Analysis of Li-ion Kinetics in LFP Cylindrical Cells Using Electrochemical Impedance Spectroscopy /

Abstract

The Lithium-ion cell is a highly complex electrochemical device which needs further study to better understand its kinetics and its degradation mechanisms for its wider applicability and useability. The lack of understanding of how external factors influence this electrochemical device hinders its operating limits. To study how and in what way these factors influence their performance and affect their behavior, an electrochemical impedance spectroscopy (EIS) technique was utilized to comprehend this phenomenon. The external temperature, the cyclability, the charge/discharge rate and the influence of State of Charge (SOC) was used to study the kinetics parameters and the degradational mechanisms. The outcomes studied were charge-transfer resistance Rct, solution resistance Rs, and the diffusion coefficient D. The study figured out that the cyclability enhances the charge transfer events as it improves the diffusional pathways. The correlation with diffusion coefficient was investigated and proved to be true. The effects of temperature and c-rates were also found to be of importance when investigating the kinetic parameters. The dependency of charge-discharge rates on solution resistance was found beneficial as it highlighted various degradation mechanisms often ignored in search of charge transfer relationship. The role of SOC highlighted the fact that cycling at lower SOCs contributes towards SEI formation and had more detrimental impact as compared to the cells cycled at higher SOCs. However, each parameter affected the cell in its own way as some were found to be detrimental upon their increase while some affect the cell upon their smaller values. These variations in trends give beneficial information regarding the activities and operating limits of the lithium-ion batteries (LIBs). Henceforth, the EIS proves to be an effective tool for deciphering the kinetics and degradational behavior of the cells.