Liquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion

Abstract

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kg water ), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure−dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apolar nature of the anions, exploring the properties of the aqueous electrolytes of lithium salts with a strongly asymmetric anion, namely, (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl) imide. Using a synergy of experimental and computational tools, we detect a remarkable level of structural heterogeneity at a mesoscopic level between anion-rich and water-rich domains. Such a ubiquitous spongelike, bicontinuous morphology develops across the whole concentration range, evolving from large fluorinated globules at high dilution to a percolating fluorous matrix intercalated by water nanowires at super-concentrated regimes. Even at extremely concentrated conditions, a large population of fully hydrated lithium ions, with no anion coordination, is detected. One can then derive that the concomitant coexistence of (i) a mesoscopically segregated structure and (ii) fully hydrated lithium clusters disentangled from anion coordination enables the peculiar lithium diffusion features that characterize water-in-salt systems.

Zinc‐Ion Hybrid Supercapacitors Employing Acetate‐Based Water‐in‐Salt Electrolytes

“…The prepeak in the SWAXS patterns of liquids made of small molecules (i.e., molecules with a radius of gyration smaller than 1 nm) is not common, with some remarkable exceptions when amphiphilic molecules are concerned. [39][40][41][42][43][44][45] In those cases, the peak always originates from the formation of a distorted L 3 liquid phase, which is often referred as "sponge-like" [46] and which is basically a bicontinuous structure. The formation of this phase is driven by the amphiphilic nature of one (or more) species, which induce a self-segregation by preferentially orienting toward each other compatible molecules or portions of molecules, that is, polar with polar and apolar with apolar.…”

Linking Electric Double Layer Formation to Electrocatalytic Activity

“…Concentrated, correlated electrolytes could provide intriguing opportunities for leveraging both co-ions and counterions to modulate interface properties and microphase separation. For example, studies of water-in-salt electrolytes, where water is dissolved into salt to form a single-phase liquid that is predominantly salt, has revealed that the emergence of microphase separation in bulk electrolytes is primarily governed by the properties of anions. − Hence, we envision that these effects will become more pronounced at electrochemical interfaces and provide promising avenues for controlling microphase separation phenomena in electrocatalytic processes.…”

Molecular Modeling of Water-in-Salt Electrolytes: A Comprehensive Analysis of Polarization Effects and Force Field Parameters in Molecular Dynamics Simulations

“…In contrast, classical force fields allow MD simulations to be run for sufficiently large system sizes and long simulation times, but they suffer from a limited accuracy as far as the chemical interactions are concerned. So far, good efforts have been made to model WiS solutions using classical MD simulations − for LiB electrolytes. However, there are only a few studies on NaB electrolytes.…”