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.