THERMAL RUNAWAY

What is Thermal Runaway?

Thermal runaway is a phenomenon when the battery enters a self-fed cycle of heating and degradation. This results in a catastrophic release of energy—usually accompanied by gas venting, sparks, and fire. In recent years, there have been a few high-profile cases of this happening with popular consumer goods, airplanes, and even electric vehicles.

A thermal runaway event is caused by a failure in an individual cell that reacts and begins breaking down the internal battery structures. This causes a thermal chain-reaction, creating a self-propagating cycle of rapid heating and deterioration.

A few examples of causes for failure are:

  • Exposure to excessive temperatures
  • Short-circuiting
  • Surges in both charging and discharging current
  • Hotspots in large packs
  • Improper electrical connections
  • Poor fail-safe software
  • Mechanical destruction, penetration, or impact

LHS Materials are helping manufacturers develop next-generation electric and hybrid vehicles by implementing the active thermal regulation in a fire-retardant matrix to overcome some of the safety and performance limitations of lithium-ion battery packs. Carefully regulating the heat fluctuations in battery packs increases the lifespan of the battery and thermal runaway prevention.

LHS Materials are engineered to be integrated thermal regulators. The matrixes are custom-designed blocks that integrate into the fire-retardant battery housing unit.

How Can We Prevent Thermal Runaway?

Thermal runaway prevention is a hot topic and the unique material properties of the LHS® battery matrixes reduce the likelihood of a thermal runaway event by regulating the individual cell temperatures. However, if a cell were to enter thermal runaway, the battery matrix isolates the incident and prevents any cascading effect.

LHS Materials actively cool the battery cell mechanically by “melting” the latent heat organics to absorb heat. However, this cooling effect is limited by the heat saturation levels of the organics: meaning the responsiveness of the electrical system in cutting off the electrical flow is vital. While both systems can consistently and effectively prevent thermal runaway, combining the two systems allows for a dynamic and actively responsive cooling system. Read more about how LHS Materials can improve performance and safety.

Lithium-Ion Energy Storage

Li-Ion batteries have a well-rounded balance of cost, weight, and capacity. This has led to their widespread adoption in high-tech and lightweight applications. Their success over other rechargeable batteries, in nearly every consumer goods market, has led to a renewed expectation of better battery performance: faster charging and discharging while lasting longer. Along with raised expectations, the rapid growth of Li-Ion technology has opened new visions for what can be made possible through having more efficient energy storage—one of which is the widespread adoption of electric vehicles.

Global electric vehicle sales have grown at a 32% compound annual growth rate over the last 4 years1. While the total market for EVs is still a small percentage of the overall vehicle sales, Bernenberg Bank predicted that EV sales will gain a solid foothold by eventually breaking 5% of total sales by 20202. This rapid growth does not include a large number of commercial fleet vehicles adopting hybrid and EV technology, for example, UPS’s adoption of hybrid technology into some of its fleet3, Telsa’s commercial electric semi-truck4, and Workhorse Group’s electric fleet pickup.

This adoption of electric and hybrid vehicles relies heavily on the progress of rechargeable technology—demanding longer vehicle range, better performance, and lower costs. Lithium-ion batteries are particularly suited for this type of application with properties including:

  • Highest energy density of any mass-produced battery
  • Low maintenance requirements
  • High degree of design flexibility
  • Relatively negligible memory effect
  • Low self-discharge rate
  • Nearly 3x the voltage capacity of the next level batteries at 3.6 V5

While Li-Ion batteries are the leading edge in rechargeable technology, the increased demands from electric vehicles have pushed the industry to increase energy density, charge/discharge capacity, and storage efficiency in these batteries. Unfortunately, as seen in the consumer goods market, these pressures have accentuated some of performance and safety limitations with Li-Ion batteries—particularly with the battery packs used in EVs. These packs have three main areas of both performance and safety limitations: regular use degradation, overheating and packing inconsistencies.

Regular Use Degradation

During thermal cycling, the charging or discharging causes internal resistance and thermal expansion which causes stress on the materials in the batteries, shortening their useful life. This puts a small amount of strain on the mechanical and material systems in the battery—which is relieved when the battery cools down to its original state. However, this cycle generates a cumulative effect of degrading the materials and putting expansion and contraction strain on mechanical system of the battery.

Pouch cells are particularly susceptible to this type of stress, whereas cylindrical cells tend to mitigate this type of life-shortening. Additionally, this thermal cycling accelerates material degradation. When the materials no longer perform with optimal materials, there is a significant loss of recoverable power and capacity.

