The safety of lithium ion batteries refers to their ability to suppress initial external disturbances and cause unsafe behavior during normal use or abuse. Cylindrical lithium ion batteries have a higher specific energy, and the battery shell is made of steel. When the battery experiences abnormal heat loss, the internal heat of the battery accumulates, generating higher pressure, and may explode, causing harm to the outside world and users. Due to differences in control panels and customer understanding of the battery, battery abuse is inevitable in actual use. Therefore, especially for cylindrical lithium-ion batteries, the safety of the battery is not only to prevent smoke, fire, liquid leakage, and explosion under various standard tests, but also to avoid personal injury caused by the above problems in the case of customer abuse.
The main production and technical advantages of cylindrical lithium ion batteries are still controlled by Japanese and Korean enterprises. Domestic enterprises that manufacture cylindrical lithium ion batteries also have a certain scale, but in terms of product performance, in addition to a certain lag in capacity, the biggest disadvantage lies in the safety performance of batteries. The competition between domestic cylindrical lithium ion battery manufacturers, as well as between domestic and foreign enterprises, is gradually pushing the cylindrical lithium ion battery industry into the era of refined production.
1. Safety evaluation of cylindrical lithium batteries
The current battery safety standards mainly include the United States UL1642 test standard, the International Electrotechnical Commission IEC test standard, Japan JIS, China GB, and other general standards. For cylindrical lithium-ion batteries, due to their technical advantages in Japanese enterprises and market advantages in Europe and the United States, there are mainly two standards that are favored in the industry: JIS and UL. Regardless of which standard, batteries are required to:
1) not exhibit abnormalities such as smoke, fire, or fluid leakage during normal customer use;
2)In the case of customer abuse, there should also be no abnormalities such as smoke, fire, liquid leakage, etc;
3) Special requirements for cylindrical batteries. Due to the fact that cylindrical lithium batteries are usually used with a large number of single batteries, which requires higher battery consistency, some customers also propose special performance tests such as deep-sea investment. Due to the use of a steel shell as the outer shell of a cylindrical lithium battery, if there is an explosion, it may cause significant harm to the human body. Therefore, the safety requirements for cylindrical lithium batteries will be higher. Even if the battery catches fire in extreme situations such as fire, no solid substances can fly out, to ensure the personal safety of users.
For cylindrical lithium-ion batteries, due to the steel shell, when the battery explodes, it can cause significant harm to the human body. Therefore, special attention should be paid to projectile testing. According to UL requirements, during the test, a fully charged battery is placed on a metal mesh, covered with a 20-hole steel mesh screen, and then heated until the battery catches fire or is destroyed. It is required that no solid particles rush out after the battery is burned, causing the mesh screen to be broken. Due to the contraction of the battery membrane under heating, the positive and negative electrode plates inside the battery lose isolation of the protective layer, resulting in a large area of short circuit.
At this time, the battery is in a fully charged state, and the battery system accumulates the highest chemical energy. A battery fire is inevitable; However, considering that the customer is likely to be near the battery when using it, it is necessary to control that no solid hard objects are ejected during the ignition process. In this state, to ensure that the battery does not explode despite fire, it requires a more reasonable design of the battery terminal components. High level terminal design can provide a smooth gas discharge channel when battery short circuit occurs to a certain extent, eliminating the continuous accumulation of heat, thereby ultimately avoiding battery explosion.
2. Analysis of factors affecting safety
In summary, safety is very important for cylindrical lithium batteries. To ensure the safety needs of terminal customers, research and development personnel need to make certain responses in battery design. Mainly reflected in the following points:
2.1 Battery material
Thermal runaway is an important factor causing potential safety hazards in cylindrical lithium batteries, and the occurrence of thermal runaway is closely related to battery materials. During the process caused by thermal runaway of a lithium battery, the SEI film decomposition provides initial heat accumulation, and thereafter the negative electrode/electrolyte decomposition reaction is connected to reach the temperature of the positive electrode/electrolyte decomposition reaction.
Subsequently, the decomposition reaction of the electrolyte ultimately leads to the occurrence of thermal runaway. Therefore, blocking any step in the process can improve the thermal stability of the battery. Improving the thermal stability of the SEI film through interface modification and controlling the reaction temperature range of the positive and negative electrode materials through material optimization can reduce the occurrence of thermal runaway of the battery and improve the overall safety of the battery.
2.2 Process control
The consistency of batteries is a prerequisite for the safety of series and parallel connection. To achieve battery consistency, production must be carefully controlled. Omissions in the details of production processes often cause serious safety issues.
2.3 Battery design
Reasonable battery design can improve the safety performance of batteries in the event of abuse. For cylindrical lithium-ion batteries, the safety design of the battery mainly considers the following two points:
(1)Selection of electrolyte additives. Overcharging is one of the common conditions of battery abuse. When the battery is overcharged, the electrolyte oxidizes and decomposes, resulting in a large amount of heat, leading to a sharp increase in the internal pressure and temperature of the battery, which poses a potential explosion hazard for cylindrical batteries. The research on improving overcharge safety by adding anti overcharge additives to electrolyte is gradually deepening.
(2) Low internal resistance design. The internal resistance of the battery refers to the resistance experienced by the current flowing through the battery during operation. Due to large internal resistance, the battery will generate a large amount of heat during operation, causing the battery temperature to rise, resulting in a decrease in the battery discharge operating voltage and a shortened discharge time, which will have a serious impact on battery performance and life, and even cause safety accidents such as explosions. The internal resistance of the battery can be reduced and the safety of the battery can be improved by optimizing the ratio and adjusting the internal layout.