Sodium/metal-chloride batteries

A. Tilley and R. BuU, The Design and Performance of Various Types of Sodium/Metal Chloride Batteries, Proc. 22nd lECEC Conf, 2 1078-1084 (1987). 

D Trickett. Current status of health and safety issues of sodium/metal chloride (ZEBRA) batteries. National Renewable Energy Eaboratory Report TP 460 25553, 1998. 

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. 

Sodium/sulfur and sodium/metal chloride technologies are similar in that sodium is the negative electrode material and beta-alumina ceramic is the electrolyte. The solid electrolyte serves as the separator and produces 100% coulombic efficiency. Applications are needed in which the battery is operated regularly. Sodium/nickel chloride cells have a higher open-circuit voltage, can operate at lower temperatures, and contain a less corrosive positive electrode than sodium/sulfur cells. Nevertheless, sodium/nickel chloride cells are projected to be more expensive and have lower power density than sodium/sulfur cells. 

A battery system closely related to Na—S is the Na—metal chloride cell (70). The cell design is similar to Na—S however, ia additioa to the P-alumiaa electrolyte, the cell also employs a sodium chloroalumiaate [7784-16-9J, NaAlCl, molten salt electrolyte. The positive electrode active material coasists of a transitioa metal chloride such as iroa(Il) chloride [7758-94-3] EeQ.25 or nickel chloride [7791-20-0J, NiQ.25 (71,72) in Heu of molten sulfur. This technology is in a younger state of development than the Na—S. 

The ZEBRA battery is a high-energy battery based on a cell with electrodes of sodium and metal chloride. The ZEBRA system was first described by Coetzer in 1986 12J. 

In contact with aluminium, disulphur dichloride provokes the instantaneous ignition of the metal. Lithium batteries contain thionyl chloride. A large number of explosions of batteries have been explained by the violent interaction of lithium with the chloride, which was assumed to be reieased through the anode. Sodium combusts in contact with thionyl chloride vapour heated to a temperature of 300°C. Finally, sulphur dichloride gives rise to explosive mixtures on impact with sodium. 

Figure 19-3 shows an electrolj ic cell using molten sodium chloride. A redox reaction between sodium and chlorine won t happen spontaneously, but the electrical energy produced by the battery provides the additional energy needed to fuel the reaction. In the process, chlorine anions cire oxidized at the anode, creating chlorine gas, and sodium is reduced at the cathode and is deposited onto it as sodium metal. 

One of the problems encountered with the Werth cell was an increase in resistance with cycling. This may have been caused in part by the /3-alumina reacting with the acidic sodium chloroaluminate melt. Coetzer had the idea of using transition metal chlorides as a positive electrode and chose a basic sodium chloroaluminate melt as the liquid electrolyte. This is compatible with /3-alumina, and a new class of secondary cells based upon the reaction between sodium metal and transition metal chloride has resulted from this work. Collectively, the term Zebra battery is used to describe this new class of cell. 

An electrolytic cell has two electrodes that dip into an electrolyte and are connected to a battery or some other source of direct electric current. A cell for electrolysis of molten sodium chloride, for example, is illustrated in Figure 18.15. The battery serves as an electron pump, pushing electrons into one electrode and pulling them out of the other. The negative electrode attracts Na+ cations, which combine with the electrons supplied by the battery and are thereby reduced to liquid sodium metal. Similarly, the positive electrode attracts Cl- anions, which replenish the electrons removed by the battery and are thereby oxidized to chlorine gas. The electrode reactions and overall cell reaction are... 

In Europe, the drive system of the Impact propelled the Opel Impuls2, a conversion vehicle based on the Opel Astra Caravan in 1991. A new, specifically developed AC induction drive unit with IGBT inverter technology was used to build a small fleet of Impuls vehicles see Figure 8.4). The fleet served as an automotive test bed for the integration of various advanced battery systems such as nickel-cadmium, nickel-metal hydride, sodium-nickel chloride, sodium-sulfur, and sealed lead-acid. 

Africa)io is a variant of sodium-sulfur technology where sulfur is replaced with a metal chloride such as NiCl2 (nickel chloride) or FeCU. It was specifically developed for applications in electric vehicles, freight transport and public transport the ZEBRA battery is more particularly intended to serve buses and utility vehicles. As with the Na-S battery, the vibrations felt in a vehicle may cause premature aging of the ceramic/metal interface. Today, such batteries are also being considered for stationary applications. 

Figure 20.19 shows a simple electrolytic cell. IMres from a battery are connected to electrodes that dip into molten sodium chloride. (NaCl melts at 801°C.) At the electrode connected to the negative pole of the battery, globules of sodium metal form chlorine gas evolves from the other electrode. The half-reactions are... 

Figure 19.2 Electrolysis of sodium chloride. Solid sodium chloride is heated past its melting point. A battery causes a flow of electric current, pumping electrons to the cathode, where sodium ions are reduced to sodium metal. Electrons are pumped from the anode, where chloride ions are oxidized to chlorine gas. 

When molten sodium chloride is electrolyzed, the products are sodium metal and chlorine gas. In this electrolytic cell, electrodes are placed in the mixture of Na and d and connected to a battery. The products are separated to prevent them from reacting spontaneously with each other. As electrons flow to the cathode, Na is reduced to sodium metal. At the same time, electrons leave the anode as Q is oxidized to CI2. The half-reactions and the overall reactions are... 

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. 

These metals are all produced by electrolysis of a mixture of molten metal chlorides the electrolyte composition is selected to minimize the process temperature and to ensure that it is the desired metal that is discharged at the cathode. The estimated annual world production of sodium and magnesium is a few hundred thousand tons while that for lithium is only a few thousand tons. The major uses are (a) sodium-manufacture of lead alkyls, isolation of titanium metal, production of several organic and inorganic substances (b) magnesium-organic synthesis, metal alloys (c) lithium - polymer initiation, organic synthesis and batteries. 

SAFT Socidte des Accumulateurs, Fixes et de Trac-lion, 156 Avenue de Metz, 93230 Romainville Secondary batteries, nickel-hydrogen, cuprous chloride, nickel-cadmium, lithium-manganese dioxide thermal cells, nickel-metal hydride secondary, sodium-nickel chloride secondary. See also SAFT (UK) and SAFT (US). (Chloride Alkad is now part of SAFT). 

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