The anode separators in rechargeable lithium-ion electrochemical cells that contain carbon lithium-intercalating anodes with copper current collectors would be modified, according to a proposal, to give some protection against the internal short-circuiting that tends to occur during overdischarge. The carbon anodes offer advantages of greater safety and chemical stability over anodes made of pure lithium, but they also introduce a need to limit discharge, as explained below. The modified anode separators would not obviate the need to limit discharge, but would help to retard or prevent internal short-circuiting when overdischarge occurs despite efforts to prevent it.
In a cell that contains a pure lithium anode, there is still plenty of lithium left in the anode, even during overdischarge. However, in a cell with a carbon anode, no more lithium is available from the anode once discharge is complete. Therefore, during overdischarge, lithium ions are not being intercalated into the cathode; instead, a new electrochemical cell is formed between the cathode and the copper current collector in the anode. As overdischarge proceeds, copper is dissolved from the anode current collector and travels through the pores in the cell separator toward the cathode. Eventually, the deposited copper forms an electrically conductive path between the anode and cathode; that is, a short circuit develops within the cell.
The proposed modification of the separator in a given cell must be tailored according to the nonuniformity of utilization of the carbon electrode and of the distribution of electric current between the cathode and anode. In other words, it is necessary to identify the location on the carbon electrode that is most likely to be susceptible to short-circuiting during overdischarge. In a prototype cell, this location is an anode tab. The essence of the proposed modification is to render nonporous the part of the anode separator bag that covers the tab, to prevent penetration by copper.
The figure illustrates how a modified separator bag for the prototype cell could be fabricated. The starting separator material would typically be a rectangular sheet of microporous polypropy lene, which would be opaque because of its porosity. The sheet would be folded along one of the shorter edges, then pressed and heated to close the micropores. Upon closure of the micropores, the folded, pressed edge region would become transparent. The heating and pressing would also cause the two layers of the fold to merge into a single transparent layer. The reason for folding before heating and pressing is that a single layer of hot-pressed material could still contain holes that would allow penetration of copper, while a double layer has proved effective in preventing penetration of copper.
The sheet would be folded again — this time in half along its larger dimension to form a separator bag. The carbon anode could then be enclosed in the bag with the tab portion visible through the hot-pressed edge region.
This work was done by Chen-Kuo Huang of Caltech for NASA's Jet Propulsion Laboratory. 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
Technology Reporting Office JPL Mail Stop 122-116 4800 Oak Grove Drive Pasadena, CA 91109 (818) 354-2240
Refer to NPO-19950