In a lithium-ion battery, the lithium-storage capacity of the carbon anode is greatly affected by a surface layer formed during the first half cycle of lithium insertion and release into and out of the carbon anode. The formation of this solid-electrolyte interface, in turn, is affected by the chemistry of the carbon surface. A study at the NASA Glenn Research Center examined the cause-and-effect relations. Information obtained from this research could contribute in designing a high-capacity lithium-ion battery and, therefore, small, powerful spacecraft.
In one test, three types of surfaces were examined: (1) a surface with low oxygen content (1.5 at.%) and a high concentration of active sites, (2) a surface with 4.5 at.% -OH or -OC type oxygen, and (3) a surface with 6.5 at.% O=C type oxygen. The samples were made from the same precursor and had similar bulk properties. They were tested under a constant current of 10 mA/g in half cells that used lithium metal as the counter electrode and 0.5 M lithium iodide in 50/50 (vol %) ethylene carbonate and dimethyl carbonate as the electrolyte.
Voltage of carbon anodes (with different surfaces) versus lithium metal. Left: During the first half cycle of lithium insertion. Right: During the first half cycle of lithium release.
For the first cycle of the electrochemical test, the graph describes the voltage of the carbon anode versus the lithium metal as a function of the capacity (amount of lithium insertion or release). From these data, it can be observed that the surface with low oxygen and a high concentration of active sites could result in a high irreversible capacity. Such a high irreversible capacity could be prevented if the active sites were allowed to react with oxygen in air, producing -OH or -OC type oxygen. The O=C type oxygen, on the other hand, could greatly reduce the capacity of lithium intercalation and, therefore, needs to be avoided during battery fabrication.
TABLE I.--CAPACITIES OF LITHIUM INSERTION AND RELEASE FOR CARBON WITH DIFFERENT SURFACES AND SIMILAR BULK STRUCTURE
| Sample | Surface oxygen | Active sites concentration | Irreversible capacity, mAh/g |
Reversible capacity, mAh/g |
||
| Chemical state | Content, at.% |
Capacity contributed by intercalation, I |
Capacity not contributed by intercalation, NI |
|||
| A | -OH or -OC | 1.5 | High | 163 | 169 | 135 |
| B | -OH or -OC | 4.5 | Low | 85 | 174 | 78 |
| C | O=C | 6.5 | Low | 66 | 0 | 95 |
Glenn contact: Dr. Ching-cheh Hung, 216-433-2302, Ching-Cheh.Hung@grc.nasa.gov
Author: Dr. Ching-cheh Hung
Headquarter program office: OSS (ATMS)
Programs/Projects: Space Power Technology
Last updated June 20, 2001, by Nancy.L.Obryan@nasa.gov
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