Conductivity in nonaqueous solutions

The techniques and apparatus which have been developed to measure electrolytic conductivities in nonaqueous solutions have been adapted from aqueous conductivity measurements with some modifications. Direct current measurements suffer the limitation of requiring reversible electrodes - a serious limitation in nonaqueous solvents. Although this problem can be circumvented U in some instances, virtually all precision conductance data have been taken using the alternating current method. General descriptions of this method are given in several sources. 2>3)... 

Lithium and sodium salts have been complexed with propylene oxide/ethylene oxide block copolymers. Conductivity was markedly increased in the complexes over that of the polymers, with the greatest increases occurring at low salt concentrations where the salt is mainly increasing Tg (175). Another study conducted in nonaqueous solution indicated that conductivity in the block copolymer complex, as well as in other complexes, was affected by the size of the metal cation and the nature of the solvent in which the complex was formed, as well as by polymer composition and structure (176). A block copolymer prepared by coupling ethylenediamine and poly(ethylene glycol) with 4,4 -diphenylmethane diisocyanate and doped with lithium perchlorate yielded high ionic conductivity (177). 

Although impedances at the anode-electrolyte and cathode-electrolyte interfaces are the limiting factor, ion transport within the bulk electrolyte is also an important consideration. Ion conductivity in nonaqueous solutions is much lower than in aqueous solutions in fact the part of the current carried by the lithium ions in the battery electrolytes is always less than half. A semiempirical rule has been observed the higher the bulk ion conductivity of the battery nonaqueous electrolyte, the more conductive the SEI formed on the electrode in this electrolyte [1]. In other words, more ionic conductivity desired in the SEI is heralded by higher lithium ion conductivity of the bulk electrol34 e. 

Barthel, J. Temperature Dependence of Conductance of Electrolytes in Nonaqueous Solutions 13... 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

One major drawback of these sulfonate salts is their poor ion conductivity in nonaqueous solvents as compared with other salts. In fact, among all the salts listed in Table 3, LiTf affords the lowest conducting solution. This is believed to be caused by the combination of its low dissociation constant in low dielectric media and its moderate ion mobil-ityi29 3 compared with those of other salts. Serious ion pairing in LiTf-based electrolytes is expected, especially when solvents of low dielectric constant such as ethers are used. ... 

From a review of the recent Russian electrochemical literature it can be concluded that intensive research on nonaqueous batteries is carried out in the U.S.S.R. Although no results of the performance of such batteries are published, papers on properties of Li solutions in solvents used in Li cells, and on the behaviour of metallic Li in nonaqueous solutions, which have appeared recently in Russian literature, can be inferred to be the tip of an iceberg of research in that field. For example, the electrodeposition of Li from dimethylformaunide solutions of its salts (40), or the behaviour of Li in aprotic solutions (4la) and the solubility and conductivity of its salts in these solutions (41b) emanate from an unnamed research institute in Moscow. 

Lower Conductivity. The equivalent conductance of nonaqueous solutions a( infinite dilution is often comparable to that of aqueous systems, but it decreases with an increase in concentration more rapidly than the corresponding aqueous systems (the effect of the lower dielectric constant). Since the specific conductivity, K (that which determines the resistance between cathode and anode) is proportional to Ac, the equivalent conductance, the IR drop between the electrodes of a cell in which deposition from nonaqueous solutions is to lake place will be greater than that in aqueous solution (see Section 4.8.7). The electricity needed to deposit a given mass of metal is proportional to the total E between the electrodes, and this includes the IR between the electrodes, which is much greater in the nonaqueous than in the aqueous cases. Hence, nonaqueous deposition will be more costly in electricity (more kilowatt hours per unit of weight deposited) than a corresponding deposition in aqueous solution. The difference may be prohibitive. 

Properties of solutions, solubility of salts, their degree of dissociation, and the resulting conductivity of nonaqueous solutions. In this respect, the effect of the solvent mixture and various possible conducting mechanisms should be discussed. 

