Self-Passivating Lithium/Solid Electrolyte/Iodine Cells

NASA’s Jet Propulsion Laboratory, Pasadena, California

Friday, December 01 2006

Passivating lithium iodide films of optimum or nearly optimum thickness form spontaneously.

Robust lithium/solid electrolyte/iodine electrochemical cells that offer significant advantages over commercial lithium/ iodine cells have been developed. At room temperature, these cells can be discharged at current densities 10 to 30 times those of commercial lithium/iodine cells. Moreover, from room temperature up to 80 °C, the maximum discharge-current densities of these cells exceed those of all other solid-electrolyte-based cells.

A cell of this type includes a metallic lithium anode in contact with a commercial flexible solid electrolyte film that, in turn, is in contact with an iodine/ graphite cathode. The solid electrolyte (the chemical composition of which has not been reported) offers the high ionic conductivity needed for high cell performance. However, the solid electrolyte exhibits an undesirable chemical reactivity to lithium that, if not mitigated, would render the solid electrolyte unsuitable for use in a lithium cell. In this cell, such mitigation is affected by the formation of a thin passivating layer of lithium iodide at the anode/electrolyte interface.

Test cells of this type were fabricated from iodine/graphite cathode pellets, free-standing solid-electrolyte films, and lithium-foil anodes. The cathode mixtures were made by grinding together blends of nominally 10 weight percent graphite and 90 weight percent iodine. The cathode mixtures were then pressed into pellets at 36 kpsi (248 MPa) and inserted into coin-shaped stainless-steel cell cases that were coated with graphite paste to minimize corrosion. The solid electrolyte film material was stamped to form circular pieces to fit in the coin cell cases, inserted in the cases, and pressed against the cathode pellets with polyethylene gaskets. Lithium-foil anodes were placed directly onto the electrolyte films. The layers described thus far were pressed and held together by stainless steel shims, wave springs, and coin cell caps. The assembled cells were then crimped to form hermetic seals.

It was found that the solid electrolyte films became discolored within seconds after they were placed in contact with the cathodes — a result of facile diffusion of iodine through the solid electrolyte material (see figure). This is fortuitous for the following reasons:

• Upon reaching the anode side, the iodine reacts with lithium from the anode to form lithium iodide, which is known as both a solid electrolyte and an effective passivating-film material for solid electrolytes in contact with lithium.
• Heretofore, it has been necessary to vacuum- deposit LiI onto a solid electrolyte to form a passivating film. The vacuum deposition process is expensive, time consuming, and difficult. If the thickness of the deposited LiI film is ≈1 mm or more, the high electrical resistivity of LiI limits the cell discharge rates. If the film is too thin, it likely contains pinholes, which act as corrosion sites and thereby degrade cell performance.
• In the present case, the LiI is neither thick enough to introduce excessive electrical resistance nor too thin to prevent formation of pinholes: The formation of LiI film is self-limiting because iodine diffuses very slowly through LiI.
  

Commercial lithium/iodine cells are primary cells (that is, they are not rechargeable, as opposed to secondary cells, which are rechargeable). Thus far, lithium/solid-electrolyte/iodine cells have been demonstrated in primary form only. However, in principle, the present approach to passivation should be applicable to secondary cells also.

This work was done by Ratnakumar Bugga, Jay Whitacre, Sekharipuram Narayanan, and William West of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena, CA 91109-8099 (818) 354-2240 E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it Refer to NPO-40789, volume and number of this NASA Tech Briefs issue, and the page number.