What is thermal runaway in energy storage?

In recent years, with the explosive growth in energy storage demand, the safety of energy storage has once again attracted attention.

Among numerous battery energy storage system safety incidents, statistical analysis has revealed that the main factors causing these incidents include: thermal runaway of lithium-ion batteries, defects in individual battery cells, mechanical damage, overheating, or external short circuits.

Thermal runaway is the most familiar and frequently searched term. So, what exactly is thermal runaway?

Thermal runaway occurs when an electrochemical battery uncontrollably raises its temperature through self-heating.

When the heat generated by a thermally runaway battery exceeds its capacity to dissipate, further heat accumulation can lead to fire, explosion, and gas release. If thermal runaway in one battery cell triggers thermal runaway in other cells within the battery system, this is called thermal runaway propagation.

So, what are the causes of thermal runaway?

The factors inducing thermal runaway in lithium-ion batteries can be categorized into three types: mechanical abuse (puncture, crush deformation, external impact), electrical abuse (overcharging, over-discharging, short circuits), and thermal abuse (failure of the thermal management system). Mechanical abuse can easily induce internal short circuits in lithium batteries, leading to thermal runaway. Electrical abuse, such as overcharging and over-discharging, can trigger internal side reactions, causing localized overheating of the battery cells and resulting in thermal runaway. External short circuits are a dangerous state of rapid battery discharge, where extremely high currents cause rapid temperature rise and may even melt the battery tabs. Under thermal abuse conditions, the thermal management system often fails, inducing internal separator shrinkage and decomposition, ultimately leading to internal short circuits and thermal runaway.

Furthermore, the battery's own condition is also a significant factor in causing thermal runaway. With the increase in the number of charge-discharge cycles and the induction of impurities during dendrite production, adverse side reactions can lead to the formation of metal dendrites that can easily puncture the separator and cause localized internal short circuits.

Research on battery thermal runaway caused by thermal abuse is based on the electrochemical-thermal coupled overcharge-thermal escape model of lithium-ion batteries established in the literature. Lithium-ion batteries typically begin to self-heat when the temperature reaches 80℃. When battery thermal management fails to effectively release excess heat, the battery temperature rises uncontrollably, spreading from individual cells to the entire battery pack, triggering a series of side reactions and leading to thermal runaway.

Thermal abuse does not occur spontaneously within the battery. It is usually caused by mechanical abuse or other reasons that raise the internal temperature of the battery to a threshold, causing localized heating and leading to thermal abuse, which further induces temperature runaway and spontaneous combustion.

Meanwhile, thermal runaway is also used as a research method to study the runaway process of experimental batteries and detect safety characteristics during thermal runaway. In 1999, KITOH et al. conducted research on monitoring the safety characteristics of thermal runaway in high-energy-density power batteries based on external heating methods. Since then, the adiabatic energy method has been widely used to test the thermal runaway temperature threshold of lithium-ion batteries. Current research on thermal abuse mainly focuses on external radiation-induced battery combustion. Liu Mengmeng established a multi-endogen transient heat generation model and an electrochemical-thermal coupling model, and studied the safety characteristics of batteries after spontaneous combustion caused by thermal abuse based on the radiation heating method. They found that battery combustion can be divided into three stages: jet combustion, stable combustion, and secondary jet combustion. LI et al. studied the effect of discharge current on temperature under the background of thermal runaway caused by thermal abuse. They found that when the discharge current is constant, the mass loss, safety characteristic parameters, thermal runaway initiation temperature, and peak temperature during thermal runaway all depend on the battery capacity.

Research on Battery Thermal Runaway Caused by Electricity Abuse: Common causes of battery thermal runaway include overcharging and over-discharging, internal short circuits, and external short circuits.

(1) Overcharging and Over-discharging: During a normal charge-discharge cycle of a lithium-ion battery, the BMS (Battery Management System) blocks the charging current based on the state of charge. When the BMS fails, overcharging can easily lead to serious spontaneous combustion accidents. Continuing to charge after reaching the SOC threshold causes lithium metal to adhere to the surface of the negative electrode active material. The adhered lithium reacts with the electrolyte at a certain temperature, releasing a large amount of high-temperature gas. Simultaneously, the positive electrode active material begins to melt due to excessive delithiation and a large potential difference with the negative electrode. Once the positive electrode potential exceeds the safe voltage of the electrolyte, the electrolyte will also undergo an oxidation reaction with the positive electrode active material. Overcharging also causes a series of side reactions such as ohmic heating and gas overflow, exacerbating the occurrence of thermal runaway. Dr. Ye Jiana discovered that the gases released during overcharging of lithium-ion batteries mainly consist of CO2, CO, H2, CH4, C2H6, and C2H4, and the gas volume and heat increase with increasing charging current. Using a combined accelerated calorimeter and battery cycle tester, experiments showed that overcharging based on constant current-constant voltage is far more dangerous than overcharging under direct constant current conditions. Ren et al., based on the overcharging performance of composite material cathodes and graphite anodes under different experimental environments, comprehensively considered the effects of charging current, separator materials, and heat dissipation systems. Their research found that the heat release during overcharging of NCM batteries is not significantly related to the charging current; the melting point of different separator materials and battery deformation and swelling are the main factors contributing to thermal runaway in lithium-ion batteries. Wang et al. analyzed the thermal propagation path and high-temperature gas escape path of lithium batteries under overcharge conditions, finding that the heat generated by the reaction of deposited lithium with the electrolyte during overcharging accounts for more than 43%. Zhang et al. studied the degradation mechanism of battery capacity based on incremental capacitance-differential voltage, finding that a single overcharge has little effect on battery capacity, but after overcharging to the point of delithiation of the positive electrode active material, it severely affects the thermal stability of the battery pack.

