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Analysis and Discussion on the Safety of Lithium Battery

by:dcfpower     2021-03-08
In current production applications, chemical power sources are widely used because of their low technical difficulty, low production cost, and better specific energy. In the process of using chemical power supply, it continuously produces directional flow of electrons through chemical reaction to provide continuous power. The chemical reaction will be accompanied by the generation of heat. When the heat cannot be diffused in time, it will lead to thermal runaway and thermal runaway. The battery is destroyed, such as violently deflated, ruptured and accompanied by fire. To use the battery safely, the safe operation of the battery must be ensured within a limited temperature range, charge rate, and charge-discharge termination voltage. u003cpu003eu003c/pu003e 1. Analysis of the causes of battery explosion The types of lithium battery pack cell explosions can be summarized as external short circuit, internal short circuit, and overcharge. The external here refers to the outside of the battery cell, including short circuits caused by poor internal insulation design of the battery pack. When a short circuit occurs on the outside of the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, which will cause part of the electrolyte to vaporize and expand the battery shell. When the internal temperature of the battery reaches 135 degrees Celsius, a good quality diaphragm paper will close the pores, the electrochemical reaction will be terminated or almost terminated, the current will drop sharply, and the temperature will slowly drop, thus avoiding an explosion. However, the pore closure rate is too poor, or the pores will not be closed at all, the battery temperature will continue to rise, more electrolyte will vaporize, and finally the battery shell will be broken, or even the battery temperature will be increased to The material burns and explodes. The internal short circuit is mainly caused by the burrs of the copper foil and the aluminum foil piercing the diaphragm, or the dendritic crystals of lithium atoms piercing the diaphragm. These tiny needle-like metals can cause micro short circuits. Because the needle is very thin and has a certain resistance value, the current is not necessarily large, and the probability of an explosion caused by a burr micro-short circuit is not high. The explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It is possible that when the temperature of the battery is not high enough to burn the material and the gas generated is not enough to break the battery case, the consumer will stop charging and take the mobile phone out. At this time, the heat generated by numerous micro-short circuits slowly raises the temperature of the battery. After a period of time, an explosion occurs. This shows that the lithium battery pack core exploded. 2. Factors affecting the safety of lithium-ion batteries Under normal circumstances, lithium-ion batteries must pass various safety performance tests in addition to the capacity, internal resistance, and high and low temperature charge and discharge performance requirements. Often the overcharge, short circuit, thermal shock, and mechanical shock of lithium-ion batteries can easily affect the safety of the battery, but the effects and effects of these factors are different. 1. The impact of needle stick, short circuit and squeeze on battery safety. When the battery has an external short circuit or is subjected to needle stick and squeeze, it is often easy to cause the internal diaphragm of the battery to rupture, causing the battery to become very large in a very short time. The current flows through, which directly causes the internal temperature of the battery to rise sharply, which triggers a series of violent reactions in a short period of time, and even causes safety problems such as combustion and explosion. For batteries with poor safety performance, this process is often completed within a few seconds. Due to the high specific energy of lithium-ion batteries, such as a 2200mAh laptop battery, the single capacity can reach about 30kJ. Once a short circuit occurs, the battery can reach a high heating power, causing the battery to rise to a very high level in a short time. Temperature, the aluminum explosion-proof valve at the battery seal is often melted. 2. The impact of thermal shock on battery safety When the battery is subjected to thermal shock, the battery's thermal conductivity is relatively slow, so it can withstand the thermal shock for a short time. Compared with short circuit and needle extrusion, if the temperature of thermal shock is low, the impact on the battery is not large. If the temperature of thermal shock is high, the SEI film on the negative electrode surface of the battery will decompose, and the negative electrode material with high lithium insertion will be An exothermic reaction will occur with the electrolyte. At this time, the battery enters a dangerous period, but whether it is dangerous depends on the rate of heat dissipation of the battery. If the rate of heat dissipation is slow, it will easily cause the internal temperature of the battery to rise further, the diaphragm will melt, and the internal occurrence of the battery Short-circuit, the temperature rises sharply, triggering the positive electrode material to participate in the reaction, and finally an explosion and other dangerous accidents. This process is generally called thermal runaway. 3. The impact of overcharging on battery safety. When the battery is overcharged, different electrode materials due to chemical Different properties have different effects. In the early stage of high-rate charging, most of the electric energy is stored through a reversible chemical reaction, and the heating power of the battery is small, but in the later stage of charging, the electric energy becomes heat due to the irreversible chemical reaction. This leads to a rapid increase in battery temperature and triggers a series of chemical reactions. 3. Measures to improve the safety of lithium-ion batteries 1. Choose raw materials with higher safety factors, choose positive and negative active materials, separator materials and electrolytes with higher safety factors The safety of the cathode material is mainly based on the thermodynamic stability of the material, the chemical stability of the material, and the physical properties of the material; the separator material should be based on the mechanical isolation performance, pore size and porosity, chemical stability of the material, and automatic shutdown Comprehensive consideration should be given to functions, small thermal shrinkage and deformation rates, material thickness, and large physical strength; the electrolyte should be chemically stable, electrochemically stable, lithium ion conductivity high, and liquid stable range Wide material.
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