CHARACTERIZATION OF THE OVERCHARGE REACTION OF LITHIUM ION RECHARGEABLE CELLS

Abstract

Lithium ion rechargeable cells are used widely in consumer applications and more recently in medical applications, specifically as the power sources for ventricular assist devices and artificial hearts. The purpose of this work was to characterize the results of overcharge of lithium ion cells and gain insight into the mechanisms that can lead to cell failure. Understanding the response to overcharge is important for appropriate device as well as cell design. Prismatic 1.5 Ah cells were systematically overcharged at increasing current rates to monitor the response. Cells were constructed with internal as well as external thermocouples to measure the temperatures generated at the case surface and the interior of the cell stack. Finally, cells with different anode to cathode ratios were constructed to gain insight into the mechanisms that can potentially lead to cell rupture. Cells that were overcharged on currents under 300 mA remained hermetic. However, cells that were overcharged at significantly higher currents could rupture on extended exposure to charging current. The study of cells constructed with internal thermocouples did reveal significant temperature differences between the cell interior and the external case walls during abuse testing. Testing of cells with different anode to cathode ratios demonstrated that the cathode plays a key role in determining the point of cell rupture.