Asymmetric Li-Ion Diffusion Profiles during Lithium Plating and Stripping in Ionic Liquid Electrolytes

Abstract

The electrochemical behavior of lithium metal in ionic-liquid based Li-electrolytes is typically characterized by an anodic peak due to the formation of an inhibition layer. In symmetric Li-Li cells, this shows up as a symmetric set of peaks in the current-cell voltage curves (Figure left), as here the current is limited by the slowest process, in this case the anodic dissolution side. Indeed, for 3-electrode measurements with a Li reference electrode and a lithium counter electrode with much larger surface area as the lithium working electrode, an asymmetric cathodic and anodic current-potential behavior is found (Figure right). Peculiar is that this behavior is reversible and thus not due to a single formation of an irreversible electrolyte decomposition layer, often referred to as a solid-electrolyte interphase or SEI layer even though the dense electronically isolating character and even ionic conductance of these decomposition layers are often doubtful. Our group has reported previously on the peculiar electrochemical behavior of ionic-liquid based electrolytes against lithium metal [1],[2]. We attributed the asymmetric behavior in the 3 electrode measurements to the formation of a temporary passivation layer on top of the usual SEI layer (dead lithium) which diffuses away when stopping the lithium dissolution, i.e., explaining the reversible behavior. As the focus of these papers was on the electrolyte material and cell performance, respectively, we did not go into more detail on the mechanism. In this paper, we will discuss the changes at the interface explaining the time dependent change in kinetics.[1] “Silica gel solid nanocomposite electrolytes with interfacial conductivity promotion exceeding the bulk Li-ion conductivity of the ionic liquid electrolyte filler” Xubin Chen, Brecht Put, Akihiko Sagara, Knut Gandrud, Mitsuhiro Murata, Julian A. Steele, Hiroki Yabe, Thomas Hantschel, Maarten Roeffaers, Morio Tomiyama, Hidekazu Arase, Yukihiro Kaneko, Mikinari Shimada, Maarten Mees and Philippe M. Vereecken, Science Advances, 6, 2, eaav3400 (2020). [2] “ High-Rate Performance Solid-State Lithium Batteries with Silica-Gel Solid Nanocomposite Electrolytes using Bis (fluorosulfonyl) imide-Based Ionic Liquid” Akihiko Sagara, Xubin Chen, Knut B Gandrud, Mitsuhiro Murata, Maarten Mees, Yukihiro Kaneko, Hidekazu Arase, Philippe M Vereecken, Journal Electrochemical Society 167 (7), 070549 (2020). Figure 1