Introduction To Lithium-Ion Battery Thermal Runaway And Material Analysis
- The division of battery thermal runaway stage
- The battery thermal runaway cause analysis
- The thermal runaway protection measures
Battery thermal runaway refers to the chain reaction phenomenon triggered by various triggers, resulting in a large amount of heat and harmful gases emitted from the battery within a short period of time, which can even cause the battery to catch fire and even explode. What will happen if battery explodes? Read this lithium ion battery explosion article to find the answer.
There are many reasons for battery thermal runaway to occur, such as overheating, overcharging, internal short circuit, collision, etc. Battery thermal runaway often starts from the decomposition of the negative SEI film in the battery cell, followed by the decomposition and melting of the diaphragm, resulting in the negative electrode and the electrolyte should occur, followed by the decomposition of both the positive electrode and electrolyte.
And it will trigger a large-scale internal short circuit, resulting in the burning of the electrolyte, which then spreads to other cells, causing a serious thermal runaway, allowing the entire battery pack to produce spontaneous combustion.
The division of battery thermal runaway stage
There are different ways of dividing the stages of battery thermal runaway, the core should be, across which point the thermal trend will not be reversed. There are theories that this point is the mass dissolution of the diaphragm.
Until then, the temperature drops, the material activity decreases and the reaction slows down. Once this point is breached, the positive and negative poles have been directly opposed to each other, and the internal temperature of the cell cannot be lowered to terminate the continuation of the reaction.
The theory of battery thermal runaway is divided into three stages, the autogenous heat stage (50 ℃-140 ℃), thermal runaway stage (140 ℃-850 ℃), thermal runaway termination stage (850 ℃ – room temperature), some literature provides the diaphragm mass melting temperature starts at 140 ℃.
Self-generated heat stage
The self-generated heat phase of the battery, also called the heat accumulation phase, begins with the dissolution of the SEI film.
The dissolution of the SEI film is clearly observed when the temperature reaches about 90°C, making the cathode and the embedded lithium carbon component contained in the cathode directly exposed to the electrolyte, and the embedded lithium carbon reacts exothermically with the electrolyte, causing the temperature to rise.
The increase in temperature in turn promotes the further decomposition of the SEI film. Without external cooling means, this process will roll forward until the SEI film is completely decomposed.
Battery thermal runaway stage
The battery thermal runaway stage of the cell is after the temperature exceeds 140°C, both positive and negative materials join the electrochemical reaction, and the increase in mass of reactants makes the temperature rise faster.
The externally observable parameter change is a sharp drop in voltage, and the process is described as follows: after reaching this temperature interval, the diaphragm begins to melt in large quantities and the positive and negative electrodes are directly connected, resulting in the occurrence of a massive short circuit. By this point, battery thermal runaway has begun and will not stop.
In a short period of time, the violent reaction generates a lot of gas while generating a lot of heat, and the heat heats up the gas, and the expanding gas breaks through the core shell, and material ejections and other phenomena occur, and the scattered material also takes away some of the heat.
The battery thermal runaway reaches its most intense state. The maximum temperature is also reached at this stage. If there are other cores around, battery thermal runaway may spread to other cores at this stage by spreading heat to the surrounding area.
The heat may be conducted through the connected conductive parts, or because of the volume expansion, the original cells that keep the spacing, at this time have been close to each other, the direct conduction of heat between the cell shell.
The battery thermal runaway cause analysis
Causes of thermal runaway battery can be divided into internal and external causes. Internal causes are often due to internal short circuits. External causes are due to mechanical abuse, electrical abuse, thermal abuse and other reasons.
Internal short circuit is the direct contact between the battery terminal, and the subsequent reactions triggered by the different degrees of contact vary greatly.
A massive internal short circuit, usually caused by mechanical and thermal abuse, will directly trigger thermal runaway. In contrast, internal short circuits that develop on their own are relatively minor, and it generates very little heat and does not immediately trigger thermal runaway.
Internal self-development commonly includes manufacturing defects, deterioration of various properties caused by battery aging, such as increased internal resistance, lithium metal deposition caused by long-term mild misuse, etc. As time accumulates, the risk of internal short circuit caused by such internal causes will gradually increase.
The mechanical abuse of lithium battery refers to the deformation of lithium battery monomer and battery pack under the action of external force, and the relative displacement of different parts of itself. The main forms against the battery cells include collision, extrusion and puncture.
The electrical abuse of lithium battery generally includes external short circuit, overcharge, over discharge several forms, which is most likely to develop into thermal runaway is overcharge. An external short circuit occurs when two conductors with differential pressure are connected outside the cell.
In contrast to punctures, the heat released from an external short circuit does not normally heat the battery. The important link between the external short circuit and the thermal runaway of the battery is the temperature reaching the point of overcharge.
It is when the heat generated by the external short circuit cannot be dissipated well that the battery temperature rises and the high temperature triggers thermal runaway. Therefore, cutting off the short circuit current or dissipating the excess heat are ways to inhibit the external short circuit from producing further damage.
During over-discharge, the cell with the lowest voltage in the pack can be forced to discharge by other cells connected in series. During forced discharge, the poles are reversed and the cell voltage becomes negative, resulting in abnormal heating of the over-discharged cell. Dissolved copper ions triggered by overdischarge migrate through the membrane and form copper dendrites with lower potential on the cathode side.
As growth continues to rise, copper dendrites may penetrate the diaphragm, resulting in a serious internal short circuit.
The thermal runaway protection measures
From the above analysis it can be seen that thermal runaway is important to prevent and monitor, as long as the need to inhibit the occurrence of thermal runaway in the self-generated heat stage, because once the thermal runaway can not be rescued, only passive protection.
In the self-generated heat stage to inhibit the occurrence of thermal runaway of the core, we have two options, one is to improve and upgrade the material of the core, the essence of thermal runaway mainly lies in the stability of the positive and negative electrode materials and electrolyte.
In the future, we also need to make higher breakthroughs in cathode material coating and modification, compatibility of homogeneous electrolyte and electrode, as well as improving the thermal conductivity of the core. Or choose the electrolyte with high safety to play the effect of flame retardant.
Secondly, it is necessary to adopt efficient thermal management solutions from the outside to suppress the temperature rise of Li-ion battery, so as to ensure that the SEI film of the cell will not rise to the dissolution temperature, and naturally, thermal runaway will not occur.
But in case it reaches the battery thermal runaway stage, the battery pack will burn violently or even explode. At this time, the internal protection of the battery pack should be carried out, so as to further prevent the occurrence of thermal spread phenomenon.
Battery thermal runaway protection can be done at two levels: module and pack. In the module, the heat transfer from the runaway cell to the surrounding cells can be slowed down by placing aerogel mats, mica flame retardant materials and ceramic insulation mats between the cells, and also by placing phase change materials inside the module, which can be used to discharge the heat generated by the runaway cell by using the heat absorption and vaporization property of the phase change materials.
In addition, when the battery thermal runaway occurs, a large number of short circuits will occur inside the cell, resulting in a sharp drop in voltage, which can be collected by voltage, in addition to the rate of temperature rise, the rapid change in air pressure inside the battery pack, smoke sensors, etc. to sense the phenomenon.
At present, the industry has generally figured out the mechanism of the occurrence of thermal runaway battery. Future research is more focused on the safety of the battery itself, thermal management, early prediction and warning of thermal runaway in the battery, late notification and transmission of obstruction.
With the scientists’ exploration, the thermal runaway problem of battery will get more comprehensive solutions in the near future.