Do All Li-Ion Batteries Explode the Same?

Do all Li-ion batteries explode the same?  The short answer is no, not all cells burn or react the same when undergoing a catastrophic thermal runaway event.  Some Li-ion chemistries and cells are more energetic, and some are more stable and “safer”.

Not a week goes by without a news story describing a Li-ion cell starting on fire and creating a hazardous event.  These include the famous Samsung Note 7 battery issues, the hoverboards that start house fires during recharge, and personal devices such as headphones, fitness trackers, e-cigarettes that cause skin burns and injuries when their batteries catch fire.  For instance, between March 1991 and May 22, 2017, the FAA documented 160 incidents at airports and in airplanes of devices smoking and catching on fire. Twenty-two (22) of these incidents alone were between Jan-May 22 of this year and this tally “should not be considered as a complete listing,” the agency says.[1]

How batteries react and burn to develop into a catastrophic thermal runaway event is dependent on a number of variables such as how the cell is damaged, the Li-ion chemistry, cell capacity, cell state of charge (SOC), quality of cell manufacturing, and the external protections put into place around the cell or in the battery pack.

All thermal runaway events are a result of a rise in cell temperature from multiple causes:

  • The use of cells in high-temperature environment
  • A defect inside the cell can result in an internal short circuit
  • A surge in the charging or discharging current
  • An improper electrical connection at the tab of a battery.
  • Mechanical damage

For instance, mechanical damage and penetration of the cell will normally create a more energetic and explosive thermal event, as compared to a short circuit or cell overheating which may only cause the cell to vent or bulge.

As an example shown in the video below, a nail penetration test conducted by Outlast Technologies of 18650 cells from the same production lot at the same 100% SOC.  As can be seen, not all cells explode the same.

 SOC also has a significant effect on cell energetics and is why it’s important to control cell charging and prevent overcharging.  Shown below in Table 1 and Figure 1 are the nail penetration results of cells at various SOC.

Table 1 reports the maximum temperatures and heat of reaction obtained from Sony US18650GR cells at different SOC.[2]

Fig. 1, displays the different degradation results of a given lot of batteries when tested at different SOC.  It is clearly seen that higher SOC yield more energetic thermal events.[3]

Various manufacturers create electrochemical cells based on different chemistries and different cell sizes and shapes.  Some chemistry provides higher energy capacity and some provide longer cell life, thermal stability and safety.  For example, Table 2 lists some common cell chemistries and their attributes.

Graph 1, shows the different energies and capacities of various chemistries.  Generally, the higher the chemistry capacity and energy, the lower the thermal stability and safety[4].

 Also, these different cell chemistries and cell sizes provide different thermal energy events due to the various stored energy, their energy density, internal chemistry reaction rates, thermal degradation mechanisms and thermal degradation rates.  For example, Table 3 below are various energy ranges of some of the different chemistries.

These results are also corroborated by nail penetration testing at Outlast Technologies of various cell sizes, capacities, and chemistries.  Graph 2 outlines the thermal curves and max temperatures of different cell sizes and chemistries.

Safety and Thermal Runaway Prevention

As described, not all systems and packs are designed the same and one measure of safety that can be employed is the use of LHS® materials.  LHS® battery matrixes are engineered to be integrated thermal regulators. The matrixes are custom-designed with unique material properties to reduce the likelihood of a thermal runaway event by regulating the individual cell temperatures. However, if a cell were to enter thermal runaway, the battery matrix isolates the incident and prevents any cascading effect.

LHS® matrixes actively cool the battery cell thermo mechanically by absorbing the reaction heat into the shape stable latent heat containing poly-organics. Therefore, it is important to understand all of the variables that go into pack design which also affect the customization of the LHS materials.  This allows the Outlast Technologies technical team to design or recommend the most effective solution for your particular requirements.

[1]www.faa.gov/about/office_org/headquarters_offices/ash/ash_programs/hazmat/aircarrier_info/media/battery_incident_chart.pdf

[2] Experimental Analysis of Thermal Runaway in 18650 Cylindrical Li-Ion Cells Using an Accelerating Rate Calorimeter, Lei, et. al, Batteries June 2017, vol. 3, issue 2, 14.

[3] The Relationship of the Nail Penetration Test to Safety of Li-ion Cells, TIAX LLC presentation at 2013 DOE Annual Merit Review Meeting.

[4] Batteryuniversity.com