Solid-Like Nano-Anion Cluster Constructs a Free Lithium-Ion-Conducting Superfluid Framework in a Water-in-Salt Electrolyte

Abstract

The ion transportation mechanism in a high-concentration solution remains unclear due to the complexity of the strong ion–ion/ion–solvent interaction, resulting in the invalidation of most ionic conducting theories based on diluted solutions. Here, a superconcentrated electrolyte (water-in-salt) is investigated by multiple experimental techniques, including advanced tools (NMR, synchrotron X-ray diffraction, and spallation neutron scattering), combined with molecular dynamics (MD) simulation to draw out its unique microstructure and uncover its intrinsic relationship with the ionic transportation. Based on the results, we firstly proposed the ionic transport model for the water-in-salt electrolyte, where the solid-like nano-anion clusters construct a superfluid framework and the lithium ion is able to move freely like in an ionic atmosphere. Our model gives a unified explanation to the unique phenomena previously discovered in water-in-salt electrolytes, including the decoupling of conductivity–viscosity and the nanophase separation between the anion and water. Our findings on the microstructure of the super-high-concentrated electrolyte and the involved unique Li-conducting mechanism can fill in the gap between a solid-state conductor and a dilute liquid electrolytic solution.

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

“…Upon increasing the salt content, local electroneutrality and increasing fluorous tail content lead to a progressive merging of these globules into a three-dimensional matrix that eventually, at the highest concentrations, will percolate across the simulation box. Our present results suggest that the low Q peak is the fingerprint of alternating anion and water domains, as the S ( Q ) decomposition into different contributions leads to water–water and anion–anion peaks out of phase with water–anion anti-peaks (data not shown). , This clearly shows that such a low Q (X-ray or neutron scattering) peak feature appears at any concentration conditions (similar to what was reported by Zhang et al and by Liu et al., but at odds with what Tan et al claimed). Such a low Q feature then represents the signature of a structural organization that is persistent in the WiS system over the whole concentration regime.…”

“…The concentration dependence of peak I amplitude is noteworthy; while our data show that the peak occurs ubiquitously in the probed concentration window, its amplitude shows a maximum at ca. c = 3 m. Accordingly, at odds with the observation done by Tan et al, the structural heterogeneities leading to the appearance of peak I are present over the whole probed concentration range. In this context, we also mention the recent report from Liu et al, where a large set of concentrations of LiTFSI–water mixtures has been studied by small-angle X-ray scattering (SAXS), confirming our present findings that peak I is stronger at more dilute conditions and, by increasing the salt content, its amplitude tends to decrease (and even vanish) and its position shifts to higher Q values …”

“…The data presented by Zhang et al do not allow detecting this behavior, as they are vertically shifted; they observe, however, a distinct shift in the peak position . Very recently, Tan et al reported neutron and X-ray scattering data from the LiTFSI–water system at c = 0.3 and 21 m . 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.…”

“…Upon increasing the salt content, conductivity reaches a maximum and, due to increased viscosity, progressively decreases when the salt content reaches concentrations of the order of a few molal (mol salt /kg solvent ). Accordingly, the highly concentrated regime that characterizes WiS systems has barely been explored in the past and only recently, more systematic studies are being developed in this new regime. − Recent reviews have addressed the nature of the structural, dynamic, and electrochemical properties of these systems. ,,,,− Much of the structural investigations have been focused on the first WiS system, namely, LiTFSI-H 2 O, mostly due to LiTFSI high solubility in water (>20 m at 25 °C) and stability against hydrolysis . The phase diagram of this binary system has been characterized, showing the existence of a eutectic LiTFSI/H 2 O = 1:1, with a melting point at ca.…”

2,2,2-Trifluoroethyl Acetate as an Electrolyte Solvent for Lithium-Ion Batteries: Effect of Weak Solvation on Electrochemical and Structural Characteristics

“…It has recently been established that the concentration of salts and the above-mentioned solvent parameters affect the design of LIB electrolytes; namely, highly concentrated electrolyte solutions widen the electrochemical window to establish 4 V-class LIBs. − In concentrated electrolyte solutions with c Li above ∼3.0 mol dm –3 , metal ions (e.g., Li ion in the case of the LIB electrolyte) exist as ion-pair complexes in solutions; however, they form a specific ordered Li-ion structure based on multiple ion pairs between Li ions and counteranions to provide a high-voltage LIB system. ,− The formation of the specific Li-ion structure in concentrated electrolytes may originate from the balance between the solvation power due to the solvent molecules and the electrostatic interaction due to anions. However, sufficient knowledge to control these contributions has not yet been reported at the molecular level.…”