With the rapid development of the electric vehicle industry, lithium-ion
batteries are widely used in the field of power batteries due to their
advantages of high energy density, no memory effect and high safety. Due to the
special nature of electric vehicles, higher requirements are also placed on the
safety of power batteries, such as in the event of a collision or other safety
accidents involving an electric vehicle.
With the rapid development of the electric vehicle industry, lithium
batteries are widely used in the field of power lithium batteries due to their
advantages of high energy density, no memory effect and high safety. Due to the
particularity of electric vehicles, higher requirements are placed on the safety
of power lithium batteries. For example, in the event of a collision or other
safety accidents in electric vehicles, the power lithium batteries must not
catch fire or explode to ensure the safety of drivers and passengers. ,
therefore, the power lithium battery safety test experiments include extrusion,
acupuncture and other tests related to the safety performance of lithium
batteries under extreme abuse conditions. Whether it can pass these stringent
safety tests is an important factor in evaluating the safety of a lithium
battery. The ultimate standard of sex.
In the extrusion test, the lithium battery shell first deforms, and then
begins to squeeze the battery core. Since the separator prepared by the current
dry stretching process has low strength in the transverse and diagonal
directions, the deformation of the battery core occurs. When it reaches a
certain level, the transverse direction of the separator will break first,
causing the positive and negative electrodes of the lithium battery to be in
direct contact, causing a short circuit, instantly releasing a large amount of
heat, causing the negative SEI film, positive electrode active material and
electrolyte to decompose, resulting in lithium Thermal runaway occurs in the
battery, eventually causing the lithium battery to catch fire and explode.
In order to prevent thermal runaway of lithium batteries during the
extrusion test and improve the safety of lithium batteries, it is necessary to
conduct in-depth research on the mechanism of thermal runaway of lithium
batteries during the extrusion test, so as to carry out targeted safety measures
for lithium batteries. 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 in the United States.
Juner Zhu and others from the Massachusetts Institute of Technology in the
United States used 18650 batteries to study the mechanism of thermal runaway of
lithium batteries during axial extrusion, and conducted simulation analysis
using a finite element analysis model. This model restored the different axial
The analysis results of the impact of axial pressure on lithium batteries were
verified by CT scans. The simulation analysis results found two reasons that can
explain the short circuit of lithium batteries during the extrusion test.
Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃
3C discharge capacity ≥70%
Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support
maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%
Since the 18650 batteries in power lithium battery packs are generally
assembled vertically, axial extrusion is an important cause of lithium battery
deformation when the battery pack is dropped, so JunerZhu has focused on the
battery deformation under axial pressure. The mechanism that causes lithium
battery short circuit. Some traditional models assume that the interior of the
lithium battery is a uniform whole, so they cannot accurately predict the test
results when predicting the 18650 battery axial compression test. This is mainly
due to the special structure of the lithium battery cell, resulting in the The
upper and lower parts are not exactly the same. At the same time, due to the
unique structure of the lithium battery upper cover (that is, the positive
electrode), when the lithium battery is subjected to axial pressure, it may
cause a short circuit in the lithium battery before an internal short circuit
occurs.
The 18650 battery mainly consists of three parts: safety valve, core and
mild steel casing. Safety valves usually consist of positive temperature
coefficient materials, aluminum safety valves, stainless steel positive
terminals, gas seals, etc. The battery core consists of positive electrodes,
negative electrodes and separators. In this test, the active material of the
positive electrode is LiCoO2. The loading speed of axial load is 5mm/min, and
all test batteries have been fully discharged (SOC=0) before the test. The test
results show that in the axial pressure test, the pressure of the 18650 battery
shows a trend of slow rise - rapid rise - slight drop - rapid rise. The voltage
test shows that the 18650 battery will fail only when the deformation reaches
4mm. , and through experiments, it was found that the voltage drop of the 18650
battery was mainly caused by the internal short circuit of the battery, rather
than 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 materials in the model mainly used an
elastic-plastic model, and the anisotropic characteristics of various materials
were taken into account. In the model Containing millions of calculation units,
the loading speed of the axial load is set to 1m/s.
The simulation results reproduce the deformation process of the 18650
battery under axial load. First, the casing in the upper cover area of the
battery begins to undergo plastic deformation. After the deformation exceeds 1
mm, the deformed casing begins to squeeze the upper part of the battery core. As
the degree of deformation increases, the battery core begins to deform, thereby
causing the pressure to increase. There is a slight decrease in the curve, and
then with the increase in the contact area between the battery case and the
cell, the pressure curve shows a rapid upward trend. The CT scan results also
well verified the above analysis. The deformation of the test battery mainly
occurred in the upper structure, and almost no deformation occurred when the
battery was lowered.
Disassembly of the 18650 battery after the test showed that although the
battery core was severely deformed, the positive and negative electrodes did not
break. Instead, a crack appeared in the separator 1.3mm from the upper edge,
which directly caused the battery to fail. A short circuit occurs, the voltage
drops, and this crack may be caused by the intrusion of a sharp edge of the
metal foil. In addition, the thickness of the diaphragm has dropped
significantly in some locations, which is mainly caused by the dented shell
squeezing the battery core.
Judging from the above analysis results, the possible reasons for the short
circuit of the 18650 battery under axial pressure include the following.
Low temperature and high energy density 18650 3350mAh-40℃ 0.5C discharge
capacity ≥60%
Charging temperature: 0~45℃ Discharge temperature: -40~+55℃ Specific
energy: 240Wh/kg -40℃ Discharge capacity retention rate: 0.5C Discharge capacity
≥ 60%
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1. The shell is in contact with the positive and negative electrodes
through the broken diaphragm.
2. The positive and negative electrodes are in contact through the ruptured
separator
3. The positive and negative electrodes are in contact through the thinned
area of the separator.
4. The safety valve is squeezed and comes into contact with the battery
core
Judging from the test results, when the axial deformation of the 18650
battery reaches 4mm, it will cause an internal short circuit, so special
consideration must be given to the safety design of the battery pack. In
addition, since deformation mainly occurs in the upper part of the 18650 battery
during axial pressure, special attention must be paid to the safety design of
the upper part of the 18650 battery.
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