Rubbery Electrolytes

Polymeric electrolytes can possibly be used to build safe, non-toxic modern battery systems, e.g. Li-batteries. In this context the understanding of the ionic conduction mechanism of dissolved alkali salts is of major importance. Besides macroscopic measurements of transport coefficients, the investigation of mobilities on a molecular level is essential to identify the relevant conduction mechanisms. 

The experiments on alkali iodides, PEOx-Nal or PEOx-Lil [316-318] were performed on PEO chains of 23 or 182 (-CH2-CH2-O-) monomers and Orion ratios between 15 and 50. The incoherent scattering from protonated polymers was measured using INI 1C, which yields the intermediate scattering function of the self-correlation. The experiments were performed in the homogeneous liquid phase where the added salt is completely dissolved and no crystalline aggregates coexist with the solution, i.e. at temperatures around 70 °C. 

The results were compared to MD-simulations [317]. Whereas the scattering function of pure PEO could be well described, the dynamics of the salt-loaded samples deviates from the predictions obtained with various electrostatic interaction models. The best but still not perfect and - at least for longer times -unphysical model assumes Hookean springs between chains to simulate the Na-ion mediated transient cross-links [317]. 

An extensive investigation on the system PPO-LiCl04 is reported by Carlsson et al. [314]. Using deuterated PPO (dPPO) with a molecular weight M =2 kg/mol 

In Fig. 6.22 the results of a viscosity scahng by f— fxT/rj (T) of the relaxation data are shown. Such a scaling is motivated by the Rouse model and should hold for the a-relaxation. The pure PPO data (right) behave according to this expectation in contrast the PP0-IiC104 curves deviate considerably. This indicates that the coupling factor between microscopic friction and viscosity depends on temperature, possibly due to transient cross-linking via Li-ions. 

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Then we address the dynamics of diblock copolymer melts. There we discuss the single chain dynamics, the collective dynamics as well as the dynamics of the interfaces in microphase separated systems. The next degree of complication is reached when we discuss the dynamic of gels (Chap. 6.3) and that of polymer aggregates like micelles or polymers with complex architecture such as stars and dendrimers. Chapter 6.5 addresses the first measurements on a rubbery electrolyte. Some new results on polymer solutions are discussed in Chap. 6.6 with particular emphasis on theta solvents and hydrodynamic screening. Chapter 6.7 finally addresses experiments that have been performed on biological macromolecules. 

Class 4 polymer-in-salt, or rubbery electrolytes, in which high-molar mass polymers are dissolved in low temperature molten salt mixtures. 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

For using lithium batteries (which generally have high energy densities) under extreme conditions, more durable and better conducting electrolytes are necessary. Salt-in-polymer electrolytes discovered by Angell et al. (1993) seem to provide the answer. Polypropylene oxide or polyethylene oxide is dissolved in low melting point mixtures of lithium salts to obtain rubbery materials which are excellent lithium-ion conductors at ambient temperatures. 

In addition to the salt in polymer approach as described above, Angell and coworkers have described preparation of polymer-in-salt materials [44] (vide infra). Lithium salts are mixed with small amounts of poly propylene oxide and poly ethylene oxide to afford rubbery materials with low glass transition temperatures. This new class of polymer electrolytes showed good lithium ion conductivities and a high electrochemical stability. 

Angell, C.A., Liu, C., Sanchez, E. 1993. Rubbery solid electrolytes with dominant cationic transport and high ambient conductivity. Nature 362 137-139. 

The mechanical properties of nanogel electrolytes were also significantly improved. The nanogels exhibited a rubbery behaviour with a storage modulus of about 1 GPa at room temperature (one order of magnitude... 

PEO, which are typical matrices for polymer electrolytes, has been reported to be 10 to 10 s at room temperature, and its temperature dependence obeys the WLF equation [24]. These features are shown in Fig. 5 [11]. The temperature dependence of the inverse of the dielectric relaxation time t(T), owing to the backbone motion of the PPO network polymer, obeys the WLF equation shown in this figure. How small ions migrate in these rubbery media is an interesting question. The percentage change in the conductivity with temperature is comparable with that in the dielectric [11,25] or mechanical relaxation time [16,26,27] of the backbone motion for the PPO-and PEO-based polymer electrolytes, when is used as reference temperature. A typical result is shown in Fig. 6 [26], in which the ratio of ionic conductivity at T, to that at T, o (Tg), and the ratio of mechanical... 

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