BATTERIES WITH MOLTEN SALT ELECTROLYTES

Most proposed battery types employed lithium-negative electrodes. In such batteries the electrolyte must contain a lithium salt and the electrode processes on the lithium electrode consist of simple transfer of the lithium ions from the crystal lattice of the metal to the melt and back. 

At the working temperature of the battery (400-600°C) pure lithium is liquid. Two main construction types of lithium electrodes have been reported liquid lithium in a porous matrix and a solid alloy of lithium with another metal. The matrix of the 

Electrochemical Power Sources Batteries, Fuel Cells, and Supercapacitors, First Edition. Vladimir S. Bagotsky, Alexander M. Skundin, and Yurij M. Volfkovich 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

Polarization of the lithium electrode is negligible both during charging and discharge record-breaking current densities, up to 40 A/cm, have been obtained with lithium electrodes. 

One of the main drawbacks of liquid lithium is its noticeable solubility in the salt melt and its capability to expel potassium from the melt  

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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]. 

Like fuel cells, batteries using molten salt electrolytes offer high performance. Molten salts have very high electrical conductivity, which permits the use of high current densities. Likewise, molten salts permit the use of highly reactive electrode materials, which cannot be used in aqueous electrolytes. For these reasons, batteries with molten salts offer very high specific energy (>100 Wh/kg). To... 

In this section we discuss three types of advanced batteries with molten salts LiAl/LiCl-KCl/FeS, LiAl/LiCl-KCl/FeS2 and Li4Si/LiCl-KCl/FeS2. They all derive from the initial battery developed at the Argonne National Laboratories in the United States [360-364], In the battery type Li/LiCl-KCl/S, both reactants and the electrolyte are in the molten state. The overall cell reaction is... 

Lithium alloy/metal sulfide batteries employ a molten-salt electrolyte and solid porous electrodes. Depending on electrolyte composition, they operate over a temperature range of 375 to 500°C. Operation at these temperatures with molten-salt electrolytes achieves high power densities, due to the high electrolyte conductivities and fast electrode kinetics. A shift from prismatic battery designs to bipolar designs enhances the power characteristics further by reducing the battery impedance. 

Elements from column III which have been studied for Li-ion anodes are essentially aluminum (Al) and gallium (Ga). Although the boron-lithium phase diagram shows several compounds [122], Li poorly reacts with B at room temperature [123]. Actually, B or rather Li-B alloys have been studied for application in the so-called thermal batteries, which are primary devices working at high temperature (350-450 °C) with molten salt electrolytes [124, 125]. 

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)... 

Because of the interest in its use in elevated-temperature molten salt electrolyte batteries, one of the first binary alloy systems studied in detail was the lithium-aluminium system. As shown in Fig. 1, the potential-composition behavior shows a long plateau between the lithium-saturated terminal solid solution and the intermediate P phase "LiAl", and a shorter one between the composition limits of the P and y phases, as well as composition-dependent values in the single-phase regions [35], This is as expected for a binary system with complete equilibrium. The potential of the first plateau varies linearly with temperature, as shown in Fig. 2. 

The molten salt, sodium aluminum chloride, fulfills two other tasks in the cell system. The ceramic electrolyte "-alumina is sensitive to high-current spots. The inner surface of the ceramic electrolyte tube is completely covered with molten salt, leading to uniform current distribution over the ceramic surface. This uniform current flow is one reason for the excellent cycle life of ZEBRA batteries. 

The molten salt electrolyte also contributes to the safety behavior of ZEBRA cells. The large amount of energy stored in a 700 g cell, which means about 30 kWh in a 300 kg battery, is not released suddenly as heat as be expected in a system with liquid electrodes such as the sodium sulfur cell. In the case of accidental destruction of ZEBRA cells, the sodium will react mainly with the molten salt, forming A1 sponge and NaCl. -The diffusion of the NaAICI ... 

Li-Al anodes have been combined in cells with CI2 in the Sohio Carb-Tek battery, operating with a molten salt electrolyte in the range of 400°-500°C. A porous carbon cathode and a BN separator were used. Addition of TeCla to the positive electrode increased the capacity in the 3.25-2.5V range. Although the battery presented many problems associated with the materials of the electrode, the casing and the seal, corrosion by CI2 being... 

Sometimes other methods of classification are also used, for example, on the basis of the application (stationary or mobile batteries), shape (cylindrical, prismatic, disk-shape batteries), size (miniature, small-sized, m ium-sized, or large-sized batteries), electrolyte type (alkaline, acidic, or neutral electrolyte, with liquid or solid (solidified), or molten salt electrolyte), voltage (low voltage or high voltage batteries), electric power generation (low power or high power batteries), and so on. 

Koura, N. lizuka, K. Idemoto, Y, Ui, K., Li and li-Al negative electrode characteristics for the lithium secondary battery with a nonflammable SOCI2, Li added, liQ saturated AICI3-EMIC molten salt electrolyte. Electrochemistry 1999, 67, 706—712. 

With thermal batteries such electrolytes are used combined with a tailor-made rapidly acting pyrotechnic heating device. Typical temperatures of operation lie between 200 and 500 °C, depending on the system. A molten salt electrolyte is used, for example, in the lithium iron disulfide battery which is described below. 

The electrolyte must have good ionic conductivity but not be electronically conductive, as this would cause internal short-circuiting. Other important characteristics are nonreactivity with the electrode materials, little change in properties with change in temperature, safety in handling, and low cost. Most electrolytes are aqueous solutions, but there are important exceptions as, for example, in thermal and lithium anode batteries, where molten salt and other nonaqueous electrolytes are used to avoid the reaction of the anode with the electrolyte. 

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