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chemical and thermal stability; and safety. The archetypal polymer electrolyte is based on the dissolution of a salt in an ion-coordinating macromolecule such as poly(ethylene oxide) (PEO). Materials of this sort were first studied by Wright et al. in 1973,10 while recognition of the potential of such systems for practical applications and much of the early development are credited to Armand and coworkers.11 The oxygen atoms in this polymer have high electron donor power and a suitable interatomic separation, thus enabling them to form multiple intrapolymer coordinate bonds with cations. The low barriers to bond rotation allow segmental motion of the polymer chain, thus providing a mechanism for ion transport. Conductivities of such polymer electrolytes are determined largely by the local mobility of the polymer segments (see the article by Ratner et al. in this issue), but the best values are typically 100–1000 times lower than those of conventional liquid electrolytes at a given temperature. This reduction may be compensated, to some extent, by forming the electrolyte into thin films, typically 100 m thick, but for highpower applications, higher intrinsic conductivities are required. Therefore, a number of new forms of polymer electrolyte have been developed. Although in practice a continuum of types exists, it is sometimes useful to consider dividing these polymer electrolytes into five classes:  Class 1: amorphous macromoleculesalt complexes, typically based on polyether hosts.  Class 2: plasticized systems, in which small amounts of low-molar-mass polar liquids are added to Class 1 polymer electrolytes.  Class 3: gel electrolytes, formed by incorporating a nonaqueous electrolyte 

solution within an inactive structural polymer matrix.  Class 4: “polymer-in-salt,” or “rubbery” electrolytes, in which high-molarmass polymers are dissolved in lowtemperature molten salt mixtures.  Class 5: composites, based on the addition of either nanoparticulate ceramics or dual-phase block copolymers. Class 1 polymer electrolytes are sometimes referred to as solvent-free or dry systems. To prevent crystallization and improve mechanical stability, sophisticated polymer synthesis is often used to develop architectures based on, for example, random and block copolymers, comb-branched copolymers, networks, interpenetrating networks, or liquidcrystal-based systems. Particular attention has been paid to the synthesis of single ionic conductors, in which the anion is linked to the polymer backbone (see the Ratner et al. article and also References 12–14). In such systems, t  1; hence in a lithium battery, concentration polarization is prevented. A plasticizer is a low-molar-mass material that may be added to increase polymer segmental motion and flexibility and thereby improve the ionic conductivity of salts dissolved in the system. Addition of suitable organic solvents can increase conductivity by a factor of almost 100. Any slight degradation of mechanical properties can be offset by subsequent cross-linking of the syste