Several modifications of the design and operation of lithium-ion rechargeable electrochemical cells have been proposed to prolong the cycle lives of the cells. As explained below, overdischarge can result in dissolution of a metal current collector in the anode of a cell, with consequent internal short-circuiting of the cell and thus loss of cycle life. The proposed modifications are intended to prevent dissolution of the metal current collector and thereby extend the cycle life.
In a typical cell of the type in question, the active cathode material is a lithiated oxide (e.g., LixCoO2) and the active anode material is carbon. These active materials are coated on different substrates or current collectors to make electrodes, and separator paper is used to prevent contact between the two electrodes. The current-collector materials must be chosen for compatibility with the cell chemistry; for example, the anode current collector can be made of copper or nickel.
A freshly fabricated cell is fully discharged: All the lithium is stored in the lithiated oxide cathode material; there is no lithium in the carbon anode material. An initial charging process is necessary to activate the cell. During the charging process, lithium becomes deintercalated from the LixCoO2cathode material and intercalated into the carbon anode material; the reverse happens when the cell is discharged.
In this context, overdischarge of a previously charged cell can be defined as continuation of discharge after all the reversible lithium has been completely depleted from the carbon anode material. Initial charging of a freshly fabricated cell in reverse polarity is equivalent to overdischarging a previously discharged cell. The figure shows the relative potentials (vs. Li) for relevant materials in a lithium-ion cell with a copper current collector in the anode. During overdischarge or reversal of polarity, the potential of the graphite anode rises above the potential of the copper current collector, causing the formation of a spurious cell between LixCoO2 and copper. The copper then begins to dissolve, causing a short circuit.
The following are the proposed modifications for preventing dissolution of the anode current collector:
- Raise the cell potential below which discharge is cut off ("discharge cutoff voltage," for short). For example, in a graphite/LixCoO2 Li-ion cell with a cathode/anode weight ratio of 3, the cycling voltage range is between 4.1 V (charge cutoff) and 3.0 V (discharge cutoff). If the discharge cutoff voltage were raised to 3.5 V, the consequent loss of capacity would be only 10 mA·h, while the cycle life would be extended.
- Configure both internal and external electrical connectors to prevent reversal of polarity in a freshly fabricated cell, and verify correct polarity of connections to test equipment before conducting a test.
- Use cathode additive(s) for protection against overdischarge, as described in "Preventing Overcharge and Overdischarge of Lithium Cells"(NPO-18343), NASA Tech Briefs, Vol. 19, No. 3 (March 1995), page 36. Low-potential cathode additives could help to reverse the potential of the spurious cell so that the copper current collector would not dissolve.
- Make the anode current collector out of carbon instead of copper. This could be done, for example, by coating a carbon-based material onto an electrically conductive polymer or onto a sheet of separator paper to make an anode.
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
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Prolonging the lives of rechargeable lithium-ions cells
(reference NPO19897) is currently available for download from the TSP library.
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