Lithium battery separator production technology—dry and wet methods. The
production process of lithium battery separators is complex. The many
characteristics of lithium battery separators and the difficulty in taking into
account their performance indicators determine that the technical barriers to
its production process are high and its research and development is difficult.
According to the difference in micropore formation mechanism, the membrane
process can be divided into two types: dry method and wet method.
The many characteristics of lithium battery separators and the difficulty
in balancing their performance indicators determine that the technical barriers
to its production process are high and its research and development is
difficult. The separator production process includes raw material formula and
rapid formula adjustment, micropore preparation technology, independent design
of complete sets of equipment, and many other processes. Among them, micropore
preparation technology is the core of the lithium battery separator preparation
process. According to the difference in micropore formation mechanism, the
separator process can be divided into two types: dry method and wet method.
1. Dry separator process
The dry separator process is the most commonly used method in the
preparation process of lithium battery separators. This process is to mix
polymers, additives and other raw materials to form a uniform melt, and form a
lamellar structure under tensile stress during extrusion. The heat-treated sheet
The crystal structure is obtained into a hard elastic polymer film, which is
then stretched at a certain temperature to form slit-like micropores, and a
microporous film is obtained after heat setting. At present, dry processes
mainly include dry uniaxial stretching and biaxial stretching.
Dry single pull
Dry single drawing uses polyethylene (PE) or polypropylene (PP) polymers
with good fluidity and low molecular weight. It uses the manufacturing principle
of hard elastic fibers to first prepare polyolefin cast sheets with high
orientation and low crystallinity. After low-temperature stretching forms
micro-defects such as silver streaks, high-temperature annealing is used to pull
the defects apart, thereby obtaining a microporous film with uniform pore size
and uniaxial orientation.
2. Wet separator process
The wet process uses the principle of thermally induced phase separation to
mix plasticizer and polyolefin resin, and utilizes the phenomenon of
solid-liquid phase or liquid-liquid phase separation that occurs during the
cooling process of the molten mixture. The film is pressed and heated to close
to the melting point. Temperature post-stretching makes the molecular chain
orientation consistent, and after a certain period of heat preservation, the
plasticizer is extracted from the film with a volatile solvent, thereby
producing an interconnected sub-micron size microporous membrane material.
The wet process is suitable for producing thinner single-layer PE
separators. It is a preparation process with better thickness uniformity, better
physical and chemical properties and mechanical properties of lithium battery
separator products. Depending on whether the orientations are simultaneous
during stretching, the wet process can also be divided into two types: wet
bidirectional asynchronous stretching process and bidirectional synchronous
stretching process.
Wet simultaneous stretching process
The process flow of wet synchronous stretching technology is basically the
same as that of asynchronous stretching technology, except that it can be
oriented in both transverse and longitudinal directions simultaneously during
stretching, eliminating the need for a separate longitudinal stretching process
and enhancing the uniformity of the thickness of the separator. However, the
problems with synchronous stretching are firstly the slow speed of the vehicle,
and secondly the slightly poor adjustability. Only the transverse stretch ratio
is adjustable, while the longitudinal stretch ratio is fixed.
Compared with dry separators, wet separators have better material
properties in terms of thickness uniformity, mechanical properties (tensile
strength, puncture resistance), air permeability, and physical and chemical
properties (wettability, chemical stability, safety). It is excellent and is
conducive to the absorption and retention of electrolyte and improves the
charge, discharge and cycle capabilities of lithium batteries, and is suitable
for high-capacity batteries. From the perspective of product strength, the
comprehensive performance of wet separators is stronger than that of dry
separators. Wet coating is the development direction of lithium battery
separators.
The main steps and principles of dry and wet lithium battery separator
processes
1. Dry method - first melt, extrusion and blowing the polyolefin resin to
form a crystalline polymer film, then perform crystallization heat treatment and
annealing operations to obtain a highly oriented film structure, and then
stretch it at high temperature , test the crystal cross-section separation to
form a porous structure battery separator. The dry process can also be divided
into uniaxial stretching and biaxial stretching.
2. Wet method - Traditional wet method preparation is mainly based on phase
inversion method, and in recent years, TIPS thermally induced phase separation
method is mainly used. The principle is to mix crystalline polymers,
thermoplastic polymers and small molecule chemical diluents with high boiling
points (such as paraffin oil) to form a homogeneous solution at high
temperature, and then lower the solution temperature to cause solid-liquid phase
separation of the mixture. Or liquid-liquid separation, extracting and removing
small molecule chemical diluents to form a porous membrane of thermoplastic and
crystalline polymers.
The main characteristics of the wet process are high cost, large
investment, high equipment requirements, long construction and production cycle,
large energy consumption during the production process, and the use of organic
solvents. However, the wet process can better control the pore size,
distribution and porosity, so it is generally used to manufacture high-end
films.
Compared with foreign companies, the separators produced by domestic
companies were mainly dry-stretched, and most of them were mid- to low-end
products. High-end wet-process separators have been monopolized by foreign
companies for many years. However, in recent years, as domestic separator
manufacturers have continued to improve their wet production processes, the
output and performance of wet separators have become closer and closer to the
levels of foreign companies. Domestic companies have rapidly expanded the
production of wet separators, and the pattern of the separator market has also
changed. Variety.
As one of the four major materials of lithium batteries, separators do not
participate in the electrochemical reactions in the battery, but key properties
such as battery capacity, cycle performance, and charge and discharge current
density are directly related to the separator.
The separator is an important part of the lithium-ion battery and an
important component that supports the electrochemical process of charging and
discharging the lithium-ion battery. It is located between the positive and
negative electrodes inside the battery, ensuring the passage of lithium ions
while blocking electron transmission. The performance of the separator
determines the interface structure, internal resistance, etc. of the battery,
and directly affects the battery's capacity, cycle, safety performance and other
characteristics. Excellent separators play an important role in improving the
overall performance of lithium batteries.
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