Impedance Analysis of the Side Reactions of Li Electrodeposition in a Quaternary Ammonium-Based Ionic Liquid Electrolyte

Abstract

Despite the high energy densities offered by secondary batteries with metallic lithium (Li) anodes, their commercialization is hindered by the problem of dendritic growth of metallic lithium at the anode. Therefore, we aim to address this problem by evaluating the morphology of the deposited lithium, which is affected by the surface film produced by various side reactions in the system. Considering the wide potential windows of quaternary ammonium ionic liquids, we study precipitation morphologies using these ionic liquids as electrolytes. In this study, we examine the side reactions of Li electrodeposition using impedance spectroscopy and transmission electron microscopy. Samples were prepared and observed by gradually decreasing the potential from 2.8 V, the open circuit potential. As a result, it was confirmed that surface films were formed at about 1.5 V (vs. Li reference electrode), even when ionic liquids with a wide potential window were used, although the direct cause of the surface film formation is not known. Importantly, the vertex frequency of the semi-circular arc derived from the surface film in the impedance measurement was 500 Hz, which is faster than the time constant of the semi-circular arc due to the dissolution and precipitation reaction of Li.

Keywords

• electrochemical ac impedance
• dendrite
• film formation
• side reaction