Abstract
Intentionally inducing worst-case thermal runaway scenarios in Li-ion cells on-demand is a definitive way to test the efficacy of battery systems in safely mitigating the consequences of catastrophic failure. An internal short-circuiting (ISC) device is implanted into three 18650 cell designs: one standard, one with a bottom vent, and one with a thicker casing. Through an extensive study of 228 cells, the position at which thermal runaway initiates is shown to greatly affect the tendency of cells to rupture and incur side-wall breaches at specific locations. The risks associated with each failure mechanism and position of the ISC device are quantified using a custom calorimeter that can decouple the heat from ejected and non-ejected contents. Causes of high-risk failure mechanisms, such as bursting and side-wall breaches, are elucidated using high-speed synchrotron X-ray imaging at 2000 frames per second and image-based 3D thermal runaway computational models, which together are used to construct a comprehensive description of external risks based on internal structural and thermal phenomena.
Highlights
The continued widespread adoption of lithium-ion (Li-ion) batteries for a plethora of applications, from e-cigarettes to manned-spacecraft, is accompanied by an urgent need for effective safety testing strategies
To determine the risks associated with overall heat generation and the distribution of heat generated from ejected and non-ejected materials, calorimetry was carried out in a custom-designed fractional thermal runaway calorimeter (FTRC) that can decouple the heat output from the casing of cells and ejected contents (Fig. 1d and e)
The radial location of the internal short circuiting (ISC) device appears to affect the fractional heat output for bottom vent (BV) cells, where more heat is ejected through the base of the cell for 6 layers deep than 3 layers deep (18.6% compared to 11.1%)
Summary
The continued widespread adoption of lithium-ion (Li-ion) batteries for a plethora of applications, from e-cigarettes to manned-spacecraft, is accompanied by an urgent need for effective safety testing strategies. We use an internal short circuiting (ISC) device that utilizes a low melting point wax to create, on-demand, an internal short circuit between the Al positive current collector and the surface of the graphite negative electrode [10] This type of short circuit is considered to be the most likely to induce thermal runaway as it connects two highly electrically conducting materials where the heat dissipation for the graphite material is relatively low due to its porous architecture [10,24]. This ISC device is selectively positioned within 18650 test-cells to determine the influence of the location of failure initiation on the risk of certain unfavorable failure mechanisms, such as bursting and local side-wall breaches. This work marks the first time that real-time internal dynamics of a cell undergoing thermal runaway are used to inform a computational model
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