Plastic crystals, ionic conductivity

McEntee, C. Wishart, J. F. (2008). Manipulating the properties of ionic liquids by synthetic design. Abstracts, 40th Middle Atlantic Regional Meeting of the American Chemical Society, Queens, NY, United States, May 17-21, (2008), MRM-052 Lewandowski, A. Galinski, M. (2004). Carbon-ionic liquid double-layer cap>acitors. Journal of Physics and Chemistry of Solids, 65,2-3, (2004), 281-286 MacFarlane, D. R. Meakin, P. Sun, J. A. N. Forsyth, M. (1999). Pyrrohdinium Imides A new family of molten salts and conductive plastic crystal phases. Journal of Physical Chemistry B, 103, 20, (1999), 4164-4170... 

A further interesting effect discovered in our laboratories is that the addition of low levels of a second component, or dopant ion, can lead to significant increases in the ionic conductivity [6, 30, 31]. Typically these dopant species, for example, Li, OH , and H" ", are much smaller than the organic ions of the matrix, and since the relaxation times characterizing the motion of these ions are more rapid than those of the bulk matrix itself, these materials may represent a new class of fast ion conductor. The dopant ion effect can be used to design materials for specific applications, for example, Li+ for lithium batteries and H /OH for fuel cells or other specific sensor applications. Finally, we have recently discovered that this dopant effect can also be apphed to molecular plastic crystals such as succinonitrile [32]. Such materials have the added advantage that the ionic conductivity is purely a result of the dopant ions and not of the solvent matrix itself. 

Rana, U.A., Vijayaraghavan, R., Doherty, C.M., Chandra, A., Efthimiadis, J., Hill, A.J., MacFarlane, D.R. and Forsyth, M., Role of defects in the high ionic conductivity of choline triflate plastic crystal and its acid-containing conpositions, J. Phys. Chem. C117 (11), 5532-5543 (2013). 

Alarco, P.J., Yaser, A.L., Ravet, N., Armand, M. (2004). Lithium conducting pyrazohum imides plastic crystals a new solid state electrolyte matrix. Solid State Ionics, 172., 1-... 

Quasi-solid-state electrolytes include gel polymer electrolytes, ionic liquids, and plastic crystal systems. It is important to distinguish polymer electrolytes and gel polymer electrolytes. In polymer electrolytes, charged cationic or anionic groups are chemically bonded to a polymer chain, while gel polymer electrolytes are solvated by a high dielectric constant solvent and are free to move. In a classical gel electrolyte, polymer and salts are mixed with a solvent, usually having a concentration above 50 wt%, and the role of the polymer is to act as a stiffener for the solvent, creating a three-dimensional network, where cations and anions move freely in the liquid phase [88]. The solid polymer electrolyte includes poly(ethylene oxide) (PEO)-based lithium ion conductors that typically show conductivities of 10 S cm while the gel polymer electrolytes have semisolid character with much higher ionic conductivities of the order 10 —10 S cm . ... 

This discussion shows that the gel electrolyte must match the use of the battery, requiring optimization of the composition of the gel polymer electrolyte, the supporting salt and its concentration, and the solvent. PAN gel electrolytes made using different solvents, lithium salts, and composition will display different behaviors with respect to the ionic conductivity, lithium-ion transference number, electrochemical window, cyclic voltam-metric behavior, and compatibility with electrodes. Table 11.1 lists the ionic conductivity at room temperature of some gel electrolytes based on PAN. Because the PAN chain contains highly polar -CN groups, which exhibit poor compatibility with lithium metal electrodes, the passivation of the interface between the gel electrolyte and lithium metal electrode is crucial. At the same time, PAN has a high crystallization tendency. At elevated temperatures, the liquid electrolyte and plasticizer will separate therefore, the polymer is modified, mainly by copolymerization and cross-linking. 

Investigation of ionic conductivity in crystalline soft solids has followed two paths crystalline polymer salt complexes and plastic crystals. Although the latter are not strictly polymer electrolytes they have similar mechanical properties and help to provide a more complete view of ion transport in non-amorphous soft solids. 

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