Direct Spectroscopic Observation of Ultraslow Ion Desolvation at an Interface

Rapid charging of batteries is an important goal in electrochemical energy storage. Many factors complicate our understanding of the kinetics of this process, including intricacies of desolvation and intercalation of ions, solid-state diffusion, interfacial electrostatics, and paucity of clear spectroscopic signatures to study these phenomena separately. Here, we have dissected this problem by focusing on desolvation of Zn2+, which is of interest in recent multivalent ion battery research. To ensure a clear signature of interfacial desolvation, our unique approach is that we have installed a molecular layer at the interface that changes its vibrational frequency upon receiving desolvated Zn2+ from the bulk. The surface-bound molecule is 4-mercaptobenzonitrile (4-MBN), which forms an MBN-Zn2+ Lewis adduct with a distinct frequency shift. Using this approach, first we report that the kinetics of desolvation from some solvents, even when saturated in Zn2+, is quite slow, ranging from 11 min for water to 2 min for dimethyl carbonate, supporting the proposal that desolvation alone can be a major barrier to the charging and discharging speed in batteries. Second, we observe that the interfacial equilibrium population of MBN-Zn2+ does not have a clear correlation with the solvent dielectric constant, donor number, or saturated Zn2+ concentration value, further supporting the premise that such bulk properties fail to predict interfacial phenomena. Our results are relevant to research in energy storage in batteries, especially for the use of multivalent ions that face large desolvation penalties.