Cation-Assisted Lithium Transport in Ionic Liquid-Plasticized Polymer Electrolytes for Improved Lithium Metal Batteries

Summary:

Polyethylene oxide(PEO)/lithium salt complexes are promising solid polymer electrolytes (SPE) for the use of the negative electrode material Li metal due to its good mechanical stability of cross linked PEO systems, their wide electrochemical stability window (ESW) and the excellent ability of PEO to dissolve Li salts.[1] However, PEO-based SPEs suffer from a low ionic conductivity for temperatures below the melting point of PEO.[2] One solution is the addition of a plasticizer to increase the segmental mobility of the PEO chains at lower temperatures. Ionic liquids (ILs) are promising in this respect as they have low vapor pressure, sufficient ESW and are less-flammable. N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1,4TFSI) containing PEO-based SPEs, namely ternary based polymer electrolytes (TSPE), are able to reach an ionic conductivity of 10-3 S/cm at 40 °C.[2] The disadvantage of this kind of IL is the low ability to coordinate Li ions. In fact, the Li ion transport occurs mainly along the PEO chains, both for PEO/salt complexes[3] and TSPEs[4]. In the latter case, however, the presence of the IL induces a dilution of these transport pathways and limit the Li ion transport gains that could be expected otherwise.
Thus, we synthesized N methyl N oligo(ethylene oxide) pyrrolidinium TFSI (Pyr1,(2O)7TFSI) with a median oligo(ethylene oxide) chain length of seven to enable the solvation of one Li cation by a single pending chain and thereby prevent transport pathways dilution and enable new conduction modes. In fact, Li ion transport occurs via several mechanisms in ILs, namely ‘vehicular’ or ‘structural’ transport,[5] that both benefit a priori from the solvating IL (whereas the transport along (long) PEO chain is structural). By the new solvating cation, the Li ion transport was enhanced by tripling the Li ion transference number, as determined by both pulsed field gradient nuclear magnetic resonance (PFG-NMR) and electrochemical methods, whereas the ionic conductivity is maintained despite the much higher viscosity of the IL.[6] These improvements can be attributed to a partly existing vehicular transport mechanism, verified via MD simulation. This, combined with favorable interfacial properties vs. Li metal, leads to significantly improved cell performance for lithium metal batteries.

References
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[5] V. Lesch, Z. Li, D. Bedrov, O. Borodin, A. Heuer, Phys. Chem. Chem. Phys. 2016, 18, 382.
[6] J. Atik, D. Diddens, J. H. Thienenkamp, G. Brunklaus, M. Winter, E. Paillard, Angew. Chem., Int. Ed. 2021, DOI: 10.1002/anie.202016716.