PNNL

Abstract

Super-concentrated aqueous electrolytes (‘water-in-salt’ electrolytes, or WiSE) 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 super-concentration (at 20 mol/g). Multiple nuclear magnetic resonance (NMR), 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 are due to deprotonation of water mole-cules 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 a SO3- ion cluster networks providing for the transport of ions and water. Thus we have discovered the structural similarity between WiSE and hydrated Nafion® membrane on the nano-meter scale.