The essence of a pouch lithium battery is simply a liquid lithium-ion battery encased in a polymer shell. Its superior performance owes much to the aluminum-plastic film within the polymer shell. Let's take a closer look at this remarkable aluminum-plastic film.
I. Introduction to Aluminum-Plastic Film
Aluminum-plastic film is a multilayer film composed of an outer nylon layer, an adhesive, a middle aluminum foil layer, and an inner heat-sealing layer. It is the encapsulation material for soft-pack lithium batteries. Compared with other commonly used plastic films, aluminum-plastic film has the following advantages:
1) Aluminum-plastic film has extremely high barrier properties: 10,000 times higher than ordinary aluminum-plastic composite film;
2) It has good cold stamping formability: it can be adapted to meet the new battery cell model according to the customer's product requirements.
3) Excellent puncture resistance: Due to the copper and aluminum mesh burrs around the polymer lithium battery cell causing pitting corrosion, electrochemical reactions are accelerated. When vacuuming and shrinking, the burrs will pierce the inner membrane, possibly even through the aluminum foil. The hydrofluoric acid inside the cell will directly corrode the aluminum foil, changing the electrolyte composition. In severe cases, it will corrode through the aluminum foil and cause leakage, which will also cause a short circuit and render the battery unusable. Therefore, puncture resistance is particularly important.
4) Excellent electrolyte stability: Many organic solvents tend to swell, dissolve, and absorb flexible packaging materials. They are excellent solvents for adhesives, damaging the bonding effect of the composite layer, altering the concentration of components in the electrolyte, and consequently affecting the battery's electrical performance. Furthermore, the presence of highly corrosive hydrofluoric acid will severely corrode the aluminum foil, causing the inner film to separate from the foil and eventually corroding the foil through, thus damaging the entire packaging. The excellent electrolyte stability of the aluminum-plastic film ensures the safety of the entire battery.
5) Aluminum-plastic film is resistant to high temperature and has strong insulation: The metal electrode sheet is about 100u thick. When heat-sealing at 170℃ and 3kg/cm2 pressure, if there is no high temperature resistant insulation layer in the inner film, the metal electrode will often be pressed onto the packaging aluminum foil, causing a short circuit, scrapping the battery and reducing the yield. Therefore, high temperature resistance and insulation are essential properties of the shell.
II. Structure and Classification of Aluminum-Plastic Film
The main components of aluminum-plastic film consist of three layers: ON, ALCPP, and PVC. The outer ON layer (nylon layer) is a decorative ON layer that also protects the ALC layer from scratches. The middle ALC layer is for shaping and preventing moisture intrusion. The inner CPP (polypropylene) layer is an electrolyte-resistant layer.
Based on different manufacturing processes, aluminum-plastic films can be divided into the following two categories. The following are typical structural diagrams of DNP (Dutch Printing) and Showa Denko aluminum-plastic films:
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Dry-process batteries have a wide range of applications; 95% of Showa ALF's production is dry-process, mainly used in high-energy, high-capacity batteries for mobile phones, MP3 players, and MP4 players. They are also widely used in high-rate, high-capacity power batteries for electric vehicles and model aircraft. Thermal-process batteries, on the other hand, are only suitable for batteries with lower capacity requirements.
III. Preparation Process of Aluminum-Plastic Film
Broadly speaking, the preparation methods of aluminum-plastic film can be divided into two categories: dry method and heat sealing.
Dry process: AL and CPP are bonded together with an adhesive and then directly pressed together. Advantages include improved molding performance, short-circuit protection, better appearance (fewer impurities, pinholes, and fisheyes), and better cutability. Additionally, it offers good resistance to electrolytes and water resistance.
Heat sealing: ALF and CPP are bonded together with MPP, and then heat-sealed under slow heating and pressurization conditions, which is a lengthy process. Furthermore, the prolonged high-temperature baking causes ALF to become brittle, leading to a deterioration in deep-drawing performance. Its advantages lie only in electrolyte resistance and water resistance, while its deep-drawing performance, short-circuit protection, appearance, and cutting performance are all unsatisfactory.
IV. The Dilemmas Faced by Domestic Aluminum-Plastic Film Manufacturers
Currently, only a very few Chinese aluminum-plastic film manufacturers can match the performance of Japanese companies in a few technical indicators. The main problems are insufficient electrolyte resistance and drawing depth. The drawing depth of domestic aluminum-plastic film is around 5mm, while that of foreign products is 8mm, or even 15mm, indicating a significant overall gap. Particularly concerning are the reaction conditions and precision control. In the manufacturing process of such high-precision materials, even the slightest error can lead to significant problems; the difficulty of process control lies precisely in the minute details. Currently, domestic aluminum-plastic film manufacturers face the following five major challenges:
1) Outdated and highly polluting aluminum foil surface treatment process: The use of water washing to remove oil and "chromium anhydride" passivation results in significant pollution. Furthermore, due to the surface tension of water, the oil in the micropores of the aluminum foil cannot be completely cleaned, leading to defects in subsequent processes.
2) The water treatment process for aluminum foil can cause hydrogen embrittlement, resulting in poor tear resistance of domestically produced aluminum-plastic film;
3) The aluminum foil surface is not coated with a UV layer, the aluminum foil is not stiff enough, it is prone to wrinkling during dry lamination, and wide-width products cannot be produced with poor yield;
4) For CPP, domestically, a coating machine is mainly used for coating. When laminated with a highly thermally conductive aluminum foil surface, it is prone to curling and layered crystal formation.
5) Due to limitations in domestic adhesive formulation technology, the product is prone to delamination and peeling.
Domestic lithium battery manufacturers are facing enormous cost pressures and are in urgent need of reducing the cost of lithium battery raw materials. As a result, the demand for import substitution and domestic production of aluminum-plastic film is becoming increasingly prominent.
