Aluminum-plastic film was jointly developed by Japan's Showa and SONY in
1999. It is a packaging material for soft-pack lithium batteries. It is a
high-strength, high-barrier multi-layer composite composed of a variety of
plastics, aluminum foil and adhesives. Aluminum plastic film has a high
technical threshold. In order to ensure the long-term stable operation of the
battery, the aluminum plastic film needs to have extremely high barrier
properties, good cold stamping formability, puncture resistance, stability and
insulation.
The main structure of the aluminum-plastic film: ON (surface layer)/AL
(aluminum layer)/Cpp (resin layer) is composed of three layers of materials, of
which the outermost nylon layer mainly plays a protective role; aluminum is the
base material, which plays a waterproof and barrier role; Polypropylene
separates the aluminum from the battery core to prevent leaked liquid from
corroding the aluminum. The adhesive bonds the nylon layer to the aluminum layer
on the one hand and the aluminum layer to the polypropylene layer on the other
hand.
The main use of aluminum-plastic film for lithium batteries is as packaging
material for battery cells. In terms of name, it is different from hard-shell
packaging materials such as steel shells and aluminum shells. Therefore, lithium
batteries packaged with aluminum-plastic films are often called soft-pack
lithium batteries. .
Aluminum-plastic film is a multi-layer film composed of an outer nylon
layer (ON), adhesive, middle aluminum foil (Al), adhesive, and inner heat
sealing layer (Cpp). It is a package for soft-pack lithium batteries. Material.
Aluminum-plastic films for batteries are required to have the following
characteristics:
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%
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(1) It has extremely high barrier properties;
(2) Has good heat sealing performance;
(3) The material is resistant to electrolyte and strong acid corrosion;
(4) It has good ductility, flexibility and mechanical strength.
The main components of aluminum plastic film are three layers of ON/AL/Cpp,
and each layer is bonded with an adhesive. According to the different composite
processes, aluminum-plastic films can be divided into two types: dry method and
thermal method, as shown in Figure 1. The dry process is that aluminum and
polypropylene are bonded with an adhesive and then directly pressed together.
The thermal process is that aluminum and polypropylene are bonded with Mpp, and
are thermally pressed by slowly increasing the temperature and pressure.
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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The main advantages of dry process aluminum plastic film are good deep
forming effect, good appearance consistency, and less prone to pinholes, fish
eyes, impurities and other defects, but its electrolyte resistance is poor. The
thermal process can improve the adhesion between the Al layer and the pp layer,
greatly improving the ability of the inner surface layer to prevent the
electrolyte from swelling and falling off. However, this specially treated Mpp
requires a higher temperature to melt it to achieve a bonding effect. , after
cooling, due to the large difference in shrinkage coefficient with PP, it is
easy to curl inward.
The functions of each part of ON/AL/Cpp:
Nylon: It can effectively prevent the penetration of air, especially
oxygen, maintain the environment inside the battery core, and ensure that the
packaging aluminum foil has good deformation ability.
Al: It can effectively prevent the penetration of moisture in the air,
maintain the environment inside the battery core, and has a certain thickness
and strength to prevent external damage to the battery core.
pp: It will not be dissolved or swollen by organic solvents in the battery
core. It is the most direct packaging protection and insulation for the internal
environment of the battery core. It effectively prevents the internal
electrolyte from contacting the Allayer and prevents the Allayer from being
corroded.
Aluminum plastic film stamping forming process
At present, the company actually uses the following two stamping processes
for production:
(1) Scalable punching: The aluminum-plastic film is under greater pressure
from the blank holder. The edge of the aluminum-plastic film is fixed and the
fixed part does not deform. It is difficult to compensate for the punching
depth. The edge part is compensated by the bottom, and the depth is adjustable.
It has poor performance and is rarely used by enterprises now;
(2) Compensatory punching: The edge pressure of the aluminum-plastic film
is adjustable, and the punching part of the aluminum-plastic film can be
compensated by the edge and the substrate. The overall movement and deformation
of the aluminum-plastic film has uniform thickness and strong punching
ability.
Aluminum-plastic film is an indispensable packaging material for soft-pack
batteries. The continued increase in demand for soft-pack batteries in the 3C,
energy storage and power fields may drive the market demand for aluminum-plastic
films.
Soft-packed batteries can be designed into different sizes according to
customer needs. After the outer dimensions are designed, corresponding molds
need to be opened to stamp the aluminum-plastic film. The forming process is
also called pit punching, which uses a forming mold to punch out a pit on the
aluminum-plastic film that can be used to roll the core.
After the aluminum plastic film is punched, it is cut and formed, generally
called a pocket bag. Generally, when the battery core is thin, you choose to
punch a single pit. When the battery core is thick, you choose to punch a double
pit. When punching the double pit, it will be due to the deformation of one
side. The assembly breaks through the deformation limit of the aluminum-plastic
film and causes rupture.
Sometimes, according to design requirements, a small pit will be made on
one side of the pit to expand the volume of the air bag.
4. Packaging side sealing and top sealing process
The packaging process includes two processes: top sealing and side sealing.
First, place the rolled core into the punched pit, and then fold the unpunched
side in half along the punched pit.
After the aluminum-plastic film is loaded into the roll core, the locations
that need to be sealed include the top sealing area, side sealing area, first
sealing area, and second sealing area.
After placing the roll core into the aluminum-plastic film, the
aluminum-plastic film can be placed in the clamp and sealed in the top sealing
and side sealing machines.
The top seal is to seal the tabs. The tabs are made of metal (aluminum for
the positive electrode and nickel for the negative electrode). How do they fit
together with the pp package? This is accomplished by a small component on the
lug - the lug glue.
During packaging, the PP in the tab glue and the PP layer of the
aluminum-plastic film melt and bond to form an effective packaging
structure.
5. Liquid injection and pre-sealing process
After the soft-packed battery core is sealed on the top side, it needs to
be x-rayed to check the parallelism of the core, and then it goes into the
drying room to remove moisture. After standing in the drying room for a certain
period of time, the liquid injection and pre-sealing process begins.
After the battery core is sealed on the top side, there is only an opening
on the side of the air bag, which is used for liquid injection. After the liquid
injection is completed, the air bag side needs to be pre-sealed immediately,
also called a seal.
After one seal is completed, the inside of the cell is completely isolated
from the external environment.
6. Resting, forming, and fixture shaping
After the liquid injection and sealing are completed, the battery core
needs to be left to rest. Depending on the production process, it is divided
into high-temperature resting and room-temperature resting. The purpose of
resting is to allow the injected electrolyte to fully infiltrate the cell, and
then You can use it to make it into a chemical.
The formation process is the first charge of the battery cell, but it will
not charge to the highest voltage, and the charging current is also very small.
The purpose of formation is to form a stable SEI film on the electrode surface,
which is also equivalent to a process of activating the battery core.
A certain amount of gas will be produced during this process, which is why
an air bag is reserved.
Some factories will use fixtures to form cells, put the cells in the
fixtures and then put them on the forming cabinet. The gas generated in this way
will be fully squeezed into the air bag, and the electrode interface after
formation will be better.
After the formation is completed, some cells, especially thick cells, will
deform to a certain extent due to large internal stress. Therefore, some
factories will set up a shaping process after formation, also called fixture
baking.
7. Second sealing process
During the second sealing, the air bag is first punctured with a guillotine
and vacuumed at the same time, so that the gas and part of the electrolyte in
the air bag will be extracted. Then immediately carry out the second sealing to
ensure the airtightness of the battery core. Finally, cut off the air bag, and a
soft-packed battery core is basically formed.
8. Subsequent processes
After cutting the air bag for the second seal, you need to trim and fold
the edges to ensure that the width of the battery core does not exceed the
standard. The folded batteries will enter the capacity classification cabinet
for capacity classification, which is actually a capacity test.
After the capacity division is completed, cells with qualified capacity
will enter subsequent processes, including appearance inspection, yellow glue
application, edge voltage detection, tab transfer welding, etc. Several
processes can be added or deleted according to customer needs, and finally OQC
inspection is performed. Then package and ship.
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