**1. Introduction**

In the wake of two notable thermal runaway (TR) episodes, the initial event at Boston's Logan Airport in January 2013, where a vacant Japan Airlines 787 ignited, followed by a similar incident 9 days later in Japan, the Federal Aviation Administration (FAA) took the step of grounding the entire Dreamliner fleet. The FAA sought to ensure necessary rectifications or remedies were put in place [1]. Over the past quarter-century, lithium-ion batteries (LIBs) have emerged as the predominant

choice for storing energy, finding widespread use in electric vehicles, handheld electronics, and renewable energy setups [2]. Their exceptional energy density, compact size, and scalability make them a preferred choice in modern technological advancements. As the demand for higher energy density and larger storage capacities grows, the number of individual cells within LIBs increases, leading to more complex battery systems. However, these advancements increase the risks associated with electrical, mechanical, thermal, electrochemical, or mixed-mode damages or failures in these batteries, emphasizing the critical importance of battery safety.

LIBs'substantial energy density renders them prone to adverse conditions like elevated temperatures, mechanical impacts, overcharging, discharging, and internal short circuits. Moreover, the combustible nature of LIB structural components, comprising the outer casing, separator, and electrolyte, heightens their susceptibility. Currently, commercial LIB operation involves the migration of lithium ions between the cathode (consisting of lithium alloy and metal oxide) and the anode (comprising graphite), a process that generates heat. While this heat is typically dissipated effectively under low-rate operation, rapid charging, and discharging can lead to heat production rates surpassing dissipation rates, resulting in excessive heat accumulation and potential thermal runaway (TR), with the risk of combustion or explosions [3]. Furthermore, battery cycling contributes to capacity and performance degradation, influenced by factors like active material properties, electrolyte composition, and the solid electrolyte interface, directly influencing TR phenomena and their propagation [4, 5].

Gradually, this series of events unfolds, releasing heat in a manner that selfperpetuates and escalates beyond control. The initial release of heat is linked to the breakdown of the solid electrolyte interface (SEI), a phenomenon commonly observed at temperatures near 100°C [6]. The decomposition and regeneration of the SEI result in a continuous release of heat, elevating the internal temperature of the battery—a phase termed heat generation. Following this, the separator, typically composed of polypropylene or polyethylene, liquefies as the battery's temperature climbs to 120–130°C. According to Joule's law, the absence of insulation between the anode and cathode causes a short circuit, producing further heat—a phase known as heat spread. Meanwhile, the electrolyte solvent breaks down into hydrogen and hydroxide radicals, potentially initiating the electrolyte combustion process [7]. Moreover, as the cathode material deteriorates, it emits oxygen, intensifying the combustion reaction. Throughout these successive events, both temperature and internal pressure escalate due to the gas generation. Upon reaching a critical threshold, the venting cap ruptures, facilitating the release of gases and preventing case rupture and potential catastrophic events [8].

The mentioned events can vary depending on the specific chemistry of the battery. An experiment was conducted by Lei et al. [9] by using an accelerating rate calorimeter (ARC) to investigate the thermal abuse conditions of various cathode chemistries. The experiment results revealed that the onset temperature was similar across all the chemistries tested, which is around 90°C. Nonetheless, variations existed in the peak temperature, rate of temperature increase, and heat production among the batteries. Notably, the lithium-nickel-cobalt-manganese-oxide (NMC) chemistry exhibited the highest values for these parameters, followed by lithium manganese oxide (LMO) and lithium-iron-phosphate (LFP) batteries. It is essential to recognize that additional factors like state of charge (SOC), state of health (SOH), and triggers for thermal runaway (TR), such as short circuits or external heating, can also influence and alter the progression and characteristics of TR [10].

### *A Critical Review on the 3D Modeling and Mitigation Strategies in the Thermal Runaway… DOI: http://dx.doi.org/10.5772/intechopen.114319*

TR studies have remained oblivious to the influence of battery aging for the most part. While many studies focus on new battery chemistries or designs aiming at higher energy densities, it is also essential to understand that the safety behavior of cells can change over time. Waldmann and Mehrens conducted a study [11] on battery aging at 0°C to examine the impact of increased lithium plating and assess safety concerns using an ARC (accelerating rate calorimeter). The research outcomes highlighted that battery aging was responsible for diminished capacity and contributed to early occurrences of thermal runaway (TR), along with intensified decomposition, culminating in the ejection of the jelly roll from the cell casing. The impact of aging on TR proves intricate, as various studies have presented conflicting observations. While some researchers have documented a decline in specific safety attributes with aging, others have observed enhancements [12]. It is widely acknowledged that the processes of aging bring about changes in the characteristics of materials present within lithiumion battery (LIB) cells [13]. Therefore, considering the influence of battery aging is crucial for a comprehensive understanding of TR behavior.

