Molten salt batteries or liquid sodium battery are a class of primary cell and secondary cell high-temperature electric battery that use molten salts as an electrolyte. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means of a high-conductivity molten salt electrolyte. They are used in services where high energy density and high power density are required. These features make rechargeable molten salt batteries a preferred energy storage to balance out environment-dependent power plants (solar, wind, etc.), and a promising technology for powering electric vehicles. Operating temperatures of 400 °C (752 °F) to 700 °C (1,292 °F), however, bring problems of thermal management and safety, and place more stringent requirements on the rest of the battery components. Some newer designs, such as the ZEBRA battery, operate at a lower temperature range of 245 °C (473 °F) to 350 °C (662 °F).
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Referred to as thermal batteries the electrolyte is solid and inactive at normal ambient temperatures. Thermally activated (“thermal”) batteries were conceived by the Germans during World War II and were used in the V-2 rockets. Dr. Georg Otto Erb is credited with developing the molten-salt battery that used the heat of the rocket to keep the salt liquid during its mission. The technology was brought to the United States in 1946 and was immediately adapted to replace the troublesome liquid-based systems that had previously been used to power artillery proximity fuzes. These batteries have been used for ordnance applications (e.g., proximity fuzes) since World War II and, subsequent to that, in nuclear weapons. They are the primary power source for many missiles such as the AIM-9 Sidewinder, MIM-104 Patriot, BGM-71 TOW, BGM-109 Tomahawk and others. In these batteries the electrolyte is immobilized when molten by a special grade of magnesium oxide that holds it in place by capillary action. This powdered mixture is pressed into pellets to form a separator between the anode and cathode of each cell in the battery stack. As long as the electrolyte (salt) is solid, the battery is inert and remains inactive. Each cell also contains a pyrotechnic heat source which is used to heat the cell to the typical operating temperature of 400 - 550C.
There are two types of design. One uses a fuze strip (containing barium chromate and powdered zirconium metal in a ceramic paper) along the edge of the heat pellets to initiate burning. The fuze strip is typically fired by an electrical igniter or squib by application of electric current through it. The second design uses a center hole in the middle of the battery stack into which the high-energy electrical igniter fires a mixture of hot gases and incandescent particles. The center-hole design allows much faster activation times (tens of milliseconds) vs. hundreds of milliseconds for the edge-strip design. Battery activation can also be accomplished by a percussion primer, similar to a shotgun shell. It is desired that the pyrotechnic source be gasless. The standard heat source typically consist of mixtures of iron powder and potassium perchlorate in weight ratios of typically 88/12, 86/14, and 84/16. The higher the potassium perchlorate level, the higher the heat output (nominally 200, 259, and 297 calories/gram, respectively).
This property of unactivated storage has the double benefit of avoiding deterioration of the active materials during storage and at the same time it eliminates the loss of capacity due to self-discharge until the battery is called into use. They can thus be stored indefinitely (over 50 years) yet provide full power in an instant when it is required. Once activated, they provide a high burst of power for a short period (a few tens of seconds) to over 60 minutes or more, with power output ranging from a few watts to several kilowatts. The high power capability is due to the very high ionic conductivity of the molten salt, which is three orders of magnitude or more greater than that of sulfuric acid in a lead-acid car battery. Older thermal batteries used calcium or magnesium anodes, with cathodes of calcium chromate or vanadium or tungsten oxides, but lithium-alloy anodes replaced these in the 1980s, with lithium-silicon alloys being favored over the older lithium-aluminium alloys. The corresponding cathode for use with the lithium-alloy anodes is mainly iron disulfide (pyrite) with cobalt disulfide being used for high-power applications. The electrolyte is normally a eutectic mixture of lithium chloride and potassium chloride. More recently, other lower-melting, eutectic electrolytes based on lithium bromide, potassium bromide, and lithium chloride or lithium fluoride have also been used to provide longer operational lifetimes; they are also better conductors. The so-called "all-lithium" electrolyte based on lithium chloride, lithium bromide, and lithium fluoride (no potassium salts) is also used for high-power applications, because of its high ionic conductivity.
These batteries are used almost exclusively for military applications i.e. "one-shot" weapons such as guided missiles. However, the same technology was also studied by Argonne National Laboratories in the 1980s for possible use in electric vehicles, since the technology is rechargeable.
