Revealing the Electrochemistry in a Voltaic Cell by In Situ Electron Microscopy

Graphical Voltaic cell : A voltaic cell was assembled in an aberration corrected transmission electron microscope with a Cu cathode, a Zn anode and an ionic liquid-based electrolyte. The detailed electrochemical reaction mechanisms of this battery were revealed at atomic level in real time. The Voltaic Pile, invented by Italian physicist Alessandro Volta in 1799, was the first electrochemical energy storage device created by mankind. It consists of alternatively stacked copper (Cu) and zinc (Zn) metal disks separated by brine soaked cloth cardboard as the electrolyte. Although fabricated more than two centuries ago, the electrochemistry of the Voltaic Cell has not been fully understood. Understanding the microscopic working principle of the Voltaic Cell not only has historic meaning but also provides a foundation for modern battery technologies, corrosion science and other electrochemical processes. Herein an in situ microscopic observation of the electrochemical reactions in a Voltaic Cell comprised of a Cu cathode, Zn anode and an ionic liquid-based electrolyte was reported. During discharge, Zn stripping initiated from pits formation at the surface of the Zn anode, which then propagated along the longitudinal direction of the nanorod. Concurrently Cu nanoparticles were electrodeposited on the Cu cathode. Atomic scale imaging revealed the formation of zinc fluoride (ZnF 2 ) solid electrolyte interphase (SEI) layer of about 2 nm, which grew semicoherently with the Zn anode and dictates the stripping kinetics. Our studies provide not only an in-depth understanding of the electrochemistry of the Voltaic Cell, but also a generic technique for in situ studies of metal plating/stripping and corrosion at nanometer to atomic resolution.