(Invited) Electrostatic Site Potential in Electrolytes As an Emerging Descriptor for Reversible Metal Electrodes

In order to maximize the electrochemical stability of a battery with metal anode, suppressing the side reaction between the metal and the electrolyte is essential. When the deposition potential positions outside of the stability window, optimization of SEI is a general approach to realize kinetic hinderance of side reactions. While another effective approach should be upshifting the deposition potential itself because it makes the side reactions much slower and milder, or even thermodynamic stability could be gained if the potential is upshifted into the stability window of the electrolyte. It is well known that electrode potentials vary significantly depending on the electrolyte. Typical example can be found in salt concentrated electrolyte, where the metal deposition potential significantly upshifts over 0.5 V upon increasing the concentration. However, the mechanism behind the potential shift has remained unclear. By careful analyses of the local coordination environment by molecular dynamics simulations and the accurate experimental measurement of the deposition potential, we revealed that the potential shift is primarily determined by electrostatic destabilization of metal cations rather than the simple Nernst-type expression depending on cation activity. The shallow site potential of metal cations is attributable to the dominant coordination to more charge-dispersive, larger size anions rather than to relatively electro-positive solvents, thus decreasing the overall Coulombic energy gain of metal cations. Such simple yet hitherto-overlooked mechanism can be a useful guideline in the development of better electrolytes for reversible metal electrode.