Ionic conducting materials

From Eq. (18) the concentration of electrons, and according to Eq. (11) the concentration of holes also, depend on the lithium activity of the electrode phases with which the electrolyte is in contact. Since anode and cathode have quite different lithium activities, the electronic concentration may vary to a large extent and an ionically conducting material may readily turn into an electronic conductor. 

The classical example of a soUd organic polymer electrolyte and the first one found is the poly(ethylene oxide) (PEO)/salt system [593]. It has been studied extensively as an ionically conducting material and the PEO/hthium salt complexes are considered as reference polymer electrolytes. However, their ambient temperature ionic conductivity is poor, on the order of 10 S cm, due to the presence of crystalUne domains in the polymer which, by restricting polymer chain motions, inhibit the transport of ions. Consequently, they must be heated above about 80 °C to obtain isotropic molten polymers and a significant increase in ionic conductivity. 

Fig. 6.7 Average shear (mechanical) and conductivity (electric modules) relaxation times, T, and for ionically conductive materials, (a) For glassy materials r, r, above 7 and t T, below Tf. (b) For polymer electrolytes ((PP0),3 NaCF3S03) shown), x, > t, for T> T..

In Chapter 3, four examples of non-stoichiometric compounds used as practical materials are described from a chemical point of view. The sections on ionic conducting materials and hydrogen-absorbing alloys concentrate on how to utilize the characteristic properties of these compounds, in relation to their non-stoichiometry. In the section on magnetic and electrical materials, methods of sample preparation, focusing on the control of non-stoichiometry, and the relation between non-stoichiometry and the properties of the compounds are presented. 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

Along the X direction, i.e., the surface of the ionic conducting material, the reaction rate-limiting step is electron transport in the product phase (D). The growth distance, x, can be expressed as ... 

Selection of an appropriate solute is important for the formulation of an effective electrolyte. Maximum conductivity, for example, seems to be associated with a size homogeneity between the substituting species and the majority cation in the cubic structure, as well as its concentration in solid solution. Figure 3 presents the effects on the ionic conductivity of stabilised zirconia at a fixed temperature, on variation of the cationic substituting species. It is evident that the optimised yttrium solid solution has a conductivity of about 0.015 S cm at 800°C, so that only a very thin electrolyte membrane can provide a technically acceptable current density at that temperature. The well-established Westinghouse SOFC system therefore operates closer to 1000°C to take advantage of the rapid increase of electrolyte conductivity with temperature (7) (see also Fig. 7). This dependance, particularly steep for YSZ, is presented for several solid ionic conducting materials in Fig. 4. 

Interpreting the experimental data in this form nowadays is a commonly employed method to obtain information about the relaxation processes in ionic conductive materials and polymer-conductivity nanoparticles composites. In this representation, interfacial polarization and electrode contributions are essentially suppressed [44, 45]. The peak in the imaginary part of M" depends on temperature, which can be related to the translational ionic motions. The corresponding relaxation time = l/(27r/p), where /p is the peak frequency, therefore is called conductivity relaxation time. 

Figure 14. Real conductivity for fructose in the temperature range from 30 C (open triangles) up to 70 °C (open squares) in steps of 2 C, exhibiting the typical behaviour foimd for ionically conducting materials. The plateau region corresponds to the d.c. conductivity (o)->-0), while the region at higher frequencies after bending is due to a.c. conductivity.

Electrochemistry involves the contact between different materials which conduct electricity. The two terminals in the electrochemical system linked to the external control device must be electronic conducting materials if the electric parameter is to be controlled, for instance using a direct current (DC) power supply. This system must also include at least one ionic conducting material. To illustrate, an electronic n/p junction... 

C. Michot, M. Armand, J.-Y. Sanchez, Y. Choquette, M. Gauthier, Int. Patent WO 95/26056, 1995. Ionic conducting material having good anticorrosive properties. 

The catalyst layer consists of the platinum catalyst, carbon as porous support material and an ionomer coating that is usually a polysulfonic acid, forming a three-phase boundary between the electrically conductive material, the ionic conductive material, and the gas phase. The hydrogen oxidation reaction (HOR)... 

Maass P, Meyer M, Bimde A (1995) Nonstandard relaxation behaviOT in ionically conducting materials. Phys Rev B 51 8164... 

Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric. Absorption of polar water molecules has a strong effect on the dielectric constant of the material. The magnitude of this effect increases with a large inner surface which can accept large amounts of water. An example of this type of dielectric is porous j8-alumina. Colloidal ferric oxide, certain semiconductors, perowskites and certain polymers have also been used. /1-alumina is characterized by ionic conductance. Materials of this type can be characterized by a complex resistance composed of real (ohmic) as well as capacitive terms. The behaviour of such solids can be symbolized by a model and an associated equivalent circuit as given in Fig. 5.8. 

The success of DuPont s Nafion spurred the development of other polymeric materials with similar chemical architecture. The most notable material developments have been the Dow experimental membrane (Dow Chemicals), Flemion (Asahi Glass), Aciplex (Asahi Kasei), as well as Hyflon Ion and its most recent modification Aquivion (SolviCore). In addition to excellent ionic conductivity, materials of the PFSA family, illustrated in Figure 2.2, exhibit exceptional stability and durability in highly corrosive acidic environments, owing to their Teflon-like backbone (Yang et al., 2008 Yoshitake and Watakabe, 2008). 

The electronic conductivity of good electronic conductors is often sufficiently close to the total conductivity. For dc or ac measurements one must be sure to use chemically compatible electrodes that do not react with the sample to form electronically insulating phases. In contrast to predominantly ionically conducting materials, one should observe Ohm s law instead of Pick s law in dc polarization measurements. Also, emf measurements may be employed in the same way as in the case of ionic conductors. The mixed conducting samples are brought into equilibrium with two different activities of the ionically mobile component. 

In reality, atoms and molecules in solid materials are far from static unless the temperature is low but even at 0 K, vibrational motion remains. For ionically conductive materials, atomic movement is the subject of major interest. allows us to simulate the dynamics of the particles in a well-defined system to gain greater insights into local structure and local dynamics - such as ion transport in solid materials. In an MD simulation, atomic motion in a chemical system is described in classical mechanics terms by solving Newton s equations of motion ... 

---