The Nanostructure of Water-in-Salt Electrolytes Revisited: Effect of the Anion Size

Abstract

The increasing interest in developing safe and sustainable energy storage systems has led to the rapid rise in attention to superconcentrated electrolytes, commonly called water-in-salt (WiS). Several works indicate that the transport properties of these liquid electrolytes are related to the presence of nanodomains, but a detailed characterization of such structure is missing. Here, the structural nano-heterogeneity of lithium WiS electrolytes, comprising lithium trifluoromethanesulfonate (LiTf) and bis(trifluoromethanesulfonyl)imide (LiTFSI) solutions as a function of concentration and temperature, was assessed by resorting to the analysis of small-angle neutron scattering (SANS) patterns. Variations with the concentration of a correlation peak, rather temperature-independent, in a Q range around 3.5−5 nm −1 indicate that these electrolytes are composed of nanometric water-rich channels percolating a 3D dispersing anion-rich network, with differences between Tf and TFSI anions related to their distinct volumes and interactions. Furthermore, a common trend was found for both systems' morphology above a salt volume fraction of ∼0.5. These results imply that the determining factor in the formation of the nanostructure is the salt volume fraction (related to the anion size), rather than its molality. These findings may represent a paradigm shift for designing WiS electrolytes.

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

“…Their study highlighted the presence of peak I (centered at 0.4 Å –1 ) in concentrated solution using both X-ray and neutron scattering, but they claim that peak I is not present in dilute solutions. Horwitz et al monitored peak I evolution between c = 4 and 21 m for LiTFSI–water by neutron scattering using D 2 O . Our present results on LiIM14–water electrolytes indicate the progressive development of the distinct low Q X-ray scattering peak I upon increasing the water content, whose position clearly shifts with the concentration.…”

“…28 The behavior is also similar to the one highlighted by Horwitz et al in their recent paper. 48 Peak positions for peak I, Q p , have been determined as a function of salt concentration by fitting the experimental data with a Gaussian function and the corresponding real space sizes estimated as D = 2π/Q p are reported in the inset of Figure 4. The linear trend of log D vs log c in the concentration range (1 ≤ c ([m]) ≤ 15) probed by the present study can be noticed.…”

“…LiTFSI was proposed as an electrolyte for aqueous lithium-ion batteries by Lux et al, 44 and its high concentration mixtures (c > 15 m) show interesting conductivity performances (5−10 mS/cm) 18 and an appreciable electrochemical stability, at least up to 2 V. 1 Nowadays, different options alternative to the LiTFSI-H 2 O WiS are being considered to enhance the resulting performance, including exploring Na-and K-based WiS 2,5,7,8,14,20,45−47 or exploiting asymmetric anions and anion mixtures (leading to the so-called water in bisalt systems). 6,9,12,16,24,45,48,49 Here, we explore an aqueous electrolyte system with a salt that is characterized by a remarkably asymmetric anion, i.e., a lithium salt with the anion being a member of the family of di(perfluoroalkyl-sulfonyl)imide, namely (trifluoromethylsulfonyl) (nonafluorobutylsulfonyl)imide (hereinafter indicated as [IM14]) (see Scheme 1). The high asymmetry of this anion makes it an ideal species to pair with cations that are prone to crystallization when paired with more conventional anions.…”

“…LiTFSI was proposed as an electrolyte for aqueous lithium-ion batteries by Lux et al, and its high concentration mixtures ( c > 15 m) show interesting conductivity performances (5–10 mS/cm) and an appreciable electrochemical stability, at least up to 2 V . Nowadays, different options alternative to the LiTFSI-H 2 O WiS are being considered to enhance the resulting performance, including exploring Na- and K-based WiS ,,,,,,− or exploiting asymmetric anions and anion mixtures (leading to the so-called water in bisalt systems). ,,,,,,, …”

“…The high asymmetry of this anion makes it an ideal species to pair with cations that are prone to crystallization when paired with more conventional anions. In fact, we recently explored a range of ionic liquid compounds based on imidazolium or other cations paired with the [IM14] anion, highlighting their high tendency to remain in the liquid state even at very low temperatures. − Very recently, a manuscript reported the role of anion size in the nanostructure of WiS systems, comparing the morphology detected in LiTFSI-based WiS with that in Li trifluoro-methanesulfonate (TfO) ones, highlighting the importance of the salt volume fraction in influencing the morphology . In this respect, the present choice for the anion represents an upper limit to the WiS systems studied so far.…”