Dynamic Response of Ion Transport in Nanoconfined Electrolytes

Structural Basis of Ultralow Capacitances at Metal-Nonaqueous Solution Interfaces

Metal-nonaqueous solution interfaces, a key to many electrochemical technologies, including lithium metal batteries, are much less understood than their aqueous counterparts. Herein, on several metal-nonaqueous solution interfaces, we observe capacitances that are 2 orders of magnitude lower than the usual double-layer capacitance. Combining electrochemical impedance spectroscopy, atomic force microscopy, and physical modeling, we ascribe the ultralow capacitance to an interfacial layer of 10-100 nm above the metal surface. This nanometric layer has a Young's modulus around 2 MPa, which is much softer than typical solid-electrolyte interphase films. In addition, its AC ionic conductivity is 4-to-5 orders of magnitude lower than that of the bulk electrolyte. The temperature dependencies of the AC ionic conductivity and thickness suggest that the soft layer is formed from metal-mediated, dipole-dipole interactions of the nonaqueous solvent molecules. The observed soft layer opens new avenues of modulating battery performance via rational design of ion transport, (de)solvation, and charge transfer in this interfacial region.

Polarization of disk electrodes in high-conductivity electrolyte solutions

We investigate the polarization of disk electrodes immersed in an electrolyte solution and subjected to a small external AC voltage over a wide range of frequencies. A mathematical model is developed based on the Debye-Falkenhagen approximation to the coupled Poisson-Nernst-Planck equations. Analytical techniques are used for predicting the spatial distribution of the electric potential and the complex impedance of the system. Scales for impedance and frequency are identified, which lead to a self-similar behavior for a range of frequencies. Experiments are conducted with gold electrodes of sizes in the range 100-350 μm immersed in a high-conductivity KCl solution over five orders of magnitude in frequency. A collapse of data on impedance magnitude and phase angle onto universal curves is observed with scalings motivated by the mathematical model. A direct comparison with the approximate analytical formula for impedance is made without any fitting parameters, and a good agreement is found for the range of frequencies where the analytical model is valid.