Origin of Unusual Acidity and Li⁺ Diffusivity in a Series of Water-in-Salt Electrolytes

Abstract

Superconcentrated aqueous electrolytes ("water-in-salt" electrolytes, or WiSEs) enable various aqueous battery chemistries beyond the voltage limits imposed by the Pourbaix diagram of water. However, their detailed structural and transport properties remain unexplored and could be better understood through added studies.Here, we report on our observations of strong acidity (pH 2.4) induced by lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) at superconcentration (at 20 mol/kg). Multiple nuclear magnetic resonance (NMR) and pulsed-field gradient (PFG) diffusion NMR experiments, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations reveal that such acidity originates from the formation of nanometric ion-rich structures. The experimental and simulation results indicate the separation of water-rich and ion-rich domains at salt concentrations ≥5 m and the acidity arising therefrom is due to deprotonation of water molecules in the ion-rich domains. As such, the ion-rich domain is composed of hydrophobic −CF 3 (of TFSI − ) and hydrophilic hydroxyl (OH − ) groups. At 20 m concentration, the tortuosity and radius of water diffusion channels are estimated to be ∼10 and ∼1 nm, respectively, which are close to values obtained from hydrated Nafion membranes that also have hydrophobic polytetrafluoroethylene (PTFE) backbones and hydrophilic channels consisting of SO 3 − ion cluster networks providing for the transport of ions and water. Thus, we have discovered the structural similarity between WiSE and hydrated Nafion membranes on the nanometer scale.

Toward the understanding of water-in-salt electrolytes: Individual ion activities and liquid junction potentials in highly concentrated aqueous solutions

“…[12][13][14] Indeed, the lack of knowledge regarding critical physical parameters such as ions activity in these highly concentrated solutions hampers accessing to the reversible potentials for reactions such as the HER. For instance, the activity of protons, equivalent to the pH, which has been subject to controversy with reports of acidic pH being measured in highly concentrated solutions of neutral salts in which it is presumably low, 15 is preventing us to estimate the cathodic stability of the electrolyte in this regime. Moreover, when moving from the ideal infinite dilution regime to a highly concentrated one, not only the potential for the HER is modified but the one of other reactions such as Li + reversible intercalation as well.…”

“…In very diluted electrolytes, the activity coefficients can be approximated through the Debye and Hückel limiting law: Chemical shifts considering an increasing activity of protons, equivalent to a decreasing pH from pH=7 at 1 mol.kg -1 of LiTFSI to pH=2 for 21 mol.kg -1 of LiTFSI. 15 with the mean activity coefficient of the salt, the cation activity coefficient, the anion activity coefficient, the ionic strength, the concentration of ion , the charge of ion , the unit charge, the vacuum permittivity, the relative permittivity of the solvent, the Boltzmann constant, the gas constant and the temperature, all given in SI units. 16 In the Debye-Hückel model, the solvent is introduced as a dielectric continuum with a fixed dielectric constant.…”

“…(a) Chemical shifts considering the activity of protons constant, equivalent to a constant pH=7. (b) Chemical shifts considering an increasing activity of protons, equivalent to a decreasing pH from pH=7 at 1 mol.kg -1 of LiTFSI to pH=2 for 21 mol.kg -1 of LiTFSI 15.…”

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

“…Despite the increasing interest in WiS electrolytes, only a couple of very recent experimental results confirm the structural heterogeneity rather than the dynamical one and, to the best of our knowledge, the few systematic studies available are mostly focused on LiTFSI 20,21 or rely on crude approximations, assuming spherical water clusters. 22 The purpose of this work is to shed some light on the morphological characteristics of this nanostructure and try to determine the driving parameters that affect it. We studied the lithium counterion effect by utilizing two typical salts (LiTf and LiTFSI) and the impact of concentration and temperature in the SANS patterns of D 2 O-based WiS.…”

Towards the Understanding of Water-in-Salt Electrolytes: Individual Ion Activities and Liquid Junction Potentials in Highly Concentrated Aqueous Solutions

“…[12][13][14] Indeed, the lack of knowledge regarding critical physical parameters such as ions activity in these highly concentrated solutions hampers accessing to the reversible potentials for reactions such as the HER. For instance, the activity of protons, equivalent to the pH, which has been subject to controversy with reports of acidic pH being measured in highly concentrated solutions of neutral salts in which the activity of protons is presumably low, 15 is preventing us to estimate the cathodic stability of the electrolyte in this regime. Moreover, when moving from the ideal infinite dilution regime to a highly concentrated one, not only the potential for the HER is modified but the one of other reactions such as Li + reversible intercalation as well.…”