Characterization of thermal cut-off mechanisms in prismatic lithium-ion batteries

Lithium-ion (Li-ion) cells that are subjected to electrical abuse, overchar ge and external short-circuit in particular, exhibit a rapid increase in ce ll temperature that could potentially lead to catastrophic disassembly of t he cell. For this reason these cells are integrated or combined with one or more safety components that are designed to restrict or even prevent curre nt flow through the cell under abusive conditions. In this work, the charac teristics of these components in several prismatic Li-ion cells are studied by monitoring the impedance (Z) at 1 kHz and the open circuit voltage (OCV ) of the discharged cells as a function of temperature. All the cells studi ed were found to use polyethylene-based shutdown (SD) separators that were irreversibly activated within a narrow temperature range between 130 and 13 5 degreesC. In some cells irreversible cut-off was also provided by a curre nt interrupt device (CID) or a thermal fuse. Both these devices had a circu it-breaker effect, causing the impedance of the cell to rise infinitely and the OCV to drop to zero. In addition to these irreversible cut-off mechani sms, some cells also contained internal or external positive-temperature-co efficient (PTC) devices that could provide current-limiting capability over a very wide temperature range. The interdependence of the thermal behavior of these components on each other and on other thermally dependant process es like cell venting, separator meltdown and weld joint failure are also di scussed. (C) 2001 Elsevier Science B.V. All rights reserved.