Characterising thermal runaway within lithium-ion cells by inducing and monitoring internal short circuits

Abstract

a Lithium-ion batteries are being used in increasingly demanding applications where safety and reliability are of utmost importance. Thermal runaway presents the greatest safety hazard, and needs to be fully understood in order to progress towards safer cell and battery designs. Here, we demonstrate the application of an internal short circuiting device for controlled, on-demand, initiation of thermal runaway. Through its use, the location and timing of thermal runaway initiation is pre-determined, allowing analysis of the nucleation and propagation of failure within 18 650 cells through the use of high-speed X-ray imaging at 2000 frames per second. The cause of unfavourable occurrences such as sidewall rupture, cell bursting, and cell-to-cell propagation within modules is elucidated, and steps towards improved safety of 18 650 cells and batteries are discussed. Broader contextFrom portable electronics to grid-scale storage, high energy density Li-ion batteries are ubiquitous in today's society. Such cells can and do fail, sometimes catastrophically, releasing large amounts of energy. To facilitate safer and more reliable cell designs, the importance of understanding failure mechanisms of Li-ion cells is widely recognised. Here, we demonstrate the application of a novel device that is capable of generating an internal short circuit within commercial cell designs, on-demand, and at a predetermined location. This enables us to test more effectively the ability of safety devices of cells and modules to withstand 'worst-case' failure scenarios. By combining the use of this device with high-speed X-ray imaging at 2000 frames per second, we characterise for the first time the initiation and propagation of thermal runaway from a known location within a Li-ion cell. The insights achieved in this study are expected to guide the design and development of safer and more reliable Li-ion cells.

Tracking Internal Temperature and Structural Dynamics during Nail Penetration of Lithium-Ion Cells

“…This suggests that gases generated within the electrode assembly in the bottom portion of the cell initially flowed toward the vacant base of the casing and thereafter changed direction toward the vent, a tortuous path for pressure relief. This buildup of pressure at the base of the cell and consequent doming is not unique to mechanical abuse scenarios; similar shifts of the electrode assembly have also been observed under thermal abuse conditions 8,17 and are expected to be the primary cause of cell rupture and ejection of their contents.…”

“…8 The direction of gas flow is apparent in Supplementary Movie 3 where fragments of broken-down material detached from within the observable electrode assembly, were carried out of the spiral-wound layers toward the base of the cell, and thereafter, toward the vent of the cell through the vacant core, as illustrated by the blue arrows in Figure 4. This suggests that gases generated within the electrode assembly in the bottom portion of the cell initially flowed toward the vacant base of the casing and thereafter changed direction toward the vent, a tortuous path for pressure relief.…”

“…This observation supports the case for the presence of an internal mandrel, which was previously shown to facilitate maintaining an opening for gas and fluidized material to escape. 8 Gas generation and the consequent uneven and dynamic pressure distribution within cells during failure is an additional important consideration for multi-physics models of battery failure, particularly since it is local pressure differences that drive the flow of fluidized material, the venting process, as well as movement of the bulk electrode assembly.…”

“…For example, when mechanical failure of the electrode assembly occurs due to applied tensile strain, electrically conducting layers from the positive and negative electrode can make contact, causing an internal short circuit. 8 Alternative methods for inducing internal short circuits by implanting devices during manufacturing have previously been described, 8,9 but have not yet been accepted as standard test methods.…”

“…High-speed X-ray computed tomography (CT) and radiography have been used in previous studies to visualize the internal breakdown and ejection of active materials following thermal runaway induced via thermal abuse, 17 electrical overcharge 18 and internal short-circuiting. 8 Failures induced via intrusive mechanical methods are expected to deviate most significantly from other in-field failures, in particular internal short-circuiting, which nail penetration is sometimes used to simulate.…”