Storage batteries based on intercalation of lithium and bromine in graphite have been proposed. Like other storage batteries, these could be recharged electrically. Optionally, these batteries could also be recharged thermally at relatively low temperatures — by use of solar or waste heat, for example. In comparison with thermocouples and thermionic devices, the proposed batteries would generate much greater potentials (about 4 V per cell versus millivolts per thermocouple or thermionic device) and would convert heat to electricity with greater efficiencies.

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Figure 1. This Rechargeable Electrochemical Cell would exploit the reversible intercalation of lithium and bromine in graphite.

A cell in a battery of this type would include a graphite anode and a graphite cathode surrounded by lithium bromide dissolved in a nonaqueous solvent like diethyl carbonate, dimethyl carbonate, dioxane, propylene carbonate, ethylene carbonate, or a mixture of two or more of these compounds. The two electrodes would be separated by an ion-exchange membrane impermeable to bromine [or Br3- ions (see Figure 1)]. It is known from prior research that graphite forms intercalation compounds with halogens and with alkali metals. It is also known from prior research that intercalation of a halogen (in this case, bromine) leads to a positive charge on the graphite and negative charge on the halogen atoms, while intercalation of alkali metal (in this case, lithium) leads to a negative charge on the graphite and positive charge on the alkali metal atoms.

In the fully charged state, the anode would be loaded with lithium, and there would be free liquid bromine in the compartment surrounding the cathode. During discharge, bromine would become intercalated into the cathode, while lithium would come out of the anode. At the end of the discharge, the anode would not contain an appreciable amount of lithium, while the cathode would contain the intercalation compound CBrx. The discharge processes at the anode and cathode could be driven electrically in reverse to charge the cell in the customary way.

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Figure 2. The Desorption of Bromine from a specimen of bromine-treated graphite was measured while the specimen was heated to progressively higher temperatures in vacuum.
The expectation that one could also recharge the cell thermally is based on the observed temperature dependence of the intercalation of bromine in graphite. Figure 2 illustrates the results of an experiment in which most of the bromine previously intercalated in a graphite specimen was driven out by heating to a temperature of 120 °C. To utilize this phenomenon to recharge the cell, one would first make an external electrical connection between the cathode and anode, then heat the cathode to drive out the bromine. The resultant electrical current through the external connection would cause lithium to become intercalated into the cathode. One would then break the external electrical connection, then allow the cathode to cool to ambient temperature, at which point the cell would be ready for discharge.

Also, during the process of thermal recharging, a voltage equal and opposite to the reversible potential of the cell must be included in the charging circuit in order to enable recharge. If such a voltage is provided by another cell of the same type, then such a two-cell system will operate as a thermally regenerative device with one cell being charged and the other undergoing discharge. The cell undergoing charge would be kept at a higher temperature compared to the cell being discharged.

This work was done by Pramod K. Sharma, Sekharipuram Narayanan, and Gregory S. Hickey of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.

This invention has been patented by NASA (U.S. Patent No. 6,042,964). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
NASA Management Office–JPL (818) 354-4770.

Refer to NPO-19824.