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Copper foil is a negative electrolytic material made from copper and a certain proportion of other metals. It is used as a conductor and is an important material in the manufacturing of copper clad laminate (CCL) and printed circuit board (pCB). Copper foil has low surface oxygen characteristics and can be attached to various substrates, such as metals, insulation materials, etc. It has a wide temperature range of use. Electronic information and lithium battery are the mainstream application fields of copper foil, and lithium battery copper foil has higher performance requirements compared to electronic copper foil.

1. Classification of copper foil

In lithium batteries, only calendered foil and electrolytic foil are generally distinguished. Below is a comparison of the production processes of calendered foil and electrolytic foil.

2. Performance requirements for copper foil for lithium-ion batteries

Copper foil serves as a carrier for negative electrode active materials in lithium-ion batteries. It is also the collection and transmission conductor of negative electrons. Therefore, there are special technical requirements for it, that is, it must have good conductivity, the surface can be evenly coated with negative electrode materials without falling off, and have good corrosion resistance.

The bonding strength of commonly used adhesives such as pVDF, SBR, pAA, etc. not only depends on the physical and chemical properties of the adhesive itself, but also has a great relationship with the surface characteristics of copper foil. When the bonding strength of the coating is high enough, it can prevent the negative electrode from powdering and falling off during the charging and discharging cycle, or peel off the substrate due to excessive expansion and contraction, reducing the cycling capacity retention rate. On the contrary, if the bonding strength does not meet the requirements, the internal impedance of the battery will increase and the cycle capacity will decline with the increase of the number of cycles. This requires copper foil for lithium-ion batteries to have good hydrophilicity.

3. The principle of hydrophilicity of copper foil

As is well known, the production methods of rolled copper foil and electrolytic copper foil are not only completely different, but more importantly, their metal structures are also completely different. Research shows that the thickness is less than 12 μ The main peak in the XRD diffraction pattern of m electrolytic copper foil is the (111) plane, and the (311) plane exhibits a certain preferred orientation. As the thickness of the copper foil increases, the diffraction peak intensity of its (220) plane continues to increase, while the diffraction intensity of other crystal planes gradually decreases when the thickness of the copper foil reaches 21 μ At m, the texture coefficient of the (220) crystal plane reaches 92%. Obviously, it is almost impossible to simply rely on the production process to achieve the same performance of electrolytic copper foil as rolled copper foil.

Water is composed of hydrogen atom and oxygen atom. The electronegativity of hydrogen is 2.1, and the electronegativity of oxygen is 3.5, so the polarity of O-H bond in water molecule is very strong. Experiments have shown that there is an angle of 104 ° 45 'between the two O-H bonds in water molecules. The dipole moment of water molecules is not equal to zero, and the "center of gravity" of positive and negative charges do not coincide, resulting in one end of the hydrogen atom being positively charged and one end of the oxygen atom being negatively charged, exhibiting strong polarity. Therefore, water molecules are highly polar molecules.

Polar molecules have a certain affinity to each other due to their electrostatic attraction, so substances composed of polar molecules must have affinity to water. Any substance that has affinity for water is called a hydrophilic substance. Metal inorganic salts and metal oxides are substances with polar structures that exhibit strong affinity with water, making them both hydrophilic substances.

Some substance molecules have symmetrical structures and therefore do not have polarity. Non polar molecules have affinity to non polar molecules, but no affinity to polar molecules, which is based on the principle of mutual solubility of substances with similar structures. A substance composed of non-polar molecules that have no affinity for water molecules is called a hydrophobic substance.

In organic chemistry, "oil" is the general term for non-polar organic liquids, so hydrophobic substances must have lipophilic properties. Introducing some polar functional groups, such as hydroxyl (- OH), amino (- NH2), carboxyl (- COOH), carbonyl (- COOH), nitro (- NO2), etc., into hydrophobic substances can give them a certain degree of polarity, resulting in hydrophilicity. The so-called hydrophilicity refers to a simple description of the affinity of a substance to water; Regarding solid substances, their hydrophilicity is generally referred to as wettability.

Regarding wetting angle, contact angle between metal and water θ Generally less than 90 °, the rougher the surface of copper foil, the better its wettability; When θ At 90 °, the rougher the solid surface, the poorer the surface wettability. With the increase in surface roughness, surfaces that are easy to wet become easier to wet, while surfaces that are difficult to wet become more difficult to wet.

4. Testing standards for hydrophilicity of copper foil

The hydrophilicity test of rolled copper foil by lithium-ion battery production companies is very simple. Simply use a brush to gently brush pure water on the surface of the copper foil and observe the occurrence of water free film rupture.

5. Factors affecting the hydrophilicity of copper foil

The relationship between the hydrophilicity of copper foil and the surface roughness of copper foil is not obvious

Hydrophilicity is related to the metallographic structure of copper foil

Through scanning electron microscopy (SEM), it can be observed that copper foil with good hydrophilicity has fine grains and relatively low surface roughness. Raw foil with low surface roughness still has good hydrophilicity after surface treatment. This is important because the finer the pellet grain size of electrolytic copper foil, the larger its true specific surface area; The larger the surface roughness, the lower the true surface area, resulting in a decrease in the hydrophilicity of the copper foil.

Hydrophilicity is related to the surface state and reaction of copper foil

If the copper foil is left in the air for a long time, non polar gas molecules N2, 02, C02, etc. in the air will adsorb on the metal surface, thereby changing the hydrophilicity of the copper foil. For example, after exposing copper foil with good hydrophilicity to air for 90 minutes, its hydrophilicity significantly decreases. This is because metal surfaces with higher surface energy are easily wetted by liquids with lower surface tension, as the wetting process reduces the free energy of the system. The specific surface energy of new metal surfaces is relatively high (about 1.0J/m2 for copper, 0.7-0.9J/m2 for aluminum and zinc), but if the surface of copper foil, especially the surface of new electrolytic copper foil, is exposed to air, it will adsorb many gas molecules and form single molecule adsorption

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