From the 40th Power Sources Conference
Molten Salt (Thermal) Batteries
By Donald Georgi
Electrochemistry in many forms were highlighted at this year’s Power Sources Conference. While the wider field of molten salts includes many inorganic compounds having acids replaced by metal or elements acting as metals, the focus here was on molten salts as applied to thermal batteries. Solid salt stored at ambient temperatures is electrically or percussively activated to ignite fuses which in turn ignite fuse trains and then heat pellets. With a rise in temperature, usually to the 500 to 700 0C region, the salt melts, allowing electrolyte conductivity to produce electricity with capabilities for high peak power density to 10 W/cm2. Thermal batteries remain inert for extended times (> 20 years), activate in 200 ms to a few seconds and can be sized to deliver Watts to kiloWatts for durations of minutes to several hours. Those of you who remember the details of NASA’s highly successful Pathfinder mission to Mars (See BD # 17, 1-5) know that at 6.2 miles above the surface of Mars, thermal batteries were activated to facilitate carrying out the deployment which resulted in the Pathfinder Lander arriving safely on the surface of Mars. The rest of its extraordinary success is now history.
The principal application of thermal batteries has been in military munitions from artillery shells all the way up to nuclear weapons, but at this conference the inherent properties were being expanded such that research could make the future stretch far beyond today’s ordinance market. One wonders when industrial and commercial requirements may someday tap the technology heretofore reserved for explosive devices.
To achieve the benefits of a thermal battery, the interior must be brought up to a temperature to melt the electrolyte and that means going into the regions of 500-600 0C. These high temperatures, if not accommodated as a system, would transfer large amounts of heat to the surrounding device both cooling the thermal battery and damaging the surroundings. The solution is to provide a package which has both mechanical strength and thermal insulating properties. To improve packaging for a one hour battery, a high efficiency container, using a four hour vacuum multifoil (VMF) construction built by InvenTek, was restructured to a size for a one hour battery. Test work by Sandia National Laboratories, analyzing a variety of structural combinations, showed that VMF could provide the necessary thermal performance for over 65 minutes. Projections for cost reductions include construction with commercial insulated coffee mug configuration which could provide acceptable performance and reduce costs to under $15 in lots of 1000.
To improve thermal battery performance, CEA Le Ripault has researchers studying iodide based salts as electrolytes due to their low melting points. The thermal decomposition of iron disulfide pyrite cathode material was characterized so that the effects of halogen electrolyte could be compared. The Fe S2 decomposition was first performed with an inert argon atmosphere and then with molten salts of iodides, bromides and chlorides. The decomposition was faster in the molten iodide, but because the process is slow, it will not interfere with the operation of the thermal battery. From the test performance it was concluded that molten iodides can be used as an electrolyte.
Another path to battery improvement may lie with nanoparticle metal disulfide cathode materials. Sandia National Laboratories presented work which first synthesized metal disulfides of iron, cobalt and nickel. In the growth process, rapid nucleation was followed by aggregation formed nanoparticles in the 10-30 nm range. Cells constructed with the nanomaterials were tested. The nickel disulfide materials showed good performance with little dependence of cell Voltage on temperature and a fairly constant polarization throughout discharge. It outperforms natural pyrite and has a higher thermal stability.
Natural pyrite (FeS2) is commonly used as a cathode material in thermal batteries. To improve the purity and produce uniform physical size and chemical properties, the fabrication of such synthetic materials may lead to more uniform and higher performing cells. US Nanocorp synthesized pyrite material with an aqueous process involving sulfate and chloride precursors. Sandia National Laboratories provided process standards, analytical evaluation and cell evaluation. Standard lithiation using 1.5% by weight of Li2O led to Voltage spikes at the start of discharge, so catholytes made with 5% Li2O eliminated the spike. In this work it was demonstrated that large quantities of synthetic pyrite can be produced. When applied to thermal cells, the electrical capacity is greater and the cell resistance is lower than that of cells produced with natural pyrite. The best overall improvement is obtained at temperatures of 400-550 OC. Battery testing is underway to validate the single-cell tests.
Normal construction of electrodes in thermal batteries involves cold pressing of powdered materials into solid electrodes. To increase energy density and power, pellet (stack of anode, separator and cathode) thickness must be reduced. If separators become too thin, catastrophic failure can result. Sandia National Laboratories with Advanced Metals has demonstrated that higher density of cathode and anode material can be obtained by hot pressing the materials in a 1,000 ton press. Unlike the cold pressed pellets, hot pressed pellets do not swell in size after removal from the press. They are stress relieved in hot pressing. The resultant pellets have greater strength and will allow for much thinner construction, adding to cell performance. A variety of hot pressed electrodes with hot and cold pressed separators were constructed for testing, first in single cells and then in 5 cell packs. Cells with cold pressed electrodes showed greater overall polarization at various temperatures, and five cell tests demonstrated significantly greater capacity. Additional optimization and alternate electrolyte investigation are planned.
