|dc.description.abstract||Lithium ion batteries hold the potential to play a key role in meeting our future and increasing energy storage needs. Lithium ion batteries have the highest energy density of all battery systems currently available. Since their introduction to the modern battery market in the mid 90’s they have evolved and become the choice system for meeting energy needs in portable electronic devices. Lithium ion batteries, however, face many challenges preventing them from being utilized to their fullest potential. They suffer from self-discharge, degradation over repeated charging, not operating well at extreme low or high temperatures, and can suffer from the deposition of metallic lithium during charging. This deposition builds up on the surface of the graphite electrode and can lead to the formation of structures called dendrites. These dendrites can cause problems like internal short-circuits, ultimately resulting in the battery catching on fire.
The focus of this work is to study the electrodeposition of lithium on graphite electrodes. Two main tools are used over the course of this study: modeling and experimentation.
The first half of this work discusses the approach through computational modeling. A simple one dimensional, needle-like dendrite model is developed. Through analysis of the concentration gradient that occurs near the surface of the electrode and evaluation of the overpotentials that develop due to applied current at the surface of the electrode a relationship is found for the tip current density. Propagation of the dendrite is calculated from determination of the tip current density and application of Faraday’s law. Appropriate verification through the method of manufactured solutions and validation by comparison to previously reported experimental data are discussed.
The second half of this work cover the experimental parts of this work to probe the electrodeposition of lithium on graphite electrodes. The development of the cell fabrication techniques and the characterization of the graphite electrode material used in these experiments is explained. The graphite used in these experiments was CMS graphite on a copper current collector with a specific capacity of 317 mAh/g.
The experimental investigation of electrodeposited lithium on graphite electrodes is studied by the development and utilization of a novel dynamic impedance measurement technique. The impedance response of the cell is captured across different states of charge of normally charged cells and of cells where electrodeposition was proven to occur. Through trends in the impedance data, and through utilization of equivalent circuit analysis, correlations of changes in the impedance with the electrodeposition of lithium are made. In the absence of lithium electrodeposition, the general impedance response of the cell is to increase with increasing SOC. However, this trend reverses under conditions where electrodeposition is occurring. The presence of electrodeposited lithium is verified using SEM imaging.||en