An Alternative Ionic Conductivity Mechanism for Plastic Crystalline Salt–Lithium Salt Electrolyte Mixtures

Authors

  • Wesley A. Henderson,

    Corresponding author
    1. Ionic Liquids & Electrolytes for Energy, Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA
    • Ionic Liquids & Electrolytes for Energy, Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA.
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  • Daniel M. Seo,

    1. Ionic Liquids & Electrolytes for Energy, Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA
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  • Qian Zhou,

    1. Ionic Liquids & Electrolytes for Energy, Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695, USA
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  • Paul D. Boyle,

    1. X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, NC 27695, USA
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  • Joon-Ho Shin,

    1. ENEA (Italian National Agency for New Technologies, Energy, and the Environment), Casaccia Research Center, Via Anguillarese 301, 00060 Rome, Italy
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  • Hugh C. De Long,

    1. Air Force Office of Scientific Research, 4015 Wilson Blvd., Arlington, VA 22203, USA
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  • Paul C. Trulove,

    1. Department of Chemistry, U.S. Naval Academy, 572 M Holloway Road, Annapolis, MD 21402, USA
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  • Stefano Passerini

    1. Westfälische Wilhelm Universität Muenster, Münster Electrochemical Energy Technology, Center & Institute of Physical Chemistry, Correns Strasse 28/30, D-48149 Muenster, Germany
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Abstract

The phase behavior and ionic conductivity of tetraethylammonium bis(trifluoromethane-sulfonyl)imide (Et4NTFSI) salt mixtures with LiTFSI have been examined. In addition, the phase behavior and crystal structure of neat LiTFSI is also reported. Two (1-x) Et4NTFSI-(x) LiTFSI (x = 0.50 and 0.67, where x is the mol fraction) mixed-salt crystalline phases form. Large variations in ionic conductivity are observed; these are attributed to solid-solid phase transitions of the neat Et4NTFSI salt creating disordered plastic crystalline phases and the formation of a low-melting eutectic composition between the neat Et4NTFSI salt and the 1/1 Et4NTFSI/LiTFSI (x = 0.50) phase. Although Et4NTFSI and LiTFSI melt at 102 and 234 °C, respectively, the two salts form a eutectic system with a melting temperature of 32 °C. Based upon the findings reported, a new conductivity mechanism is proposed for plastic crystalline salt-lithium salt electrolytes which is not ascribed to solid-state diffusion/conduction.

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