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The Majority of Batteries are Lithium-ion Batteries
Wind power facilities, solar power farms, microgrids, data centers, and telecommunication facilities all share at least one common feature: they rely on battery energy storage systems (BESS) composed of thousands of lithium-ion batteries.
While lithium-ion batteries offer numerous advantages, they also come with some drawbacks. On one hand, lithium-ion battery energy storage systems require complex Battery Management Systems (BMS) to maintain safe operating parameters for voltage, temperature, and charging. Improper management or misuse of the batteries can lead to failure, resulting in venting or overheating. If a battery catches fire (thermal runaway), it can quickly escalate into a catastrophic fire, and even lead to explosions. Such fires are extremely challenging to extinguish and can propagate rapidly to neighboring batteries in a domino effect.
What is Battery Thermal Runaway
Battery thermal runaway refers to a cumulative amplification of current and battery temperature during constant voltage charging, resulting in gradual damage to the battery. In typical batteries, due to the absence of gaps between the positive and negative plates filled with liquid, oxygen generated at the positive electrode during the charging process cannot reach the negative electrode, making it prone to hydrogen generation along with the escape of oxygen from the battery.
Cause of Battery Thermal Runaway
The reasons for causing battery thermal runaway stem from inadequate battery selection and thermal design, or external factors such as short circuits leading to elevated battery temperatures, loose cable connections, etc. The solution should address both battery design and battery management.
Thermal runaway triggered by overcharging specifically occurs when the battery management system itself lacks circuit safety features for overcharging, causing the battery’s BMS to lose control while it is still charging.
Internal short circuits can also lead to thermal runaway, and battery manufacturing impurities, metal particles, expansion and contraction during charge-discharge cycles, lithium plating, and so on can potentially cause internal short circuits. These internal short circuits develop slowly, over a very long period of time, and it’s unpredictable when they might result in thermal runaway.
Furthermore, collisions are a typical mechanical trigger for thermal runaway. For instance, Tesla has experienced multiple fire incidents due to this reason. Experts have revealed that a joint analysis of Tesla’s collision accidents in the United States was conducted in cooperation with Tsinghua University and MIT. When simulating collisions in the laboratory, the closest analogy is a puncture.
How to Detect Battery Thermal Runaway
China’s new technology has developed high-precision, multimodal integrated optical fiber sensors that can be implanted inside batteries, pioneering the accurate analysis and early warning of the entire commercial battery thermal runaway process.
By identifying the characteristic turning points and common patterns in the chain reaction triggering battery thermal runaway, it has enabled precise discrimination of the internal microscopic ‘irreversible reactions’ within batteries. This provides an essential means to rapidly interrupt the chain reaction of battery thermal runaway and ensure the safe operation of batteries within their designated range.
In the future, given the small size, flexible shape, resistance to electrical interference, and remote operational capabilities of optical fiber sensors, along with the suitability for mass production using standard manufacturing techniques, it will be possible to simultaneously monitor various critical parameters such as temperature, pressure, refractive index, gas composition, and ion concentration at multiple positions within the battery using a single optical fiber.
How to Prevent Battery Thermal Runaway
On an individual battery basis
🔔From the perspective of battery material design, materials can be developed to prevent thermal runaway and inhibit the reactions leading to it. From a battery management standpoint, different temperature ranges can be predicted to define various safety levels, enabling a graded warning system.
🔔Due to battery aging, there is poor battery consistency and overcharging. So, by using the ‘first parallel, then series’ combination method for battery packs to address individual consistency issues, they have the same capacity as the smallest single cell. With this consistency, the capacity has been restored, and it also helps prevent overcharging.
🔔Identify reputable battery manufacturers, select battery and cell capacities, perform safety predictions for internal short circuits, and proactively identify individual cells with internal short circuits before thermal runaway occurs.
🔔Construct a thermoelectric coupling model to inhibit battery heat propagation and utilize insulation layers for blocking and suppression.
On the battery unit level
For battery units, a greater emphasis is placed on innovative thermal management solutions tailored to various application scenarios to prevent battery thermal runaway. The first aspect is thermal design (which includes)
🔔Determining the goals and requirements of the thermal management system
🔔Measure or estimate module heat generation and heat capacity.
🔔Initial assessment of the thermal management system (including the selection of heat transfer media, design of heat dissipation structures, etc.).
🔔Predict the thermal behavior of modules and battery packs.
🔔Initial design of the thermal management system for.
🔔Design the thermal management system and conduct experiments.
🔔Optimization of the thermal management system.
A brief analysis of the current mainstream heat exchange methods:
👉Air cooling is a method that involves passing air over the surface of the battery pack to dissipate heat and prevent battery thermal runaway. For example, mild hybrid cars like the Prius and Insight in Japan use both series and parallel ventilation methods to remove heat from the surface of the traction battery.
Air-based thermal management systems for power batteries, while cost-effective and relatively simple with low implementation difficulty, face challenges in meeting the heat dissipation requirements of power batteries in scenarios involving a high number of batteries, limited placement space, high power usage, harsh operating conditions (such as climbing or braking), and misuse (overcharging, over-discharging, overheating, overcurrent, etc.).
👉Heat pipe heat transfer – this approach is also used to prevent battery thermal runaway. Heat pipes, as efficient heat transfer components, can rapidly and effectively transfer thermal energy between two objects. In the thermal management systems of electric vehicles, many researchers both domestically and abroad have applied heat pipes as heat transfer components for power battery heat dissipation. Compared to traditional forced convection cooling systems, the heat dissipation system incorporating heat pipes not only keeps the power battery within its normal operating temperature range but also ensures uniform temperature distribution among individual battery cells, which cannot be achieved by forced cooling systems. However, heat pipes come with the drawbacks of excessive weight and volume and have heat transfer limitations.
👉Due to the limitations of air cooling technology and heat pipe space, power battery thermal management techniques using liquid media for cooling have begun to be applied in battery systems. Power battery thermal management systems employing liquid media and forced convection, utilize devices or components (such as pumps, liquid cooling channels, valves, etc.) to transport the liquid medium to the battery surface. They make use of liquid media flow heat transfer technology to heat or cool the power battery, ensuring that the power battery operates within the ideal temperature range.
Liquid cooling technology provides more even temperature control for batteries compared to air cooling, but liquid cooling systems are more complex, require additional energy consumption, and are susceptible to leaks, which can lead to battery short circuits. For instance, Tesla uses serpentine tubing with attached thermal materials to form a liquid cooling system around cylindrical batteries. The cooling fluid consists of 50% water and 50% ethylene glycol, and the temperature differential can be controlled within 2%. In order to further save space and improve efficiency, liquid cooling enclosures have also started to gain market presence (widely used in commercial vehicles and energy storage containers as well).
👉Furthermore, phase change material (PCM) heat exchange systems use phase change materials as the heat transfer medium, utilizing their ability to absorb (or release) battery heat during phase change reactions. This cooling technology provides good temperature control and uniform temperature distribution but is expensive due to material costs. To address material limitations, some researchers have introduced metal additives into the PCM, such as thin aluminum sheets, carbon fibers, carbon nanotubes, etc., to enhance thermal conductivity.
Battery thermal runaway is the most undesirable and highly avoided battery safety incident. Improving battery safety and preventing thermal runaway requires a combined effort in battery formulation design, structural design, and thermal management design for the battery packs. Together, these measures enhance battery thermal stability and reduce the likelihood of thermal runaway occurrences.
