Among the factors that cause internal short circuits in lithium-ion
batteries, lithium dendrites are the most common and dangerous factor. The
reason why lithium dendrites are common is due to the characteristics of
lithium-ion batteries. For example, during low-temperature charging or high-rate
charging, due to poor kinetic conditions of the negative electrode, it is very
easy to cause metal lithium to form a coating on the surface of the negative
electrode. As the coating The development will eventually form lithium
dendrites. When the lithium dendrites grow to a certain extent, they may pierce
the separator and cause a short circuit in the lithium-ion battery. The
separator is the most important line of defense against lithium dendrites. After
the lithium dendrites grow to a certain extent, they will meet the separator and
form a backlog and acupuncture on the separator. This will eventually lead to
mechanical failure of the separator and cause a short circuit between the
positive and negative electrodes. .
Currently, common separators on the market are mainly divided into three
categories: dry stretched separators, wet process separators and non-woven
process separators. Among them, the separators prepared by the dry stretching
process have obvious anisotropy and have longitudinal characteristics. Very high
tensile strength, the tensile strength in the transverse direction is
significantly weaker than that in the transverse direction. While the wet method
has similar tensile strength in all directions, the non-woven separator has poor
tensile strength in all directions.
In order to solve the safety issues caused by lithium dendrites, people are
also developing multi-functional composite separators with the function of
inhibiting lithium dendrites. Kai Liu and others from Stanford University
developed a three-layer composite separator in which the middle layer of the
separator is nano-SiO2 particles. , the main function of nano-SiO2 particles is
to react with the lithium dendrites that pierce the separator, consume the
lithium dendrites, and thereby prevent the continued growth of the lithium
dendrites. This separator can promptly block the growth process of lithium
dendrites after they are produced in the negative electrode, preventing the
lithium dendrites from piercing the separator and causing short circuits between
the positive and negative electrodes.
A common method to study the separator's resistance to lithium dendrites is
to make a test battery with both positive and negative electrodes made of
metallic lithium, and repeatedly charge and discharge the battery until a short
circuit occurs in the battery. The better the lithium dendrite performance. In
order to understand the in-depth mechanism of separator failure caused by
lithium dendrites, Fu Sun and others from the Berlin University of Technology
conducted an in-depth study of the mechanism of separator failure caused by
lithium dendrites using online X-ray phase contrast imaging methods. The
experiment revealed how lithium dendrites are produced and grown, and
demonstrated the process of destruction of a three-layer composite separator by
lithium dendrites, providing important guidance for modified separators.
In the experiment, Fu Sun used the 2352 model separator from celgard. The
separator is composed of one layer of PE (melting point 135°C) and two layers of
PP (melting point 165°C). The longitudinal tensile strength of the separator is
1900kg/cm2, and the longitudinal tensile strength of the separator is
1900kg/cm2. It is only 135kg/cm2 and the puncture strength is 300g/cm2. The
principle of this experiment is shown in the figure below.
The state of the cross section of the battery at the beginning is shown in
Figure b below. It can be seen that the surfaces of the two electrodes are very
smooth. Then the battery is charged and discharged. The charge and discharge
curve is shown in Figure D below. After charging for 13.5 hours, the voltage
drops rapidly (red tip). The indicated position), indicating that the battery
has a short circuit.
The three-dimensional structure of the battery after short circuit is shown
in the figure below. It can be seen from the image that the initially flat and
smooth electrode surface has transformed into a rough surface containing a large
number of holes. During the first discharge process, a charge transfer of
approximately 0.986C occurred, and approximately 10-5 mol of Li was transferred
from the negative electrode to the positive electrode, thus creating a vacancy
at the negative electrode. Therefore, the separator was pushed under the action
of the growing Li microstructure of the positive electrode. towards this vacancy
(Fig. b-e). It can then be seen that the growth of the lithium microstructure
generates great stress, causing the three-layer composite separator to begin to
crack, and the middle part melts due to the high temperature generated by the
short circuit, which also indicates that the current dry stretching The
three-layer composite separator prepared by the process cannot withstand the
huge stress caused by the growth of lithium microstructure.
The study found that the deposition process of lithium is very uneven. The
lithium structure produced in the middle of the positive electrode was produced
during the first discharge process, while during the first charge process, some
lithium microscopic structures were mainly produced on the surface of the
negative electrode around it. structure. In order to illustrate this phenomenon,
Fu Sun created a function with r as the variable and the volume change as the
output, as shown in the figure below. As can be seen from the blue solid line in
the figure below, for the positive electrode, the closer to the middle position,
the greater the amount of lithium deposition. As can be seen from the blue
dotted line, the lithium deposition of the negative electrode mainly occurs at
the surrounding positions.
So in summary, the short circuit situation in the battery should be like
this. First, although during the charging process, lithium deposition mainly
occurs around the negative electrode, lithium deposition also occurs in the
middle of the negative electrode. Since the diaphragm in the middle position has
been severely deformed after discharge at this time, the strength of the
diaphragm here is very fragile, so the diaphragm is pierced, and the positive
and negative electrodes are short-circuited. Since the diameter of the
short-circuit point is only 2um, the moment The current density reached
470A/cm2. The huge heat generated by the short circuit caused the decomposition
of the SEI film, the melting of the separator and the metal Li, the voltage
dropped rapidly, and then the current was cut off and the temperature dropped
rapidly. This experiment also showed that during the process of internal short
circuit, due to the excessive heat generated instantaneously, the three-layer
composite separator was melted at the same time. Therefore, the obturator
mechanism of the three-layer composite separator is difficult to play a
protective role in the process of internal short circuit. . Therefore, in the
subsequent development and improvement of the separator, we need to further
improve the tensile strength and puncture strength of the separator to avoid the
occurrence of internal short circuit caused by lithium dendrites penetrating the
separator. Secondly, we also need to improve the thermal stability of the
separator (above 165°C) to ensure that the battery does not suffer from thermal
runaway when an internal short circuit occurs. For example, the current
ceramic-coated separator and all-ceramic separator are a good choice. Active
Multifunctional composite separators that inhibit the growth of lithium
dendrites are also a common method to improve the safety of lithium-ion
batteries.
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