Liquid electrolytes ionic conductivities

For liquid electrolytes, ionic conductivity, self-diffusivity, and viscosity are three key properties. Though originally based on dilute aqueous electrolyte solutions, the Walden rule [52] has been proposed as a tool to provide insight to the proton transfer and ion association. The rule suggests that the molar cmiductivity of an electrolyte, A, is proportional to the fluidity, which can be expressed as the inverse of the shear viscosity i/. In other words, the product of the molar conductivity and viscosity of an electrolyte is a constant, as shown in (3.10). 

In a broad sense, electrochemical phenomena involve electron transfer processes through a two-dimensional boundary (interface) separating the electrode (metal-type conductor) and the electrolyte (ionically conducting). In the study of such phenomena, one can distinguish between electrodics, focused on the heterogeneous elec-trode/electrolyte charge transfer process, and ionics, devoted to the study of ionically conducting liquid or solid phases (Bockris and Reddy, 1977). 

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. 

Hayamizu, K., Tsuzuki, S., Seki, S., Ohno, Y, Miyashiro, H. and Kobayashi, Y, Quaternary ammonium room-temperature ionic liquid including an oxygen atom in side chain/lithium salt binary electrolytes ionic conductivity and H, Li, and NMR studies on diffusion coefficients and local motions, J. Phys. Chem. B 112,1189-1197 (2008). 

More recently, as a consequence of the use of ionic liquids (TLs) in lithium batteries, a new class of solid polymer electrolytes was proposed by Ohno and co. [76]. Basically, these SPEs, better known as polymeric ionic liquids (PILs), are prepared through the radical pol3unerization of ILs by properly combining different cations, anions and polymer backbones [77]. PILs show the potential advantage to combine the benefits of ionic liquids (high ionic conductivity, high thermal... 

Two modifications of plasticizers are generally applied. One is to use materials that have similar structures with the plasticizers mentioned in Table 11.1. As shown in Figure 11.35, phosphate can be used as a spacer to separate EO units. The other modification uses ionic liquids as plasticizers. Inorganic plasticizers are prepared by replacing the organic plasticizers in the gel polymer electrolyte with ionic liquids. For example, in the case of P(TFE-HFP) copolymer, it is mixed with an ionic liquid. The ionic conductivity at room temperature exceeds 10 S/cm, and rises to more than 10 S/cm at 100°C. As discussed in Section 9.7, the preparation of ionic liquid is simple. 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

Figure 1 shows the temperature variation of the ionic conductivities for several polymer-electrolyte systems. At room temperature they are typically 100 to 1000 times less than those exhibited by a liquid or the best ceramic- or glass-based electrolytes [6,8], Although higher conductivities are preferable, 100-fold or 1000-fold... 

Influence on Electrolyte Conductivity In porous separators the ionic current passes through the liquid electrolyte present in the separator pores. Therefore, the electrolyte s resistance in the pores has to be calculated for known values of porosity of the separator and of conductivity, o, of the free liquid electrolyte. Such a calculation is highly complex in the general case. Consider the very simple model where a separator of thickness d has cylindrical pores of radius r which are parallel and completely electrolyte-filled (Fig. 18.2). Let / be the pore length and N the number of pores (all calculations refer to the unit surface area of the separator). The ratio p = Ud (where P = cos a > 1) characterizes the tilt of the pores and is called the tortuosity factor of the pores. The total pore volume is given by NnrH, the porosity by... 

In polymer electrolytes (even prevailingly crystalline), most of ions are transported via the mobile amorphous regions. The ion conduction should therefore be related to viscoelastic properties of the polymeric host and described by models analogous to that for ion transport in liquids. These include either the free volume model or the configurational entropy model . The former is based on the assumption that thermal fluctuations of the polymer skeleton open occasionally free volumes into which the ionic (or other) species can migrate. For classical liquid electrolytes, the free volume per molecule, vf, is defined as ... 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

For a battery to give a reasonable power output, the ionic conductivity of the electrolyte must be substantial. Historically, this was achieved by the use of liquid electrolytes. However, over the last quarter of a century there has been increasing emphasis on the production of batteries and related devices employing solid electrolytes. These are sturdy and ideal for applications where liquid electrolytes pose problems. The primary technical problem to overcome is that of achieving high ionic conductivity across the solid. 

All cells comprise half-cells, electrodes and a conductive electrolytethe latter component separates the electrodes and conducts ions. It is usually, although not always, a liquid and normally has an ionic substance dissolved within it, the solid dissociating in solution to form ions. Aqueous electrolytes are a favourite choice because the high dielectric constant e of water imparts a high ionic conductivity k to the solution. 

Noda, A., Susan, M., Abu Bin, H., Kudo, K., Mitshushima, S., Hayamizu, K. and Watanabe, M. 2003. Bronsted acid-base ionic liquids as proton-conducting nonaqueous electrolytes. Journal of Physical Chemistry B 107 4024 033. 

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