Dance Your PhD 2012: Electrochemical Characterization of Passivating Films in Lithium-Ion Battery Electrolytes

Quick Key:
Navy: electrons (e-)
Blue/Orange: ferrocenium / ferrocene (Fcn+/ Fcn)
Red: lithium ions (Li+)
Yellow: solvent (Sol)


Side reactions in lithium-ion batteries are a major problem for battery safety and lifetime. My research tries to determine what these side reactions are and how to control them. To do this, I developed a new method to characterize side reactions by using the molecule ferrocene. When ferrocene is in its reduced (electrically neutral) form, it is orange. When it is in the oxidized (positively charged) form, known as ferrocenium, it is blue-green.

The main reaction in this system is the reduction of lithium ions (red dancers). When lithium ions are reduced by electrons (navy blue dancers), the resulting lithium intercalates into the electrode. However, solvent molecules (yellow dancers) are reduced at the same potential. The insoluble solvent reduction products precipitate as a film on the electrode surface. This film passivates the electrode, blocking further reaction.

Solvent reduction is much more complex than the single-electron reaction shown here, and the properties of such thin films are hard to measure. To better understand the films, we add ferrocenium (blue dancer). Because ferrocenium reduction occurs at a different potential than lithium or solvent reduction, its rate is much easier to measure. As the film grows thicker and less porous, through-film ferrocenium reduction becomes more difficult. This method can thereby determine film parameters such as the thickness and porosity.

This dance was performed with the help of Lindy on Sproul, our campus swing dance club. The song, "What'cha Know Joe?", reflects both the poorly-understood nature of side reactions and the mental state frequently experienced during a Ph. D.