The net effect is to reduce the blockage caused by precipitation of LiF, thereby maintaining a greater degree of access of electrolyte to the cathode.
The fluorinated boron-based anion receptor most commonly used heretofore — tris(pentafluorophenyl) borane — has been found to sharply increase the viscosities of electrolyte solutions. Increases in viscosities generally contribute to reductions in discharge capacities. In contrast, the present additive — tris(hexafluoroisopropyl) borate — can be mixed with such conventional lithium- cell solvents as propylene carbonate and dimethoxyethane to obtain solutions that have much lower viscosities and accept greater concentrations of LiF.
The promise of tris(hexafluoroisopropyl) borate as an additive was demonstrated in an experiment on two Li-(CF)n cells in standard commercial button-style packages. The cells were identical with one exception: the electrolyte solution in one cell contained 24 weight percent of tris(hexafluoroisopropyl) borate. The cells were tested at discharge currents of 1 mA. As shown in the figure, the cell containing the additive outperformed the one without the additive by a wide margin.
This work was done by Jay Whitacre and William West of Caltech for NASA's Jet Propulsion Laboratory.
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Refer to NPO-42346, volume and number of this NASA Tech Briefs issue, and the page number.