What is lithium ion battery failure?

Lithium ion battery failure refers to the battery performance degradation or abnormal service performance caused by some specific essential reasons. It may occur in any link of production, transportation and use, which will not only affect the performance of the battery, but also cause safety problems such as fire and explosion.

Capacity attenuation failure

Generally, in the standard cycle life test, after 500 cycles, the battery capacity shall not be lower than 90% of the initial value, and after 1000 cycles, it shall not be lower than 80% of the initial value. If the capacity does not meet the standard and excessive attenuation occurs, it belongs to capacity attenuation failure. The capacity attenuation failure of lithium ion battery can be divided into reversible capacity attenuation and irreversible capacity attenuation. Reversible attenuation can recover the lost capacity by adjusting the charging and discharging system of the battery and improving the service environment of the battery. Irreversible attenuation cannot be saved because irreversible changes have taken place in the battery, resulting in unrecoverable capacity loss.

The main reason of battery capacity attenuation lies in material failure, which is also inseparable from objective factors such as battery manufacturing process and service environment. From the perspective of materials, the causes of capacity attenuation failure include cathode material failure, SEI transition production on the surface of cathode, electrolyte failure, collector failure and so on.

Internal short circuit

Internal short circuit often causes self discharge, capacity attenuation, local thermal runaway and safety accidents of lithium-ion batteries. During the short circuit inside the battery, the two electrode materials are interconnected electronically, resulting in local high current density. The internal short circuit in lithium-ion battery may be caused by the formation of lithium dendrite or compression shock. Long time internal short circuit will lead to self discharge and local temperature rise. The impact of local temperature rise is very significant, because if the temperature exceeds a certain threshold, the electrolyte may begin to decompose through exothermic reaction, resulting in heat out of control, which has potential health and safety hazards.

Increased internal resistance

The internal resistance of lithium-ion battery is related to the internal electron transport and ion transport process of the battery, which is mainly divided into ohmic resistance and polarization internal resistance. The polarization internal resistance is mainly caused by electrochemical polarization, which is also divided into electrochemical polarization and concentration polarization. When the internal resistance of the battery increases, there are also failure problems such as the decrease of energy density, the decrease of voltage and power, and the heat production of the battery. The main factors affecting its generation are the key materials of the battery and the service environment of the battery, but the abnormality of the key materials is the fundamental factor affecting the increase of internal resistance.

Thermal runaway

"Thermal runaway" is a positive feedback cycle of energy: the rising temperature will cause the system to heat up, and the system will heat up and raise the temperature, which in turn makes the system hotter. Thermal runaway of lithium battery refers to the rapid rise of local or overall temperature inside the battery, the heat can not be dissipated in time, a large amount of heat accumulates inside, and induces further side effects. Cobalt oxide chemicals in lithium batteries are involved in the "thermal runaway" reaction. When the chemical is heated to a certain temperature, it begins to heat itself, and then develops into fire and explosion. In some cases, the release of pressure from this organic electrolyte can lead to battery rupture. It may also burn if exposed to high temperatures or sparks. In order to prevent the occurrence of thermal runaway, PTC, safety valve, thermal conductive film and other measures are generally adopted, but it is more important to improve the design, manufacturing technology and application methods of the battery.

Gas production

There are two kinds of gas production of lithium-ion battery, namely normal gas production and abnormal gas production. The gas production phenomenon caused by consuming electrolyte to form a stable SEI film in the process of battery formation is normal gas production, and the phenomena such as excessive consumption of electrolyte to release gas or oxygen release of cathode material are abnormal gas production. After the lithium battery is assembled, a small amount of gas will be produced in the pre formation process. These gases are inevitable and are also the source of the so-called irreversible capacity loss of the cell. In the first charge and discharge process, after the electrons reach the negative electrode from the external circuit, they will have redox reaction with the electrolyte on the surface of the negative electrode to generate gas.

Lithium evolution

During the charging process of lithium-ion battery, lithium ions will be de embedded from the positive electrode and embedded into the negative electrode. However, when an abnormal condition occurs that the lithium ions de embedded from the positive electrode cannot be embedded into the negative electrode, lithium ions can only precipitate on the surface of the negative electrode and form a layer of gray material, which is lithium precipitation. There are many reasons for lithium precipitation, including insufficient negative electrode margin, uneven coating surface of positive and negative electrodes, charging in low temperature environment, large rate charging and so on. At present, lithium evolution failure is mainly inhibited by adding electrolyte additives, artificial SEI, high salinity electrolyte, structured negative electrode and optimizing battery structure.

Summary: lithium battery failure can be divided into performance failure and safety failure according to different impact types. There are also two main reasons for its occurrence, namely internal and external causes: internal causes include physical failure, chemical changes, etc., and external causes include high temperature, impact, acupuncture, man-made damage, etc. strict production and high-quality raw materials can avoid such situations to the greatest extent, so there is no need to panic.