Analysis of Power Battery Safety Factors

In the field of new energy vehicles, lithium battery-powered batteries have been widely used because of their relatively high energy density and output power. However, the performance and life of lithium-ion power cells will decay with continuous use, and more importantly, there will be different conditions in different operating environments. For example, at cold and low temperatures, it is easy to have low capacity and serious decay. At high temperatures, there is a hidden danger that thermal runaway causes spontaneous combustion and spontaneous explosion.

These potential safety hazards have led to a lack of confidence among consumers of new energy vehicles. Therefore, it is very necessary to pay attention to the safety of lithium-ion power cells and carry out research and improvement. It is helpful to reduce the safety hazards of batteries and the frequency of accidents, and reduce or avoid the harm caused by the safety problems of power cells.

In simple terms, lithium-ion power cells are mainly composed of positive poles, negative poles, diaphragm, electrolytes, and battery shells. If it is divided into positive materials, it is mainly divided into lithium cobalt acid, lithium manganese acid, lithium iron phosphate and lithium nickel cobalt manganese acid ternary materials. According to the structural shape of the core, it is mainly divided into three types: cylindrical and square and soft package.

The advantages and disadvantages of different materials selection and structure design batteries are very obvious. Therefore, the safety of lithium-ion power batteries is closely related to the nature and structure design of battery materials, and is also closely related to battery preparation technology and operating environment. From the production of lithium-ion power cells to the final application, the factors that affect the safety of lithium-ion power cells run through the process of material selection and design of the core, the integration of modules, and the use of the environment.

(1) Selection and Assessment of Core Materials

The nature and safety of the core are largely determined by the selection of the core material. If the raw materials are not thoroughly evaluated when selecting the core material, the safety of the core will inevitably be insufficient in the first stage.

The specific capacity and specific energy of the battery are mainly determined by the positive electrode material. More importantly, its safety is affected by the intrinsic electrode potential of the positive electrode material, such as the safety difference between lithium iron phosphate and ternary. Therefore, it is necessary to improve the type of the core material by selecting and doping the elements, and to select the material with less reaction exoheat that matches the electrochemistry window of the electrolyte as much as possible to improve the core safety.

The influence of negative active material on safety performance mainly comes from the growth of lithium dendrites and the reaction of electrolytes. In the process of rapid charging, once the speed of lithium ions passing through the SEI membrane is slower than that of lithium deposited on the negative electrode, the dendrites of lithium will continue to grow with the charge and discharge cycle, which may lead to an internal short circuit that causes the electrolyte to react and heat out of control. Therefore, the safety of the core can be improved by improving the thermal stability of the SEI membrane.

The solvents commonly used in electrolytes are organic carbonate compounds that are chemically active and easily flammable. Positive materials are strongly oxidizing when they are in a charged state, while positive materials in a strong oxidizing state are generally less stable and easily release oxygen, while carbonates easily react with oxygen, releasing a large amount of heat and gas. Once the heat is out of control, the heat generated will further accelerate the decomposition of the positive pole, produce more oxygen, and promote more exothermic reactions.

The main role of the diaphragm is to separate the positive and negative poles of the battery, play a function of closing and blocking the channel, and allow lithium ions to pass freely, while electrons can not pass through. Once the diaphragm breaks down and other conditions will cause the positive and negative pole contact short circuit leading to thermal runaway, so the mechanical strength, porosity, thickness uniformity and temperature resistance of the diaphragm are highly required.

(2) Structural design and production process

The safety of lithium-ion power cell is also related to the structure of the battery. In particular, the capacity and size of the battery have an important influence on the safety of the battery. High-capacity batteries usually place more heat, while large-sized batteries have relatively difficult heat dissipation and heat is more likely to accumulate, thus increasing the probability of thermal runaway.

In order to ensure that lithium ion batteries do not have problems during use, a safety valve will be installed on the surface of the lithium battery shell to prevent internal pressure from being too high. There are many potentially dangerous parts in the structure of the core that cause internal short circuits. Therefore, necessary measures or insulation should be installed at these key locations to prevent internal short circuits in the battery under abnormal circumstances.

The basic steps of the electric core production and manufacturing process are divided into mixing, coating, roller pressure, cutting, winding or stacking, Polar ear welding, injection, sealing, converting, exhaust, and capacity sharing. The safety of the electric core may be caused by improper operation processes at each step.

In the testing stage of the core raw material, if it is not strictly in accordance with the standards or the poor environment at the time of production, it is easy to guide the core to be mixed with impurities, which has a great impact on the safety of the battery. In addition, if more water is mixed into the electrolyte, side reactions may occur and increase the internal pressure of the battery, which will have an impact on safety. In the production process of the electric core, due to the limitation of the process level, there are slight differences in the thickness of the battery plate, the micropore rate, and the activation degree of the active material.

Inconsistency in the internal structure of the battery makes it impossible for the voltage, capacity, and internal resistance of the same type of battery to be manufactured in the same batch to be completely consistent. The product can not achieve good consistency and may have a negative impact on the safety of the core.

(3) External environment and conditions of use

The environment of new energy vehicles in use is constantly changing. Once a collision occurs, the battery system will bear a huge impact load and may be damaged by extrusion, puncture, etc., causing serious risks such as battery combustion and explosion. On the other hand, long-term bumps on the road surface can easily lead to battery fixation and loosening, causing some mechanical damage and some problems caused by loosening the connectors.

Complex environment for power cells

The battery box shell serves as the first protective layer of the battery and the waterproof level needs to reach IP67. On the basis of not affecting the waterproof level, it must have a cooling system at the same time to ensure that the temperature in the confined space is not too high, effectively protecting the safety and service life of the battery. Its structure must ensure that sufficient strength is satisfied on the basis of a large capacity of accommodation space to ensure that the battery inside is protected from heavy extrusion in an abnormal collision.

In addition to the attention and external protection conditions in the process of core manufacturing, the management function of BMS is also highly required. BMS mainly detects the status of cells and the status of individual cells in the battery, and adjusts the cell(group) control strategy according to the status of the cell(group). It can accept the control information of the upper control module and make the necessary response. To realize the charge and discharge management of the power cell(group) to ensure the safe and stable operation of the power cell system. Therefore, a fully functional BMS can improve the safety and reliability of power cells during use.

summary

The safety performance of lithium-ion power batteries determines its market and future in the field of new energy vehicles. In order to ensure the safety of new energy vehicles, each enterprise needs to continuously improve the safety of lithium battery core through improving the technology and technology. It needs to continuously optimize the structure and design analysis of the power battery system. In addition, users also need to use the power battery system correctly to prevent mechanical abuse, thermal abuse and electrical abuse to ensure the safety and reliability of the battery.

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