AA rechargeable battery

　　AA rechargeable battery safety technology is very important, these aspects must be considered

　　This article reviews the dangers of lithium-ion batteries and analyzes the causes of safety problems in lithium-ion battery products. From the three perspectives of materials, design and process, some commonly used safety technologies are briefly introduced, and some examples of safety design are proposed.

　　1. The dangers of lithium-ion batteries

　　Lithium-ion batteries are a potentially dangerous chemical power source due to their chemical characteristics and system composition.

　　1 High chemical activity

　　Lithium is a main group I element in the second period of the periodic table of elements and has extremely active chemical properties.

　　2 High energy density

　　Lithium-ion batteries have extremely high specific energy (≥140Wh/kg), which is several times that of secondary batteries such as nickel-cadmium and nickel-hydrogen. If a thermal runaway reaction occurs, it will release a high amount of heat and easily lead to unsafe behavior.

　　3 Using organic electrolyte system

　　The organic solvent of the organic electrolyte system is hydrocarbon, which has a low decomposition voltage, is prone to oxidation, and the solvent is flammable. If leakage occurs, the battery will catch fire, or even burn or explode.

　　4. High probability of side reactions

　　During normal use of lithium-ion batteries, chemical positive reactions in which electrical energy and chemical energy are converted into each other are carried out internally. 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 this side reaction intensifies, it will seriously affect the performance and service life of the battery, and A large amount of gas may be produced, causing the pressure inside the battery to rapidly increase and then explode and catch fire, causing safety issues.

　　5 The structure of the electrode material is unstable

　　The overcharging reaction of lithium-ion batteries will change the structure of the cathode material, causing the material to have a strong oxidation effect, causing the solvent in the electrolyte to undergo strong oxidation; and this effect is irreversible, and if the heat caused by the reaction accumulates, There is a risk of thermal runaway.

　　2. Analysis of the causes of safety issues in lithium-ion battery products

　　After 30 years of industrial development of lithium-ion battery products, safety technology has made great progress, effectively controlling the occurrence of side reactions in the battery and ensuring battery safety. However, as lithium-ion batteries are used more and more widely and their energy density becomes higher and higher, incidents such as explosions and injuries or product recalls due to safety hazards have occurred frequently in recent years. We summarize the main reasons causing safety problems in lithium-ion battery products as follows:

　　1 Battery core material issues

　　The materials used in the battery core include: positive active material, negative active material, separator, electrolyte and shell, etc. The selection of materials and the matching of the composed system determine the safety performance of the battery core. When selecting positive and negative active materials and separator materials, the manufacturer did not conduct a certain assessment of the characteristics and matching of the raw materials, resulting in inherent deficiencies in the safety of the battery cells.

　　2 Production process issues

　　The raw materials of battery cells are not strictly tested and the production environment is poor, which leads to the mixing of impurities in production, which not only has a great disadvantage on the capacity of the battery, but also has a great impact on the safety of the battery; in addition, if too much is mixed into the electrolyte Moisture may cause side reactions to increase the internal pressure of the battery, affecting safety; due to limitations in the production process level, the product cannot achieve good consistency during the production process of the battery cells, such as poor flatness of the electrode base, Problems such as falling off of electrode active materials, mixing of other impurities in active materials, weak tab welding, unstable welding temperature, burrs on the edges of pole pieces, and failure to use insulating tape on key parts may all affect the safety of the battery core. negative effect.

　　3. Battery core design defects and reduced safety performance

　　In terms of structural design, many key points that have an impact on safety are not taken seriously by the manufacturers, such as the absence of insulating tape in key parts, the separator design leaving no margin or insufficient margin, the positive and negative electrode capacity ratio design is unreasonable, and the positive and negative electrode activity The material area ratio design is unreasonable, the tab length design is unreasonable, etc., which may pose hidden dangers to the safety of the battery. In addition, in the production process of battery cells, in order to save costs and improve performance, some battery manufacturers try to save and compress raw materials, such as reducing the diaphragm area, thinning copper foil, aluminum foil, and not using pressure relief valves and insulating tapes, etc. , these will reduce the safety of the battery.

　　4 Energy density is too high

　　Currently, the market is pursuing higher-capacity battery products. In order to increase product competitiveness, manufacturers continue to increase the volume specific energy of lithium-ion batteries, which greatly increases the risk of batteries.

　　3. Security technology

　　Although lithium-ion batteries have many hidden dangers, under specific conditions of use and by adopting certain measures, the occurrence of side reactions and violent reactions in the cells can be effectively controlled to ensure their safety. The following is a brief introduction to several commonly used safety technologies for lithium-ion batteries.

　　1 Choose raw materials with higher safety factor

　　Choose positive and negative active materials, separator materials and electrolytes with higher safety factors.

　　a) Selection of cathode materials

　　The safety of cathode materials is mainly based on the following three aspects:

　　1. Thermodynamic stability of the material;

　　2. Chemical stability of materials;

　　3. Physical properties of materials.

　　b) Selection of diaphragm materials

　　The main function of the separator is to separate the positive and negative electrodes of the battery to prevent the positive and negative electrodes from contacting and causing a short circuit. At the same time, it has the ability to allow electrolyte ions to pass through, that is, it has electronic insulation and ion conductivity. The following points should be noted when selecting separators for lithium-ion batteries:

