High-energy-density primary (nonrechargeable) electrochemical cells capable of relatively high discharge currents at temperatures as low as –40 °C have been developed through modification of the chemistry of commercial Li/CFx cells and batteries. The commercial Li/CFx units are not suitable for high-current and low-temperature applications because they are current limited and their maximum discharge rates decrease with decreasing temperature.
The term “Li/CFx” refers to an anode made of lithium and a cathode made of a fluorinated carbonaceous material (typically graphite). In commercial cells, x typically ranges from 1.05 to 1.1. This cell composition makes it possible to attain specific energies up to 800 Wh/kg, but in order to prevent cell polarization and the consequent large loss of cell capacity, it is typically necessary to keep discharge currents below C/50 (where C is numerically equal to the current that, flowing during a charge or discharge time of one hour, would integrate to the nominal charge or discharge capacity of a cell). This limitation has been attributed to the low electronic conductivity of CFx for x ≈ 1. To some extent, the limitation might be overcome by making cathodes thinner, and some battery manufacturers have obtained promising results using thin cathode structures in spiral configurations.
The present approach includes not only making cathodes relatively thin [≈2 mils (≈0.051 mm)] but also using subfluorinated CFx cathode materials (x <1) in conjunction with electrolytes formulated for use at low temperatures. The reason for choosing sub-fluorinated CFx cathode materials is that their electronic conductivities are high, relative to those for which x >1. It was known from recent prior research that cells containing subfluorinated CFx cathodes (x between 0.33 and 0.66) are capable of retaining substantial portions of their nominal low-current specific energies when discharged at rates as high as 5C at room temperature. However, until experimental cells were fabricated following the present approach and tested, it was not known whether or to what extent lowtemperature performance would be improved.
For the experimental cells, cathodes were fabricated by spray deposition of multiple layers of cathode mixtures onto roughened 1-mil (≈0.025-mm)-thick aluminum- foil current collectors. Each cathode mixture consisted of a CFx powder and carbon black suspended in a binder/solvent solution of poly(vinylidene fluoride) in N-methyl-2-pyrrolidinone. For some of the cells, the CFx was sub-fluorinated by various amounts (x = 0.53 or x = 0.65). For other cells, used as controls, a fully fluorinated industrial CFx (x = 1.08) was used.
Each resulting cathode structure, 1 to 3 mils (about 0.025 to 0.076 mm) thick, was vacuum furnace dried, then incorporated into a standard coin cell case along with a separator, lithium foil anode, and an electrolyte consisting of LiBF4 dissolved at a concentration if 0.5 M in an 80/20 DME/PC (dimethoxy ethane/propylene carbonate) solvent mixture. The cells were tested in galvanostatic discharges at room temperature and –40 °C at currents from 2C to C/40. The fully fluorinated and sub-fluorinated cells performed comparably at rates as high as 2C at room temperature. At –40 °C, the sub-fluorinated cells exhibited approximately 3 times the specific capacities of the fully fluorinated cells when discharged at C/10 and C/5 discharge rates (see figure).
This work was done by Jay Whitacre, Ratnakumar Bugga, Marshall Smart, G. Prakash, and Rachid Yazami of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/ Computers category.
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Refer to NPO-43219, volume and number of this NASA Tech Briefs issue, and the page number.