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In recent years, with the continuous growth of the number of EV in China, the frequency of EV fire accidents has also increased year by year. The cause of the fire is a variety of factors.
Although the flame temperature of lithium-ion batteries is low when burning, when battery fire is completely extinguished, the battery temperature does not quickly drop to a safe level, and a large number of toxic gases are produced.
In this article, the fire mechanism is elaborated, and the changes of the main characteristic parameters during thermal runaway are analyzed. Finally, some suggestions are provided for the development of car battery safety in the future. This article aims to provide a basis for car battery safety.
1. Background to the importance of car battery safety
According to news reports, fire accidents at charging stations or charging piles ranked first with about 26%, including direct fires, charging equipment failures, overcharging and other reasons caused by fires during charging.
In recent years, fire accidents caused by charging problems have been particularly frequent, and also the top 15 power battery companies in the world all come to the conclusion that we should pay attention to the car battery safety during charging.
In addition to accidents caused by charging, fires during shutdowns also account for a high proportion, about 13%. When the vehicle is switched off and stopped, most car fires are caused by short circuits in the batteries.
Internal short circuits do not necessarily produce immediate thermal runaway. However, when the vehicle leaves the charging point or the operation stops, the operating temperature continues to increase as the internal short circuit intensifies and continues to generate heat.
Once the temperature exceeds the thermal runaway threshold, thermal runaway occurs. Therefore, this has higher requirements for car battery safety in the state of shutdown and power failure.
2. Internal short circuit
The working conditions of on-board power batteries are very complex. High current charge and discharge, high and low temperature operating environment, rain immersion, vibration and even overcharge and overdischarge are all easy to induce internal short circuits.
Internal short circuit refers to the destruction of the internal separator, the cathode and anode contact each other, forming a potential difference, resulting in continuous discharge and heat generation. Internal short circuit is considered to be the main cause of spontaneous combustion of power batteries.
Mechanical misuse, thermal misuse, electrical misuse, mechanical fabrication are the main trigger forms of internal short circuit.
- Mechanical misuse is a component of external forces that usually occur in accidents.
- Thermal misuse refers to the internal short circuit caused by direct contact between the cathode and the anode in a high heat state.
- Battery overheating can be caused by mechanical misuse, electrical misuse, and connector connection point failure, and connector contact loss will lead to an increase in resistance within pack, which in turn leads to local overheating.
Power abuse is caused by overcharging or overdischarging the battery, which is not a short-term process. During the evolution of internal short circuits, the internal temperature of the battery increases.
This leads to complex chemical reactions between electrodes, electrolytes and separators, mostly exothermic reactions, which further aggravates the severity of internal short circuits. When a large area of the battery is short-circuited, the electrical energy is released at a high rate, and the temperature rises sharply and causes thermal runaway.
3. External short circuit
The external short circuit may be triggered by accidental water leakage or oil stains entering the battery pack, battery deformation due to external force, loose connection wire of the connecting plate caused by vibration.
Because the resistance of the connection is small, it will cause the current to be extremely high during discharge and the battery temperature to rise rapidly, which will cause thermal runaway and car battery safety problems.
4. Overcharge mechanism
Under charging conditions, some pouch cells will continue to charge after exceeding the cut-off voltage of charging, resulting in an overcharge phenomenon.
During the overcharge process, dendrite is formed in the anode separator of the battery, which has an accelerated effect on the exothermic reaction in the batteries and the car battery safety.
The continuous growth of dendrites between the diaphragms will aggravate the phenomenon of micro-short circuits. Causing the temperature of the cell to rise rapidly and will cause a series of side reactions, further affecting the car battery safety.
5. Overdischarge mechanism
Because the inconsistency of this battery is inevitable, when the voltage of a cell is lower than the specified discharge cut-off voltage, it continues to discharge, resulting in overdischarge of the batteries.
The cell with the lowest voltage in the batteries pack will be forcibly discharged by other cells connected in series when overdischarged, and the electrode may be reversed during the forced discharge process. In this process, the car battery voltage increases, and the over-discharge cell will produce abnormal heat generation.
Overdischarge causes this batteries capacity to decrease, and the solid electrolyte interface (SEI) of the anode breaks down. When the faulty monomer is charged again, a large amount of lithium ions will be lost, and these losses are irreversible and will also seriously affect car battery safety.