A radioisotope thermal generator, e.g. pellets of 90SrTiO4, can be used for long-term delivery of heat for the battery after activation, keeping it in molten state.[1]
Since the mid-1960s much development work has been undertaken on rechargeable batteries using sodium (Na) for the negative electrodes. Sodium is attractive because of its high reduction potential of -2.71 volts, its low weight, its non-toxic nature, its relative abundance and ready availability and its low cost. In order to construct practical batteries, the sodium must be used in liquid form. Since the melting point of sodium is 98 °C (208 °F) this means that sodium based batteries must operate at high temperatures, typically in excess of 270 °C (518 °F).
Sodium-sulfur battery and lithium sulfur battery comprise two of the more advanced systems of the molten salt batteries. The NaS battery has reached a more advanced developmental stage than its lithium counterpart; it is more attractive since it employs cheap and abundant electrode materials. Thus the first commercial battery produced was the sodium-sulfur battery which used liquid sulfur for the positive electrode and a ceramic tube of beta-alumina solid electrolyte (BASE) for the electrolyte. Corrosion of the insulators was found to be a problem in the harsh chemical environment as they gradually became conductive and the self-discharge rate increased. A further problem of dendritic-sodium growth in Na/S batteries led to the development of the ZEBRA battery. The possibility of construction of Potassium-ion battery by molten electrolyte has been recently patented.
ZEBRA Na-NiCl2 battery, Museum Autovision, Altlußheim, Germany |
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| specific energy | 90 Wh/kg[1] |
|---|---|
| energy density | 160 Wh/l[1] |
| specific power | 155 W/kg, peak power 335 C [2] |
| Energy/consumer-price | 3.33 Wh/US$ |
| Time durability | >8 years |
| Cycle durability | ~3000 cycles |
| Nominal cell voltage | 2.58 V |
The ZEBRA battery operates at 245 °C (473 °F) and utilizes molten sodium aluminumchloride (NaAlCl4), which has a melting point of 157 °C (315 °F), as the electrolyte. The negative electrode is molten sodium. The positive electrode is nickel in the discharged state and nickel chloride in the charged state. Because nickel and nickel chloride are nearly insoluble in neutral and basic melts, intimate contact is allowed, providing little resistance to charge transfer. Since both NaAlCl4 and Na are liquid at the operating temperature, a sodium-conducting β-alumina ceramic is used to separate the liquid sodium from the molten NaAlCl4. This battery was invented in 1985 by the Zeolite Battery Research Africa Project (ZEBRA) group led by Dr. Johan Coetzer at the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa, hence the name ZEBRA battery. In 2009, the battery had been under development for more than 20 years. The technical name for the battery is Na-NiCl2 battery.
The ZEBRA battery has an attractive specific energy and power (90 Wh/kg and 150 W/kg). For comparison, LiFePO4 lithium iron phosphate batteries store 90–110 Wh/kg and the more common LiCoO2 lithium ion batteries store 150–200 Wh/kg. Nano Lithium-Titanate Batteries store 72 Wh/kg energy and can provide a power of 760 W/kg .[3] The ZEBRA's liquid electrolyte freezes at 157 °C (315 °F), and the normal operating temperature range is 270 °C (518 °F) to 350 °C (662 °F).
The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium aluminumchloride. The primary elements used in the manufacture of ZEBRA batteries, Na, Cl and Al have much higher worldwide reserves and annual production than the Li used in Li-ion batteries.[4] Lifetimes of over 1500 cycles and five years have been demonstrated with full-sized batteries, and over 3000 cycles and eight years with 10- and 20-cell modules.
Vehicles powered by ZEBRA batteries have covered more than 2 million km. Modec Electric Van uses ZEBRA batteries for the 2007 model and the IVECO daily 3.5 ton delivery vehicle was announced in mid 2010 The Th!nk City also offers a ZEBRA battery option.[5] In 2011, the US Postal Service began testing five delivery vans that had been converted to all-electric power, one of which uses a ZEBRA battery.[6]
When not in use, ZEBRA batteries are typically left under charge so that they will remain molten and be ready for use when needed. If shut down and allowed to solidify, a reheating process must be initiated that may require up to two days to restore the battery pack to the desired temperature and impart a full charge. This reheating time varies depending on the state-of-charge of the batteries at the time of their shut down, battery-pack temperature, and power available for reheating. After a full shut down of the battery pack, three to four days will usually elapse before a fully charged battery pack loses enough energy to cool and solidify.