of homogeneous, transparent mixtures of LiCl04 PMMS-8 and PAGS-12 exhibit distinct maxima at ratios of ethylene oxide units to lithium (EO/Li of 20-25 (Figure 2). This behavior is typical for amorphous polymer electrolyte complexes (7, 8). An increase... 

One of the important properties of a polymer electrolyte leading to its development activity is the ionic conductivity. Temperature dependence on the conductivity of amorphous polymer electrolytes generally follows the Vogel-Tammann-Fulcher [VTF] equation [14] ... 

Figure 1.5 Temperatxu"e variation of the conductivity for selected amorphous polymer electrolytes. Superscript numbers are the literature references...

It is clear from the analysis of Figs 5.1-5.3 that the materials characterized in this study show a non-linear variation of log conductivity with IIT in the range between 25 and 100 °C. This behavior is characteristic of amorphous polymer electrolytes. 

FRECH R, CHINTAPALLI s, BRUCE p and VINCENT c A, Structure of an amorphous polymer electrolyte, poly(ethylene oxide)3 LiCF3S03 , Chem Commun, 1997, 157-158... 

However, in the presence of motion, the efficacy of REDOR is severely limited the dynamic processes average the dipolar interaction, consequently leading to biased, overestimated interatomic distances. Thus, when studying amorphous polymer electrolytes or the amorphous parts of a heterogeneous system, REDOR NMR spectroscopy must be performed at temperatures below the T, where all the mobility is suppressed. ... 

Arrhenius plots of the ionic conductivity of amorphous polymer electrolytes, such as PPO-based electrolytes, frequently do not lie on a simple straight line, but rather, on a positively curved line (Fig. 3) [11]. Such curves are well represented by a Williams-Landel-Ferry (WLF) type equation [13] ... 

The ionic conductivity of many amorphous polymer electrolytes frequently exhibits a maximum at a certain salt concentration (see Fig. 2). 

Figure 11.9. Conductivity vs temperature plot for two ionically conducting crystals and for a polymer electrolyte, LiTf-aPtO40, which is based on amorphous poly(ethylene) oxide (after Ratner...

Figure 1. Temperature variation of the conductivity for a cross-section of polymer electrolytes. PESc, poly (ethylene succinate) PEO, polyethylene oxide) PPO, polypropylene oxide) PEI, poly(ethyleneimine) MEEP, poly(methoxyethoxy-ethoxyphosphazene) aPEO, amorphous methoxy-linked PEO PAN, polyacrylonitrile PC, propylene carbonate EC, ethylene carbonate.

The polymer electrolytes discussed so far suffer from a number of disadvantages. Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline or glassy) electrolytes at or below room temperature. The best all-amorphous systems have conductivities less than 10"4 S cm-1 at room temperature. These ambient... 

Addition of both ion-conducting and inert ceramics enhances the conductivity of a polymer electrolyte. This increase is attributed to an increase in volume fraction of the amorphous phase [133-136]. No... 

Therefore, an ideal polymer electrolyte must be flexible (associated with a low Tg), completely amorphous, and must have a high number of cation-coordination sites to assist in the process of salt solvatation and ion pair separation (see Table 11). A review on this subject has been recently published by Inoue [594]. 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

The mobility of the ions in polymer electrolytes is linked to the local segmental mobility of the polymer chains. Significant ionic conductivity in these systems will occur only above the glass transition temperature of the amorphous phase, Tg. Therefore, one of the reqnirements for the polymeric solvent is a low glass-transition temperature for example, Tg = —67°C for PEO. 

One method of reducing crystallinity in PEO-based systems is to synthesize polymers in which the lengths of the oxyethylene sequences are relatively short, such as through copolymerization. The most notable hnear copolymer of this type is oxymethylene-linked poly(oxyethylene), commonly called amorphous PEO, or aPEO for short. Other notable polymer electrolytes are based upon polysiloxanes and polyphosphazenes. Polymer blends have also been used for these applications, such as PEO and poly (methyl methacrylate), PMMA. The general performance characteristics of the polymer electrolytes are to have ionic conductivities in the range of cm) or (S/cm). 

Numerous amorphous room temperature polymer electrolytes are also known. A variant of PEO which interspaces methylene moieties with ethylene moieties is also amorphous at room temperature. Its repeat structure is (—O—CH2—O—C2H4—). Another intensively studied Li-ion conductive amorphous polymer is known as MEEP. This acronym stands for methoxy-ethoxy-ethoxy-phosphazene. The polymer structure is a repeating (—N=PR2—) phosphazene unit with two alkoxy chains dangling from the phosphorous atoms, i.e., (—N=P—(O—C2H4 —O—C2H4—O—CH3)2—). 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

Polymer electrolytes conduct cations by segmental motions of the polymer backbone that carry the cations from one complexation site to the next.60-62 Since this requires significant fluidity, the polymers are conductive only in the amorphous state, i.e. above the crystalline to gel transition temperature. For the bulk polymer of PHB, this temperature is in the range of 0° to 10 °C. Accordingly, single molecules of PHB dissolved in fluid lipid bilayers should be capable of considerable segmental motions at physiological temperatures. 

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