Solvent-free polymer electrolytes

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

Glasses and polymer electrolytes are in a certain sense not solid electrolytes but neither are they considered as liquid ones. A glass can be regarded as a supercooled liquid and solvent-free polymer electrolytes are good conductors only above their glass transition temperature (7 ), where the structural disorder is dynamic as well as static. These materials appear macroscopically as solids because of their very high viscosity. A conductivity relation of the Vogel-Tamman-Fulcher (VTF) type is usually... 

Fish D, Khan IM, Smid J (1986) Poly[(methoxyheptaethylene oxide)methylsiloxane]/lithium perchlorate complexes as solvent-free polymer electrolytes for high energy density storage devices. Polym Prepr (Am Chem Soc, Div Polym Chem) 27 325-326... 

Lithium solid-state electrolytes can be roughly divided into three main categories (i) ceramic (CE), (ii) glasses (GL), (iii) solvent-free polymer electrolytes (SPEs). Indeed, the most appealing class is CE, which has been the object of recent good reviews [6-8]. These electrolytes can easily offer a relatively high conductivity (up to 10 cm ), and have the further advantage of a thermal expansion... 

Oh JS, Kang YK, Kim DW (2006) Lithium polymer batteries using the highly porous membrane filled with solvent-free polymer electrolyte. Electrochim Acta 52 1567-1570... 

S.2 Solvent-Free Polymer Electrolytes 637 (c) Intrachain movement via ion cluster... 

The polymer electrolytes discussed so far suffer from a number of disadvantages. Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline or glassy) electrolytes at or below room temperature. The best all-amorphous systems have conductivities less than 10 S cm at room temperature. These ambient temperature conductivities may be insufficient in some cases for the power required by a lithium battery. Secondly, the interfacial impedances present at both the lithium anode (passivation) and composite cathode (passivation, contact) are in addition to the ohmic losses in the electrolyte. Thirdly, the lowness of cation transference number, although similar to the values in liquid systems, is a major issue since the total conductivity is lower and could limit the use of solvent-free polymer electrolytes except in the form of extremely thin films or above room temperature. 

In order to investigate the dynamics of an electrolyte system, the line width of the central Li transition as a function of P(E0-EC)/LiCF3S03 electrolyte content (P(EO-EC) = poly(ethylene oxide-co-ethylene carbonate)) as well as temperature were measured by Jeon and Kwak on solvent-free polymer electrolyte materials based on poly(vinylidene fluoride) (PVdF) membranes fllled with P(EO-EC). As seen in Fig. 7.8 a typical temperature dependence is observed for this system with broad spectral lines below and narrow Unewidths above Tg, indicating Li ion mobilities closely associated with segmental motion of P(EO-EC) polymer. 

SHIN, J.H., HENDERSON, W.A., TIZZANI, c., et al. Characterization of solvent-free polymer electrolytes consisting of ternary PEO-LiTFSI-PYR14TFSI,/. Electrochem. Soc., 2006,153, A1649-A1654. 

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