Cation valency in water-in-salt electrolytes alters the short- and long-range structure of the electrical double layer

Author:

Berlinger Sarah A.1ORCID,Küpers Verena2,Dudenas Peter J.3ORCID,Schinski Devin1,Flagg Lucas3,Lamberty Zachary D.1ORCID,McCloskey Bryan D.14,Winter Martin25,Frechette Joelle14ORCID

Affiliation:

1. Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720

2. Münster Electrochemical Energy Technology, University of Münster, Münster 48149, Germany

3. Polymer Processing Group, Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899

4. Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

5. Helmholtz-Institute Münster Ionics in Energy Storage, Münster 48149, Germany

Abstract

Highly concentrated aqueous electrolytes (termed water-in-salt electrolytes, WiSEs) at solid-liquid interfaces are ubiquitous in myriad applications including biological signaling, electrosynthesis, and energy storage. This interface, known as the electrical double layer (EDL), has a different structure in WiSEs than in dilute electrolytes. Here, we investigate how divalent salts [zinc bis(trifluoromethylsulfonyl)imide, Zn(TFSI) 2 ], as well as mixtures of mono- and divalent salts [lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) mixed with Zn(TFSI) 2 ], affect the short- and long-range structure of the EDL under confinement using a multimodal combination of scattering, spectroscopy, and surface forces measurements. Raman spectroscopy of bulk electrolytes suggests that the cation is closely associated with the anion regardless of valency. Wide-angle X-ray scattering reveals that all bulk electrolytes form ion clusters; however, the clusters are suppressed with increasing concentration of the divalent ion. To probe the EDL under confinement, we use a Surface Forces Apparatus and demonstrate that the thickness of the adsorbed layer of ions at the interface grows with increasing divalent ion concentration. Multiple interfacial layers form following this adlayer; their thicknesses appear dependent on anion size, rather than cation. Importantly, all electrolytes exhibit very long electrostatic decay lengths that are insensitive to valency. It is likely that in the WiSE regime, electrostatic screening is mediated by the formation of ion clusters rather than individual well-solvated ions. This work contributes to understanding the structure and charge-neutralization mechanism in this class of electrolytes and the interfacial behavior of mixed-electrolyte systems encountered in electrochemistry and biology.

Funder

National Science Foundation

Deutsche Forschungsgemeinschaft

Publisher

Proceedings of the National Academy of Sciences

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