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Internal short-circuit is the most dangerous abusive condition for Li-ion batteries and has been the root cause for several catastrophic accidents involving Li-ion batteries in recent years. Large-format Li-ion batteries are particularly vulnerable to internal short-circuits because of high energy content. Nail penetration test is commonly used to study the internal short-circuits, but the test results usually have poor reproducibility and offer limited insight. In this work, a 3 D multiscale electrochemical-thermal coupled model is used to investigate the nail penetration process in a large-format Li-ion cell. A parametric study is carried out and the results reveal strong coupling of the cell thermal response and electrochemical behaviour, which is influenced substantially by key parameters including shorting resistance, nail diameter, nail thermal conductivity, and cell capacity. The present study provides some insight that will help design more reliable experimental internal short-circuit testing protocols and improve the abuse tolerance of Li-ion cells. With the increasing interest in large-format Li-ion batteries for automotive applications, safety has become a primary concern due to the high energy density of Li-ion batteries and wide-ranging working conditions for electric vehicles compared with electronic applications. Safety must be maintained for electric vehicles. Even a single accident due to battery failure could turn public opinion against electric mobility and set back industry development for years. 1 Several abusive conditions have been identified which have the potential to cause safety issues in Li-ion batteries. Typical examples of battery abusive conditions are: internal short-circuit, external short-circuit, over-charging and over-heating. Internal short-circuit in Li-ion batteries is usually caused by manufacturing defects and is intrinsically more hazardous than other abusive conditions. During the short-circuit process, a large current passing through the cell and shortcircuit spot produces enormous amount of Joule heat, which easily triggers exothermic reactions of active materials and electrolyte, leading to thermal runaway. In addition, common protective equipment, such as PTCs, in Li-ion batteries cannot protect the battery undergoing internal short-circuit. The active material will continuously react until depletion during an internal-short circuit process. Large-format batteries are more vulnerable to internal short-circuit because of its high energy content.Nail penetration tests have been widely adopted by battery manufacturers to emulate the internal short-circuit process in Li-ion cells. In a nail penetration test, an electrically conductive rod (e.g. stainless steel rod) with its end tapered to a sharp point is used to pierce through the testing cell (Figure 1). The typical nail diameters range from 3 mm to 20 mm and the penetration speed is typically 8 cm/s.2 The orientation of the penetration should be perpendicular to the cell electrodes. In addition to video monitoring, s...

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Safety has become an increasingly pressing issue in large-format, energy-dense Li-ion batteries for automotive applications. Among various abusive scenarios for Li-ion batteries, internal short-circuit is most dangerous and has been the root cause for several highly publicized catastrophic accidents in recent years. Nail penetration and crush tests are commonly used as experimental proxy for internal shorting, but fail to truly emulate the internal short-circuits seen in field accidents. Also, experimental methods only give a simple pass/fail result, providing little insight into fundamental mechanisms governing the battery thermal and electrochemical response during internal shorting. In this study, a 3D electrochemical-thermal coupled model is used to scrutinize the internal shortcircuit process in a large-format Li-ion cell with a stacked-electrode design. The model reveals the 3D electrochemical and thermal processes inside the battery cell during internal shorting. A parametric study is carried out, showing that the short-circuit resistance and the number of shorted electrode layers have the most significant influence on cell electrochemical and thermal behavior. Novel experimental methods, designed to precisely control these key parameters, must be developed in order to advance the understanding and improvement of Li-ion battery safety. As energy and power densities of Li-ion batteries rise ever higher to meet requirements of sustainable energy applications such as electric transportation and grid energy storage, risk of battery failure with potential for catastrophic incidents makes safety a primary concern. The highest specific energy available in today's commercial Li-ion rechargeable batteries is about 240 Wh/kg, almost 20% of the energy content of TNT at 4.61 MJ/kg.1 Under normal discharge (charge), the chemical (electrical) energy is converted to electrical (chemical) energy with minimal heat generation. However, when the battery is exposed to abusive conditions, a large amount of heat could be generated due to unmanaged energy release or absorption. This increased heat generation will trigger a series of exothermic reactions that generate more heat, eventually leading to a thermal runaway. Catastrophic consequences will usually follow battery thermal runaway.Internal short-circuit is the most common and dangerous abusive condition and is the cause of most field incidents involving Li-ion batteries. It occurs when a current path develops within the battery cell. It is usually caused as a result of manufacture defects or physical damages to the battery cell. The fundamental reason that internal short-circuit is so dangerous is that very high localized heating results from current flow through a short-circuit object (SCO).This localized heating is difficult to dissipate and the current flow is large, especially in large-format cells. The localized heating then triggers rapid temperature rise and thermal runway of batteries. In addition, common protective devices installed externally on Li-ion batteries...

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