Ionic motion, polymer electrolytes

Figure 4. Two representations (on the left) of cation motion in a polymer electrolyte assisted by polymer chain motion only, and two (on the right) showing cation motion taking account of ionic cluster contributions.

The classical example of a soUd organic polymer electrolyte and the first one found is the poly(ethylene oxide) (PEO)/salt system [593]. It has been studied extensively as an ionically conducting material and the PEO/hthium salt complexes are considered as reference polymer electrolytes. However, their ambient temperature ionic conductivity is poor, on the order of 10 S cm, due to the presence of crystalUne domains in the polymer which, by restricting polymer chain motions, inhibit the transport of ions. Consequently, they must be heated above about 80 °C to obtain isotropic molten polymers and a significant increase in ionic conductivity. 

The most important deficiency in the models developed so far concerns the failure to take account of interactions between the mobile ions. As the ionic concentration in polymer electrolytes is frequently greater than 1.0moldm and the mean distance between ions of the order of 0.5-0.7 nm, then relatively stong coulombic interactions exist which must affect ion motion. Ratner and Nitzan have begun to address this problem from a theoretical viewpoint (Ratner and Nitzan, 1989) although it has not been fully developed yet to give a complete description of conduction in ion associated polymer electrolytes. The interactions between ions which lead to ion association are discussed further in the following section. 

There are two classes of materials which may be used as electrolytes in all-solid-state cells polymer electrolytes, materials in which metal salts are dissolved in high molar mass coordinating macromolecules or are incorporated in a polymer gel, and ceramic crystalline or vitreous phases which have an electrical conductance wholly due to ionic motion within a lattice structure. The former were described in Chapter 7 in this... 

Systems composed of polymers which have functional groups that can solvate ions, wherein the charge separation required for ionic motion is obtained by dispersion and dissolution of electrolytes in the polymeric matrix. 

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]. 

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. 

To achieve high conductance, both reasonable conductivity and mechanical stability in a thin film form are required. Semi-crystalline polymers have superior mechanical characteristics but vastly inferior conductivity properties to those which are fully amorphous (and well above their Tg), since ionic motion does not occur in the crystalline regions. The design of an optimized electrolyte for battery use is in fact more strongly dictated by morphological considerations (which are affected by the choice of dissolved salt) than by the selection of a system containing a solute with a high cationic transference number. 

The motion of ions (i.e. conductivity) in polymer electrolytes appears to occur by a liquid-like mechanism in which the movement of ions through the polymer matrix is assisted by the large amplitude segmental motion of the polymer backbone. Ionic conductivity primarily occurs in the amorphous regions of the polymer [4,5]. The temperature dependence of the conductivity of polymer electrolytes is best related by the Vogel-Tamman-Fulcher (VTF) equation... 

Poly (ethylene oxide) (PEO) - LiX complexes appear to be the most suitable electrolytes for lithium polymer batteries, however, the local relaxation and segmental motion of the polymer chains remain a problem area (Armand et al., 1997). Therefore, the PEO-based electrolytes show an appreciable ionic conductivity only above 100°C (Gorecki et al., 1986). This is, of course, a drawback for applications in the consumer electronic market. On the other hand, the gel polymer electrolytes although offer high ionic conductivity and appreciable lithiiun transport properties it suffers from poor mechanical strength and interfacial properties (Croce et al., 1998 Gray et al., 1986 Kelly et al., 1985 Weston et al., 1982). Recent studies reveal that the nanocomposite polymer electrolytes alone can offer safe and reliable lithium batteries (Appetecchi... 

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