Understanding the Microscopic Structure of a “Water-in-Salt” Lithium Ion Battery Electrolyte Probed with Ultrafast IR Spectroscopy

Abstract

“Water-in-salt” electrolytes have been demonstrated to have potential applications in the field of high-voltage aqueous lithium ion batteries (LIBs). However, the basic understanding of the structure and dynamics of the concentrated “water-in-salt” electrolytes at the molecular level is still lacking. In this report, the structural dynamics of the concentrated lithium bis­(tri­fluoro­methane sulfonyl)­imide (LiTFSI) aqueous solutions were investigated using Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. The OD stretches of the water molecule and the thiocyanate (SCN–) anionic solute were utilized as the IR probes to monitor the local structure of the concentrated LiTFSI aqueous solutions. The concentration-dependent IR spectra and vibrational relaxation dynamics of the SCN– anion were systematically measured and analyzed. It was found that the vibrational population of the SCN– anion decayed at a much faster rate as the LiTFSI bulk concentration increased. From the static IR spectra and vibrational relaxation dynamics of SCN–, it was firmly confirmed that the SCN– anion interacted with the Li+ cations surrounded with domains of bulk-like water in the concentrated LiTFSI aqueous solutions. The rotational dynamics and spectral diffusion measurements of the SCN– anion further showed that the rotation of the SCN– anionic probe is strongly confined and restricted by the specific network structure formed by a framework of the TFSI– anions and the associated interfacial water molecules. Macroscopic conductivity and viscosity measurements were also performed for the LiTFSI aqueous solutions in order to elucidate a full picture of the liquid structure. The rotational dynamics of the SCN– anion decoupled from the viscosity of the solution are suggested to be segregated from the dynamics of the heterogeneous domains formed by the extended ion network of the TFSI– anions. All of these results demonstrated that the structural dynamics of the concentrated LiTFSI aqueous solutions can be revealed from the viewpoint of the anionic solute. And the heterogeneous structures in the LiTFSI solutions proposed by the molecular dynamic (MD) simulation were supported by experiment. The results in this work are expected to provide a basic understanding of the microscopic structure and transport mechanism of lithium ions in the water-in-salt electrolytes of LIBs.

Comment on “Critical Role of Anion–Solvent Interactions for Dynamics of Solvent-in-Salt Solutions”

“…On p 8460 of ref , the FTIR data (Figure 4a) presented are inconsistent with the previously reported results. , In the FTIR spectra of LiTFSI system, at the highest salt concentration, two peaks centered at 2430 and 2630 cm –1 can be resolved. This sounds a bit strange.…”

Physicochemical and Electrochemical Properties of Water‐in‐Salt Electrolytes

“…This broadening is attributed to increased viscosity of the electrolyte [23] . The 17 O peak related to the salt only starts changing once the concentration of LiTFSI salt rises above 10 m , meaning that only at these concentrations, structure of TFSI − anions is affected by lower population of water molecules and anions start forming three‐dimensional ion networks forming heterogeneous domains [28] . Similar broadening at higher concentrations (Figure 2a) has been reported for 17 O NMR peak in mixed cation (potassium and lithium) acetate electrolytes, [23] where broadening of the peak becomes clear at concentrations of 17.8 m KOAc+4.4 m LiOAc and more.…”

“…The high conductivities actually stem from water‐ion structures [41] . Water‐solvated Li + ions can move via bulk‐like water domains or nanometric water channels (diameter of about 1.2 nm) network formed by TFSI anions at a fast rate via vehicular‐type ion transport [28,43,44] . The type of ion transport is believed to change to more of a hopping‐type process due to lack of sufficient bulk‐like water, [15] when the concentration of the salt increases to values where the water/salt ratio gets close to 2 or lower, such as Li(TFSI) 0.7 (BETI) 0.3 ⋅ 2H 2 O hydrate melt [32] and Li 0.2 K 0.8 OAc ⋅ 1.3H 2 O [23] .…”

Intercalated water in aqueous batteries

“…Such a structure enables the preferential reduction of the anion instead of water and the formation of stable interphase, thus expanding the voltage window 30 . Moreover, it has been demonstrated that the cation transport in a concentrated electrolyte is closely associated with the structural dynamics of water molecules 31 . Despite the high viscosity, the ionic conductivity is not compromised, due to the presence of both bulk‐like and interfacial water molecules.…”

“…30 Moreover, it has been demonstrated that the cation transport in a concentrated electrolyte is closely associated with the structural dynamics of water molecules. 31 Despite the high viscosity, the ionic conductivity is not compromised, due to the presence of both bulk-like and interfacial water molecules. Such a feature builds an intertwined high-way of dissolved ions and nanometric bulk water channels that conducts the cation fast.…”