Beneficial properties of films that form on carbon anodes are enhanced.

Alkyl pyrocarbonates have been found to be useful as electrolyte additives for improving the low-temperature performances of rechargeable lithium-ion electrochemical cells. The beneficial effects of these and other additives have been investigated, along with various electrolyte formulations, in continuing research directed toward extending the range of practical operating temperatures from the present lower limit of –20 °C down to –40 °C, and even lower if possible. This research at earlier stages was reported in a number of NASA Tech Briefs articles; namely, “Update on Electrolytes for Low-Temperature Lithium Cells” (NPO-20407), Vol. 24, No. 1, (January 2000), page 56; “Lithium Alkoxide Electrolyte Additives for Lithium-Ion Cells” (NPO-20607), Vol. 25, No. 6 (June 2001), page 52; “Aliphatic Ester Electrolyte Additives for Lithium-Ion Cells” (NPO-20601), Vol. 25, No. 6 (June 2001), page 53; and “Ethyl Methyl Carbonate as a Cosolvent for Lithium-Ion Cells” (NPO-20605), Vol. 25, No. 6 (June 2001), page 53.

These Alkyl Pyrocarbonates, when used as additives to an optimized electrolyte formulation, have been found to improve the low-temperature performances of rechargeable lithium-ion cells by contributing to the formation of protective SEIs with increased ionic conductivities.

To recapitulate from the cited prior articles: the loss of performance with decreasing temperature is attributable largely to a decrease of ionic conductivity and the increase in viscosity of the electrolyte. What is needed to extend the minimum operating temperature from –20 °C down to –40 °C is a stable electrolyte solution with relatively small low-temperature viscosity, a large electric permittivity, adequate coordination behavior, and appropriate ranges of solubilities of liquid and salt constituents. Whether the anode is made of graphitic or non-graphitic carbon, the surface film acts as a solid/electrolyte interface (SEI), the nature of which is critical to low-temperature performance. Desirably, the surface film should exert a chemically protective effect on both the anode and the electrolyte, yet should remain conductive to lithium ions to facilitate intercalation and deintercalation of the ions into and out of the carbon during discharging and charging, respectively.

One previously reported optimized electrolyte formulation is a 1.0 M solution of LiPF6 in a ternary solvent that consists of equal volume parts of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Also previously reported is the use of quaternary additives to this baseline optimized formulation to enhance low-temperature performance.

The present alkyl pyrocarbonate additives (see figure) to the baseline optimized electrolyte formulation promote the formation of protective and conductive SEIs on carbon anodes. The formation of such SEIs is believed to be facilitated by products (e.g., CO2) of the decomposition of these additives. These decomposition products are believed to react to form Li2CO3-based films on the carbon electrodes. The improvement (relative to the baseline formulation) in interfacial properties resulting from the use of these additives is more evident at low temperature, where enhanced kinetics of intercalation and deintercalation of Li, higher ionic transport across SEIs, and increased discharge capacities with low overpotentials are observed. Also, the SEIs that form in the presence of these additives are more stable toward any further reduction of the electrolyte and thus more stable against growth to greater thicknesses; hence, they contribute to the cycle lives of the anodes.

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

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Refer to NPO-20775, volume and number of this NASA Tech Briefs issue, and the page number.

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