Tris(2,2,2-trifluoroethyl) borate as an electrolyte additive increases low-temperature capacity.

Some progress has been reported in continuing research on the use of anion-receptor compounds as electrolyte additives to increase the sustainable rates of discharge and, hence, the discharge capacities, of lithium-poly(carbon monofluoride) [Li-(CF)n, where n >1] primary electrochemical power cells. Some results of this research at a prior stage were summarized in “Increasing Discharge Capacities of Li(CF)n Cells” (NPO-42346), NASA Tech Briefs, Vol. 32, No. 2 (February 2008), page 37. A major difference between the present and previously reported results is that now there is some additional focus on improving performance at temperatures from ambient down to as low as –40 °C.

Two Pairs of Li-(CFx)n Cells containing an electrolyte in the form of 0.5 M LiBF4 in a non-aqueous solvent were discharged at a rate C/2.5 at a temperature of –40 °C. The cells not containing the electrolyte additive were essentially nonfunctional; those containing the additive were functional, retaining approximately half of their room-temperature discharge capacities.
To recapitulate from the cited prior article: During the discharge of a Li-(CF)n cell, one of the electrochemical reactions causes LiF to precipitate at the cathode. LiF is almost completely insoluble in most non- aqueous solvents, including those used in the electrolyte solutions of Li-(CF)n cells. LiF is electrochemically inactive and can block the desired transport of electrons at the cathode, and, hence, the precipitation of LiF can form an ever-thickening film on the cathode that limits the rate of discharge. An anion-receptor electrolyte additive helps to increase the discharge capacity in two ways:

  • It renders LiF somewhat soluble in the non-aqueous electrolyte solution, thereby delaying precipitation until a high concentration of LiF in solution has been reached.
  • When precipitation occurs, it promotes the formation of large LiF grains that do not conformally coat the cathode.
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