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Coordinating Anions “to the Rescue” of the Lithium Ion Mobility in Ternary Solid Polymer Electrolytes Plasticized With Ionic Liquids
Authors:Jan-Philipp Hoffknecht  Alina Wettstein  Jaschar Atik  Christian Krause  Johannes Thienenkamp  Gunther Brunklaus  Martin Winter  Diddo Diddens  Andreas Heuer  Elie Paillard
Institution:1. Institute for Inorganic and Analytical Chemistry, University of Münster, Corrensstrasse 28/30, D48149 Münster, Germany

MEET Battery Research Center, University of Münster, Corrensstrasse 46, D48149 Münster, Germany;2. Institute for Physical Chemistry, University of Münster, Corrensstrasse 28/30, D48149 Münster, Germany;3. Helmholtz Institute Münster – Forschungszentrum Jülich GmbH (IEK 12), Corrensstrasse 46, D48149 Münster, Germany;4. MEET Battery Research Center, University of Münster, Corrensstrasse 46, D48149 Münster, Germany;5. MEET Battery Research Center, University of Münster, Corrensstrasse 46, D48149 Münster, Germany

Helmholtz Institute Münster – Forschungszentrum Jülich GmbH (IEK 12), Corrensstrasse 46, D48149 Münster, Germany

Abstract:Lithium salts with low coordinating anions such as bis(trifluoromethanesulfonyl)imide (TFSI) have been the state-of-the-art for polyethylene oxide (PEO)-based “dry” polymer electrolytes for 3 decades. Plasticizing PEO with TFSI-based ionic liquids (ILs) to form ternary solid polymer electrolytes (TSPEs) increases conductivity and Li+ diffusivity. However, the Li+ transport mechanism is unaffected compared to their “dry” counterparts and is essentially coupled to the dynamics of the polymer host matrix, which limits Li+ transport improvement. Thus, a paradigm shift is hereby suggested: the utilization of more coordinating anions such as trifluoromethanesulfonyl-N-cyanoamide (TFSAM), able to compete with PEO for Li+ solvation, to accelerate the Li+ transport and reach a higher Li+ transference number. The Li–TFSAM interaction in binary and ternary TFSAM-based electrolytes is probed by experimental methods and discussed in the context of recent computational results. In PEO-based TSPEs, TFSAM drastically accelerates the Li+ transport (increases Li+ transference number by a factor 6 and the Li+ conductivity by 2–3) and computer simulations reveal that lithium dynamics are effectively re-coupled from polymer to anion dynamics. Last, this concept of coordinating anions in TSPEs is successfully applied in LFP||Li metal cells leading to enhanced capacity retention (86% after 300 cycles) and an improved rate performance at 2C.
Keywords:ionic liquids  Li + transport mechanism  lithium metal batteries  PEO-based polymer electrolytes
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