Safety analysis of Lithium ion Power Battery
Big data statistics:
According to public information, there were eight fire accidents in the first half of 2018 (January-June), which was almost the same as in 2017. Among them, 7 accidents occurred after May, indicating that the fire of new energy vehicles mainly occurred in summer.
From the perspective of molding and brand, pure electric cars, such as Tesla and Jiangling, are all on the list. However, according to the latest data, since August and September, there have been as many as 12 public fires, which is much higher than the number of public fires in the whole year of 2016 and 2017.
Because the main reason for electric vehicle fire is that the battery is overheated, heat can not be released in time caused by high temperature battery fire, so in high temperature, heavy rain and other weather frequent summer is also reasonable.
Based on the accident data of the first half of the year, from the point of view of fire causes, there were 5 fire accidents caused by charging, accounting for 50% of the total, which became the first cause of fire accidents.
Followed by collisions and spontaneous combustion of fire, each of two cases, each accounting for 20%. From the view of vehicle status, static and charging are classified, and fire accidents may occur in the static and use of batteries.
As the energy source of pure electric vehicle, the main cause of lithium-ion battery ignition is the thermal runaway caused by the overheating of the battery, which is most likely to occur in the process of battery charging and discharging. Lithium-ion batteries have a certain internal resistance, which can produce a certain amount of heat while outputting electric energy to provide power for pure electric power, making their own temperature higher. When their own temperature exceeds their normal operating temperature range, it will damage the life and safety of the battery. In the pure electric vehicle, the power battery system is composed of several power battery cells. During the working process, a large amount of heat accumulates in the narrow battery box. If the heat can not be released quickly in time, the high temperature will affect the life of the power battery and even lead to thermal runaway, thus causing fire and explosion accidents. In principle, the causes of thermal runaway are mainly in the following four aspects:
(1) mechanical abuse
It mainly happens in automobile collision, because of the effect of external force, the lithium battery monomer and battery pack are deformed, the relative displacement of different parts of the battery itself, resulting in tearing of battery diaphragm and internal short circuit; flammable electrolyte leakage eventually triggers fire. In the abuse of machinery, puncture injury is the most serious, it may lead to the conductor inserted into the battery body, resulting in a direct short circuit between positive and negative electrodes. In contrast, collision, extrusion and so on, but the probability of internal short circuit, puncture process heat generation is more intense, resulting in a higher probability of thermal runaway.
(2) electrical abuse
Electricity abuse is mainly caused by improper use of batteries, including external short circuit, overcharge and overdischarge. However, the growth of copper dendrite caused by overdischarge will reduce the safety of the battery and increase the probability of thermal runaway. External short circuit is the result of two conductors with voltage difference connected to the outside of the battery. When the external short circuit occurs, the heat generated by the battery can not be well dissipated, the battery temperature will also rise, high temperature triggered thermal runaway.
Overcharging is the most dangerous form of electrical abuse. Because of excessive lithium insertion, the lithium dendrite grows on the anode surface. Secondly, excessive lithium removal leads to cathode structure collapse due to heating and oxygen release (oxygen release from NCA cathode). The release of oxygen accelerates the decomposition of electrolytes and produces a large amount of gas. As the internal pressure increases, the exhaust valve opens and the battery starts to exhaust. After contacting with air, the active substance in the battery reacts violently, releasing a large amount of heat, which leads to the combustion of the battery pack.
(3) heat abuse
Heat abuse mainly refers to partial overheating in batteries, which rarely exists independently. It is often developed through mechanical abuse and electrical abuse, and it is a situation that eventually directly triggers thermal runaway and other accidents. Heat abuse is usually caused by the short circuit caused by the excessive heat of the battery caused by the high external environment or the failure of the temperature control system, which leads to thermal runaway. In terms of reasons, the causes of heat abuse are the most complicated. Collision and damage of battery packs, internal structure and performance of battery, or failure of other thermal management systems and air conditioning systems may lead to heat abuse.
(4) internal short circuit
Internal short circuit is a direct contact between the positive and negative electrodes of the battery, of course, the degree of contact, the subsequent reactions are also very different, usually by mechanical and heat abuse caused by large-scale internal short circuit will lead to direct thermal abuse. The causes of internal short circuit are also complex, such as over-charging of lithium-ion batteries, dendrite accumulation to a certain extent leading to piercing the battery diaphragm, resulting in internal short circuit or collision, piercing injury directly lead to positive and negative contact and lead to thermal runaway. Compared with the internal short circuit caused by external factors, the internal short circuit caused by the spontaneous defects in the battery manufacturing process is relatively mild, and the inherent internal short circuit generates little heat, and does not immediately trigger thermal runaway. Moreover, this inherent defect will evolve to a lighter internal short circuit after a period of time.