A Combined Aging and Abuse Study of Li-Ion Cells

Abstract

Lithium-ion batteries have been powering a wide range of electrical and electronic devices, from consumer products and medical equipment to automotive systems and space applications. Some highly publicized failures of products powered by lithium ion cells, such as laptops, cellphones and electronic toys, have been reported recently. A portion of these failures has been linked to overheating of the battery resulting in fire and explosion. Overall, these incidents have catalyzed a great deal of research and design activities aimed at trying to understand the causes of such failures and help guide safer cell designs. In this work, the degradation mechanisms in commercial 18650 lithium ion cells, due to long term cycling under nominal-use charge discharge cycling as well as under electric vehicle drive cycling, are studied. In addition, studies were carried out after subjecting the cells to overcharge and external short circuit testing on fresh as well as cycled cells. During the external short test, the cells were subjected to a low resistance load (50 mΩ). Actual results indicate that for a very short time the cell experiences a very high current causing a rise in the cell temperature (>70 oC). In the overcharge test, the cell is charged up at a constant C-rate until the test is stopped due to either an abnormal event or the cell protection activation. Preliminary results have shown lithium metal deposition, as expected, in the negative electrode once that the intercalation rate has been overcome by the diffusion rate. Also, cell temperature has shown a significant increment during the overcharge process. Finally, the electrodes and electrolyte from the cycled cells were subjected to a similar analysis to understand the degradation mechanisms in long term use as well as to understand their behavior under abuse conditions after long term use. Understanding what causes this temperature rise and its effect on the electrodes could provide some information about how to prevent thermal runaway in cells and batteries used in various applications for long periods. The cells under these tests have been characterized by conducting a Destructive Physical Analysis (DPA). The study is being accomplished through charge/discharge cycling, internal resistance measurement, capacity fading, and impedance spectroscopy. Cell temperatures are monitored in all the experiments. The morphological changes are studied by extracting the electrodes from the abused cells and analyzing them by microscopy and spectroscopy techniques. This analysis will help to obtain the changes in the electrochemical properties of the cell as well as its morphology.