Analysis of the cause of ignition of lithium ion battery
As a source of energy for pure electric vehicles, the main cause of ignition of lithium-ion batteries is the 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, when the output electric energy is powered by pure electric power, a certain amount of heat is generated, so that its own temperature becomes high, and when the temperature exceeds its normal operating temperature range, the entire battery will be damaged. Life and safety. In a pure electric vehicle, the power battery system is composed of a plurality of power battery cells. During the working process, a large amount of heat is generated in a small battery box. If the heat cannot be quickly dissipated in time, the high temperature will affect the power. The battery life may even be out of control, causing accidents such as fire and explosion. In principle, there are four main reasons for the loss of control:
(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, puncture damage is the most serious, it may insert a conductor into the battery body, causing a direct short circuit between the positive and negative electrodes. In contrast, collision, extrusion, etc., only probabilistic occurrence of internal short circuit, the generation of heat in the puncture process is more intense, and the probability of causing thermal runaway is higher.
(2) Electrical abuse
Electrical abuse is mainly caused by improper use of the battery, and 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 over-discharge will reduce the safety of the battery and increase the probability of thermal runaway. The external short circuit is the result of the connection of two conductors with differential pressure outside the cell. When the external short circuit occurs, the heat generated by the battery cannot be dissipated well, and the battery temperature will rise accordingly. 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, producing a large amount of gas. As the internal pressure increases, the exhaust valve opens and the battery begins to vent. After the active material in the cell contacts the air, a violent reaction occurs, and a large amount of heat is released, thereby causing combustion of the battery pack.
(3) Thermal 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 ultimately triggers accidents such as thermal runaway. Thermal abuse is generally caused by a short circuit caused by a high external environment or a high battery heat caused by the temperature control system not functioning, thereby causing thermal runaway. For reasons, the cause of heat abuse is the most complicated. The collision and damage of the battery pack, the internal structure of the battery, the performance or the failure of other thermal management systems and air conditioning systems may lead to heat abuse.
(4) Internal short circuit
The internal short circuit is directly contacted by the positive and negative electrodes of the battery. Of course, the degree of contact is different, and the subsequent reactions caused are also very different. Large-scale internal short circuits usually caused by mechanical and thermal abuse will directly lead to thermal 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 separator to penetrate. Therefore, an internal short circuit or a collision or a puncture injury directly causes the positive and negative electrodes to contact, 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, and the heat generated by the inherent internal short circuit is small, and the thermal runaway is not immediately triggered. Moreover, this inherent defect will evolve over a period of time to a lesser degree of internal short circuit.
From : Energy storage Website of China