Mitigating Thermal Runaway Risks and Lithium-Ion Battery Fires

By Chris Wright

Let’s address an uncomfortable truth of energy storage technology today: lithium-ion (Li-ion) batteries, especially those using Lithium Nickel Manganese Cobalt (NMC) chemistry, have an inherent risk of thermal runaway.

Since lithium-ion batteries are used throughout the world, from residential back-up systems to multi-megawatt systems providing reliability services to the grid, the risk of these types of battery fires can be severe.

Just how big is this risk? In the United States, fortunately, there have only been a few fires that made the headlines, but there have been over 23 battery system fires in South Korea just since 2018. These incidents have caused more than just property and financial damage - they have also caused significant new project and expansion delays.

Battery system fires in South Korea Source: Clean Energy Associates (CEA)

Battery fires have damaged the reputations of the developers, asset owners, and manufacturers, and detrimentally affected public perception that Li-ion energy storage is a safe solution for the energy transition.

Causes of Thermal Runaway and Energy Storage System Fires

The causes of energy storage fires and explosions are complex and can be influenced by many factors. These factors can range from errors in manufacturing, design, and installation, to human errors, but the primary cause of an energy storage system fire is caused by having a battery cell go into thermal runaway.

During thermal runaway, the heat generated within the battery cell exceeds its ability to dissipate. When this occurs, the battery cell in thermal runaway can cause adjacent battery cells to overheat, potentially driving them into thermal runaway. If this chain reaction occurs across multiple battery cells, it can spread from modules to the rack and result in a significant fire.

Thermal runaway is primarily caused by the cell undergoing external abuse conditions. These conditions include external heating, over-charging, over-discharging, high-current charging, nail penetration, crushing or the occurrence of an external short-circuit. These external abuse conditions lead to plating of the Lithium which can lead to internal short circuit of the cell.

This energized-state of the internal cell can lead to exothermic reactions within the cell such as electrode-electrolyte reactions, chemical decompositions, or other electrochemical reactions. If the heating-rate as a result of these exothermic reactions is greater than the possible dissipation rate of the cell, it may lead to thermal runaway.

During a thermal runaway event, heat inside the battery can also cause the battery’s electrolyte to evaporate out of the battery cell, creating a mixture of volatile gases that can explode if the battery system is not properly vented.

These issues are susceptible to any type of lithium-ion battery, but the risk is higher for NMC chemistry due to the higher energetics. As battery cells achieve higher and higher energy density, a thermal runaway fire may become even more intense and last longer with higher levels of stored energy to dissipate.

Industry Adapting Towards Thermal Runaway Mitigation and Prevention

While the recent energy storage fires have been a step back for the energy storage industry and project development, these cases have inspired varying solutions to reduce the risks of these fires and the severity of thermal runaway events.

Improved Thermal Management Systems

Energy storage companies are developing improved thermal management systems that can help reduce the risk of a single cell going into a thermal runaway event resulting in a rack-level fire. Historically these have been very simple forced air convection systems using ambient or cooled air blowing across the battery racks. More modern thermal management systems are incorporating liquid cooling technologies that are more effective for removing heat from the cell and provide improved module temperature control.

Thermal Runaway Detection Technologies

Large container-based energy storage systems are also developing new thermal runaway detection technologies that detect volatile gas generation. Some of these systems can detect certain components of the evaporating electrolyte being released from the battery during a thermal runaway event. After detection, various safety systems attempt to mitigate the thermal runaway and buildup of volatile gasses, or they can quickly communicate to operators that an event has occurred to prevent further damage.

Built-In Thermal Fuse Safety Systems

Battery manufacturers are also engineering cells to be safer, adding built-in thermal fuse safety systems. When a cell’s thermal runaway event is detected, the thermal fuse will open the circuit, disconnecting it from neighboring cells or modules. From there, the system's new liquid cooling, gas detection, and communication systems can help cool and help slow the event from spreading.

Reducing Thermal Runaway Risks with Technical Due Diligence

Even with these new trends in safety systems, a robust technical due diligence program can significantly reduce the risk of your energy storage project. These programs include evaluating various battery technologies, assessing factory quality and manufacturing systems, assessing the supply chain for risks, and evaluating the safety mitigation systems surrounding them. These programs should include reviews of the system’s design, as well as assessing the codes and standards required by the Authorities Having Jurisdiction (AHJ).

While we’ve seen that energy storage fires are possible due to many factors, preventing thermal runaway events and having a dedicated quality assurance program protects the investors' assets, our clean energy industry’s reputation, and people’s lives.

CEA provides a full suite of services that support project development, due diligence, detailed engineering, system sizing and performance assessments of energy storage projects - ensuring both project competitiveness and success.

View our recent webinar on energy storage safety trends to get an overview of designing battery systems with critical energy storage safety codes and standards.