Credit: Reprinted with permission from Nano Letters. Copyright 2015. American Chemical Society.
Scientists used a combination of techniques to reveal unprecedented insights into the changes that occur at the molecular level in an operating battery. The far right image shows the chemical changes in an operating battery during the charging process: positively charged lithium ions (red dots) are attracted by the negatively charged copper (Cu) electrode, while negative ions (green dots) migrate toward a positive electrode (outside of the image). The electrochemical reaction creates a layer—referred to as the solid electrolyte interphase (SEI) layer—near the copper electrode that is enriched in solvent molecules (blue). The chemistry of the SEI layer is critical to battery performance. (The techniques used in this research were in situ liquid secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM).)