Revealing Water and Ion Transport Under Extreme Confinement for Sustainable Batteries

Fluidic transport under extreme confinement is an essential process in energy storage systems such as sustainble aqueous batteries. Understanding the structure and intermolecular bonding of confined fluids lays the foundation for improving battery performance. However, probing confined water and proton experimentally at the length scale of intermolecular and surface forces has remained a challenge. Here, we report direct probing of bonding in confined water/proton at the nanoscale first in a simplified platform carbon nanotube and then in a realistic aqueous battery. We first reveal changes in H-bonding environment with nanoscale resolution and the underlying molecular structure of confined water inside individual carbon nanotubes which serves as a simplified platform to mimic confined fluids at the cathode. H-bonding is directly probed through the O-H stretch frequency with vibrational electron energy-loss spectroscopy and compared to spectra from molecular-dynamics simulations based on density-functional-theory. Experimental spectra show that water in larger carbon nanotubes exhibit the bonded O-H vibrations of bulk water, but at smaller diameters, the frequency blueshifts to near the ‘free’ O-H stretch found in water vapor and hydrophobic surfaces without bimodal distribution, indicating a total disruption of the H-bond network of confined water. The matching simulations reveal that, in addition to steric confinement, vibrating tube as a molecular-level flexible channel plays a key role in breaking up the H-bond network through host-guest interactions, resulting in an orientationally-dispersed, non-H-bonded phase. Furthermore, we expand the work to directly investigate the mechanism of ion transport on cathode in aqueous battery. This research demonstrates the potential of unveiling molecular-level structure and bonding in confined fluids for sustainable battery applications.