According to public information, there were 8 fire accidents in the new energy battery vehicles in China from January to June 2018, which was basically the same as in 2017. Among them, 7 accidents occurred after May, indicating that the new energy vehicles are mainly concentrated in the summer. The fire caused by charging accounts for half of the fire, and it becomes the first cause. Secondly, it is self-igniting during collision and driving. The battery may be in a fire accident during standing and use. From the perspective of molding and brand, Tesla, Jiangling, etc. Pure electric vehicles are on the list. Since August and September, there have been as many as 12 incidents of fire, which is much higher than the number of fires in the previous two years.
January-June 2018 electric vehicle fire statistics
Analysis of the cause of ignition of lithium ion battery
As a source of energy for pure electric vehicles, the cause of ignition of lithium-ion batteries is mainly due to thermal runaway caused by overheating of the battery, which is most likely to occur during battery charging and discharging.
Since the lithium ion battery itself has a certain internal resistance, it will generate a certain amount of heat while outputting electric energy, and its own temperature becomes high. When the temperature exceeds its normal working temperature range, it will damage the life and safety of the whole battery.
(1) Mechanical abuse
Mainly occurs in the collision of a car. Due to the external force, the lithium battery cell and the battery pack are deformed, and the relative displacement of the different parts of the battery occurs, causing the battery separator to be torn and an internal short circuit; the flammable electrolyte leakage eventually causes a fire.
In mechanical abuse, the puncture injury is the most serious. It may insert the conductor into the battery body, causing the positive and negative poles to be directly short-circuited. In contrast, collision, extrusion, etc., only probabilistic internal short-circuit; the heat generation during the puncture process is more Intense, the probability of causing thermal runaway is higher.
(2) Electrical abuse
Electrical abuse is mainly caused by improper use of the battery. There are several types of external short circuit, overcharge and over discharge. Among them, the transient discharge causes the least harm, but the growth of copper dendrites due to overdischarge will reduce the safety of the battery. Sexuality increases the chance of thermal runaway.
An external short circuit is the result of two conductors with a differential pressure being connected outside the cell. When an external short circuit occurs, the heat generated by the battery does not dissipate well, and the battery temperature rises. out of control.
Overcharging is one of the most harmful of electrical abuse. Lithium dendrites grow on the surface of the anode due to excessive lithium intercalation. Second, excessive deintercalation of lithium causes the cathode structure to collapse due to heat and oxygen release (oxygen release from the NCA cathode). .
The release of oxygen accelerates the decomposition of the electrolyte and generates a large amount of gas. Due to the increase of internal pressure, the exhaust valve opens and the battery begins to vent. After the active material in the battery contacts the air, it reacts violently and releases a large amount of heat. Cause the battery pack to burn and catch fire.
(3) Heat abuse
Heat abuse mainly refers to local overheating in the battery, rarely existed independently, often developed through mechanical abuse and electrical abuse, and is a situation that eventually directly triggers accidents such as thermal runaway.
Thermal abuse is generally caused by a high external environment or a short circuit caused by excessive heat of the battery caused by the temperature control system not functioning, thereby causing thermal runaway.
For reasons of reason, the cause of heat abuse is the most complicated, battery pack collision, damage, internal structure of the battery, performance or other thermal management systems, failure of the air conditioning system may lead to heat abuse.
(4) Internal short circuit
The internal short circuit is directly contacted by the positive and negative terminals of the battery. Of course, the degree of contact is different, and the subsequent reactions caused by the battery are also very different. The large-scale internal short circuit caused by mechanical and thermal abuse will directly lead to heat abuse.
The cause of the internal short circuit is also complicated. For example, the lithium ion battery is overcharged, and the dendrite accumulates to a certain extent, causing the battery to be pierced, resulting in an internal short circuit or collision. After the puncture damage, the positive and negative electrodes are directly contacted, resulting in thermal runaway.
Compared with the internal short circuit generated by external factors, the internal short circuit caused by the spontaneous defects in the battery manufacturing process is relatively mild. The congenital internal short circuit generates little heat and does not immediately trigger the thermal runaway. Defects will evolve over a period of time to a lesser degree of internal short circuit.
The main method to solve the thermal runaway of power battery
In view of the thermal runaway of lithium-ion batteries, the current domestic mainstream solutions are mainly improved from external protection and internal improvement. External protection mainly refers to system upgrades, and internal improvement is the improvement of the battery itself.
(1) Improvement of cooling method
The thermal management system is mainly responsible for controlling the temperature to ensure that the battery is always at a reasonable operating temperature. Usually, the thermal management system is controlled by the vehicle controller. When the battery pack temperature is abnormal, the air conditioning system performs heat dissipation or heating in time to ensure the battery. Safety and longevity.
The heat dissipation method of the battery is divided into four according to the heat conduction method and the medium: air cooling (air cooling), liquid cooling (water cooling), phase change material (solid), and combined cooling (air cooling / water cooling + solid cooling) Kind.
(2) Improvement of internal materials and structures
The internal improvement is to modify the material structure inside the battery core, so that the battery has better heat resistance and heat dissipation performance. In the current research hotspot, develop solid electrolyte; structural modification of positive and negative electrodes; More safe diaphragm materials are one of the mainstream methods for improving the thermal performance of batteries from the inside.
