New State-Diagram of Aqueous Solutions Unveiling Ionic Hydration, Antiplasticization, and Structural Heterogeneities in LiTFSI–H₂O

Abstract

Here, we report a new state-diagram for aqueous solutions based on concentration-dependent glass-transition temperatures of concentrated and ice freeze-concentrated solutions. Different from the equilibrium phase diagram, this new state-diagram can provide comprehensive information about the hydration numbers of solutes, nonequilibrium vitrification/cold-crystallization, and vitrification/devitrification processes of aqueous solutions in three distinct concentration zones separated by two critical water-content points of only functions of the hydration number. Based on this new state-diagram, we observe the comparable hydration ability of LiTFSI to LiCl and an atypical concentration-dependent cold-crystallization behavior of the LiTFSI–H2O system. These results unveil the negligible hydration ability of TFSI– in a water-rich solution, characterize the antiplasticizing effect of water induced by the strengthened Li+–TFSI––H2O interaction when only hydration water and confined water are present, and confirm the increasing fraction of water-rich domains with the decrease in water content when the cation and anion become incompletely hydrated on average. These results highlight the novel water-content-mediated interactions among the anion, cation, and H2O for LiTFSI–H2O.

Anion type-dependent confinement effect on glass transitions of solutions of LiTFSI and LiFSI

“…Our research group has recently developed a new state-diagram for this purpose. [28,29] Unlike a traditional phase diagram, which describes the equilibrium state of matter during a slow heating process, the statediagram plots the concentration-dependent T g of solutions and of the freeze-concentrated phase in water-rich solutions. Here, the term 'freeze-concentrated phase' typically denotes the liquid phase that experiences a concentration increase as a result of the crystallization of ice when cooling dilute aqueous solutions.…”

“…With the help of this new state diagram, the hydration number n H of LiTFSI can be accurately quantified as n H = 7. [29] This means that, in a water-rich solution, the solute together with water at a molar ratio of 1:7 can easily vitrify even under moderate and slow cooling rates. Considering the difficulty of bulk water vitrification, the number of these easily vitrified water is reasonably defined as n H .…”

“…Another exceptional characteristic of LiTFSI•nH 2 O is the antiplasticizing effect of water observed in zone II. [29] Normally, T g of bulk water is lower than that of solutes or concentrated solutions. Therefore, adding water to concentrated solutions would lower T g of the system, as demonstrated for LiCl solutions (Fig.…”

A MOF-derived hollow Co3O4/NiCo2O4nanohybrid: a novel anode for aqueous lithium ion batteries with high energy density and a wide electrochemical window

“…17 In the charge-discharge test of aqueous LIBs in this voltage range, it is hard to get a large energy density and specific capacity with an unstable electrolyte. In order to enlarge the electrochemical window of aqueous LIBs, lithium bis(trifluoromethane sulfonyl)imide (LiTFSI), [18][19][20][21] an ultra-high concentration aqueous electrolyte whose solubility in water can reach above 20 mol kg −1 at room temperature, could be useful for improving the factor of the electrochemical window. When the concentration of LiTFSI is high enough, the LiTFSI in water can form a "water-in-salt" electrolyte, 22 which can resist hydrolysis and stabilize the battery reaction.…”