**1. Introduction**

Lithium-ion batteries (LIBs) are extensively used in various applications, including from small-scale portable electronics to large-scale application in electric vehicles, along with other applications in the field of drones, airplanes, and robots [1]. LIBs have shown efficient and practical applications in domestic as well as hand-held devices [2]. The LIBs are preferred over other chemistries owing to their strong advantages of high-energy density, low self-discharge, fast charging ability, enhanced cycle life, among many other advantages [3].

However, LIBs are likely to fail in case of over-charging and over-discharging with the possibility of a severe accident in case of overheating or internal-external short circuit [4]. The thermal abuse of LIBs can be caused by penetration, external-short, overheating, or over-charge [5]. In the case of a thermal abuse incident, the available chemical energy in LIBs rapidly converts to heat energy, generating a large amount of heat with the possibility of thermal runaway—fire accompanied by the occasional explosion [6]. Owing to high-energy density and flammable components, LIBs are prone to safety issues. Physical abuse of LIBs could lead to a temperature rise to the levels of 100 to 150 ◦C,

which could trigger cascading exothermic electrochemical reactions along with chemical disintegration, resulting in the rise of temperature to the levels of 500 ◦C in a few seconds. Although many safety methods, including effective cooling, have been installed, several cases of hazards have been reported over the years [7,8]. The recent events of LIB failure events leading to fire and explosion accidents have focused research attention on LIB safety [9]. For example: iPod caught fire due to overheated LIB [10] and three fire accidents of Boeing 747 at different locations [11]. Although, recently, LIB safety issues have received attention due to increased instances of hazards in electrical vehicle application, the safety concerns were admitted for many years [12,13].

In the case of continuous increase of temperature, rapid reactions start to occur. At around 110 ◦C, the graphite anode reacts with the electrolyte solvent. At around 165 ◦C, cathode material decomposes and releases oxygen. At around 180 ◦C, electrolyte solvent decomposes, and flammable components undergo combustion leading to thermal runaway [14]. The mechanical abuse in terms of penetration, or crush leading to thermal abuse, is an important safety concern and has received increased attention in recent years [15]. In many applications, the overheating and overcharge is controlled as a safe temperature and cut-off voltage are maintained. However, mechanical abuse leading to a short circuit is a challenging threat, as LIB can be subjected to crushing or metal penetration in unpredictable situations, such as accidents. The penetration provides a low-resistance path, drawing extremely high levels of currents with a rapid discharge of available chemical energy and simultaneously converting it into thermal energy. In the absence of resistive load, with the short circuit providing a low-resistance path, the energy is dissipated as joule heat, resulting in a rapid temperature increase [16].

Chen et al. developed an electro-thermal model to investigate the thermal performance during normal discharge and internal short circuit. The authors investigated the effect of an internal short circuit at the center of the battery on the temperature rise of the battery, and suggested that the increasing thermal conductivity of a separator can be an effective method to reduce the heat accumulation at the location of the internal short circuit [17]. Shi et al. conducted an experimental study on the exothermic behavior of LIB under mechanical abuse and suggested that dibenzylamine (DBA) could be used to prevent thermal runaway. The authors suggested that under normal conditions DBA does not affect the battery performance, and during mechanical abuse, DBA is released, which increases electrolyte resistivity preventing thermal runaway [18]. Vyroubal et al. presented a finite element model of nail penetration into the lithium-ion battery and showed that shorting resistance has a significant influence on the cell electrochemical-thermal process of batteries [19]. Noelle investigated internal short circuit on LIB by conducting direct current internal resistance, extremal shorting, and nail penetration experiment, and presented an electrolyte resistance model with experimental validation [20]. Fang et al. developed a 3D electrochemical-thermal model for internal short in Li-ion cell, and predicted that discharge rate was higher in Anode-Aluminum short as compared to Anode-Cathode owing to lower short resistance [21]. Mao et al. investigated the failure mechanism of the lithium-ion battery during nail penetration tests and presented the influence of penetration position and depth on temperature of the jelly-roll-type cylindrical cell. The authors pointed out that maximum temperature was observed when the nail was penetrated at the center of the cylindrical cell with the region of thermal runaway covering the entire cell. According to the micro short-circuit cell model presented by authors, if the temperature reaches between 90–120 ◦C, heat can spread to whole battery. However, reactions cannot lead to thermal runaway due to the unavailability of oxygen. But if the temperature reaches up to more than 233 ◦C, then the separator shrinks, thus leading to a larger short-circuit area, with the start of a reaction between cathode and electrolyte releasing a large amount heat and combustion byproducts in terms of gases [22]. Zao et al. conducted external and internal short circuit tests on batteries with different capacities. The authors showed that for external short test, with lower internal resistance, heat was accumulated between clamps and battery tabs, whereas, in case of nail penetration, heat accumulation increases with battery capacity and may catch fire. The authors suggested hydrogel-based thermal management to prevent thermal runaway [23]. There are few studies on how to mitigate the effect of internal short circuit and prevent thermal runaway. For example, Wang et al. suggested a

