Competitive Salt Precipitation/Dissolution During Free-Water Reduction in Water-in-Salt Electrolyte

International audienceWater-in-salt electrolytes based on highly concentrated bis(trifluoromethyl)sulfonimide (TFSI) promise aque-ous electrolytes with stabilities nearing 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. Now, two distinct reduction potentials are revealed for the chemical environments of free and bound water and that both contribute to SEI formation. Free water is reduced about 1 V above bound water in a hydrogen evolution reaction (HER) and is 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. Water-in-salt (WIS) electrolytes have recently emerged as new promising electrolytes owing to their high electrochem-ical stability window (ESW) of about 3 V. [1, 2] The drastic widening of the ESW could increase the energy of an electrochemical cell fourfold compared to conventional aqueous electrolytes, making WIS attractive, safe alternatives to hazardous organic electrolytes. [3-5] However, poor cycling stability hinders practical application for the critical case of low-voltage negative electrodes that operate below about 1.9 V vs. Li/Li +. [6-8] ESWs reported to date with sufficient cycling stability are hence far below the expected 3 V. [2, 5, 9-12] Ever since the promise of WIS electrolytes has been realized, extensive efforts have been devoted to understanding the origin of ESW stabilization. The high salt concentration in WIS significantly changes the liquid structure and alters interfacial reactivity. [2, 4] At the positive electrode, it is widely agreed that anions accumulate at the interface, forming a dense hydrophobic layer that prevents contact between water and electrode, [13-15] which raises the onset of oxygen evolution from 3.8 V (at pH 7) to about 4.9 V vs. Li/ Li +. [1, 2, 6, 16, 17] At the negative side, most studies assigned the improved cathodic stability to SEI formation from TFSI reduction, [4, 14] which hinders water molecules to access the surface while allowing for Li + diffusion. [2, 8, 14] The SEI kinetically suppresses the hydrogen evolution reaction (HER) and shifts the cathodic limit from 2.6 V (at pH 7) to about 1.8 V vs. Li/Li +. [2, 8] Since the SEI forms only beyond salt concentrations at 21m (molal, mol per kg solvent), it was argued to arise from bound water molecules having a lower reduction potential than TFSI. [7-9] However, the assignment of the reduction wave to either HER or TFSI reduction is unclear and several studies have reported on a rather wide range of potentials for TFSI reduction between 2 and 2.5 V vs. Li/ Li +. [8, 14, 18] Two conflicting explanations were recently brought forward by Suo et al. [14] and Dubouis et al. [19] The first reported the competitive reduction of water, dissolved O 2 , CO 2 , and TFSI during formation of an SEI comprising Li 2 CO 3 and LiF. They concluded that the TFSI in the ionic clusters and water are reduced at the same potential, where H 2 evolution is considered as a parasitic reaction for the SEI formation. Dubouis et al. [19] found simultaneous decomposition of water and TFSI with TFSI being chemically decomposed by products from the HER and not electrochemically as suggested previously. These conflicting explanations raise questions about what truly causes the enhanced reductive stability and what nevertheless limits it. If direct TFSI reduction forms the Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi