Lithium iron battery separator production process, lithium battery
separator production process principle. As one of the four core materials of
lithium batteries, separators are an important part of determining the
performance, safety and cost of lithium batteries. The production process of
lithium battery separators is divided into two categories: dry production
process and wet production process. Dry production process It is further
subdivided into one-way stretching process and two-way stretching process.
Lithium iron battery separator production process
The production process of lithium iron battery separators is mainly divided
into dry stretching and wet stretching. The many characteristics that lithium
battery separators need to possess put forward special requirements for its
production process, which includes raw material formula and rapid formula
adjustment, micropore preparation technology, and independent design of complete
sets of equipment.
The dry uniaxial stretching process is a method of producing hard elastic
fibers to prepare highly oriented polypropylene or polyethylene films with low
crystallinity. During the high-temperature annealing process, a film with high
crystallinity is obtained. This film is first stretched at low temperatures to
form micro-defects, and then at high temperatures the defects are pulled apart
to form micropores. At present, more than one-third of China's production
capacity uses the dry double drawing process, and its products occupy a large
proportion of the mid- to low-end market.
Wet production process, also known as phase separation method or thermal
phase separation method, wet process mixes liquid hydrocarbons or some small
molecular substances with polyolefin resin. After heating and melting, a uniform
mixture is formed, and then the temperature is cooled for phase separation and
pressing. The membrane is obtained, and then the membrane is heated to a
temperature close to the melting point, biaxially stretched to orient the
molecular chains, and finally kept warm for a certain period of time. The
residual solvent is eluted with volatile substances to prepare interconnected
microporous membrane materials.
The wet production process can not only prepare interconnected microporous
membrane materials, but also produce lithium battery separators with high
longitudinal and transverse strength. At present, the wet production process is
mainly used to produce single-layer lithium battery separators.
Lithium battery separator production process principle
1. Dry method - The polyolefin resin is first melted, extruded and blown to
form a crystalline polymer film, then crystallized heat treatment and annealing
operations are performed to obtain a highly oriented film structure, and then
stretched 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 lithium battery separator 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.
The raw material of dry process is generally PP, while the raw material of
wet process is generally PE. Generally speaking, the melting temperature of PP
is around 170°C, and the melting temperature of PE is around 140°C. Therefore,
although the separator produced by the wet process is thinner, the lower melting
temperature makes the separator easy to shrink at high temperatures, causing a
short circuit in the battery, and the safety of the battery cannot be
guaranteed. Coating a layer of inorganic nanoparticles or
high-temperature-resistant organic chemicals on the surface of the separator can
improve the high-temperature safety performance of the separator and can well
make up for this shortcoming of the wet process.
Lithium battery separator dry single drawing process flow
Feeding: After preprocessing raw materials such as PE or PP and additives
according to the formula, they are transported to the extrusion system.
Casting: The pretreated raw materials are melted and plasticized in the
extrusion system and then the melt is extruded from the die. The melt forms a
base film with a specific crystal structure after casting.
Heat treatment: The base film is heat treated to obtain a hard elastic
film.
Stretching: The hard elastic film is cold stretched and hot stretched to
form a nano-porous film.
Slitting: Cut the nano-microporous membrane into finished membranes
according to customer specifications.
Wet asynchronous stretching process
Feeding: PE, pore-forming agent and other raw materials are pretreated
according to the formula and transported to the extrusion system.
Casting: The pretreated raw materials are melted and plasticized in a
twin-screw extrusion system and then the melt is extruded from the die. After
casting, the melt forms a cast thick sheet containing a pore-forming agent.
Longitudinal stretching: Longitudinal stretching of cast thick sheets.
Transverse stretching: The cast thick sheet after longitudinal stretching
is stretched transversely to obtain a base film containing a pore-forming
agent.
Extraction: The base film is extracted with a solvent to form a base film
without pore-forming agents.
Shaping: The base film without pore-forming agent is dried and shaped to
obtain a nano-porous membrane.
Slitting: Cut the nano-microporous membrane into finished membranes
according to customer specifications.
The performance of diaphragm products is affected by the base material and
manufacturing process. The stability, consistency, and safety of the separator
have a decisive impact on the discharge rate, energy density, cycle life, and
safety of lithium batteries. . From the perspective of product strength, the
comprehensive performance of wet separators is stronger than that of dry
separators.
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