Sulfur-Sodium Storage Batteries

In the course of discharge, sodium is anodically oxidized to sodium ions Na , which penetrate the solid electrolyte and act as current carriers in it. Sulfur is reduced on the positive electrode and reacts with sodium ions from the electrolyte giving rise to various sodium polysulfides, NajS. The overall current-producing reaction can be divided into two stages  

Stage (14.2) gives rise to molten Na2S5 which does not mix with molten sulfur so that a two-phase liquid system is formed. At stage (14.3) when free sulfur has previously been consumed, the system consists of one phase. The melting points of polysulfides with m between 3 and 5 lie in the range of 235-285 C. 

The high working temperature of the sulfur-sodium batteries is necessitated not only by the desire to increase the conductivity of electrolyte but also by the need to operate with molten reactants and intermediate compounds. 

The stability of the electrolyte during operation presents considerable difficulties. In the early battery prototypes, the electrolyte developed microcracks metallic sodium leaked through them resulting in a battery failure. The cracks were caused not 

The batteries are mostly of cylindrical constmction, with the electrolyte shaped as a tube of length 20-50 cm, diameter 1.5-3.5 cm, and wall thickness about 1 mm. One reactant is inside the tube and the other is on the outside. Molten sulfur and polysulfides are typically inside the tube. Molten sodium is in the gap between electrolyte and the batteries container wall. A stock of sodium is stored in a container in the upper and lower part of the batteries. 

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In the sodium—sulfur storage battery above 300°C, the overall chemical reaction occurs between molten sodium metal and sulfur to form sodium polysulfide. The cell voltage is related to the activity of the sodium ( Aia) sulfide relative to its activity in the metal. 

Though sodium-sulfur batteries have been under development for many years, major problems still exists with material stability. It is likely that the first commercial uses of this batteiy will not be for electric vehicles. Sodium-sulfur storage batteries may be more well-suited for hybrid electric vehicles or as part of a distributed energy resources system to provide power ill remote areas or to help meet municipal peak power requirements. 

The properties of sodium hydroxide and potassium hydroxide are very similar. These hydroxides are prepared by the electrolysis of aqueous NaCl and KCl solutions (see Section 19.8) both hydroxides are strong bases and very soluble in water. Sodium hydroxide is used in the manufacture of soap and many organic and inorganic compounds. Potassium hydroxide is used as an electrolyte in some storage batteries, and aqueous potassium hydroxide is used to remove carbon dioxide and sulfur dioxide from air. 

Other types of batteries have been introduced to serve specific needs (electric vehicles, electricity storage for grid support, etc.). For this discussion, we have chosen to focus on sodium-sulfur (Na-S) batteries and nickel-chloride-based batteries, which are both so-called high temperature battery systems, and lastly redox flow systems. 

Acids and bases have many uses in the chemical industry. Sulfuric acid, H2SO4, is the world s most widely produced chemical. It is used to produce fertilizers and plastics, to manufacture detergents, and to conduct electricity in lead-acid storage batteries for automobiles. The base sodium hydroxide, NaOH, is used in the production of pulp and paper, in the manufacture of soaps, in the textile industries, and in the manufacture of glass. 

To transport people and material growing transportation systems are needed. More and more of the energy for these systems is drawn from secondary batteries. The reason for this trend is economic, but there is also an environmental need for a future chance for electric traction. The actual development of electrochemical storage systems with components like sodium-sulfur, sodium-nickel chloride, nickel-metal hydride, zinc-bromine, zinc-air, and others, mainly intended for electric road vehicles, make the classical lead-acid traction batteries look old-fashioned and outdated. Lead-acid, this more than 150-year-old system, is currently the reliable and economic power source for electric traction. 

The sodium/beta battery system includes designs based on either the sodium/sulfur or the sodium/metal chloride chemistries (see Chapter 40). The sodium/sulfur technology has been in development for over 30 years and multi-kW batteries are now being produced on a pilot plant scale for stationary energy storage applications. At least two 8 MW/40 MWh sodium/sulfur batteries have been put into service for utility load leveling by TEPCO in Japan. 

Consider the battery in Fig. 7.18. The sodium beta alumina barrier allows sodium ions formed at the anode to Row across to the sulfur compartment, where, together with the reduction products of the sulfur, U forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 CC to keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At the present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times the power on a weight basis. 

American Electric Power is installing a 6 mW wind farm with battery storage for 27 million, or at a unit cost of 4,500/kW, using NGK Insulator s sodium-sulfur batteries made in Japan. The rationale for the installation is that although the wind turbines operate mostly at night when the value of electricity is low, by storing the electricity generated until the next peak period, its value is much increased. 

The availability of solar energy is subject to diurnal-, seasonal-, and weather-related variations. Therefore, if solar energy is to meet continuous energy demands, it must be stored. On small installations, such storage can be provided by high-density batteries. (NGK Insulators Ltd. of Japan, for example, manufactures sodium-sulfur batteries that can store 7 mWh.) On midsized installations, pumped hydrostorage can be considered. 

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