Feb 13, 2018 | Uncategorized
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.
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.
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.
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.
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.
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.
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.
 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.
 The Relationship of the Nail Penetration Test to Safety of Li-ion Cells, TIAX LLC presentation at 2013 DOE Annual Merit Review Meeting.