| dc.description.abstract | Lithium ion batteries are quite ubiquitous in terms of their market spread. They represent an extremely compact energy storage device. In addition to making them a lucrative choice for a diverse set of applications, this high energy density and larger terminal voltage also make them quite dangerous if not handled properly. In extreme events, they can also catch fire. To ensure continuous and safe operation, cell manufacturers specify a voltage window of operation. This voltage window describes the lowest discharge voltage and highest charge voltage. Intuitively speaking, the cell stability should not be specified just in terms of one voltage value. Rather, it should be a function of cell temperature as well as charging current. In order to gain insights into the cell operation during and after overcharge, commercial 18650 cells were used for electrochemical cycling. Three major sets of tests were performed on these to answer the following questions: 1) How do Li-ion cells behave if the electrochemical window is manipulated? 2) How does the charging rate affect the overcharge behavior of these cells? 3) Finally, is there a way to track changes occurring during each state of overcharge and perhaps, elucidate the reactions in the cell contributing to overcharge? The results showed that even if the upper voltage limit of Li-ion cells is extended and a higher capacity is gained, the cycle life of the cell diminishes considerably. Secondly, as expected, the charging rate is found to have a significant effect, leading to the hypothesis that overcharge of Li-ion cell is not solely dependent on the upper voltage limit, but also on the charging rate (current). Based on destructive physical analysis (DPA) and electrochemical impedance spectroscopy measurements, the resistance from the cell separator was found to be a leading presence during overcharge of the cell. Finally, based on overcharge tests and the supporting DPA analysis, it was concurred that overcharge is a cathode dependent process as opposed to the popular belief of anodic dependence. | en |
