What is the hydrophilic property of copper foil for lithium-ion batteries?

1. The concept of copper foil

Copper foil is a cathode electrolytic material made of copper and a certain proportion of other metals. It is used as a conductor and is an important material for the manufacture of copper clad laminates (CCL) and printed circuit boards (PCB). Copper foil has low surface oxygen characteristics and can be attached to various substrates, such as metals, insulating materials, etc., and has a wide temperature range. Electronic information and lithium batteries are the mainstream application fields of copper foil. Compared with electronic copper foil, lithium battery copper foil has higher performance requirements.

2. Classification of copper foil

Lithium batteries generally only distinguish between rolled foil and electrolytic foil. The following is a comparison of the production process of rolled foil and electrolytic foil.

3. Performance requirements of copper foil for lithium-ion batteries

Copper foil is both a carrier of negative electrode active materials in lithium ion batteries. It is also the collector and conductor of negative electrode electrons. Therefore, it has special technical requirements, that is, it must have good electrical conductivity, the surface can be evenly coated with the negative electrode material without falling off, and it must have good corrosion resistance.

Currently commonly used adhesives such as PVDF, SBR, PAA, etc., their bonding strength not only depends on the physical and chemical properties of the adhesive itself, but also has a great relationship with the surface characteristics of the 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 cycle, or peeling off the substrate due to excessive expansion and contraction, reducing the cycle capacity retention rate. Conversely, if the bond strength is not too high, as the number of cycles increases, the internal resistance of the battery increases due to the heavy peeling of the coating, and the cycle capacity attenuation increases. This requires copper foil for lithium ion batteries to have good hydrophilicity.

4. The principle of hydrophilicity of copper foil

As we all know, rolled copper foil and electrolytic copper foil are not only completely different in production methods, but more importantly, their metal structures are also completely different. Studies have shown that the main peak in the XRD diffraction pattern of electrolytic copper foil with a thickness of less than 12μm is the (111) plane, and the (311) plane shows a certain preferred orientation. With the increase of the thickness of the copper foil, the diffraction peak intensity of the (220) plane With continuous improvement, the diffraction intensity of other crystal planes gradually decreases. When the thickness of the copper foil reaches 21μm, 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 as the rolled copper foil.

Water is composed of hydrogen atoms and oxygen atoms. The electronegativity of hydrogen is 2.1 and the electronegativity of oxygen is 3.5. Therefore, the O-H bond in water molecules is very polar. Experiments show that the angle between the two O-H bonds in the water molecule is 104°45'. The dipole moment of the water molecule is not equal to zero, and the "center of gravity" of the positive charge does not coincide with the "center of gravity" of the negative charge, so that one end of the hydrogen atom is positively charged, and the end of the oxygen atom is negatively charged, showing strong polarity. Water molecules are very polar molecules.

Polar molecules have a certain affinity due to their mutual electrostatic attraction, so substances composed of polar molecules must have an affinity for water. Any substance that has an affinity for water is called a hydrophilic substance. Metal inorganic salts and metal oxides are all substances with a polar structure. They have a strong affinity with water, so they are all hydrophilic substances.

The molecular structure of some substances is symmetrical and therefore not polar. Non-polar molecules have affinity for non-polar molecules, but have no affinity for polar molecules. This is a conclusion based on the principle of mutual dissolution of substances with similar structures. A substance composed of non-polar molecules, whose molecules 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. Some polar functional groups, such as hydroxyl (-OH), amino (-NH2), carboxyl (-COOH), carbonyl (-COH), nitro (-NO2), etc., are introduced into hydrophobic substances to make them have a certain Polarity and therefore hydrophilicity. The so-called hydrophilicity is a simple description of the affinity of a substance to water; for solid substances, its hydrophilicity is generally called wettability.

Regarding the wetting angle, the contact angle θ between metal and water is generally less than 90°, so the rougher the copper foil surface, the better the wettability; when θ>90°, the rougher the solid surface, the worse the surface wettability. As the surface roughness increases, the easily wettable surface becomes easier to wet, and the hard to wet surface becomes more difficult to wet.

5. Test standard for hydrophilicity of copper foil

Lithium-ion battery manufacturers are very simple to test the hydrophilicity of rolled copper foil. They only use a brush to gently brush pure water on the surface of the copper foil to observe whether there is any water film rupture.

6. Factors affecting the hydrophilicity of copper foil

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

6.2 Hydrophilicity is related to the metallographic structure of copper foil

Scanning electron microscopy (SEM) shows that copper foil with good hydrophilicity has fine grains and relatively low surface roughness. The raw foil with low surface roughness has good hydrophilicity after surface treatment. This is mainly due to the finer the pellet grains of the electrolytic copper foil, the larger its real specific surface area; and the larger the surface roughness, the lower its real surface area, which leads to a decrease in the hydrophilicity of the copper foil.

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

If the copper foil is placed in the air for a long time, the non-polar gas molecules N2, 02, CO2 in the air will be adsorbed on the metal surface, thereby changing the hydrophilicity of the copper foil. For example, after exposing a copper foil with good hydrophilicity to the air for 90 minutes, its hydrophilicity decreases significantly. This is because metal surfaces with high specific surface energy are easily wetted by liquids with low surface tension, because the wetting process reduces the free energy of the system. The specific surface energy of the new metal surface is higher (the specific surface energy of copper is about 1.0 J/m2, and that of aluminum and zinc is about 0.7-0.9 J/m2), but if the surface of the copper foil is especially the surface of the new electrolytic copper foil When exposed to the air, it will adsorb many gas molecules to form a single molecule adsorption layer. The presence of surface pressure significantly reduces the wettability of the copper foil surface.

In addition to non-polar gas molecules, the surface of copper foil may also absorb dust and organic oil in the air, making it more hydrophobic. Therefore, the packaging of copper foil for lithium ion batteries must adopt vacuum packaging to reduce the oxidation of the copper foil surface and maintain the hydrophilicity of the copper foil.