　　1. It has electronic insulation and ensures the mechanical isolation of the positive and negative poles;

　　2. It has a certain pore size and porosity to ensure low resistance and high ionic conductivity;

　　3. The diaphragm material has sufficient chemical stability and must be resistant to electrolyte corrosion;

　　4. The diaphragm must have the function of automatic shutdown protection;

　　5. The thermal shrinkage and deformability of the diaphragm should be as small as possible;

　　6 The diaphragm must have a certain thickness;

　　7 The diaphragm must have strong physical strength and sufficient resistance to puncture.

　　c) Selection of electrolyte

　　Electrolyte is an important component of lithium-ion batteries and plays a role in transporting and conducting current between the positive and negative electrodes of the battery. The electrolyte used in lithium-ion batteries is an electrolyte solution formed by dissolving an appropriate lithium salt in an organic aprotic mixed solvent. It should usually meet the following requirements:

　　1. Good chemical stability, no chemical reaction with electrode active materials, current collectors and separators;

　　2. Good electrochemical stability and wide electrochemical window;

　　3 Lithium ions have high conductivity and low electronic conductivity;

　　4. Wide liquid temperature range;

　　5. Safe, non-toxic and environmentally friendly.

　　2 Strengthen the overall safety design of the battery core

　　The battery core is the link that combines various materials in the battery. It is the integration of the positive electrode, negative electrode, separator, tab and packaging film. The structural design of the battery core not only affects the performance of various materials, but also affects the overall performance of the battery. Electrochemical performance and safety performance have an important impact. The selection of materials and the structural design of the battery core are in a local and overall relationship. In the battery core design, the material characteristics should be combined to formulate a reasonable structural model.

　　In addition, some additional protection devices can also be considered in the AA rechargeable battery structure. Common protection mechanism designs include the following:

　　1. Using switching elements, when the temperature inside the battery rises, its resistance rises accordingly. When the temperature is too high, the power supply will be automatically stopped;

　　2. Set up a safety valve (that is, the vent hole on the top of the battery). When the internal pressure of the battery rises to a certain value, the safety valve will automatically open to ensure the safety of the battery.

　　The following are some examples of the safety design of battery cell structures:

　　a) Capacity ratio of positive and negative electrodes and design size of chip

　　Select the appropriate positive and negative electrode capacity ratio based on the characteristics of the positive and negative electrode materials. The ratio of the positive and negative electrode capacities of the battery core is an important link related to the safety of the lithium-ion battery. If the positive electrode capacity is too large, metallic lithium will appear on the surface of the negative electrode. deposition, and if the negative electrode is too large, the battery capacity will suffer a greater loss. Generally speaking, N/P=1.05~1.15, and make appropriate selections based on actual battery capacity and safety requirements. Design the size of the piece so that the position of the negative electrode paste (active material) covers (is larger than) the position of the positive electrode paste. Generally, the width should be 1 to 5 mm larger and the length should be 5 to 10 mm larger.

　　b) Allowance for diaphragm width

　　The overall principle of separator width design is to prevent internal short circuits caused by direct contact between the positive and negative electrodes. Due to the thermal shrinkage of the separator during the charging and discharging process of the battery and in thermal shock and other environments, the separator will deform in the length and width directions. In the wrinkled area, the distance between the positive and negative electrodes increases, resulting in increased polarization; in the stretched area of the separator, the possibility of micro short circuit increases due to the thinning of the separator; shrinkage of the edge area of the separator may cause the positive and negative electrodes to directly Internal short circuit occurs due to contact, which will cause the battery to be in danger due to thermal runaway. Therefore, when designing a battery, the shrinkage characteristics of the separator must be considered when using the area and width. The separator is larger than the anode and cathode. Taking into account process errors, the isolation film must be at least 0.1mm longer than the outer edge of the pole piece.

　　c) Insulation treatment

　　Internal short circuit is an important factor in the safety hazard of lithium-ion batteries. In the structural design of the battery cell, there are many potentially dangerous parts that can cause internal short circuit. Therefore, necessary measures or insulation should be installed at these key locations to prevent them from occurring under abnormal circumstances. If a short circuit occurs within the battery, for example: keep a necessary distance between the positive and negative electrode ears; apply insulating tape on the finishing single side without paste in the middle, and cover all the exposed parts; apply insulating tape between the positive aluminum foil and the negative active material; application Insulating tape covers all the welding parts of the tabs; insulating tape is used on the top of the battery core.

　　d) Set up a safety valve (pressure relief device)

　　Lithium-ion batteries are often in danger because the internal temperature or pressure is too high, causing explosions and fires; setting up a reasonable pressure relief device can quickly release the pressure and heat inside the battery when danger occurs, reducing the risk of explosion. A reasonable pressure relief device is required to not only meet the internal pressure of the battery during normal operation, but also automatically open to release the pressure when the internal pressure reaches the dangerous limit. The setting position of the pressure relief device needs to consider the impact caused by the increase in internal pressure of the battery shell. Designed based on deformation characteristics; the design of safety valves can be achieved through sheets, edges, seams, and notches.

　　3. Improve technology level

　　Efforts should be made to standardize and standardize the battery cell production process. In the steps of mixing, coating, baking, compacting, slitting and winding, standardization should be formulated (such as diaphragm width, electrolyte injection volume, etc.), and process methods should be improved (such as low-pressure injection method, centrifugal loading, etc.) Shell method, etc.), do a good job in process control, ensure process quality, and narrow the differences between products; set up special process steps in key steps that have an impact on safety (such as deburring the electrode plate, sweeping powder, and using different welding methods for different materials) methods, etc.), implement standardized quality monitoring, eliminate defective parts, and exclude defective products (such as pole piece deformation, diaphragm puncture, active material falling off, electrolyte leakage, etc.); keep the production site tidy and clean, and implement 5S management and 6 -sigma quality control to prevent impurities and moisture from being mixed during production and to minimize the impact of accidents on safety during production.

　　Author/Peng Qi

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