These potential safety hazards have caused insufficient confidence among consumers of new energy vehicles. Therefore, it is necessary to pay attention to the safety of lithium-ion battery and research and improvement, which will help reduce the safety hazard of the battery and the frequency of accidents, and reduce or avoid the harm caused by the safety of the power battery.
Briefly, a lithium ion power battery is mainly composed of a positive electrode, a negative electrode, a separator, an electrolyte, a battery case, and the like. According to the positive electrode material, it is mainly divided into lithium cobaltate, lithium manganate, lithium iron phosphate and lithium lanthanum manganese ternary materials. According to the structural shape of the core, it is mainly divided into three types: cylindrical and square, and soft pack.
The advantages and disadvantages of different material selection and structural design are very obvious. Therefore, the safety of lithium ion power battery is closely related to the material properties and structural design of the battery, and is closely related to the battery preparation technology and the use environment. From the production of lithium-ion power batteries to the final application, the factors affecting the safety of lithium-ion power batteries are throughout the process of material selection and design production, module integration and use environment.
(1) Battery material selection and evaluation
The nature and safety of the battery core is largely determined by the choice of the battery material. If the battery material is selected without in-depth evaluation of the raw material, the safety of the battery core will inevitably occur 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 selection of the material of the battery core and the doping of the element, and to select a material whose potential is closely matched with the electrochemical window of the electrolyte and which has less heat release, so as to improve the safety of the battery core.
The influence of the negative active material on the safety performance mainly comes from the growth of lithium dendrites and the reaction of the electrolyte. In the process of fast charge, once the speed of lithium ions passing through the SEI film is slower than the speed at which lithium is deposited on the negative electrode, the dendrites of lithium will grow with the charge and discharge cycle, which may cause an internal short circuit and cause the electrolyte to react to cause heat. out of control. Therefore, the safety of the cell can be improved by improving the thermal stability of the SEI film.
Solvents commonly used in electrolytes are organic carbonate compounds which are chemically active and highly flammable. When the positive electrode material is in a charged state, it is strongly oxidizing, while the positive electrode material in a strong oxidizing state generally has poor stability, and it is easy to release oxygen, and the carbonate is easily reacted with oxygen to release a large amount of heat and gas. Once heat is generated, the heat generated will further accelerate the decomposition of the positive electrode, generate more oxygen, and promote more exothermic reactions.
Battery separator
The main function of the diaphragm is to separate the positive and negative electrodes of the battery, which functions to close and block the passage, allowing lithium ions to pass freely and electrons not to pass. Once the diaphragm is broken or the like, the positive and negative contact short-circuit will cause thermal runaway, so the mechanical strength, porosity, thickness uniformity and temperature resistance of the diaphragm are highly demanded.
(2) Structural design and production process
The safety of lithium-ion battery is also related to the structure of the battery. In particular, the battery capacity and size have an important impact on the safety of the battery. A battery with a high capacity usually emits a large amount of heat, and a battery with a large volume is relatively difficult to dissipate heat, and heat is more likely to be accumulated, so that the probability of thermal runaway is greater.
In order to ensure that the lithium-ion battery does not have any problems during use, a safety valve is placed on the surface of the lithium battery battery case to prevent the internal pressure from being too high. There are many potential dangers in the structural design of the battery core that cause internal short circuits. Therefore, necessary measures or insulation should be set at these critical positions to prevent short circuit in the battery under abnormal conditions.
The basic steps of the cell manufacturing process are divided into mixing, coating, rolling, cutting, winding or lamination, tab welding, injecting, sealing, forming, venting, filling, etc., in every step. There is a possibility that the safety of the battery core may be caused by improper operation.
In the raw material testing stage of the battery, if it is not strictly in accordance with the standard or the production environment is poor, it is easy to cause impurities in the battery core, which has a great impact on the safety of the battery. Further, if a large amount of water is mixed in the electrolytic solution, a side reaction may occur to increase the internal pressure of the battery, which may affect safety. In the production process of the battery core, there are slight differences in the thickness of the battery plate, the microporosity, the activation degree of the active material, and the like due to the limitation of the process level.
The inconsistency in the internal structure of the battery makes it impossible to completely match the voltage, capacity, internal resistance, etc. of the same type of battery shipped from the same batch. Failure to achieve good product consistency can adversely affect the safety of the battery.
(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 be subjected to huge impact loads and may be damaged by extrusion, puncture, etc., causing serious dangers such as burning and explosion of the battery. On the other hand, bumping on the road for a long time can easily lead to loosening of the battery, causing some mechanical damage and loosening of the connecting parts.
Power battery use environment is complex
The battery case is the first protective layer of the battery and the waterproof rating is IP67. On the basis of not affecting the waterproof level, it must also have a heat dissipation system to ensure that the temperature in the confined space is not too high, effectively protecting the safety and service life of the battery. The structure must ensure sufficient strength on the basis of a large-capacity accommodation space to ensure that the battery inside is protected from being strongly squeezed under abnormal collision.
In addition to the attention and external protection conditions in the manufacturing process of the battery core, the management functions of the BMS are also highly demanded. The BMS mainly detects the state of the battery and the state of each single cell in the battery pack, and performs appropriate control strategy adjustment on the battery (group) according to the state of the battery (group), and can accept the control information of the upper control module and make necessary In response, charge and discharge management of the power battery (group) is realized to ensure safe and stable operation of the power battery system. Therefore, a full-featured BMS can improve the safety and reliability of the power battery during use.
to sum up
The safety performance of lithium-ion battery determines its market and future in the field of new energy vehicles. In order to ensure the safety of new energy vehicles, companies need to continuously improve the safety of lithium battery cells through improved processes and technologies. It is necessary to continuously optimize the structure and design of the power battery system. In addition, users also need to use the power battery system correctly, to prevent mechanical abuse, heat abuse and electrical abuse to ensure the safety and reliability of the battery.
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