Axial pressure causes 18650 lithium battery to fail -Lithium - Ion Battery Equipment
With the rapid development of the electric vehicle industry, lithium batteries are widely used in the field of power lithium batteries due to their high energy density, no memory effect and high safety. Due to the particularity of electric vehicles, higher requirements are also placed on the safety of power lithium batteries. For example, in the event of a safety accident such as a collision with an electric vehicle, the power lithium battery must not catch fire or explode to ensure the safety of drivers and passengers. Therefore, the safety test of power lithium battery includes extrusion, acupuncture and other tests involving the safety performance of lithium battery under extreme abuse conditions. Whether it can pass these strict safety tests is to evaluate the safety of a lithium battery. The ultimate standard of sexuality.(Lithium - Ion Battery Equipment)
In the extrusion test, the lithium battery first deforms the outer casing, and then begins to extrude the cell. Due to the low strength of the separator prepared by the current dry stretching process in the lateral and diagonal directions, the cell is deformed. When it reaches a certain level, the transverse direction of the separator will first break, causing the positive and negative electrodes of the lithium battery to directly contact, resulting in a short circuit, and a large amount of heat is released instantaneously, resulting in a decomposition reaction of the negative electrode SEI film, the positive electrode active material and the electrolyte, resulting in lithium Thermal runaway of the battery eventually caused the lithium battery to catch fire and explode.
In order to prevent the thermal runaway of the lithium battery in the extrusion test and improve the safety of the lithium battery, it is necessary to conduct in-depth research on the mechanism of the thermal runaway of the lithium battery in the extrusion test, so as to carry out targeted safety measures for the lithium battery. designed to improve the safety of lithium batteries in extrusion tests. Let's take a look at the relevant research results of the Massachusetts Institute of Technology.
JunerZhu et al. of the Massachusetts Institute of Technology used 18650 batteries to study the mechanism of thermal runaway of lithium batteries in the process of axial extrusion, and used a finite element analysis model for simulation analysis, which restored different axial The influence of the pressure on the lithium battery was verified by the CT scan. The simulation analysis results found two reasons that could explain the short circuit of the lithium battery during the extrusion test.
Since the 18650 battery is generally assembled vertically in the power lithium battery pack, the axial extrusion is an important reason for the deformation of the lithium battery when the battery pack is dropped. Therefore, JunerZhu has mainly studied the deformation of the battery under the axial pressure. Mechanisms that lead to short circuits in lithium batteries. Some traditional models cannot accurately predict the test results when predicting the axial compression test of the 18650 battery because the interior of the lithium battery is assumed to be a uniform whole. The upper part and the lower part are not exactly the same. At the same time, due to the unique structure of the upper cover (that is, the positive electrode) of the lithium battery, when the lithium battery is subjected to axial pressure, it may cause a short circuit in the lithium battery before the internal short circuit occurs.
The 18650 battery is mainly composed of three parts: safety valve, coil core and mild steel casing. The safety valve is usually composed of a positive temperature coefficient material, an aluminum safety valve, a stainless steel positive terminal, and a gas seal. The loading speed of the axial load was 5 mm/min, and all the test cells were fully discharged (SOC=0) before the test. The test results show that the pressure of the 18650 battery in the axial pressure test shows a trend of slow increase - rapid increase - slight decrease - rapid increase, while the voltage test shows that the 18650 battery will fail when the deformation reaches 4mm. , and through experiments, it was found that the voltage dip of the 18650 battery was mainly caused by the internal short circuit of the battery, not the open circuit of the internal structure. In order to study the failure mechanism of 18650 under axial pressure, JunerZhu also used finite element software to analyze it. The material in the model mainly adopts the elastic-plastic model, and considers the characteristics of anisotropy of various materials. Including millions of computing units, the loading speed of the axial load is set to 1m/s.
In the extrusion test, the lithium battery first deforms the outer casing, and then begins to extrude the cell. Due to the low strength of the separator prepared by the current dry stretching process in the lateral and diagonal directions, the cell is deformed. When it reaches a certain level, the transverse direction of the separator will first break, causing the positive and negative electrodes of the lithium battery to directly contact, resulting in a short circuit, and a large amount of heat is released instantaneously, resulting in a decomposition reaction of the negative electrode SEI film, the positive electrode active material and the electrolyte, resulting in lithium Thermal runaway of the battery eventually caused the lithium battery to catch fire and explode.
In order to prevent the thermal runaway of the lithium battery in the extrusion test and improve the safety of the lithium battery, it is necessary to conduct in-depth research on the mechanism of the thermal runaway of the lithium battery in the extrusion test, so as to carry out targeted safety measures for the lithium battery. designed to improve the safety of lithium batteries in extrusion tests. Let's take a look at the relevant research results of the Massachusetts Institute of Technology.
JunerZhu et al. of the Massachusetts Institute of Technology used 18650 batteries to study the mechanism of thermal runaway of lithium batteries in the process of axial extrusion, and used a finite element analysis model for simulation analysis, which restored different axial The influence of the pressure on the lithium battery was verified by the CT scan. The simulation analysis results found two reasons that could explain the short circuit of the lithium battery during the extrusion test.
Since the 18650 battery is generally assembled vertically in the power lithium battery pack, the axial extrusion is an important reason for the deformation of the lithium battery when the battery pack is dropped. Therefore, JunerZhu has mainly studied the deformation of the battery under the axial pressure. Mechanisms that lead to short circuits in lithium batteries. Some traditional models cannot accurately predict the test results when predicting the axial compression test of the 18650 battery because the interior of the lithium battery is assumed to be a uniform whole. The upper part and the lower part are not exactly the same. At the same time, due to the unique structure of the upper cover (that is, the positive electrode) of the lithium battery, when the lithium battery is subjected to axial pressure, it may cause a short circuit in the lithium battery before the internal short circuit occurs.
The 18650 battery is mainly composed of three parts: safety valve, coil core and mild steel casing. The safety valve is usually composed of a positive temperature coefficient material, an aluminum safety valve, a stainless steel positive terminal, and a gas seal. The loading speed of the axial load was 5 mm/min, and all the test cells were fully discharged (SOC=0) before the test. The test results show that the pressure of the 18650 battery in the axial pressure test shows a trend of slow increase - rapid increase - slight decrease - rapid increase, while the voltage test shows that the 18650 battery will fail when the deformation reaches 4mm. , and through experiments, it was found that the voltage dip of the 18650 battery was mainly caused by the internal short circuit of the battery, not the open circuit of the internal structure. In order to study the failure mechanism of 18650 under axial pressure, JunerZhu also used finite element software to analyze it. The material in the model mainly adopts the elastic-plastic model, and considers the characteristics of anisotropy of various materials. Including millions of computing units, the loading speed of the axial load is set to 1m/s.
