Sodium ion conduction

This cell reaction necessitates a so-dium-ion-conductive electrolyte. At present, the best and most stable sodium ion conductor is / "-alumina. This electrolyte has sufficient high sodium ion conductivity at temperatures of about 300 °C. The ft"-alumina electrolyte is normally designed as a tube closed at one end with a negative... 

In the Na/S system the sulfur can react with sodium yielding various reaction products, i.e. sodium polysulfides with a composition ranging from Na2S to Na2S5. Because of the violent chemical reaction between sodium and sulfur, the two reactants have to be separated by a solid electrolyte which must be a sodium-ion conductor. / " -Alumina is used at present as the electrolyte material because of its high sodium-ion conductivity. 

Sodium ion conduction appears to be common because of the well-known properties of the beta-aluminas and, to a lesser extent, the NASICONs (see Section 2.12.1), Table 2.1. There are, however, relatively few other examples of high Na ion conductivity, especially at room temperature. In contrast to Ag, the usual coordination number of Na is high, often 7-9, and the sites may be distorted. The bonding of Na in such structures is much more ionic than that of Ag, therefore. 

Abel, E. Maguire, G. E. M. Meadows, E. S. Murillo, O. Jin, T. Gokel, G. W., (1997) Planar bilayer conductance and fluorescent studies confirm the function and location of a synthetic sodium-ion-conducting channel in a phospholipid bilayer membrane J. Am. Chem. Soc. 119, 9061-9062. 

Nasicon is a generic term for sodium ion conducting oxides containing elements such as Zr, Si, and P. The values of their conductivities are similar to that of /1-alumina. 

The great acceleration of interest in solids with high ionic conductivity was stimulated by the report of Yao and Kummer in 1967 that sodium /3-alumina has a sodium ion conductivity at room temperature comparable to that of an aqueous sodium chloride solution. The simultaneous invention of the sodium//3-alumina/sulfur battery by the same group intensified interest in the commercial application of solids with high ionic conductivity. 

Local anesthetics prevent the voltage-dependent increase in sodium ion conductance and thus block the initiation and propagation of action potentials. This occurs via two mechanisms. Firstly, non-specific activity on the membrane surface causes the membrane to swell, physically preventing sodium ions getting through the membrane pores. Secondly, blockade of sodium channels occurs. 

With the conductivity of an aqueous electrolyte (e.g., IN KCl) serving as a reference, comparable conductivities can be achieved in solid electrolytes under certain conditions. Some of the best solid ionic conductors, commonly referred to as superionic conductors , have resistivities comparable to those of aqueous electrolytes at room temperature (e.g., RbAg4l5 and single crystal MgO-stabilized 6"-alumina). However, they are either in the form of single crystals, which is impractical for most applications, or composed of very expensive and relatively unstable materials. Resistivities comparable to those of aqueous electrolytes can be achieved in solid electrolytes at higher temperatures in both superionic conductors like 6"-alumina (i.e., 300°C) and normal ionic conductors such as stabilized zirconia (800-1000°C), stabilized cerium oxide (>800 C), and stabilized bismuth oxide (>600°C). Sodium ion conducting glasses are much less conductive than polycrystalline 8 -alumina. 

The optimum sodium ion conductivity in lithia-stabilized /8"-alumina occurs at z 0.33, which corresponds to Na20 0.2 Li20 6.43AI2O3 with approximately 13.1 mol % Na20... 

Divalent ion-stabilized "-alumina is represented by the general composition Nai + jMjAll 1 -yOi7, where M can be Mg, Ni, or Zn. When y = 1, all the sodium ion sites in the conduction plane are filled. For optimum sodium ion conductivity, y k 0.67, which corresponds to NaaO 0.8 MgO 6.I9AI2O3 with approximately 12.5 mol % NaaO and 10 mol% MgO. MgO is incorporated into the /0"-alumina crystal structure by the following defect reaction... 

Grain boundaries offer some impedance to sodium ion conduction (i.e., approximately a factor of five increase for polycrystalline jS -alumina at 300 C) " . They also increase the activation energy for conduction by approximate factors of 1.6-2 depending on the temperature and grain size. Sodium ion grain boundary conduction is dominant in polycrystalline 8"-alumina when the grain size is very small (< 1-2 pm) and when the temperature is below 100°C. 

Sodium ion conduction is anisotropic in single crystals, and large shapes with the proper orientation are very difficult and expensive to fabricate,... 

The desired results after sintering and annealing are 100% conversion to the ) " phase for maximum sodium ion conductivity maximum mass density to eliminate porosity, particularly large pores, in the microstructure uniform, fine grain size below 10 pm, to maximize strength and minimize sources or singularities for fracture and no loss of NaaO to control composition and eventually the sodium ion conductivity. 

Sodium ion conduction in /S"-alumina is anisotropic and ceramic electrolytes produced from this material not only can be sensitive to exposure to moisture but also are somewhat difficult to densify (e.g., because of problems related to NaaO evaporation, conversion to single-phase / -alumina, and control of exaggerated grain growth). For these reasons. Serious attempts have been made to find alternative materials that have Na" ion conductivities comparable to or higher than / "-alumina but are easier and/or potentially more economical to fabricate. Here we review attempts to develop alternatives. They fall into two classes of materials the crystalline NASICON family of compounds and conducting glasses. 

Table 1 lists sodium ion resistivities at 300°C for polycrystalline ceramic conductors (e.g., 6"-alumina, and NASICON) and sodium ion conducting glasses (e.g., NASIGLAS and a sodium borosilicate glass). (The terms NASICON and NASIGLAS were defined... 

The data presented in Table 1 permit several conclusion to be drawn on the potential use of these materials as electrolytes in electrochemical devices with practical values for the area-specific resistance. Only the j0"-alumina and NASICON electrolytes possess sufficient conductivities for use as membranes with thicknesses of 1 mm or greater. The sodium ion conductivities of polycrystalline NASICON and /l"-alumina are comparable. The glassy electrolytes must be used in the form of thin films ( < 50 pm and possibly under 10-15 pm) or as capillary tubes with very thin walls (10-50 pm). 

---