modified current collector with surface-notch, instead of a flat current collector, leading to a negligible temperature increase associated with internal sorting [24]. However, safety concerns still remain for the accumulation of large amounts of heat leading to thermal runaway in LIB during internal short circuit, and more understanding is still needed of the thermal behavior of LIB during internal shorting. temperature increase associated with internal sorting [24]. However, safety concerns still remain for the accumulation of large amounts of heat leading to thermal runaway in LIB during internal short circuit, and more understanding is still needed of the thermal behavior of LIB during internal shorting.

current collector with surface-notch, instead of a flat current collector, leading to a negligible

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 3 of 23

The goal of the present study is to investigate the thermal abuse behavior of the LIR2450 coin cell with internal short circuit by penetrating element. The experimental coin cell discharge study is conducted and validated with numerical study within ±5.0%. The effect of penetrating element size, location of penetrating element, state of charge (SOC), discharge rate, short-circuit resistance, and heat transfer coefficient on maximum temperature and heat generation rate are presented. The study provides comprehensive insights on the thermal behavior of LIBs during thermal abuse condition with internal short circuit by penetrating element. The goal of the present study is to investigate the thermal abuse behavior of the LIR2450 coin cell with internal short circuit by penetrating element. The experimental coin cell discharge study is conducted and validated with numerical study within ±5.0%. The effect of penetrating element size, location of penetrating element, state of charge (SOC), discharge rate, short-circuit resistance, and heat transfer coefficient on maximum temperature and heat generation rate are presented. The study provides comprehensive insights on the thermal behavior of LIBs during thermal abuse condition with internal short circuit by penetrating element.

#### **2. Experimental Study 2. Experimental Study**

Figure 1 shows the schematic for the experimental study. The LIB under consideration is the LIR2450 coin cell with 120 mAh capacity, and the specifications of the coin cell are presented in Table 1. The coin cell was connected to KIKUSUI electronic load PLU-150 for evaluating and maintaining current, voltage, and power. The discharge test of the coin cell was conducted under constant current condition. The coin cell was fully charged to 4.2 V and discharge tests were conducted with a cut-off voltage of 2.75 V. The thermocouple was attached to a coin cell positive tab for measuring the surface temperature. The second thermocouple was attached to measure the ambient temperature. All the experiments were conducted in a room set at a constant temperature. The temperature of the room was controlled and kept constant at 25 ◦C. The details of the equipment used in the experimental study are presented in Table 2. Figure 1 shows the schematic for the experimental study. The LIB under consideration is the LIR2450 coin cell with 120 mAh capacity, and the specifications of the coin cell are presented in Table 1. The coin cell was connected to KIKUSUI electronic load PLU-150 for evaluating and maintaining current, voltage, and power. The discharge test of the coin cell was conducted under constant current condition. The coin cell was fully charged to 4.2 V and discharge tests were conducted with a cut-off voltage of 2.75 V. The thermocouple was attached to a coin cell positive tab for measuring the surface temperature. The second thermocouple was attached to measure the ambient temperature. All the experiments were conducted in a room set at a constant temperature. The temperature of the room was controlled and kept constant at 25 °C. The details of the equipment used in the experimental study are presented in Table 2.

**Figure 1. Figure 1.** Experimental setup. Experimental setup.





The measured parameter uncertainty is presented based on the accuracy of the instrument. The measured parameters were voltage, current, and temperature. The accuracy for current measurement was calculated based on Equation (1). The maximum uncertainty in measuring current was 0.45%. The accuracy of the voltage measurement was calculated based on Equation (2). The maximum uncertainty in voltage measurement was 0.08%. The accuracy in temperature measurement was calculated based on Equation (3). The maximum uncertainty in temperature measurement was 1.25%.

$$I\_{\rm U} = \pm (0.2\% \text{ of set} + 0.2\% \text{ of full scale}) \, + \,\text{V}\_{\rm in}/500 \,\text{k}\Omega \tag{1}$$

$$V\_{U} = \pm (0.2\% \text{ of set} + 0.2\% \text{ of full scale}) \tag{2}$$

$$T\_{\rm II} = \pm (0.1\% \text{ of reading} + 0.5^{\circ} \text{C}) \tag{3}$$
