What are the requirements of polymer Nickel Metal Hydride No. 5 battery for soft packaging materials?

(1) Extremely high barrier properties

The water vapor transmission rate should be less than 1×10-4~1×10-6g/m2

d1atm; the oxygen transmission rate should be less than 1×10-1~1×10-3g/m2

d1atm; this requirement is about 10,000 times higher than the barrier property of ordinary aluminum-plastic composite materials.

It can be seen that the thickness of aluminum foil of general soft packaging materials must be greater than 30um to prevent the slow penetration of water vapor. Therefore, it is difficult for ordinary composite materials to meet the requirements. Generally, thicker aluminum foil is used, and a composite structure of 3 to 7 layers is required, and various composite methods are applied in combination. The structures of soft packaging materials are generally the following three types:

a:ON/AL/CPP; b:PET/ON/AL/CPP; c:PET/AL/PET/EMAA

Structures a and b are more suitable for molding packaging. Structure c is only suitable for general packaging.

(2) Good heat sealing performance

Polymer lithium-ion batteries are sensitive to high temperatures and are generally used at temperatures below 60°C. This requires that the heat sealing temperature of the soft packaging material should be as low as possible while meeting the heat sealing strength requirements. Generally speaking, the heat sealing strength should not be less than 35N/15mm and the heat sealing temperature should not be higher than 150°C. If a higher temperature is used, cooling measures must be taken. The heat sealing time should generally not exceed 3s to prevent heat radiation and heat conduction from damaging the battery cell.

(3) The inner layer material does not react with the electrolyte. The inner layer material of the soft packaging material cannot be dissolved or swollen by the electrolyte. If the inner layer material is dissolved by the electrolyte, the dissolved components will undergo a chemical reaction and produce gas; if the inner layer material is swollen by the electrolyte, the concentration or component ratio of the electrolyte will change and affect the performance of the battery cell.

The electrolyte used for polymers is an organic electrolyte composed of a variety of lipids. The electrolyte lithium salt in the electrolyte will decompose into highly acidic hydrofluoric acid in the presence of water. This requires that the bonding layer between the inner layer material and the aluminum foil is not affected by the electrolyte and the hydrofluoric acid produced by the electrolyte to prevent the inner layer material and the aluminum foil from peeling off and affecting the barrier properties of the packaging material. Many Japanese manufacturers now use a special adhesive that is resistant to electrolyte and hydrofluoric acid for the composite layer of the inner layer material and the aluminum foil of the soft packaging materials to prevent the material from peeling off after being soaked in the electrolyte. The peel strength between the aluminum foil and the inner layer material directly affects the sealing performance of the battery.

(4) Good ductility, flexibility and mechanical strength The production process of polymer lithium-ion batteries places high demands on the flexibility of packaging materials. The widely used cold press molding packaging method also places high demands on the ductility of packaging materials. Safety assurance during use also places high demands on the mechanical strength of soft packaging materials.

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