Polyethylene oxide ionic conductivity

An example of an ionically conductive polymer is polyethylene oxide containing LiC104, which is used as a solid phase electrolyte in batteries. 

Although the motion of protons does not lead to electrical conduction in the case of benzoic acid, electronic and even ionic conductivity can be found in other molecular crystals. A well-studied example of ionic conduction is a film of polyethylene oxide (PEO) which forms complex structures if one adds alkaline halides (AX). Its ionic conductivity compares with that of normal inorganic ionic conductors (log [cr (Q cm)] -2.5). Other polymers with EO-units show a similar behavior when they are doped with salts. Lithium batteries have been built with this type of... 

A second type of solid ionic conductors based around polyether compounds such as polyethylene oxide) (PEO) has been discovered and characterized. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as L1CIO4 to produce conducting solid solutions. The use of these materials in rechargeable lithium-batteries has been proposed. 

On the other hand, polyphosphazene 3.79 and its variants are non-crystalline. Then-ionic conductivities at room temperature are 1,000 times or more greater than that of polyethylene oxide).164 166 For battery type applications, 3.79 must be cross-linked lightly to prevent slow liquid-like flow, but this can be accomplished by radiation techniques without lowering the conductivity. An analogous type of polymer, with a poly-siloxane backbone and oligoether side groups, is being studied for similar applications. 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

Haidar, B., Singru, R.M., Maurya, K.K., Chandra, S. (1996) Temperature dependence of positron-annihilation lifetime, free volume, conductivity, ionic mobility, and number of charge carries in a polymer electrolyte polyethylene oxide complexed with NH4CIO4 . Phys. Rev. B. 54, 7143. 

A third type of ionic conduction occurs in polymer electrolytes, such as polyethylene oxide. The mechanism of ionic conduction in polymer electrolytes is not entirely understood. However, it is believed to involve rapid polymer segmental motion which creates regions of an elastomeric nature. These elastomeric regions have relaxation times similar to liquids, and, thus, allow a higher ionic mobility than would be concluded from the polymer s macroscopic properties [2]. 

H-S. Xu and C-Z. Yang, The ionic conductive properties of sulfonated polyethylene oxide polyurethane, J. Polym. Sci., Polym. Phys. Ed., 1995, 33, 745-751 G. Robila, T. Buruiana and E.C. Buruiana, Sulfonated polyurethane ionomers in solution, Angew. Makromol. Chem., 1995, 231, 25-33, etc. 

The WLF formula shows that the ionic conductivity of the polymer electrolyte is shown in the temperature range higher than Tg. Ionic conductivity decreases rapidly if its temperature goes below that of Tg. The EO unit is recognized as the most excellent structure from the ionic dissociation viewpoint. The ion is transported coupled with the oxyethylene chain motion in amorphous polymer domain. However, oxyethylene structure easily becomes crystalline. Therefore, in order to accelerate the quick molecular motion of the polymer chain and quick ion diffusion, it is important to lower the crystallization of polymer matrixes. The methods for inhibiting the crystallization of the polymer are, for example, to introduce the polyethylene oxide chain into the low Tg polymer such as polysiloxane and phosp-hazene, or to introduce the asymmetric units such as ethylene oxide/propylene oxide (EO/PO) into polymer main chain. 

Depending on the type of charge transport carrier, conductive polymers can be classified into two main groups of ionically and electronically conductive polymers (Fig. 6.1). Polyethylene oxide with lithium perchlorate (LiCl04) is an example of an... 

Solid polymer electrolytes made of polyethylene oxide (PEO) and polypropylene oxide (PPO) are considered because of their strong thermal conduction and electrochemical properties over a wide operating temperature range [114,115]. However, the low room temperature ionic conductivities exhibited by PEO and PPO solid state polymer electrolytes prevents successful application in ESs. When PEO was incorporated into a gel electrolyte to boost conductivity, the result indicated that PEO and PPO are actually found inferior compared to PVA and PVdF for gel electrolytes because the oxygen atoms in the polymer backbone limit ion mobility [115]. 

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