Competitive salt precipitation/dissolution during free‐water reduction in water‐in‐salt electrolyte Bouchal, Roza Li, Zhujie Bongu, Chandra Le Vot, Steven Berthelot, Romain Rotenberg, Benjamin Favier, Frederic Freunberger, Stefan Alexander ; https://orcid.org/0000-0003-2902-5319 Salanne, Mathieu Fontaine, Olivier ddc:540 ddc:541 Water-in-salt electrolytes based on highly concentrated bis(trifluoromethyl)sulfonimide (TFSI) promise aqueous electrolytes with stabilities approaching 3 V. However, especially with an electrode approaching the cathodic (reductive) stability, cycling stability is insufficient. While stability critically relies on a solid electrolyte interphase (SEI), the mechanism behind the cathodic stability limit remains unclear. Here, we reveal two distinct reduction potentials for the chemical environments of ‘free’ and ‘bound’ water and that both contribute to SEI formation. Free-water is reduced ~1V above bound water in a hydrogen evolution reaction (HER) and responsible for SEI formation via reactive intermediates of the HER; concurrent LiTFSI precipitation/dissolution establishes a dynamic interface. The free-water population emerges, therefore, as the handle to extend the cathodic limit of aqueous electrolytes and the battery cycling stability. Wiley 2020 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/8057 https://research-explorer.ista.ac.at/download/8057/8401 Bouchal R, Li Z, Bongu C, et al. Competitive salt precipitation/dissolution during free‐water reduction in water‐in‐salt electrolyte. <i>Angewandte Chemie</i>. 2020;132(37):16047-16051. doi:<a href="https://doi.org/10.1002/ange.202005378">10.1002/ange.202005378</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1002/ange.202005378 info:eu-repo/semantics/altIdentifier/issn/0044-8249 info:eu-repo/semantics/altIdentifier/e-issn/1521-3757 https://creativecommons.org/licenses/by/4.0/ info:eu-repo/semantics/openAccess