Theory of Coupled Ion-Electron Transfer Kinetics in Ion Intercalation Materials

Abstract

Intercalation materials are commonly used in energy storage applications and more specifically in Li-ion batteries. At every cycle, Li ions are transferred between the anode and cathode of the battery. The intercalation process involves the transfer of solvated Li ions in the intercalation matrix and the electrochemical reduction of the reaction site. Until recently, electrochemical ion intercalation has been described using the phenomenological Butler-Volmer (BV) equation. However, there have been cases where its predictions deviate from the experimentally observed rates. In this work, we consider a novel theoretical approach, where we describe the intercalation process with coupled ion-electron transfer kinetics. More specifically, the classical picture of Marcus electron transfer is generalized to describe the ion insertion kinetics in solids. This coupling leads to ion concentration dependencies on the activation barrier of ion intercalation. We validate our theoretical prediction against Tafel plot measurements on Li intercalation in layered oxides (LiCoO2, NCM111), and also on rate capability measurements for several intercalation materials used in Li-ion batteries. This work opens the way to better understand and engineer the interfacial kinetics in energy storage applications.