Local Structure of Li⁺ in Superconcentrated Aqueous LiTFSA Solutions

Abstract

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li + , which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li + , the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of 6 Li/ 7 Li and H/D. The Li + local structure changes with the increase of the LiTFSA concentration; the oligomer ([Li p (TFSA) q ] (p-q)+ (q > 2) forms at the molar fraction of LiTFSA (x LiTFSA ) > 0.25. The average structure can be determined in which two water molecules and two oxygen atoms of TFSA anion(s) coordinate to the Li + in the superconcentrated LiTFSA aqueous solution (LiTFSA) 0.25 (H 2 O) 0.75 . In addition, the intermolecular interaction between the neighboring water molecules was not found, and the hydrogen-bonded interaction in the solution should be significantly weak. According to the coordination number of the oxygen atom (TFSA or H 2 O), a variety of TFSA − and H 2 O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li + .

Experimental Determination of Relationship between Intramolecular O–D Bond Length and Its Stretching Vibrational Frequency of D2O Molecule in the Liquid State

“…26,27 Indeed, it has been revealed by means of recent neutron diffraction with the H/D isotopic substitution method that intermolecular hydrogen bonds among water molecules are extensively collapsed in highly concentrated aqueous 25 mol % LiTFSA solution. 28 In the present paper, we describe the results of time-of-flight neutron diffraction and ATR-IR (ATR: Attenuated Total Reflection) spectral measurements on pure liquid D 2 O and six kinds of concentrated aqueous solutions in D 2 O. The ν OD −r OD relationship in the liquid state has been directly obtained from the r OD value determined from the least-squares fitting analysis of the neutron interference term in the high-Q region and the ν OD value derived from the gravitational center of the O−D stretching band observed in the ATR-IR spectra.…”

“…In order to investigate the ν OD – r OD relationship in the liquid state experimentally, it is necessary to employ aqueous solutions with a wide range of average ν OD values (corresponding to various r OD values). The ClO 4 – ···water interaction is known to exhibit a higher frequency O–D band component corresponding to weak interaction between the O atom of ClO 4 – and the D atom of neighboring HDO. − Recently, highly concentrated aqueous solutions involving LiTFSA, NaTFSA (TFSA – : N­(SO 2 CF 3 ) 2 – ) and NaFSA (FSA – : N­(SO 2 F) 2 – ) have indicated significantly higher O–D stretching frequencies reflecting weaker anion–water interaction as well as a breakdown of intermolecular hydrogen bonds among water molecules in the solution. , Indeed, it has been revealed by means of recent neutron diffraction with the H/D isotopic substitution method that intermolecular hydrogen bonds among water molecules are extensively collapsed in highly concentrated aqueous 25 mol % LiTFSA solution …”

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

“…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.…”

“…The atomistic level description will be obtained by comparison between structural and spectroscopic information with MD results, providing a clear description of the role played by anion hydrophobicity in determining a nanoseparated morphology and of the enduring presence of fully hydrated lithium ions with no anions coordinating them, even at the most concentrated conditions. Despite the importance of chaotropic anions such as IM14 in enabling efficient WiS systems to be developed, so far, very little structural information exists on the organization of WiS based on salts different from LiTFSI. ,, This work aims at expanding the spectrum of available salts that can be envisaged as electrochemically appealing WiS candidates, providing a novel insight into the structural role of long fluorous tails of the anions in affecting the ubiquitous structural heterogeneities in these systems.…”

“… 17 − 36 Recent reviews have addressed the nature of the structural, dynamic, and electrochemical properties of these systems. 2 , 3 , 8 , 15 , 37 − 42 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. 1 The phase diagram of this binary system has been characterized, 18 showing the existence of a eutectic LiTFSI/H 2 O = 1:1, with a melting point at ca.…”