TY - JOUR
T1 - Origin of Unusual Acidity and Li+Diffusivity in a Series of Water-in-Salt Electrolytes
AU - Han, Kee Sung
AU - Yu, Zhou
AU - Wang, Hui
AU - Redfern, Paul C.
AU - Ma, Lin
AU - Cheng, Lei
AU - Chen, Ying
AU - Hu, Jian Zhi
AU - Curtiss, Larry A.
AU - Xu, Kang
AU - Murugesan, Vijayakumar
AU - Mueller, Karl T.
N1 - Publisher Copyright:
Copyright © 2020 American Chemical Society.
PY - 2020/6/25
Y1 - 2020/6/25
N2 - 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 -CF3 (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 SO3- 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.
AB - 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 -CF3 (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 SO3- 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.
UR - http://www.scopus.com/inward/record.url?scp=85087110404&partnerID=8YFLogxK
U2 - 10.1021/acs.jpcb.0c02483
DO - 10.1021/acs.jpcb.0c02483
M3 - Article
C2 - 32484675
AN - SCOPUS:85087110404
SN - 1520-6106
VL - 124
SP - 5284
EP - 5291
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 25
ER -