Risk and safety technology of lithium ion battery (1)

1. Risk of lithium ion battery

Lithium ion battery is a potentially dangerous chemical power source due to its chemical characteristics and system composition.


(1)High chemical activity

Lithium is the main group I element in the second period of the periodic table, with extremely active chemical properties.


(2) High energy density

Lithium ion batteries have very high specific energy (≥ 140 Wh/kg), which is several times that of nickel cadmium, nickel hydrogen and other secondary batteries. If thermal runaway reaction occurs, high heat will be released, which will easily lead to unsafe behavior.


(3) Adopt organic electrolyte system

The organic solvent of organic electrolyte system is hydrocarbon, with low decomposition voltage, easy oxidation and flammable solvent; In case of leakage, the battery will catch fire, even burn and explode.


(4) High probability of side effects

In the normal use process of lithium ion battery, the chemical positive reaction of mutual conversion between electrical energy and chemical energy takes place in its interior. However, under certain conditions, such as overcharging, over discharging or over current operation, it is easy to cause chemical side reactions inside the battery; When the side reaction is aggravated, it will seriously affect the performance and service life of the battery, and may produce a large amount of gas, which will cause explosion and fire after the pressure inside the battery increases rapidly, leading to safety problems.


(5)The structure of electrode material is unstable

The overcharge reaction of lithium ion battery will change the structure of the cathode material and make the material have a strong oxidation effect, so that the solvent in the electrolyte will have a strong oxidation; And this effect is irreversible. If the heat caused by the reaction accumulates, there will be a risk of causing thermal runaway.


2. Analysis of safety problems of lithium ion battery products

After 30 years of industrial development, lithium-ion battery products have made great progress in safety technology, effectively controlled the occurrence of side reactions in the battery, and ensured the safety of the battery. However, as lithium ion batteries are used more and more widely and their energy density is higher and higher, there are still many incidents such as explosion injuries or product recalls due to potential safety hazards in recent years. We conclude that the main reasons for the safety problems of lithium-ion battery products are as follows:


(1) Core material problem

The materials used for the electric core include positive active materials, negative active materials, diaphragms, electrolytes and shells, etc. The selection of materials and the matching of the composition system determine the safety performance of the electric core. When selecting positive and negative active materials and diaphragm materials, the manufacturer did not conduct a certain assessment on the characteristics and matching of raw materials, resulting in a congenital deficiency in the safety of the cell.


(2) Production process problems

The raw materials of the cell are not strictly tested, and the production environment is poor, leading to impurities in the production, which is not only harmful to the capacity of the battery, but also has a great impact on the safety of the battery; In addition, if too much water is mixed in the electrolyte, side reactions may occur and increase the internal pressure of the battery, which will affect safety; Due to the limitation of the production process level, during the production of the electric core, the product cannot achieve good consistency, such as the poor flatness of the electrode matrix, the falling off of the active electrode material, the mixing of other impurities in the active material, the insecure welding of the electrode lug, the unstable welding temperature, the burrs on the edge of the electrode piece, and the absence of the use of insulating tape in key parts, which may adversely affect the safety of the electric core.


(3)The design defect of the electric core reduces the safety performance

In terms of structural design, many key points that have an impact on safety have not been paid attention by the manufacturer. For example, there is no insulating tape at the key parts, no margin or insufficient margin is left in the diaphragm design, the design of the capacity ratio of positive and negative electrodes is unreasonable, the design of the area ratio of positive and negative active substances is unreasonable, and the design of the lug length is unreasonable, which may lay hidden dangers to the safety of the battery. In addition, in the production process of the cell, some cell manufacturers try to save and compress raw materials in order to save costs and improve performance, such as reducing the area of the diaphragm, reducing copper foil, aluminum foil, and not using the pressure relief valve or insulating tape, which will reduce the safety of the battery.


(4)Too high energy density

At present, the market is in pursuit of battery products with higher capacity. In order to increase the competitiveness of products, manufacturers continue to improve the volume specific energy of lithium ion batteries, which greatly increases the risk of batteries.

Post time: Nov-06-2022