Electrolytes beta”-alumina

Like the Li/FeSx system, which is presently the most advanced rechargeable battery system based on a molten salt electrolyte, the Na/S system is presently the most advanced rechargeable battery system based on a solid electrolyte (beta-alumina) It operates at about 300 C. 

The Prospects For Solid Electrolyte SBs. For reasons discussed above, the Agl-based cells, being useful for some special types of primary batteries, are not very promising for secondary ones. The beta-alumina cells, on the contrary, have already been developed to the pilot-plant stage and their prospects are fairly good to become commercialized. They are the most advanced among the candidate batteries for traction. The high operating temperature could be lowered if a solid... 

Another solid electrolyte that may lead to important practical applications is sodium beta alumina. Its unusual name comes from a misidertificalion and an uncer-... 

Fig. 7.18 Sodium/sulfur battery with a sodium beta alumina solid electrolyte.

Sodium-sulphur batteries with /3-alumina electrolyte ( beta batteries )... 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

Sodium hydroxide, hydrogen, and chlorine can be produced concurrently in a cell where a sodium chloride-zinc chloride mixture is separated from sodium hydroxide with a beta-alumina diaphragm. In such a cell (which to date has simply been bench tested), pure molten sodium hydroxide and dry chlorine are produced. Because of the higher temperature, the cell operates at lower overvoltage and ohmic loss than the conventional aqueous electrolytic processes (38). 

Advances in electrochemical systems rest in large measure with the evolution of new materials that exhibit chemical stability in severe environments, high electrocatalytic activity, rapid ion conductivity, etc. Examples include RuOx-TiOy-Ti electrocatalysts, the polymer Nafion, yttrium-stabilized zirconate and beta-alumina electrolytes, and metastable alloys produced by rapid solidification processing. 

For research into the manufacture of gas sensors of a potentiometric type, we have tested different solid electrolytes. The unique property of these sensors is that the two different metalhc electrodes ate located in the same gaseous phase. This property has prompted us to study particularly the beta-alumina and calcium sulfate. 

Nevertheless, there are comparable studies on solid electrolytes, more specifically beta-alumina. 

The three boundary point is a specific zone that has its own adsorption sites different from the sites of the sohd electrolyte and of the metal. If we note by p the adsorption sites at the surface of the beta-alumina, Si those present at the level of the three boundary point and S2 those of the metal, the different steps of the formation of the oxygen species present in the system can be expressed as follows ... 

Electrolytes are distinguished from pure electronic conductors by the fact that the passage of an electric current is only insured by displacement of charged species called ions and hence accompanied by a transfer of matter. Therefore, electrolytes are entirely ionic electrical conductors without exhibiting any electronic conductivity (i.e., no free electrons). They can be found in the solid state (e.g., fluorite, beta-aluminas, yttria-stabilized zirconia, and silver iodide), liquid state (e.g., aqueous solutions, organic solvents, molten salts and ionic liquids), and gaseous state (e.g., ionized gases and plasmas). The ions (i.e., anions or cations)... 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

Some examples of materials which conduct by both electrons and electron holes are the Ca0-Zr02 electrolyte studied by Patterson et al. [12], the silver beta alumina electrolyte studied by Whittingham and Huggins [13] and silver chloride coexisting with silver as one electrode [14]. Under these conditions, it is... 

They have been employed as solid electrolytes in small primary batteries, especially for applications where long life and freedom from self discharge are important. Sodium beta alumina has a comparably high conductivity at 300-400 C and is of interest as electrolyte for the sodium/sulphur battery. Several reviews on e evelopment of solid electrolyte batteries have been written... 

A further useful classification of solid electrolytes is based upon whether the fast ion-conduction process is three-dimensional throughout the crystal lattice, as in the high temperature electrolytes and the silver salts, or is confined to two-dimensional layers, as in the beta alumina family of compounds, or to onedimensional tunnels, as in the hollandite materials such as... 

Ag, T1 and NO also form beta aluminas, although the sodium compounds are the only ones of technical significance as potential battery electrolytes. 

The principal battery which is under development with a beta-alumina electrolyte is the sodium/sulphur system which operates at 300-400 C and has liquid electrodes. This is described in my second lecture to the summer school. The interest in this battery stems from its projected application as a traction battery for vehicles and as a load levelling device for power stations. For such applications the energy and power requirements are many orders of magnitude larger than for the solid state batteries described above and current densities of 200 mA/cm are commonplace. Liquid electrodes are therefore necessary to avoid electrode polarisation effects. The practical problems of developing these high temperature, high power level batteries are formidable and a substantial world-wide effort is involved. 

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