Salt electrolytes, molten

Fig. 13.51. Theoretical specific energy plotted against the equivalent weight for various batteries. The present commercial battery systems are in the lower right corner. Types of electrolytes , molten salt or ceramic o, aqueous. (Reprinted from K. Kordesch, in Comprehensive Treatise of Electrochemistry, J. O M. Bockris, B. E. Conway, E. Yeager, and R. E. White, eds., Vol. 3, p. 123, Plenum, 1981.)...

A molten salt battery is a primary or secondary battery that uses a molten salt as its electrolyte. Molten salt batteries are a class of primary cell and secondary cell high-temperature electric battery. These types of batteries are used where high energy density and high power density are required. Their energy density and power density give them potential for use in electric vehicles. 

Roughly fuel cells can be classified into low-temperature (operation temperature below 500 °C) and high-temperature (operation temperature up to 1,000 °C) cells [13]. As electrolytes molten salts or ionically conductive solids are used. Concerning the fuels, electrodes, electrolytes, and constructive details, in recent years a variety of different fuel cells of both types have been developed, e.g., ... 

It is accurate for simple low valence electrolytes in aqueous solution at 25 °C and for molten salts away from the critical point. The solutions are obtained numerically. A related approximation is the following. 

The principal use of AIF. is as a makeup ingredient in the molten cryoflte, Na.. AIF AI2O2, bath used in aluminum reduction cells in the HaH-Haroult process and in the electrolytic process for refining of aluminum metal in the Hoopes cell. A typical composition of the molten salt bath is 80—85%... 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Lithium Chloride. Lithium chloride [7447- 1-8], LiCl, is produced from the reaction of Hthium carbonate or hydroxide with hydrochloric acid. The salt melts at 608°C and bods at 1382°C. The 41-mol % LiCl—59-mol % KCl eutectic (melting point, 352°C) is employed as the electrolyte in the molten salt electrolysis production of Hthium metal. It is also used, often with other alkaH haHdes, in brazing flux eutectics and other molten salt appHcations such as electrolytes for high temperature Hthium batteries. 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

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. 

Chemical Production. Electrolytic production of chemicals is conducted either by solution (water) electrolysis or fused-salt electrolysis. Fluorine, chlorine, chlorate, and manganese dioxide are Hberated from water solutions magnesium and sodium are generated from molten salt solutions. 

The melt is heated by passing a large elecuical cunent between two electrodes, one of which is tire metal rod to be refined, and the otlrer is the liquid metal pool standing in a water-cooled copper hearth, which collects the metal drops as tlrey fall tluough the molten electrolyte. This pool tlrerefore freezes at the bottom, forming the ingot. Under optimum chcumstances tire product billet takes the form of a cylindrical solid separated from the molten salt by... 

In 1963, Major (Dr.) Lowell A. King (Figure 1.1) at the U.S. Air Force Academy initiated a research project aimed at finding a replacement for the LiCl/KCl molten salt electrolyte used in thermal batteries. 

U.S. Air Force Academy in 1961. He was an early researcher in the development of low-temperature molten salts as battery electrolytes. At that time low temperature meant close to 100 °C, compared to many hundreds of degrees for conventional molten salts. His work led directly to the chloroaluminate ionic liquids. 

Early in their work on molten salt electrolytes for thermal batteries, the Air Force Academy researchers surveyed the aluminium electroplating literature for electrolyte baths that might be suitable for a battery with an aluminium metal anode and chlorine cathode. They found a 1948 patent describing ionically conductive mixtures of AICI3 and 1-ethylpyridinium halides, mainly bromides [6]. Subsequently, the salt 1-butylpyridinium chloride/AlCl3 (another complicated pseudo-binary)... 

For a review of salts formerly thought of as low-temperature ionic liquids, see Mamantov, G., Molten salt electrolytes in secondary batteries, in Materials for Advanced Batteries (Murphy, D. W., Broadhead, J., and Steele, B.C. H. eds.). Plenum Press, New York, 1980,... 

The early history of ionic liquid research was dominated by their application as electrochemical solvents. One of the first recognized uses of ionic liquids was as a solvent system for the room-temperature electrodeposition of aluminium [1]. In addition, much of the initial development of ionic liquids was focused on their use as electrolytes for battery and capacitor applications. Electrochemical studies in the ionic liquids have until recently been dominated by work in the room-temperature haloaluminate molten salts. This work has been extensively reviewed [2-9]. Development of non-haloaluminate ionic liquids over the past ten years has resulted in an explosion of research in these systems. However, recent reviews have provided only a cursory look at the application of these new ionic liquids as electrochemical solvents [10, 11]. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

There are five classes of fuel cells. Like batteries, they differ in the electrolyte, which can be either liquid (alkaline or acidic), polymer film, molten salt, or ceramic. As Table 1 shows, each type has specific advantages and disadvantages that make it suitable for different applications. Ultimately, however, the fuel cells that win the commercialization race will be those that are the most economical. 

Pt electrodeposits may also be produced from molten salt electrolytes. Such a high-temperature process has the advantage that the deposits are diffusion bonded to the titanium substrate and thus have good adhesion, and, if necessary, thick deposits can be produced. However, they have the disadvantage that because of the complexity of the process there is a limitation on the size and shape of the object to be plated, and the resultant deposits are softer and less wear resistant than those from aqueous solutions... 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

Attention has been given for some time to the use of lithium alloys as an alternative to elemental lithium. Groups working on batteries with molten salt electrolytes that operate at temperatures of 400-450 °C, well above the melting point of lithium, were especially interested in this possibility. Two major directions evolved. One involved the use of lithium-aluminium alloys [5, 6], whereas another was concerned with lithium-silicon alloys [7-9]. 

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