This paper decomposes studying on the failure mode of power lithium-ion battery system is of great significance to improve the battery life, the safety and reliability of electric vehicles and reduce the service cost of electric vehicles. Various failure modes are considered in the design of power lithium-ion battery system to improve the safety of power lithium-ion battery.

Power lithium ion battery system is usually composed of cell, battery management system and pack system, including functional components, harness, structural parts and other related components. The failure modes of power lithium-ion battery system can be divided into three different levels, namely cell failure mode, battery management system failure mode and pack system integration failure mode.

Cell failure mode

The failure modes of cell can be divided into safety failure mode and non safety failure mode. The following points are important for the safety failure of the cell:

1.Short circuit between positive and negative electrodes inside the cell:

The short circuit in the battery is caused by the inside of the cell. There are many reasons for the short circuit in the battery, which may be caused by defects in the cell processing process or by the deformation of the cell caused by long-term vibration and external force. Once a serious internal short circuit occurs, the control cannot be stopped, and the external insurance cannot be used, smoke or combustion will certainly occur.

In case of such a situation, all we can do is to inform the personnel on board to escape at the first time. With regard to the internal short circuit of the battery, so far, the battery manufacturer has no method to screen out 100% of the cells that may have internal short circuit when leaving the factory, and can only do enough testing in the later stage to reduce the probability of internal short circuit.

2.Cell leakage:

This is a very dangerous and infrequent failure mode. Many electric vehicle fire accidents are caused by battery leakage. The causes of battery leakage are: external force damage; The sealing structure is damaged due to collision and non-standard installation; Manufacturing reasons: poor sealing performance caused by welding defects and insufficient amount of sealing glue.

After battery leakage, the insulation of the whole battery pack fails, and the single point insulation failure is not a big problem. If there are two or more insulation failures, external short circuit will occur. From the actual use, compared with the plastic case cell, the metal case monomer is more prone to liquid leakage, resulting in insulation failure.

3.Lithium evolution of battery negative electrode:

Improper use of the battery, overcharge, low-temperature charging and high current charging will lead to lithium evolution from the negative electrode of the battery. Lithium iron phosphate or ternary batteries processed by most domestic manufacturers will decompose lithium when charged below 0 ℃. Above 0 ℃, they can only be charged with small current according to the characteristics of the cell. After lithium precipitation from the negative electrode, the lithium metal cannot be reduced, resulting in irreversible attenuation of battery capacity. Lithium precipitation reaches a certain degree of severity, forms lithium dendrites, pierces the diaphragm and produces internal short circuit. Therefore, the power lithium-ion battery should not be charged at low temperature.

4.Cell inflation:

There are many reasons for flatulence. The most important reason is that gas occurs due to side reactions inside the battery. The most typical side reaction is with water. Flatulence can be prevented by strictly controlling moisture during cell processing. In case of battery inflation, liquid leakage will occur.

The non safety failure of the cell only affects the service performance. The following points are important:

1.Poor capacity consistency:

The inconsistency of power lithium-ion batteries usually refers to that the residual capacity difference and voltage difference of batteries in a group of batteries are too large, resulting in the deterioration of battery endurance. There are many reasons for the poor consistency between batteries, including the processing and manufacturing process of the battery, the storage time of the battery, the temperature difference during the charge and discharge of the battery pack, the charge and discharge current, etc.

At present, the key solution is to improve the control level of battery processing and manufacturing process, ensure the consistency of batteries as far as possible from the processing level, and use the same batch of batteries for matching. This method has a certain effect, but it can not be cured. The problem of poor consistency of battery pack will occur after it is used for a period of time. After the inconsistency of battery pack, if it can not be solved in time, the problem will become more serious and even dangerous.

2.Excessive self discharge:

The irreversible reaction caused by micro short circuit caused by impurities during battery manufacturing is the most important reason for the large self discharge of individual batteries. In most battery manufacturers, the self discharge micro hour of the battery can be ignored. Due to the chemical reaction with the environmental conditions during the long-term charging, discharging and shelving of the battery, the battery has a large self discharge phenomenon, which reduces the battery power, low performance and can not meet the use demand.

3.Low temperature discharge capacity reduction:

With the decrease of temperature, the low-temperature performance of electrolyte is poor, the participation in reaction is not enough, and the conductivity of electrolyte decreases, resulting in the increase of battery resistance, the decrease of voltage platform and the decrease of capacity. At present, the discharge capacity of batteries of various manufacturers at – 20 ℃ is basically 70% ~ 75% of the rated capacity. At low temperature, the battery discharge capacity is reduced and the discharge performance is poor, which affects the service performance and driving range of electric vehicles.

4.Battery capacity attenuation:

The attenuation of battery capacity mainly comes from the loss of active lithium ion and the loss of electrode active material. The regularity of the layered structure of the positive active material decreases, the passivation film is deposited on the negative active material, the graphitization degree decreases, and the porosity of the diaphragm decreases, resulting in the increase of the charge transfer impedance of the battery. The de intercalation capacity decreases, resulting in the loss of capacity.

Battery capacity attenuation is an unsuspecting problem of batteries. However, at present, battery manufacturers should first deal with the previous safety failure and battery consistency, and then consider extending the cycle life of the battery on this basis.

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