Electrical conductivity is a critical issue in nonaqueous electrochemistry, since the use of nonaqueous solvents, which are usually less polar than water, means worse electrolyte dissolution, worse charge separation, and, hence, worse electrical conductivity compared with aqueous solutions. In this section, a short course on electrical conductivity in liquid solutions is given, followed by several useful tables summarizing representative data on solution conductivity and conductivity parameters. 

A nonactive electrode may include noble metals such as gold, silver, and platinum, the so-called sp-metals such as In, Ga, Cd, Bi, as well as transition (or d) metals such as nickel or cobalt. Carbon electrodes and semiconductors such as indium tin oxide [1], diamond [2], and conducting polymers may fall into the category of nonactive electrodes in appropriate solutions, as do composite materials that contain metal oxides or chalcogenides. The behavior of active electrodes in nonaqueous solution is discussed separately in the next chapter. 

Tahon [4] prepared poly(3,4-aIkoxythiophene), (III), derivatives to enhance the conductivity in nonaqueous printing. The process for preparing aqueous and nonaqueous solution dispersions of this agent are described by Louwet [5]. 

It has been seen that reliable conductivity values are known only at low electrolyte concentrations. Under these conditions, even conductance equations for models such as the McMillan-Mayer theory (Sections 3.12 and 3.16) are known. However, the empirical extension of these equations to high concentration ranges has not been successful. One of the reasons is that conductivity measurements in nonaqueous solutions are still quite crude and literature values for a given system may vary by as much as 50% (doubtless due to purification problems). 

Thus, because of the lower dielectric constant values, the effect of an increase of electrolyte concentration on lowering the equivalent conductance is much greater in nonaqueous than in aqueous solutions. The result is that the specific conductivity of nonaqueous solutions containing practical electrolyte concentrations is far less than the specific conductivity of aqueous solutions at the same electrolyte concentration (Table4.26 andFig. 4.107). 

Tafel bigb field case, in ionic conduction, 469 Triple ions, in nonaqueous solution, 552... 

Reactions can be conveniently monitored by H NMR spectroscopy or UV/vis spectrophotometry, and when conducted in nonaqueous solvents proceed without detectable core degradation. In aqueous solution at neutral and basic pH values, ligand substitution seems to be the first step in hydrolytic core degradation (68, 69). Therefore, when these reactions are carried out in an aprotic solvent, or in an aprotic-aqueous solvent mixture. 

The principles governing conductivity in nonaqueous solvents are the same as those for aqueous solutions, of course. The dependence of the conductivity on the viscosity of the solvent was discussed in Section 31.11. However, in solvents having low dielectric constants, there is a lessening of the degree of ionization of many substances. Electrolytes that are completely dissociated in water may be only partially dissociated in a low dielectric constant solvent. Hydrochloric acid is completely dissociated in water HCl is a strong acid. In ethyl alcohol, however, HCl is a half-strong acid, with a dissociation constant of about 1.5 X 10. ... 

H. S. Lee, X. Q. Yang, C. L. Xiang, J. McBreen, J. Electrochem. Soc. 1998,145, 2813-2818. The synthesis of a new family of boron-based tmion receptors tmd the study of their effect on ion pair dissociation and conductivity of lithium stilts in nonaqueous solutions. 

The vast majority of electrochemical studies have been conducted in aqueous solution, followed by studies in a limited number of high-dielectric nonaqueous solutions (e.g., acetonitrile, DMSO, propylene carbonate, dimethyl formamide) and, more recently, in ionic liquids. Nevertheless, electrochemists have always been interested in the possibilities offered by more unusual media, including the opportunity to have a wider potential window and to study electrochemical reactions at higher potentials, to extend the scope of electroanalysis to new analytes and media, or the deposition of a wider range of materials. To some extent, electrochemistry at extreme conditions of temperature [1] or pressure [2] offers some of these same challenges and possibilities. 

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