Renowned for their cylindrical design spanning 18 mm (roughly 0.71 inches) in diameter and 65 mm (approximately 2.56 inches) in length, 18,650 cells are extensively utilized across diverse sectors, including electric vehicle technology and aerospace endeavors [14]. The capacity of 18,650 cells has been progressively growing over time, with current commercially available cells achieving capacities as high as 3 Ah. However, along with increased capacity comes heightened heat and gas generation during cell failure. One particular issue revolves around the buildup of gas within the rigid casing of an 18,650 cell, essentially converting it into a pressurized container. Failure to relieve internal pressure in a controlled manner can result in violent rupture, potentially culminating in explosion. More innovative engineering solutions are necessary to address these risks and mitigate the potential for catastrophic failure [15]. Various integrated safety devices have been developed to address the potential issues related to gas buildup and pressure within 18,650 cells [16]. Among these tools are pressure relief vents, positive temperature coefficients (PTC), current-limiting switches, and current interrupt devices (CIDs). They are designed to thwart gas accumulation and, when required, effectively regulate pressure release in a secure manner.

By tradition, when evaluating the safety of LIB cells, the experimental approach is seen as the most accurate method. This means that conducting real-world tests and experiments on the batteries has been the primary way to assess their safety. It has been the go-to approach because it allows researchers to observe and measure the battery's performance under different conditions, such as temperature extremes or high loads. However, a limited number of experiments can be done as it is expensive and time-consuming. Additionally, estimating the performance of large battery packs in a statistically sound manner poses significant challenges. Moreover, the variation in size and capacity of batteries across different applications introduces complexities in scaling up the battery systems, as certain physical phenomena become more prominent at larger scales, which are less significant in single or small battery systems [17–27].

TR is a well-documented phenomenon in LIBs, which is not only equivalent to irreversible severe damage or complete failure of a LIB but also accompanied by dangerous fire risks and even explosions. It is safer to assume TR is inevitable at some point in time or under certain conditions, and all efforts must be made to ensure the battery design is not susceptible to catastrophic TR events. On top of that, TR of one cell can lead to a cascading runaway effect on neighboring cells and cause thermal or

electrical damage in those cells. Even though the heat released by a single cell can be anywhere between 50 and 500 kJ, the entire energy of a pack could be one thousand times greater, and consequently, the propagation of a single point failure to the rest of the system is a daunting outcome that cannot be overlooked [28]. Capacities of modern 18,650 cells are well over 3 Ah, and it only takes 2 s for them to generate >6 L of an essentially flammable gas mixture during a typical TR event [29]. This is also accompanied by the release of almost 70 kJ of energy. At the same time, the surface temperature can exceed 600°C. Inadequate cooling provisions or the absence of effective pressure relief mechanisms during a thermal runaway (TR) incident can escalate the likelihood of side-wall breaches. These breaches occur when thermal melting or pressure-induced splits lead to ruptures in the casings of cells. Side-wall breaches are regarded as the most severe type of breach for the sturdy casings typically utilized in standard 18,650 cells. Breaches with flares may exert enough force to surpass heat-sink materials and disrupt or collide with neighboring cells, consequently facilitating the spread of TR from cell to cell [30, 31].

Cascading failure can be due to the propagation of a premixed flame at the singlecell level. While an insignificant reactant diffusion is expected at this level, single-cell instabilities are usually linked to thermally induced diffusion. On the other hand, the interference of gaps between the cells makes the cell-to-cell propagation unsteady, and subsequent damage initiation in other cells resembles that of ignition under external heat flux. Thermal management systems should mitigate the effects of TR and simultaneously prevent cell-to-cell propagation when batteries are stacked or packed together. Specific considerations are of paramount importance when it comes to battery design and implementation of battery thermal management systems: randomness of TR events widens the range of possible outcomes. It can be arduous to determine or predict the onset, acceleration, trigger, trigger cell peak, and neighbor cell peak temperatures. Calculating the total amount of energy released through the sides and top of the battery cell is also tricky. It is also challenging to predict the exact location of such failures (e.g., top vs. side), analyze the pressure increase, identify the type of evolving gases, and the type/trajectory of ejecta material. Different active and passive cooling systems and countermeasures against damage propagation have been developed and tested. For instance, utilizing Al and Cu barriers is a simple approach for passive thermal management, which also decreases or eliminates the risk of cascading failures. Although this phenomenon has been a subject of intense scientific scrutiny, there is still plenty of room to propose novel, safer individual cells and battery pack designs. Intelligent battery management systems (BMS), battery thermal management systems (BTMS), software controls, and modular designs are becoming more focused on the safety concerns related to TR. Although LIB is not the only type of energy storage system prone to thermal runaway, its lower runaway temperature underscores the importance of thermal management or even the implementation of fire suppression systems [32]. As energy density, size, and the number of individual cells in currently used LIBs and future designs increase, so do the magnitude and probability of risks associated with electrical, mechanical, thermal, electrochemical, or mixed-mode permanent damages or failures in these batteries.

Since the inception of the Doyle, Fuller, and Newman (DFN) pseudo-2D (P2D) model in 1994, battery simulation techniques have undergone rapid advancement. MATLAB/Simulink emerges as a predominant commercially available simulation software tool in today's market, especially for vehicle system simulation. Prominent examples such as ADVISOR and Autonomie are developed on the MATLAB/Simulink platform. Additionally, Simcenter Amesim and GT-Suite are either utilized
