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.

As the energy density of lithium‐ion cells and batteries increases, controlling the outcomes of thermal runaway becomes more challenging. If the high rate of gas generation during thermal runaway is not adequately vented, commercial cell designs can rupture and explode, presenting serious safety concerns. Here, ultra‐high‐speed synchrotron X‐ray imaging is used at >20 000 frames per second to characterize the venting processes of six different 18650 cell designs undergoing thermal runaway. For the first time, the mechanisms that lead to the most catastrophic type of cell failure, rupture, and explosion are identified and elucidated in detail. The practical application of the technique is highlighted by evaluating a novel 18650 cell design with a second vent at the base, which is shown to avoid the critical stages that lead to rupture. The insights yielded in this study shed new light on battery failure and are expected to guide the development of safer commercial cell designs.

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SUMMARYProper understanding of heat generation and design of heat dissipation paths are critical for ensuring the safety of lithium ion modules during abuse events such as external shorts. Additionally, the behavior of positive thermal coefficient (PTC) current limiting devices-generally effective at the single-cell level-can be difficult to predict for a multi-cell module. To help guide battery pack design, a coupled thermal/electrical model of a commercial 18 650-size cell and a module with 16 cells in parallel (16P) are developed. Cell electrical response is modeled using an equivalent circuit, including the temperature-dependent behavior of the PTC. Cell thermal response is modeled with a highresolution thermal model from which a simpler 5-node thermal circuit model is extracted. Cell models are integrated into a module-level model considering cell-to-cell electrical and thermal interactions via conduction, convection, and radiation. The module-level model is validated with a 16P external short experiment and applied in a parametric study to assess thermal safety margin.