In recent years, the new energy automobile industry has developed rapidly, and safety issues, as the foundation of the healthy development of the industry, have also received increasing attention from the industry. With the explosive growth of new energy vehicles, the number of new energy vehicles in China has exceeded 2 million, and safety issues have become more sensitive. If the occurrence of security accidents cannot be effectively controlled, the development of the new energy vehicle industry will be affected.
According to the information from the General Administration of Market Supervision and Administration, more than 40 new energy vehicle fires have occurred in China since 2018. And most new energy vehicle safety accidents are due to problems with power batteries. In the industry, there is a professional saying that a power battery fails, which is called battery failure. Today, I will take everyone to understand what is the failure of lithium-ion batteries.
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Lithium-ion battery failure refers to the degradation of battery performance or abnormal use performance caused by some specific essential reasons.It may occur in any link in production, transportation, and use.It will not only affect battery performance, but even cause fire, Safety issues such as explosions. Lithium battery failure can be divided into performance failure and safety failure according to the type of impact. The main reasons for its occurrence are also divided into two types, internal and external causes: internal causes include physical failure and chemical changes, and external causes include high temperature, impact, acupuncture, and man-made damage. The following will introduce in detail several common lithium battery failures, including capacity attenuation failure, internal short circuit, internal resistance increase, thermal runaway, etc.
Capacity decay failure
Generally, when performing a standard cycle life test, after the number of cycles reaches 500, the battery capacity should not be lower than 90% of the initial value, and after 1,000 times, it should not be lower than 80% of the initial value. If the capacity does not meet the standard The phenomenon of excessive attenuation is a capacity attenuation failure. The capacity decay failure of lithium-ion batteries is divided into reversible capacity decay and irreversible capacity decay. Among them, the reversible attenuation can restore the lost capacity through methods such as adjusting the battery’s charge and discharge system and improving the battery’s use environment. The irreversible attenuation is caused by irreversible changes inside the battery, which leads to irrecoverable capacity loss, so it cannot be rescued.
The main reason for battery capacity degradation is material failure, which is also inextricably linked to objective factors such as the battery manufacturing process and the use environment. From a material point of view, the causes of capacity attenuation failure are the failure of the positive electrode material, the SEI transition production on the negative electrode surface, the failure of the electrolyte, the failure of the current collector, and so on.
Short circuit
Internal short-circuits often cause self-discharge, capacity degradation, local thermal runaway and safety accidents of lithium-ion batteries. During a short circuit inside the battery, the two electrode materials are electronically interconnected internally, resulting in a locally high current density. Internal short circuits in lithium-ion batteries may be caused by lithium dendrite formation or compression shock. A long internal short circuit will cause self-discharge and local temperature rise. The effect of local temperature rise is very significant, because if the temperature exceeds a certain threshold, the electrolyte may begin to decompose through an exothermic reaction, causing thermal runaway, which has potential health and Security risks.
Increased internal resistance
The internal resistance of a lithium-ion battery is related to the internal electron transmission and ion transmission process of the battery. It is mainly divided into ohmic resistance and polarization internal resistance. The polarization internal resistance is mainly caused by electrochemical polarization, and it is divided into electrochemical polarization and concentration pole. Into. When the internal resistance of the battery is increased, failure problems such as reduced energy density, reduced voltage and power, and heat generation of the battery are also accompanied. The main factors affecting its production are the battery key materials and the battery use environment, but the abnormality of key materials is the fundamental influencing factor of the increase in internal resistance.
Thermal runaway
“Thermal runaway” is a positive energy feedback loop process: an increased temperature will cause the system to become hot, and the system will become hotter and increase the temperature, which in turn will make the system hotter. Lithium battery thermal runaway refers to the rapid rise of local or overall temperature inside the battery, and the heat cannot be dissipated in time, and a large amount of it accumulates inside, and induces further side reactions. Participating in the “thermal runaway” reaction are cobalt oxide chemicals in lithium batteries. When this chemical is heated to a certain temperature, it begins to self-heat and then develops into fire and explosion. In some cases, this organic electrolyte release pressure can cause the battery to rupture. It may also burn if exposed to high temperatures or if it encounters sparks. In order to prevent the occurrence of thermal runaway, measures such as PTC, safety valve, and thermal film are generally used, but it is more important to improve the technology of battery design, manufacture, and use.