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 stations has drawn attention to the safety of lithium batteries. Among them, the most dangerous factor is the thermal runaway of lithium batteries. In this case, what thermal runaway problems do lithium batteries have that cannot be ignored? Let’s learn about them with Grepp’s technicians!
1.Principle of thermal runaway of lithium battery
Stage 1: 125°C, the beginning of thermal runaway. The SEI film reacts and decomposes, and the decomposition of SEI exposes the negative electrode to the electrolyte, which promotes the reaction between the electrolyte and the lithium in the negative electrode and generates gas.
Research on the fire hazard and thermal runaway conditions of lithium-ion batteries
The second stage: 125~180 ℃, the gas release and temperature increase in the battery are accelerated. At this stage, the gas production rate is accelerated, and the cathode material is decomposed, such as: LiCoO2 decomposes to generate O2. Lithium salts also decompose, such as LiPF6 decomposes to form LiF and Lewis acid PF5. The Lewis acid reacts with the electrolyte at high temperature to generate a large amount of gas.
Stage 3: Above 180°C, thermal runaway occurs. At this stage, the exothermic reaction between the positive/negative electrode material and the electrolyte and the decomposition reaction rate of the electrolyte increase sharply, and the internal temperature of the battery also rises sharply accordingly, and the pressure relief valve opens or causes spontaneous combustion.
2.Reasons for thermal runaway of lithium batteries
1）Mechanical abuse, such as extrusion, collision, acupuncture, etc., will cause the lithium battery (cell) to deform under the action of external force, the diaphragm will be destroyed, and the short circuit between the positive and negative electrodes will induce thermal runaway.
2）Heat abuse, lithium batteries work for a long time in a high temperature environment. The main heat sources in the whole process are: the external high temperature environment, the heat of polarization, reaction heat, and decomposition heat generated during use.
3）Abuse of electricity, overcharging of the lithium battery leads to the destruction of the active material structure, the decomposition of the electrolyte to produce gas, and the increase of the internal pressure of the battery. In addition, it also includes over-discharge, high-rate (exceeding the specification) charging, etc.
Review on Thermal Runaway of Li-ion Batteries for Vehicles
3.Thermal runaway mechanism of lithium batteries at different ambient temperatures
1） At low temperature: The risk factors mainly come from the formation of lithium precipitation and lithium dendrites on the negative electrode side.
2）Under normal temperature: The risk factors mainly come from the heat generation of polarization (ohmic polarization, electrochemical polarization, etc.), or the heat generation under high-rate charge/discharge.
3）Under high temperature: The risk factors mainly come from the failure of materials, including: decomposition of SEI, shrinkage of diaphragm, etc.
4.Preventive measures for thermal runaway of lithium batteries
1) Set the safety valve, but the pressure value range of the safety valve needs to be strictly controlled.
2) Install thermistor to prevent battery overcharge or short circuit.
3) BMS precise thermal management, using water cooling, air cooling, etc. to cool the battery during battery use.
4) The use of additives in the electrolyte reduces the flammability of the electrolyte.
5) Improve the quality of SEI film formation, such as: adding LiCF3SO3 to the electrolyte, so that there are more inorganic components in the SEI.
6) Prevent the reaction between the positive electrode material and the electrolyte, such as the use of additives in the electrolyte or the coating of the positive electrode material.
7) Increase the melting point of the diaphragm, such as: coating a ceramic layer on both sides of the diaphragm.
8) Standardize the use of lithium batteries to reduce or eliminate human factors such as overcharge and overdischarge.