Hopping polymer electrolytes

These materials are introduced in Chapter 5 and only brief mention of them is necessary here. It is important to appreciate that polymer electrolytes, which consist of salts, e.g. Nal, dissolved in solid cation coordinating polymers, e.g. (CH2CH20) , conduct by quite a different mechanism from crystalline or glass electrolytes. Ion transport in polymers relies on the dynamics of the framework (i.e. the polymer chains) in contrast to hopping within a rigid framework. Intense efforts are being made to make use of these materials as electrolytes in all solid state lithium batteries for both microelectronic medical and vehicle traction applications. 

For application to the surface of the device, the polymer and electrolyte are codissolved in a spreading solvent. After evaporation of the solvent, the polymer electrolyte remains as a thin film. The concentration of the salt in the polymer is expressed as a ratio of the number of polymer repeat units per unit of salt. In these studies, the MEEP/LiCF3S03 ratio is 4 1 or 5 1. The ionic conductivity of these polymer electrolytes is thought to be due to ion hopping between Lewis base sites along and between chains. This process is facilitated by small amounts of a coordinating solvent. The devices described here are operated in the presence of some solvent vapor. Usually 20 ll of THF per 50 ml N2 atmosphere over the device is added as a plasticizer. 

Percolation theory has been successfully applied to such diverse phenomena as hopping conductivity in semiconductors [130], gelation in polymer melts [131], permeability of porous rocks [129, 130], spreading of epidemics [132], and spreading of wildfires [133]. In PEFC research, percolation theory has been employed for establishing relations between water uptake of polymer electrolyte membranes and their proton conductivities [98]. Moreover, percolation concepts... 

As soon as the redox hopping front across the polymer reaches the polymer/electrolyte interface, the me-... 

In addition, several KMC simulation models have been developed to study in more detail the diffusion processes in Li-ion batteries, since the distribution and kinetics of the Li-ion motion have direct consequences in terms of the battery performance and stability. These models include work on Li-ion hopping in polymer electrolytes, parameterized by polarization energy calculations, and a KMC study " of ambipolar Li-ion and electron-polaron (e ) diffusion in nanostructured Ti02 (which investigated the simultaneous diffusion of both LC and e in the electrode). 

Solvent effects may in certain cases contribute to the charge conduction mechanism with polymer films, however, as the work of Kaufman et al (24) has shown. On the basis of spectral data, these workers have concluded that mixed valence states of tetrathiafulvalene (TTp2+, TTF" ", etc.) are not essential to charge-conduction in poly TTF films, but that electron hopping modulated by the solvent-induced pendent group collisions is. An additional phenomenon related to the electrolyte noted by this group is that ion flow into the polymer phase appeared to limit the kinetics of oxidation reactions in these films. 

---