Early experiments in a continuing research program have demonstrated that thermal batteries made from powdered solid electrode and electrolyte materials can be improved by use of smaller (nanometer vs. micrometer size) cathode powder grains. The improvements include the possibility of fabricating and using thinner cathodes, plus increases in mechanical robustness, thermal stability, and overall power density.

Figure 1. A Basic Single-Cell Thermal Battery contains electrode and electrolyte disks. Unlike power cells used in many commercial products, this cell must be activated by heating to melt the electrolyte.

A thermal battery is a primary battery that is activated by heating to melt the solid electrolyte to supply electrical power for a limited time. Thermal batteries are highly reliable energy sources with high power densities and long shelf lives. They are particularly useful for supplying short-term power in expendable weapons (e.g., torpedoes and projectiles) and exploratory spacecraft; likely future commercial applications could include generating emergency power in aircraft and providing startup power for automobiles with weak batteries.

A single-cell thermal battery of the type under study in this research program (see Figure 1) contains an electrolyte disk stacked between an anode and a cathode disk. Each disk is made by cold-pressing the appropriate cathode, electrolyte, or anode powder. Heretofore, thermal batteries have been manufactured by techniques that impose lower limits on disk thicknesses needed to ensure adequate mechanical strengths. Accordingly, achievable power densities and other performance characteristics have been limited, and progress toward miniaturization and toward enhancement of activation characteristics and of safety has been impeded.

Figure 2. Voltages Generated by Cells of the types described in the text were measured during constant-current (0.4 A) discharges at a temperature of 400°C.

The anode material used in the experiments was an alloy of 44 percent Li + 56 percent Si, supplied as a powder of micron-size particles. The anode disks were formed by pressing this powder in a 2-cm-diameter round steel die at 6,000 psi (41 MPa). The electrolyte material included a powder of eutectic salt comprising 45 percent LiCl + 55 percent KCl. To strengthen the electrolyte disks, the eutectic salt powder was blended with 35 percent of MgO powder. The blended powder was pressed at 4,000 psi (28 MPa) to form electrolyte disks.

The cathodes were made from a blend of 68 percent FeS2 powder, 30 percent of the eutectic salt powder, and 2 percent of SiO2. The blend was pressed into disks at 4,000 psi (28 MPa). To provide a basis for comparison, the FeS2 powder used to make some cathode disks had particle sizes of the order of 1 µm, while that used to make the other cathode disks had an average particle size ≈25 nm. The nanostructured FeS2 powder was made by ball-milling the micron FeS2 powder.

For equal weights of blended powder and identical pressing conditions, the cathode disks made from nanostructured FeS2 came out 23 percent thinner and thus 30 percent denser than the cathode disks made from micron-scale FeS2. The nanostructured disks were found to be more robust than the others by comparison of degrees of shattering in a simple drop test. Thus, it was demonstrated that thinner, more robust cathode disks can be made by use of nanostructured instead of micron-scale FeS2 powder.

In thermogravimetric tests of the thermal decomposition of FeS2 into FeS + S, the nanostructured-FeS2 cathodes were found to be more stable. Finally, the discharge electrical performances of batteries containing nanostructured-FeS2 cathodes were found to be superior to those containing micron-scale-FeS2 cathodes (see Figure 2): The discharged electrical energy per unit mass averaged over all the cell material (electrodes + electrolyte) was found to be 109 J/g in the nanoscale case and 58 J/g in the micron-scale case.

This work was done by Ming Au, Yabin Lei, and Tapesh Yadav of Nanomaterials Research Corp. for Lewis Research Center. No further documentation is available. Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135.

Refer to LEW-16698.

NASA Tech Briefs Magazine

This article first appeared in the April, 1999 issue of NASA Tech Briefs Magazine.

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