In addition, in the state of overdischarge, the anode potential continues to rise. In the high potential state, the deposition of the current collector on the surface of the anode, and the binding force between the active material and the current collector and the anode current collector's ability to transfer electrons will be damaged, reduce car battery safety.
6. Battery thermal runaway
When the battery thermal runaway is often accompanied by a large number of physical and chemical reactions, such as SEI membrane decomposition, anode active material reaction with electrolyte, cathode active material reaction with electrolyte, electrolyte decomposition, anode active material and adhesive reaction. It may be that multiple chemical reactions occur together.
At high temperature, the capacity of the lithium-ion battery is attenuated, the internal resistance of the batteries increase, and the increased value of the anode resistance is higher than that of the cathode.
The cathode and electrolyte will generate a series of chemical reactions to produce a large amount of gas. The gas produced quickly collects and expands, and the pressure inside the batteries rises rapidly, forming an injection when the internal pressure exceeds the car battery safety standard.
7. Measures to improve car battery safety
① Improve battery materials
The cathode and anode materials, separators, electrolytes of the batteries are all factors that affect the car battery safety. Since the anode material of the car battery is of importance, many companies like the top 10 lithium ion battery anode material companies have set up the plan.
When the temperature in the batteries show signs of abnormal rise, it will cause the separator to shrink and melt, resulting in electrolyte leakage, resulting in internal short circuit and thermal runaway.
The general material of the batteries separators are polypropylene or polyethylene, and its high temperature resistance is relatively poor. Polyimide material has strong heat resistance, can be used as a cell separator substrate, or added inorganic nano coating to the separator, can effectively improve the high temperature resistance and stability of the separator.
Melamine-based porous organic polymer composite separator has strong flame retardancy, forming a good expansion of protective carbon between the cathode and anode in the event of fire, increasing internal resistance and enhancing the car battery safety.
Electrolyte combustion will cause great harm and risk, and the corresponding flame retardant additives can be added to the electrolyte to reduce the risk of combustion. When the batteries overheat, it is accompanied by a large number of side reactions. Flame retardant additives break down phosphorus-containing radicals at high temperatures, which react with them to improve car battery safety.
② Improvement of battery pack housing
When an electric vehicle has a mechanical collision, the batteries are crushed and collided, or when the electric vehicle catches fire, the batteries housing is critical to the mechanical protection of the batteries pack.
Enhance the flame retardant performance of the shell, and the high flame retardant shell has the function of suppressing the spread of fire. Improving the sealing performance of the battery has a significant impact on preventing the batteries pack from flooding, gas leakage and combustion diffusion.
A flame retardant coating is applied to the surface of the housing to enhance the flame resistance of the batteries enclosure. Fireproof felt materials can be added between modules, cells and batteries housing to effectively block the chain reaction between single cells, and improve fire performance.
High-strength materials can also be used in the structure of the batteries case to absorb the force of partial collision, protect the batteries pack from damage, and reduce the risk of short circuit caused by mechanical collision.
③ Improved cooling system
The car battery pack is composed of several single cells connected in series and parallel, and the single battery of the same specification model may have inconsistencies after forming the battery pack, so the heat generated by the battery is also different.
Under the current working conditions, the inconsistency of the heat dissipation conditions of each cell will aggravate the inconsistency of the batteries pack, which will affect the charge and discharge rate and capacity decay of the cell.
Therefore, improving the heat dissipation system is of great help to improve the safety of car batteries. Phase-change heat dissipation material is used. The morphology of the phase change material changes with temperature, and a large amount of energy is absorbed or released during the phase change to ensure that the temperature of the battery pack is constant.
Reasonable structure of gas pressure relief valve. It can discharge the gas generated inside in time and control the air pressure in the batteries pack. Design fast diffusion channels for flammable gases. It makes flammable gases more effective in discharging the pack, reducing the chain reaction caused by gas and heat generated affecting adjacent batteries.
8. Conclusion
This article talks about car battery safety issues analysis and how to improve. The causes of fire were analyzed, focusing on the mechanisms of internal short circuit, external short circuit, overcharge and overdischarge.
Trigger forms such as mechanical abuse, electrical abuse, and thermal abuse are explained in detail. In terms of car battery safety recommendations, we will improve battery materials, battery case housings, and heat dissipation systems.
At present, car battery safety is still a research hotspot in the future. With the establishment of big data platforms, there will be better technology to detect car battery safety.
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