Nominal small thermal battery sizes are over an inch tall and an inch in diameter. If those sizes could be reduced by an orderof magnitude, applications to small munition fuse self-destruct applications could reduce the safety and handling problems during manufacture. The U.S. Army Research Laboratory has a program to develop such a miniature cell. An experimental model was constructed which provided transient heat transfer information and discharge data. Performance showed a Voltage profile which provided between 1.5 Volts and 1 Volt for 8 seconds. Cells based on mathematically optimized designs employed plasma-sprayed thin film electrodes, producing a cell 0.20 inches in diameter and 0.25 inches tall. In production units, the internal vacuum would have to be maintained for 20 years, requiring better chemical processing and the addition of gettering agents.
The greater the understanding of a system, the greater the ability to characterize it with a model. Models begin as gross approximations, moving to better and better descriptions through performance validations. Armed with better models, better batteries can be made because differences between important factors ca be quantified. While the look-back accuracy of models is a foundation, the real benefit of models is to project new materials and methods for better batteries.
Aerosapatiale Batteries has produced ‘Ether’ (for Electro-THERmal modeling,) a lumped-parameter model for thermal batteries which includes a thermal/electrical - network, thermal/electric coupling and electrochemical modules. The program, developed from many years of work, seeks to provide a full thermal battery preliminary design calculation, including cost, directly from input of specifications on the basis of already validated materials. It is anticipated that the model will continue to improve so that electrodes can be examined separately and thermal modeling relative to the input data can be consolidated.
Thermal batteries produced over the last 20 years employ lithium/iron disulfide chemistry and are activated from an external electrical pulse which:
Fires an igniter, which
Ignites a heat paper, which
Ignites the heat pellets, which
Heats the electrolyte to melting temperatures, which
Activates the battery.
Depending on the design, this activation can take from 200 ms in small batteries to several seconds in large batteries. When the application requires very fast activation times, such as in short range missiles or aircraft ejector seats, a starter battery with low energy rapidly activates until a main battery can be brought up to temperature. This means having two batteries with size, weight and reliability penalties.
QuinetiQ Ltd. presented information on a single cell which replaces the sequential ignition system with a heating wire that ignites heating pellets, thus activating the cell in 20-40 ms. The wires, 25 microns thick, were fabricated from chromium/nickel and iron. A 10 Amp, 50 ms pulse initiated the activation. Prototype batteries showed activation times of 151 and 162 ms.
Shock encountered in munitions launch is a very specialized environment, but work continues to produce a thermal battery which will survive a 20,000 g pulse. Eagle Picher is beginning their work with component analysis to remove failure sites. Present testing has successfully undergone testing to 13,000 g’s. Improvements continue to be pursued.
Plasma Sprayed Anodes
Constructing anodes with thinner, more rugged and flexible features has been the target of research using plasma-spray techniques. The combination of Creare Inc. and Eagle Picher reported on the development of fabrication processes and buildup of multi-cell batteries for performance testing, the results of which have moved this technology out of the research area and into the development category.
Present fabrication of anodes is accomplished by combining binders with powders and pressing them into circular disks, limited to 5-7 cm in diameter and 0.7-2 mm thickness. Thicker anodes limit lithium utilization while binders retard diffusion.
Moving to thin stainless steel substrates as small as 75 microns thick, a plasma spray of Lithium-silicon can be deposited with coating thickness of over 300 microns, reducing the diffusion problems which limit performance. The resulting anodes are also less fragile and display more consistent performance based on a repeatable process. Drop testing at three feet showed no chipping. The anode area can be increased to 2.6-4.3 cm in diameter.
When assembled into a cell, the plasma sprayed anodes show the same Voltage characteristic as the pellet anode. Plasma sprayed anodes produce 2/3 of the e theoretical capacity while pellet anode cells only deliver 1/3 of the theoretical capacity. Cell resistance of the plasma sprayed cells was roughly 2/3 that of the pellet cells at low capacity and provided performance at capacities beyond the capabilities of pellet cells. All these improvements (smaller package size, improved ruggedness and higher electrical performance) point the way for further investigation of spray methods to co-spray the electrolyte.
Separator mechanics impose a limitation on cell size because the common pressed magnesium oxide powder separator is limited to 50 mm diameters. When ceramic fiber separators (CFS) are considered, the size increased to 90 mm in diameter and thickness could be reduced below the 10 mil thickness of the magnesium oxide separator. Many benefits come from the CFS including higher power cells, lower manufacturing cost, lower scrap rate, greater reliability in manufacture, and better performance in automatic handling. InvenTek Corp. has developed a CFS which, when incorporated in thermal batteries, shows superior pulse performance with the ability to increase power and energy density. Projections are to increase pulse power by 50%
Eagle-Picher Technologies’ batteries are found to be OK by Pentagon review team. The review team revealed there were no excessive failure rates or improper or inadequate production and testing of the batteries. Boeing Co., maker of the Joint Direct Attack Munition ( JDAM), utilizing these thermal batteries, said it has not experienced any problems with Eagle-Picher batteries and that a safety feature prevents the bomb’s release if the battery is defective. The review resulted after Rick Peoples , a former Eagle-Picher employee, accused the company of making batteries which did not meet specifications. (See BD 71-6 for more background.)(04-02 BD73-7)