In recent years, lithium-ion batteries have become ubiquitous in our daily lives, powering everything from smartphones and laptops to electric vehicles and renewable energy storage systems. While these batteries offer numerous benefits, including high energy density and rechargeability, they also pose significant safety risks if not properly managed or manufactured. One of the most critical risks associated with lithium-ion batteries is thermal runaway, a phenomenon that can lead to intense fires and explosions. This article delves into the mechanics of thermal runaway, its causes, consequences, and the measures to mitigate such hazardous events.

What is Thermal Runaway?

Thermal runaway (TR) refers to a self-sustaining, exothermic reaction within a battery that leads to a rapid increase in temperature and pressure. This dangerous chain reaction can result in a battery emitting toxic gasses, catching fire, or even exploding. The process begins when an increase in temperature triggers successive reactions, intensifying the heat in a feedback loop that the battery’s safety mechanism cannot contain.

What Causes Thermal Runaway?

Several factors can trigger thermal runaway in lithium-ion batteries, including:

  • Overcharging: Charging a battery beyond its specified limits can cause excessive heat.
  • Short-circuiting: Internal or external short circuits can produce significant heat quickly.
  • Physical damage: Damage to a battery’s structure can lead to internal short circuits or exposure to reactive materials, both of which can trigger thermal runaway.
  • Manufacturing defects: Imperfections in the manufacturing process, such as contaminants inside the battery, can lead to internal short circuits and, as a result, thermal runaway.
  • Overheating: External sources of heat beyond the battery’s operating temperature range.

Risk Mitigation Strategies

To mitigate the risks of thermal runaway, manufacturers must take several precautions:

Battery Management Systems (BMS): These systems monitor the battery’s state, including its charge level, temperature, and health, to prevent conditions that could lead to thermal runaway.

  • Precise Monitoring: Improved sensors and algorithms for real-time monitoring of voltage, current, and temperature at the cell level can detect anomalies early.
  • Intelligent Charging Protocols: Implementing adaptive charging algorithms that adjust the charging rate based on the battery’s condition and history can prevent overcharging and overheating.
  • State-of-Health Estimation: Advanced BMS can assess the degradation and health of batteries, predicting potential failures before they lead to hazardous conditions.

Safe Design and Manufacturing:

  • Quality Control: Implementing stringent quality control measures to detect and eliminate defects, such as metallic contaminants or electrode misalignment, can reduce internal short-circuit risks.
  • Thermal Insulation and Venting: Designing batteries with thermal insulation between cells, such as intumescent polymers, and incorporating venting mechanisms can help dissipate heat more effectively and prevent the spread of thermal runaway from one cell or module to others. As we will talk about more, LithiumPrevent is an example of a highly effective thermal barrier for isolation cells or modules.
  • Robust Separators: Developing and using more robust separators that can withstand higher temperatures and prevent short circuits between the anode and cathode.
  • Electrical Insulation: Proper insulation of high voltage components, such as busbar insulation, helps prevent accidental short circuits. A short circuit can rapidly elevate temperatures within the battery pack. Insulating materials around busbars and other high-voltage components ensure electrical isolation from the battery casing and other conductive components. This isolation is critical to prevent electrical faults that could lead to sparks or heating, initiating a thermal event. The materials used for insulating busbars and other electrical connections must possess high thermal resistance and dielectric strength. In the event of an increase in temperature within the battery pack, these materials must maintain their insulating properties to prevent electrical failures and the spread of thermal energy. Fire-resistant plastics, such as LithiumPrevent, which is an intumescent thermoplastic composite, are ideal for this application, given their ability to be manufactured in complex parts in high volumes.

Advanced Materials and Technologies: Researchers are continually developing safer battery materials and designs, including non-flammable electrolytes and thermal management systems, to reduce the risk of thermal runaway.

  • Non-Flammable Electrolytes: The development of solid-state electrolytes or other non-flammable alternatives can significantly reduce the fire hazard associated with liquid electrolytes.
  • Thermal Management Technologies: Innovations in cooling technologies and thermal management systems can help dissipate heat more effectively and prevent the onset of thermal runaway.

Thermal Runaway Containment Strategies

Thermal runaway can still occur despite engineering teams having taken all precautions necessary to mitigate its risk. The next line of defense in this scenario is a propagation containment strategy. Standards like UL 9540A in stationary energy storage and UN ECE Regulation No. 100 (R100 Rev.3) for electric vehicles outline requirements for the containment of thermal runaway propagation. The most critical factors in this defense include:

Isolation of Affected Cells

  • Thermal Barriers: Incorporating thermal barriers between individual cells or groups of cells can help contain heat and prevent the propagation of thermal runaway to adjacent cells, modules, units, or to the passenger compartment in the case of electric vehicles. In the context of containing and mitigating the propagation of thermal runaway in lithium-ion batteries, the choice of thermal barrier materials is crucial. These materials must possess high thermal resistance and stability, be non-flammable, and have the ability to absorb or dissipate heat effectively. Passive fire protection solutions, such as LithiumPrevent, are an ideal solution in this context.
  • Physical Separation: Designing battery packs with physical spacing or isolation features between cells allows for heat dissipation and reduces the risk of propagation.

Active Cooling Systems

  • Liquid Cooling: Some battery systems are equipped with liquid cooling circuits that can be activated to remove excess heat from the battery pack. This system can be particularly effective if sensors detect a rapid temperature rise indicative of thermal runaway.
  • Air Cooling: Forced air cooling can also be used to dissipate heat. While generally less effective than liquid cooling, it can be a simpler, lighter,  and more cost-effective solution in some applications.

Venting Mechanisms

  • Pressure Relief Vents: Batteries can be designed with built-in pressure relief vents that open to release gasses and pressure build-up in the event of thermal runaway. While this does not stop the reaction, it can prevent the battery casing from rupturing explosively.
  • Vent Pathways and Headroom: Ensuring hot gasses have enough room and an effective pathway to vent is critical. Moving heat out of the battery pack to address “the oven effect” is necessary in order to avoid propagation in high energy density battery packs. This is also a consideration related to cooling the pack.

Fire Suppression Systems

  • Automatic Fire Suppression: In larger installations, such as energy storage systems or EVs, automatic fire suppression systems can be installed. These systems can detect thermal runaway and release non-conductive fire suppression agents that cool the affected area and smother flames.
  • Use of Non-Flammable Electrolytes: Although not a containment strategy per se, the development and use of non-flammable electrolytes can significantly reduce the risk of fire when thermal runaway occurs.

Design for Safe Failure

  • Fault Tolerant Design: Battery systems can be designed to ensure that even in the case of thermal runaway, the system as a whole can fail safely without causing harm to users or significant property damage.
  • Redundant Safety Systems: Incorporating multiple, independent safety systems ensures that even if one system fails, others can still activate to mitigate the consequences of thermal runaway.

Implementing these strategies requires a multi-disciplinary approach, combining advanced materials science, mechanical and electrical engineering, and a deep understanding of the chemistry underlying lithium-ion batteries. While it’s impossible to eliminate the risk of thermal runaway entirely, these measures can significantly reduce the likelihood of severe incidents and mitigate their impact when they do occur.

 Mitigate and Contain Thermal Runaway: Solutions & Expertise from Pyrophobic

Don’t let thermal runaway become a safety hazard for your lithium-ion battery pack. Pyrophobic Systems offers a comprehensive suite of intumescent thermoplastic safety solutions. Contact our team of fire safety experts today for a free consultation. Our team can help you assess your specific needs and recommend the most effective thermal runaway mitigation strategies for your application.