Quantifying the Impact of Overdischarge on Large Format Lithium Ion Cell Safety

Abstract

Lithium-ion battery safety has always been a concern; however, safety is becoming increasingly important due to the proliferation of high capacity, high power cells and modules utilized in electric vehicle, grid, aerospace, and military applications. While safety measures such as battery management algorithms, thermal management, fuses, and sensors can reduce the risk of catastrophic failure, not all of the risk can be managed while the battery is active. Self-discharge of lithium ion cells below a zero state of charge voltage, overdischarge, is one such case. Overdischarge has generally been considered more of a performance issue than a safety concern unless coupled with recharge. However, with battery-powered devices potentially left for long periods of time in storage, DoD interest in extending the shelf life of devices in the field, and proliferation of battery packs composed of large numbers of cells, there is an increased interest in determining limits of low voltage recharge and developing a methodology for determining safety limits. Furthermore, overdischarge is a class of safety risk that is inherent to the cell and cannot be prevented with passive protection devices. This talk will cover safety concerns associated with overdischarge of large format lithium-ion batteries including gas generation, copper dissolution at low voltage and precipitation upon recharge, and health of the Solid Electrolyte Interphase layer. While the gas generation behavior of lithium-ion cells has been studied extensively[1], a methodology for assessing the gas generation prior to full thermal runaway has not been developed. It has been demonstrated that oxygen can be released from the electrode structure due to a destabilization of the crystal lattice.[2] Additionally, electrolyte decomposition, typically during charging, contributes to gas buildup within the cell that can eventually cause rupturing of the casing.[3] We discuss methods for assessing the amount of gas generated at intermediate stages of cell abuse. Finally, we present recent data on the safety implications of copper dissolution in overdischarged large format cells. [1] E. Roth, C. Crafts, D. Doughty, and J. McBreen, “Advanced technology development program for lithium-ion batteries: thermal abuse performance of 18650 li-ion cells,” Sandia National Laboratory Report, SAND2004-0584, 2004 [2] W. Kong, H. Li, X. Huang, and L. Chen, “Gas evolution behaviors for several cathode materials in lithium-ion batteries,” Journal of Power Sources Vol. 142, 2005, pp. 285-291. [3] K. Kumai, H. Miyashiro, Y. Kobayashi, K. Takei, and R. Ishikawa, “Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell,” Journal of Power Sources Vol. 81-82, 1999, pp. 715-719.