Influence of Water and Other Contaminants in Electrolyte Solutions on Lithium Electrodeposition

Abstract

Electrochemistry of a nickel electrode in propylene carbonate [PC] containing LiClO₄, LiCF₃SO₃, LiPF₆ was studied through a micro electrode (φ =25 μ m) techniques in the wide potential range between +4.5 and -0.2 V vs. Li/Li⁺. Common pronounced peaks were observed in the potential range positive to lithium electrodeposition on nickel in all electrolyte solutions examined. Thus, these peaks can be attributed to reactions related to Li⁺ or commonly contained contaminants such as water and acids. In particular, the peak which appeared at the most negative potential seemed to be underpotential deposition (UPD) of lithium.

To prove this hypothesis a nickel electrode in highly dried PC (water content: 3 – 8 ppm) intentionally contaminated with a small amount of water and CF₃COOH was examined via cyclic voltammetry. Changing the content of water and acid (and its ratio) in PC resulted in a variety of voltammograms and one of them was identical to the one observed in PC containing lithium electrolytes. These facts preclude the existence of UPD of lithium on nickel in the electrolyte solutions. Instead, the existence of NiOH on nickel and its redox reaction mechanism have been postulated. The mechanism is consistent with the experimental facts: a nickel electrode passivates in PC with a small amount of water, and a small amount of acid, CF₃COOH, can prevent passivation. The vicinity of the electrode surface may be exposed to an alkaline atmosphere owing to the reduction product of water. This seems to be the cause of troubles we run into with the electrodes at cathodic potentials