Technical characteristics of battery foil
What we usually call battery foil now refers to the non-modified aluminum foil used in lithium battery current collectors, which means it should be distinguished from other foils used on batteries. On the one hand, it is the electrode of the current collector, and on the other hand, it serves as the carrier of the positive or negative electrode material of the lithium battery, that is, the lithium battery material is coated on it. Therefore, a more accurate definition should be the aluminum foil for the current collector of the lithium battery.
Properties that current collector aluminum foil should have
Due to its special use conditions, current collector aluminum foil has very strict requirements on its various technical indicators. It is the leader in the aluminum foil family. Some specific requirements are as follows:
Foreign matter: Lithium batteries must have high safety during use, so foreign matter (impurities) on the surface should be strictly controlled. If foreign matter control is not carried out, it cannot be used as a qualified battery foil. The foreign matter can be aluminum powder or magnetic substances ( iron powder), and other substances. The entire production process must be protected to reduce the possibility of introducing foreign matter. In the finished product process, a device for detecting foreign matter must be set up and accurately measured. Foreign matter should be ≤50mg/300,000m2.
Wettability: Wetting performance mainly depends on the liquid contact angle. The liquid contact angle is when a liquid droplet does not fully expand on the solid surface. At the meeting point of the gas, liquid, and solid phases, the contact angle θ is used to determine the wetting of the solid by the liquid (the horizontal line of the solid-liquid interface and the gas-liquid interface The angle θ between the interface tangents is called the contact angle): when θ < 900, it is called wetting: when θ > 90*, it is non-wetting: when θ = 00 or does not exist, it is called complete wetting; θ = At 1800 it is completely non-wetting. θ=900 is the dividing line between wetting and non-wetting.
Since the oil on the surface of aluminum foil generally cannot be completely removed, and the uneven surface wrinkles will have a great impact on the surface tension, it is not practical to directly measure the contact angle, so the dyne value is used in production practice to reflect the lubrication. Wetting ability, the higher the dyne value, the better the wetting performance. However, some high-end customers, especially those in Japan and other places, still use the contact angle method to measure the wettability of aluminum foil surfaces. In order to be consistent with the customer's testing method, a small number of domestic manufacturers currently use the contact angle method. However, this method is greatly affected by various factors, measuring instruments and measurement methods, and the measurement data obtained can only be used as a reference. At present, most manufacturers use the surface dyne value measurement method.
Dyne value is a popular name. To be precise, it should be the surface tension coefficient. It mainly expresses the size of surface tension, that is, the force pulling each other between two adjacent parts of the liquid surface within unit length. The unit of surface tension is N/m in the SI system, but dyn/cm (dyn/cm) is still commonly used, 1dyn/cm=1mN/m. Generally, the wettability of finished aluminum foil is required to be ≥32mN/m.
Surface wetting tension is one of the most important technical indicators of battery foil. It affects the bonding quality of the coating, especially the coating firmness of the carbon-coated foil. When the dyne value is low, the aluminum foil and the adhesive material will not bond well. Defects such as tightness and missing paint. The dyne value of the aluminum foil surface must be >32 dyn/mm, and some carbon-coated products with high requirements may even reach 34 dyn/mm.
At present, although the national standards have clear regulations on the detection methods of dyne value, in the actual operation process, different companies and enterprises have different detection methods, so the detection results will often vary greatly. With the rapid development of the battery foil market, it is particularly important to further standardize and unify dyne value detection methods and judgment standards.
Plate shape: Plate shape is a professional term in the non-ferrous metal industry, but in the battery foil industry, it is called tension. This difference in name is mainly due to different industries. But whether it is plate shape or tension, it expresses one meaning, which is the straightness of the product. In the battery industry, two terms are used to express the product shape quality: tension and sag. Although this is a completely unscientific term, it will not change as it has been widely recognized in the battery industry. For plate shape, the battery industry has formed its own set of standards and measurement devices, so When defining terms in the battery industry, it changed the original definition of the aluminum processing industry.
In short, the tension is the shape of the plate, and the amount of sagging is the specified tension and the sagging amount of the edge of the product under the specified length.
Shape quality is a key technical indicator for battery foil. In the future, with the development of the industry and the continuous improvement of battery product accuracy, users will have higher and higher requirements for the shape of aluminum foil products. How to optimize the process through process And technological innovation to continuously improve the shape quality of products is an important issue facing the aluminum foil industry.