Over-discharge causes much less harm; early over-discharge is unlikely to trigger battery thermal runaway, but it does affect battery capacity. Zhou et al. studied the discharge characteristics of nickel-cobalt-manganese (NCM) ternary lithium batteries after over-discharge. During static discharge, the internal short circuit degree of the NCM lithium battery decreases, the resistance increases, and the discharge current decreases. Experiments show that the greater the depth of discharge, the greater the degradation of individual cells within the battery pack. Ma et al. found in lithium battery over-discharge experiments that over-discharge does not change the structure of the battery active material, but it causes the negative electrode current collector to dissolve, increases the SEI film thickness, and accelerates battery aging.

External Short Circuit

External short circuits are also an important cause of thermal runaway in power batteries. Chen et al. established a new electrothermal coupling model based on a heat generation, distribution, and propagation model. Studies show that the peak temperature of lithium-ion batteries under external short circuit conditions exists at the edge of the tab. Ma et al. found that under external short-circuit conditions in power batteries, the heat generated by side reactions is far less than the heat generated by electrochemical processes. Furthermore, the electrochemical heat generation is positively correlated with the initial state of charge (SOC) but negatively correlated with the peak temperature thermal stress.

Internal Short Circuit

Internal short circuits, occurring inside the battery, are difficult for the BMS system to detect and are a major cause of thermal runaway in lithium-ion batteries. When the battery is overcharged or over-discharged, lithium dendrites gradually grow until they penetrate the SEI film, triggering an internal short circuit and rapidly leading to uncontrollable temperature rise and thermal runaway. In addition, lattice damage caused by rough battery manufacturing processes or burrs in the current collector can also cause internal short circuits.

Huang et al. created an internal short circuit by embedding a low-melting-point alloy in the separator and puncturing it. They used a K-type micro-thermocouple to measure the local temperature and collected data on the heat spread distribution caused by the internal short circuit. Zhang et al. embedded a low deformation temperature threshold nickel-titanium alloy in the separator or current collector and heated it until it deformed and punctured the separator, thus achieving an internal short circuit. Experiments revealed that the primary heat source for thermal runaway occurred during the reaction between the positive current collector and the negative electrode, leading to a short circuit and subsequent rapid temperature rise. Internal short circuits between the positive and negative electrodes, aside from partial charring, did not cause severe thermal runaway.

Research on Battery Thermal Runaway Due to Mechanical Abuse

Automotive power batteries inevitably suffer mechanical failures due to accidents during use. If the battery pack is deformed by external forces such as puncture or compression, it can trigger internal structural changes, even leading to direct contact between the positive and negative electrodes under extreme stress conditions, causing an internal short circuit and resulting in thermal runaway. Therefore, research on battery thermal runaway due to mechanical abuse is essential. Researchers such as Fan Wenjie and Xu Huiyong have conducted studies on thermal runaway caused by mechanical abuse based on finite element modeling and numerical monitoring analysis.

WANG et al. conducted research based on the changes in the cross-sectional area of ​​a pouch lithium-ion battery pack after a collision. Puncture experiments revealed that numerous localized deformations and shear fracture layers appeared within the battery pack during the puncture process. The tearing of the current collector and positive electrode active material, and the puncture of the separator due to internal structural rearrangement of the battery pack, were the root causes of internal short-circuit thermal runaway. Lamb et al. studied the deformation state of 18650 cylindrical lithium-ion batteries under puncture conditions using computed tomography (CT) technology. Experiments revealed that the penetration phenomenon between the positive and negative electrodes exacerbated internal short circuits, and the attached aluminum foil melted during the short circuit, forming numerous metal beads at the puncture crack. Li et al. established finite element analysis models for various states of mechanical abuse based on puncture and compression, and developed a learning algorithm to predict the thermal runaway process of batteries using parameters from spent batteries. They analyzed the impact of mechanical abuse on lithium-ion battery safety from eight types of parameters, including impact force, collision angle, and deformation range, significantly reducing computational load.

Mechanical abuse in real-world applications is far more complex than simple puncture or compression experiments. Experimental simulations alone cannot provide a deep understanding of the safety characteristics of battery mechanical abuse. The fundamental solution lies in optimizing battery installation positions, establishing reliable BMS systems, and optimizing the overall vehicle frame design during the design of power battery packs to minimize deformation and compression of the power battery pack during collisions.