Liquid electrolytes ionic conduction principles

The development of solid state conducting solids that are on a par with liquid electrolytes has revolutionized the design of batteries and other solid state ionic devices (SSIs) in recent years, and this section explains the operating principles behind some of these devices. Figure 5.15 is a simple schematic diagram which we can use to explain the operation of several different types of electrochemical device. 

LSE, the classical electrochemistry, is concerned with electrochemical cells (ECs) based on liquid ionic-conductors (liquid electrolytes (LEs)). Solid-state electrochemistry is concerned with ECs in which the ionic conductor (electrolyte) is a solid. Both fields are based on common thermodynamic principles. Yet, the finer characteristics of ECs in the two fields are different because of differences in the materials properties, conduction mechanisms, morphology and cell geometry. Differences that come immediately to mind are (1) The lack of electronic (electron/hole) conduction in most LEs, while electronic conduction exists to some extent in all solid electrolytes (SEs). (2) In LEs both cations and anions are mobile, while in SEs only one kind of ions is usually mobile while the other forms a rigid sublattice serving as a frame for the motion of the mobile ion. An... 

The number of studies which utilize ionic liquid electrol54e in redox capacitor system is still small, probably due to the difficulty to reproduce the pseudo-capacitive reaction in ionic liquid media. While the principle of pseudo-capacitance of conductive polymer electrodes permits to utilize ionic liquid electrolytes, high viscosity and rather inactive ions of ionic liquid may make their pseudo-capacitive reaction slow. The combination of nanostmctured conductive polymer electrode and ionic liquid electrolyte is expected to be effective [27]. It is far difficult that ionic liquids are utilized in transition metal-based redox capacitors where proton frequently participates in the reaction mechanisms. Some anions such as thiocyanate have been reported to provide pseudo-capacitance of manganese oxide [28]. The pseudo-capacitance of hydrous ruthenium oxide is based on the adsorption of proton on the electrode surface and thus requires proton in electrolyte. Therefore ionic liquids having proton have been attempted to be utilized with ruthenium oxide electrode [29]. Recent report that 1,3-substituted imidazolium cations such as EMI promote pseudo-capacitive reaction of mthenium oxide is interesting on the viewpoint of the establishment of the pseudo-capacitive system based on chemical nature of ionic liquids [30]. 

FIGURE 1.14 Proton conductivities of various solid-state compounds, liquid electrolytes, and composite materials. (Reprinted from Solid State Ionics, 125 (1-4), T. Norby, Solid-state protonic conductors Principles, properties, progress and prospects, 1-11, Copyright (1999),... 

The concept of SPE dates back to 70s, when Armand firstly proposed a new ion conductor based on a lithium salt properly complexed by a polar and aprotic polymer matrix without the use of any liquid component (additives or liquid electrolytes) [65]. At the beginnings, the research on SPEs was exclusively focused on poly(ethyleneoxide) (PEO) as the complexing polymer [66]. Ever since, a lot of polymer/salt systems were deeply investigated, such as those based on PMMA, PAN, PVDF [66-69]. In principle, SPEs must satisfy some basic requirements (i) ionic conductivity higher than 10 " S/cm at room temperature, (ii) good thermal, chemical and mechanical stability, (iii) lithium transport number close to the unity, and (iv) compatibility with the electrodes and consequently wide electrochemical windows [67]. 

The principle of ionic conduction is directly related to ion mobility, in liquid electrolytes and it is consequence of a three-dimensional random movement of ions. Such movement allows a uniform concentration of ions throughout the solution in absence of external electric field by a mass-transport mechanism called diffusion. On the other hand, in case of application of external forces such as electric fields, ions acquire a nonrandom component of velocity directly proportional to such external influence [9]. 

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