Introduction
With the rapid development of the new energy industry, lithium-ion batteries require increasingly higher energy density. The negative electrode current collectors for lithium batteries are moving toward thinner, higher-tensile, and higher-ductility designs.
In the field of copper foil for lithium batteries, core technical parameters include copper foil thickness, tensile strength, elongation, roughness, elastic modulus, and surface wettability.
Technical Specifications
Industry practice typically classifies lithium battery copper foil by tensile strength into categories: ordinary tensile (300-400 MPa), medium tensile (400-500 MPa), high tensile (500-600 MPa), and ultra-high tensile (>600 MPa).
Elongation performance must be considered in combination with actual foil thickness. For example, 6 μm lithium battery copper foil generally has conventional elongation >4% and high-elongation >6%; 8 μm commonly has conventional elongation >5% and high-elongation >8%.
It is worth noting that some suppliers report elongation for high-elongation 6 μm copper foil in the 10-20% range, but downstream manufacturers often measure much lower elongation because of differences in cutting dies. The elongation figures referenced here follow downstream battery manufacturer standards.
Because there is no unified industry standard for high-elongation, high-tensile lithium battery copper foil, specifications are typically defined according to downstream requirements.
Tensile Mechanism
Tensile strength of copper foil is influenced by both thickness and grain size. With constant thickness, tensile strength increases as grain size decreases. For comparable grain sizes, tensile strength is proportional to thickness. The relationship varies across different thickness-to-grain-size ratios: when the thickness-to-grain-size ratio is small (<4), tensile strength is proportional to thickness; when the ratio is large (>15), tensile strength is inversely related to thickness.
At constant thickness, the increase in tensile strength with decreasing grain size can be explained by grain refinement strengthening: grain boundaries have higher free energy relative to grain interiors and act as barriers to dislocation motion. Under external loading, significant shear stress must develop at grain boundaries to produce shear deformation in adjacent grains.
The transfer of slip from a plastically deformed grain to neighboring grains depends on whether the stress concentration produced by dislocation pile-ups near grain boundaries can activate dislocation sources in adjacent grains. Grain refinement increases the number of grain boundaries; if boundary structure remains unchanged, greater external force is required to form dislocation pile-ups, thereby strengthening the material.
Therefore, tensile strength under the influence of grain size is proportional to the degree of dislocation pile-up.
Engineering High Mechanical Performance at the Microstructure Level
Research shows that during electrodeposited foil manufacturing, adding specific additives can optimize grain size, morphology, texture, and internal stress of copper foil microstructure, enabling controlled production of high mechanical performance copper foil. Measures such as grain refinement, selecting appropriate crystallographic texture, and improving bath cleanliness can significantly increase the tensile strength and elongation of high-tensile copper foil.
For example, a (220) texture is favorable for improving tensile strength, while a (200) texture helps increase elongation. Maintaining high bath cleanliness and adopting equiaxed grains or nano-twin structures can further enhance mechanical properties.
Application Advantages
After outlining the theory behind high-tensile and high-elongation lithium battery copper foil, the specific application advantages of high-tensile, high-ductility copper foil include:
- Improved coating and calendaring efficiency at downstream battery factories, reducing web breaks and increasing production efficiency.
- Ability to increase negative electrode active material loading, which improves battery energy density.
- Enhanced electrode compaction density and reduced electrode thickness, contributing to higher lithium-ion battery energy density.
- Better suppression of deformation caused by active material expansion and contraction during electrochemical cycling, enhancing battery durability.
