Lithium Electrodeposition Process in Various Hydrogenated Electrolyte Solutions
Authors: Yorita Asumi , Maeda Kazuno , Yamada Suzumi , Kondo Toshihiro 
Published: December 2024

Introduction: The lithium metal anode, which is attracting attention as a high-capacity next-generation anode material, is hindered in practical application by the formation of dendritic deposits known as lithium dendrites during charging (electrodeposition). One method to achieve dendrite-free electrodeposition of Li involves the addition of a small amount of water to an electrolyte containing LiPF6, leading to the formation of HF through hydrolysis. During electrodeposition, LiF precipitates along with lithium, resulting in the formation of columnar lithium, which is known to inhibit dendrite formation [1]. In this study, therefore, we investigated the influence of solvent properties on the columnar structure of lithium by adding a small amount of water to electrolytes containing LiPF6 in three different solvents with varying properties, comparing the mass changes on the electrode surface and the composition of the deposits during lithium electrodeposition.

Experiment: Three solvents (pure dimethyl carbonate (DMC), a mixture of ethyl carbonate (EC) and DMC (v/v = 1:1), and a mixture of EC and diethyl carbonate (DEC) (wt/wt = 1:1)) were used for the experiments after adding 1 M LiPF6 and approximately 50 ppm water, ensuring sufficient progress of hydrolysis. Copper foil or copper-coated quartz crystal microbalance (QCM) electrodes were employed as the working electrode, and lithium foil was used as the counter electrode. Lithium electrodeposition was conducted at a constant current density of -0.5 mA cm-2 for 2 hours, during which the mass changes and impedance variations were monitored using QCM and electrochemical impedance spectroscopy (EIS), respectively. Additionally, composition analysis of the deposited lithium after electrodeposition was performed using X-ray photoelectron spectroscopy (XPS).

Results and Discussion: The rate of hydrolysis of LiPF6 by the added water varied significantly depending on the solvent, with durations of 2 days for DMC alone, 8 days for EC/DMC, and 20 days for EC/DEC. This difference is seemed to be due to variations in the dissociation of LiPF6. When a current was passed through the copper electrode, a plateau region of cell voltage due to HF reduction was maintained for 10 to 20 seconds around 2 V, followed by a decrease in voltage and a constant voltage of ca. -0.2 V, indicating the onset of lithium deposition. The ratio of mass increase during lithium deposition (mass per electron; MPE) was in the reverse order of the rate of hydrolysis, with EC/DEC > EC/DMC > DMC. Additionally, the resonant resistance increased significantly in the order of DMC > EC/DMC > EC/DEC. Based on the mass changes and the morphology of the deposition mentioned later, it is highly probable that an increase in viscosity and density of the electrolyte due to side reactions, or an increase in surface area due to dendrite formation, occurred. In EC/DEC and EC/DMC, a characteristic blue-colored precipitate indicative of columnar lithium structures was observed, while in DMC, it was gray, showing lithium dendrite. XPS analysis revealed that LiF was dominant on the surface of the deposited lithium in all electrolytes, but there were significant differences in the amount of surface carbon, with EC/DEC > EC/DMC > EC/DEC. These results suggest that differences in EC content lead to variations in the dissociation and viscosity of the electrolyte, which in turn affect the morphology of the deposited lithium.

References:

[1] R. M. Kasse, et al., J. Mater. Chem. A, 8, 16960 (2020).