Alkoxides of lithium have been found to be useful as electrolyte additives to improve the low-temperature performance of rechargeable lithium-ion electrochemical cells. As explained below, an additive of this type exerts beneficial electrochemical effects both within the bulk of the electrolyte and on the surface of the carbon anode, such that the low-temperature electrical characteristics of the cell are improved.
The discovery of lithium alkoxide electrolyte additives was made during 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 "Update on Electrolytes for Low-Temperature Lithium Cells" (NPO-20407), NASA Tech Briefs, Vol. 24, No. 1, (January, 2000), page 56. To recapitulate: 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.
The electrolytes investigated intensively at earlier stages of this research were made of LiPF6 mixed with various proportions of aliphatic carbonates. One optimal formulation that was found to yield excellent room- and low-temperature performance is a 1.0 M solution of LiPF6 in a solvent that consists of equal volume parts of ethylene carbonate, dimethyl carbonate, and diethyl carbonate. Prototype cells that contained this electrolyte exhibited high charge and discharge capacities at temperatures from -20 to -40 °C, capability for discharge at high rates, and high cycle lives at both low and room temperatures.
A lithium alkoxide additive helps to improve the low-temperature performance of a cell in the following two ways:
- A film forms on the surface of the carbon anode. Whether the anode is made of graphitic or nongraphitic carbon, the surface film acts as a solid/electrolyte interface, the nature of which is critical to low-temperature performance. Desirably, the surface film would exert a protective effect yet would remain conductive to lithium ions to facilitate intercalation of lithium. The effect of the lithium alkoxide additive is to render the surface film more favorable in these respects.
- This results in the formation of asymmetric carbonates via a disproportionation reaction, such as ethyl methyl carbonate in the electrolyte solution described above. Thus, this approach represents a novel method of introducing asymmetric carbonates, which have been identified as being beneficial to low-temperature characteristics, into electrolyte formulations.
This work was done by Marshall Smart, Ratnakumar Bugga, and Subbarao Surampudi of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
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-20607, volume and number of this NASA Tech Briefs issue, and the page number.