Lithium-ion batteries store a large amount of energy in a compact volume. While this enables long runtimes, if something goes wrong the stored energy can be released rapidly as heat. As battery packs become larger and more energy-dense: particularly for high-energy applications, the associated safety risks also increase.

Once a lithium-ion battery fire begins, it can be difficult to extinguish. The battery effectively becomes its own fuel source. Consequently, lithium-ion batteries are not permitted in checked baggage on airplanes, and addressing thermal safety has become one of the most critical and increasingly important engineering challenges.

When a battery fails, it doesn’t fail quietly. Batteries, like us, love a little drama.

Lithium-ion batteries can catch fire in a process known as thermal runaway, which is triggered by a series of heat-releasing (exothermic) reactions.

A thermal event usually begins when the battery experiences abnormal heating. If heat is generated faster than it can be dissipated, the temperature rises. Once the temperature exceeds a critical level, chemical reactions inside the battery start releasing additional heat. This further increases the temperature, creating a chain reaction that leads to thermal runaway.

During this process, the battery may release gas, smoke, or flames. The heat and gases produced can also spread to nearby cells. Because large battery packs often contain hundreds or even thousands of cells, a single thermal runaway event can quickly propagate and become severe.

In other words, one failing cell can become a neighborhood problem very quickly.

Preventing thermal events and stopping their propagation are major focuses of battery safety research and engineering.

Thermal events in batteries are usually triggered by abuse conditions or internal defects. The most common abuse conditions include:

Internal short circuits - Manufacturing defects, contamination, or damage to the separator inside the battery can cause the positive and negative electrodes to come into contact.

Mechanical damage - Crushing or puncturing can physically compromise internal layers.

Electrical abuse - Overcharging or high current. Charging a battery beyond its voltage limit can destabilize the materials inside the cell.

Overheating - High temperatures from external sources, such as hot environments or nearby failing cells, can initiate internal reactions.

Batteries behave beautifully within their operating window. Step outside it, and the chemistry starts writing its own script.

Lithium-ion batteries contain multiple layers of materials, including electrodes, a separator, and a flammable liquid electrolyte. These components enable ion transport and electrochemical reactions that store and release energy.

When a battery experiences one of the abuse conditions mentioned above, it begins to heat up internally.

As the temperature rises, the solid electrolyte interphase (SEI) layer on the anode starts to break down, releasing additional heat. The SEI normally protects the electrode and stabilizes the interface between the electrode and electrolyte. Once it begins to degrade, unwanted reactions accelerate.

With further temperature increase, the electrolyte begins to decompose, generating both heat and flammable gases.

When the cell temperature approaches the melting point of the separator, the separator can shrink or fail, leading to an internal short circuit between the electrodes and causing rapid heating of the battery.

As the temperature continues to rise, the response depends on the cathode chemistry. In cathode materials such as lithium cobalt oxide (LCO) or nickel manganese cobalt oxide (NMC), the cathode can begin to decompose and release oxygen. This oxygen reacts with the electrolyte and other cell components, accelerating the temperature rise and eventually leading to thermal runaway.

At that point, the battery contains heat, oxygen, and fuel (electrolyte): the classic ingredients for combustion.

And the system is no longer under the control of the battery management system or the user.

Battery safety is a complex issue influenced by multiple factors that determine the thermal behavior of a battery.

Battery design and materials
The materials used in the electrodes, electrolyte, and separator play a critical role in determining battery safety. Their thermal and chemical stability influences how the battery responds to elevated temperatures or abnormal operating conditions.

Cell quality
The quality of cell manufacturing is equally important. Manufacturing defects (such as metal particle contamination, misalignment of electrode layers, uneven coating, or cracks) can lead to internal short circuits.

In practice, many battery failures are not chemistry problems; they are manufacturing problems.

Batteries are subjected to various abuse tests to evaluate how they behave under extreme conditions and to ensure safe and reliable performance. Common tests involve overheating, overcharging, or mechanical damage. “Nail pen” short for nail penetration refers to punching a nail through the cell to simulate a hard short, and watching how dramatically the stack lights up.

Careful monitoring and control of cell temperature play a critical role in reducing the likelihood and severity of thermal events, as abnormal heating often initiates battery failures. 

Battery management systems widely known as “BMS” play an important role in battery safety  by continuously monitoring parameters such as voltage, current, and temperature, allowing the system to detect abnormal conditions and respond before they develop into serious safety events. 

While new materials and cell designs are constantly being proposed to improve battery safety, making batteries completely risk-free remains extremely challenging. Even if a battery chemistry or its components are considered safer in theory, their safety must still be validated through extensive testing.

Because with batteries, the chemistry always gets the final say.