Experimental materials made from mixtures of nickel and tin powders have shown promise for use as the negative electrodes of rechargeable lithium-ion electrochemical power cells. During charging (or discharging) of a lithiumion cell, lithium ions are absorbed into (or desorbed from, respectively) the negative electrode, typically through an intercalation or alloying process. The negative electrodes (for this purpose, designated as anodes) in state-of-the-art Li-ion cells are made of graphite, in which intercalation occurs. Alternatively, the anodes can be made from metals, in which alloying can occur. For reasons having to do with the electrochemical potential of intercalated lithium, metallic anode materials (especially materials containing tin) are regarded as safer than graphite ones; in addition, such metallic anode materials have been investigated in the hope of obtaining reversible charge/discharge capacities greater than those of graphite anodes. However, until now, each of the tin-containing metallic anode formulations tested has been found to be inadequate in some respect.
In preparation for making experimental electrodes, Ni and Sn powders were mixed in various proportions and suspended in a solution of [poly(vinylidene fluoride)-hexafluoropropylene dissolved in 1-methyl-2-pyrrolidinone]. (A carbon powder was also incorporated into some of the suspensions, but subsequent tests revealed that better electrode performances were obtained without the carbon.) The experimental electrodes were then made by coating nickel foils with the suspensions and drying them in air.
The experimental electrodes were characterized through several tests, including cyclic charge/discharge tests in electrochemical cells of several different types that differed in their counter-electrode or positive-electrode materials. The electrolyte in each cell was a 1-M solution of LiPF6 in a mixture of 1 part ethylene carbonate with 3 parts ethyl methyl carbonate. In each cell, the positive electrode was separated from the experimental electrode by a sheet of microporous polyethylene. Positive electrodes were made, variously, of the experimental electrode materials themselves or of lithium-metal, LiNi1-xCoxO2, or LiMn2O4. Of the electrodes tested, the best results were obtained with those made from a mixture of 41.6 weight percent Ni powder and 58.4 weight percent Sn powder: These electrodes exhibited high specific charge/discharge capacity (>0.3 A·h/g), capacity density >2.5 A·h/mL (more than three times that of Li-ion-cell negative-electrode materials now commercially available), nearly zero capacity fade, low irreversible capacity (45 to 76 mA·h/g), and long cycle life.
This work was done by Grant M. Ehrlich and Christopher Durand of Yardney Technical Products, Inc., for Johnson Space Center. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Yardney Technical Products, Inc.
82 Mechanic Street
Pawcatuck, CT 06379
Refer to MSC-23114, volume and number of this NASA Tech Briefs issue, and the page number.