Impedance Spectroscopy of Lithium Intercalation into Cathode Materials in Coin Cells

Understanding the internal reactions in Li-ion batteries is crucial to analyze them more accurately and improve their efficiency since they are involved in almost every aspect of everyday life. Electrochemical impedance spectroscopy is a valuable research technique to investigate such batteries, as it reveals sensitive properties and essential information about cell reaction mechanisms and kinetics. Physical understanding of the electrochemical process and system of a battery can be analyzed using equivalent electric circuits (EECs) with rational selection of electric circuit elements and their combination. However, impedance analysis of a battery is often conducted using oversimplified EEC models in practice due to the complexity and difficulty of the physics and mathematics of the modeling. This study proposes and verifies an EEC model that represents a three-stage mechanism for intercalation-type materials. For the systematic model study and verifications, we investigated cathode half cells using four different layered structured cathode materials, namely, LiCoO₂, LiNi_1/3Mn_1/3Co_1/3O₂, LiNi_0.9Mn_0.05Co_0.05O₂, and Ni_0.815Co_0.15Al_0.035O₂. Parametric analysis of the impedance fittings for the four different cathode materials showed similar behavior depending on the states of charge. We also provided the complete set of parameters of the four systems: charge transfer resistance, double-layer capacitance, and solid-electrolyte interphase (SEI) resistance and capacitance. Lastly, we explain how different electrochemical processes, such as intercalation and alloying, can be analyzed and modeled in EEC models.