Electrolytes comprising LiPF6 dissolved in alkyl carbonate/fluoroester mixtures have been found to afford improved low-temperature performance and greater high-temperature resilience in rechargeable lithium-ion electrochemical cells. These and other electrolytes comprising lithium salts dissolved mixtures of esters have been studied in continuing research directed toward extending the lower limit of operating temperatures of such cells. This research at earlier stages, and the underlying physical and chemical principles, were reported in numerous previous NASA Tech Briefs articles.
The purpose of the present focus on high-temperature resilience in addition to low-temperature performance is to address issues posed by the flammability of the esters and, at temperatures near the upper end (about 55 °C) of their intended operating temperature range, by their high chemical reactivity. As used here, “high-temperature resilience” signifies, loosely, a desired combination of low flammability of an electrolyte mixture and the ability of a cell that contains the mixture to sustain a relatively small loss of reversible charge/discharge capacity during storage in the fully charged condition at high temperature. The selection of fluoroesters for study as candidate electrolyte solvent components to increase high-temperature resilience was prompted in part by the observation that like other halogenated compounds, fluoroesters have low flammability.
The fluoroesters investigated in this study include trifluoroethyl butyrate (TFEB), ethyl trifluoroacetate (ETFA), trifluoroethyl acetate (TFEA), and methyl pentafluoropropionate (MPFP). Solvent mixtures were prepared by mixing these fluoroesters with two other esters: ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The specific solvent mixtures were the following:
1EC + 3EMC + 1TFEB
1EC + 2EMC + 2TFEB
1EC + 1EMC + 3TFEB
1EC + 3EMC + 1ETFA
1EC + 2EMC + 2TFEA
1EC + 3EMC + 1TFEA
1EC + 3EMC + 1MPFP
where the numbers indicate the relative volume proportions of the constituents.
Electrolytes were prepared by dissolving LiPF6 at a concentration of 1.0 M in these solvents. In addition, baseline (non-fluoroester-containing) electrolytes were prepared by dissolving LiPF6 at a concentration of 1.0 M in the following solvent mixtures:
1EC + 1DEC + 1DMC
1EC + 4EMC
where “DEC” signifies diethyl carbonate and “DMC” signifies dimethyl carbonate.
Rechargeable carbonanode/LiNi0.8 Co0.2O2-cathode cells containing these electrolytes were assembled and subjected to charge-discharge cycling tests at various temperatures from room temperature (23 °C) down to –60 °C. The cells were also evaluated with respect to high-temperature resilience by measuring the fractions of initial reversible capacity retained after storage for 10 days at a temperature of 55 °C. In these tests, the cell containing the electrolyte 1.0 M LiPF6 in (1EC + 3EMC + 1TFEB) exhibited the greatest overall improvements in both low-temperature performance (see figure) and high-temperature resilience over the cells containing the baseline electrolytes.
This work was done by Marshall Smart, and Ratnakumar Bugga of Caltech and G. K. Surya Prakash, Kiah Smith, and Pooja Bhalla of the University of Southern California for NASA’s Jet Propulsion Laboratory.
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:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-44626, volume and number of this NASA Tech Briefs issue, and the page number.