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What Are the Thermal Runaway Problems of Lithium Batteries That Cannot Be Ignored?

With the vigorous development of new energy vehicles and electrochemical energy storage, the installed capacity of lithium batteries is increasing day by day. However, the frequent occurrence of accidents in new energy vehicles and energy storage power plants has made the safety of lithium batteries a major concern. Among them, the most dangerous factor is the lithium battery thermal runaway problem, in that case, what exactly are the lithium battery thermal runaway problems that can not be ignored? Here with us to learn more about it!

Ⅰ. The principle of thermal runaway of lithium batteries

Stage 1: 125 ℃, the beginning of the thermal runaway stage, SEI membrane reaction decomposition, SEI decomposition of the negative electrode exposed to the electrolyte, prompting the electrolyte and lithium reaction in the negative electrode and generate gas.

Stage 2: 125~180℃, gas release and heating up inside the battery is accelerated. The rate of gas production accelerates in this stage, and the cathode material decomposes, e.g. LiCoO2 decomposes to produce O2. lithium salt also decomposes, e.g. LiPF6 decomposes to produce LiF and Lewis acid PF5. and Lewis acid will react with electrolyte at high temperature to produce large amount of gas.

Stage 3: Above 180°C, thermal runaway occurs. At this stage, the exothermic reaction between positive/negative electrode materials and electrolyte and the rate of electrolyte decomposition reaction increases sharply, and the internal temperature of the battery increases sharply accordingly, and the pressure relief valve opens or spontaneous combustion is triggered.

Ⅱ. The causes of thermal runaway of lithium batteries

1. Mechanical abuse, such as: extrusion, collision, pinprick, etc., under the action of external forces lead to lithium battery (cell) deformation, diaphragm is destroyed, the positive and negative electrode short circuit and induced thermal runaway.

2. Thermal abuse, lithium batteries work for a long time in a high-temperature environment, the main heat sources throughout the process are: the external high-temperature environment, the use of polarization heat, reaction heat, decomposition heat, etc.

3. Electrical abuse, lithium battery overcharge leads to the destruction of the active material structure, electrolyte decomposition and gas production, resulting in an increase in internal pressure of the battery. In addition, it also includes over-discharge, large multiplier (over specification) charging, etc.

Ⅲ. The mechanism of thermal runaway of lithium batteries under different ambient temperatures

1. Low temperature: risk factors mainly from the negative side of the lithium precipitation and lithium dendrite generation.

2. At room temperature: risk factors mainly from polarization (ohmic polarization, electrochemical polarization, etc.) heat generation, or large multiplier charge / discharge heat generation.

3. At high temperature: risk factors mainly from the failure of the material, including: SEI decomposition, shrinkage of the diaphragm, etc.

Ⅳ. The lithium battery thermal runaway preventive measures

1. Set the safety valve, but the safety valve pressure value range needs to be strictly controlled.

2. The installation of thermistors to prevent overcharging or short-circuiting of the battery.

3. BMS precise thermal management, the use of water cooling, air cooling and other battery cooling during the use of the battery.

4. The use of additives in the electrolyte to reduce the flammability of the electrolyte.

5. Improve the quality of SEI film formation, such as: adding LiCF3SO3 etc. to the electrolyte to make more inorganic components in SEI.

6. Prevent the reaction between positive electrode material and electrolyte, such as: the use of additives in electrolyte or the coating of positive electrode material.

7. Increase the melting point of the diaphragm, such as: coating ceramic layer on both sides of the diaphragm.

8. Regulate the use of lithium batteries, reduce or eliminate overcharging, overdischarging and other human factors.

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