Li-Ion thermal run away is a set of chemical reactions and shorts that happen internally in a cell. Excessive heat from inside or outside the cell triggers them. The chemical reactions cause extra heat, which causes a positive feedback cycle to intensify the heat and then the reactive agents get depleted within the cell.
How Does It Happen?
In Lithium-ion batteries, a thin polyethylene separator separates cathode and anode. When this separator ruptures, a short circuit happens which initiates the process called thermal runaway.
Thermal runaway happens usually during charging when it is normal for the temperature to get raised to the melting point of the metallic lithium causing an explosive reaction.
Sometimes microscopic metal particles coming in touch with different parts of the battery trigger a short circuit leading to thermal runaway. When too many microscopic metal particles collect, electrical shorts can develop and current flow develops between the positive and negative plates. This leads to surge in temperature, leading to a thermal runaway; this is often given a terminology called ‘venting with flame.’
Why and How Is It Harmful?
Heat from failing cells during a thermal runaway process can spread to other cells causing a thermal instability that can trigger a chain reaction. This leads to disintegration of the cells in due course of time.
Representation Of A Thermal Run Away in Li-Ion Battery
Thermal run away can physically appear like smoldering or a fire with a flame. In the following figure we can see a moderate size battery pack after a single 2.2 Ah 18650 Li-Cobalt Oxide cell experienced thermal run away. It can be marked that the thermal run away reaction was intense enough to cause an eruption in the battery pack case.
Prevention Of Thermal Run Away
Introducing a Flame Retardant
Using a thermal fluid, which contains a flame retardant, helps in combating thermal runaway that occurs from punctures and improper charging. A flame retardant is a compound that inhibits, suppresses or delays the production of flames or prevents the spread of fire.
Using Damage initiating Devices
There are certain shutdown mechanisms that can be initiated mechanically or thermally when damage just starts to trigger in Li-ion battery. This can proactively take care of the thermal leakage problem and stop it from spreading further.
Alteration in Battery Design as proposed by Tesla
To minimize the impact of thermal runaway damage Tesla had filed a patent where a novel battery design was proposed. This design presented a narrow release path through a portion of the cell in the event of thermal energy escaping during a thermal runaway, thereby minimizing the chances of a rupture in an undesirable location.