Ionic conductivity liquid/polymer gels

Ogihara, W. et al., Ionic conductivity of polymer gels deriving from alkali metal ionic liquids and negatively charged polyelectrolytes, Electrochim. Acta, 49,1797,... 

Many approaches have been developed for the production of ionic liquid-polymer composite membranes. For example, Doyle et al. [165] prepared RTILs/PFSA composite membranes by swelling the Nafion with ionic liquids. When 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate was used as the ionic liquid, the ionic conductivity ofthe composite membrane exceeded 0.1 S cm at 180 °C. A comparison between the ionic liquid-swollen membrane and the liquid itself indicated substantial proton mobility in these composites. Fuller et al. [166] prepared ionic liquid-polymer gel electrolytes by blending hydrophilic RTILs into a poly(vinylidene fiuoridej-hexafluoropropylene copolymer [PVdF(HFP)] matrix. The gel electrolytes prepared with an ionic liquid PVdF(HFP) mass ratio of 2 1 exhibited ionic conductivities >10 Scm at room temperature, and >10 Scm at 100 °C. When Noda and Watanabe [167] investigated the in situ polymerization of vinyl monomers in the RTILs, they produced suitable vinyl monomers that provided transparent, mechanically strong and highly conductive polymer electrolyte films. As an example, a 2-hydroxyethyl methacrylate network polymer in which BPBF4 was dissolved exhibited an ionic conductivity of 10 S cm at 30 °C. 

An alternative to lithium batteries with liquid electrol5des are those with solid polymer electrolytes. Solid polymer electrodes are generally gel type electrolytes which trap solvent and salt in pores of the polymer to provide a medium for ionic conduction. Typical polymer electrolytes are shown in Table 15.8. 

A gel electrolyte is a polymer gel that confines a liquid electrolyte. Gel electrolytes have better ionic conductivity than polymer electrolytes. Wang et al. have studied poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) gel electrolyte with plasticizers such as EC-DMC, and PC-EC-DEC for Li-S batteries at room temperature. The ionic conductivity is improved significantly to about 1.2 X 10 Scm, which is much higher than that of PEO-based polymer electrolyte. High discharge capacity and utilization of sulfur are obtained with PAN-S [70, 71] and C-S [46, 69] composite electrodes. However, much lower utilization of sulfur is obtained when PVDF [134] or PVDF-HFP [119] are combined with TEGDME as a plasticizer. When TEGDME combines with EC as the plasticizer for microporous PVDE-HEP gel electrolytes, a better performance is observed in Hthium-sulfur batteries [31]. 

An interesting approach consists of the addition of nanoscale inorganic fillers, to improve the mechanical, interfacial and conductivity properties of the (gel) polymer electrolytes. Since the pioneering work by Scrosati and co-workers, addition of T102 and other nanoparticles has been extensively employed to improve the ionic conductivity of polymer electrolytes. It is well known that the presence of such nanoparticles changes the conduction mechanisms assigned to the ions introduced in the polymer however, how these nanoparticles actually act is stiU unknown. These materials can also improve the mechanical properties of gel electrolytes and ionic liquid-based electrolytes. However, their effect on the mechanical stability can result in a loss in electrolyte penetration. 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, poly(tetrafluoroethylene), and cellulose acetate, have been used for porous substrates for supported-liquid membranes. The PVdF coated polyolefin-based microporous membranes used in gel—polymer lithium-ion battery fall into this category. Gel polymer... 

To address the zinc dendrite problem in nickel-zinc cells, eVionyx claims to have developed a proprietary membrane system that is nonporous, has very high ionic conductivity, is of low cost, and can block zinc dendrite penetration even in high concentrations of KOH. The polymeric membrane has an ionic species contained in a solution phase thereof. The ionic species behaves like a liquid electrolyte, while at the same time the polymer-based solid gel membrane provides a smooth impenetrable surface that allows the exchange of ions for both discharging and charging of the cell. 

However, radical polymerization of common vinyl monomers in situ in ionic liquids [16] produced new polymer-electrolytes called ion gels. These ion gels have completely compatible combinations of ionic liquids and the resulting network polymers. The ion gels can softly bend, and they showed fast ionic conductivity of 10 S cm at room temperature. 

A variety of dimensionally stable solid electrolytes consisting of a mixture of organic plasticizers such as EC, PC etc., along with structurally stable polymers such as poly( acrylonitrile) (PAN) or poly( vinyl sulfone) (PVS), or polyvinyl pyrrolidine (PVP) or polyvinyl chloride (PVC) and several lithium salts have been tested and found to have excellent ionic conductivities at ambient temperatures [155-156]. In these gel type electrolytes the primary role of the polymers PAN, PVS, PVP or PVC is to immobilize the lithium salt solvates of the organic plasticizer liquids. However, with polymers such as PAN a coordination interaction with Li+ is also quite likely. 

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