# Layered Gradient Grain Structure Enhances Mechanical Properties of Ultra-Thin Copper Foil

## TL;DR

This paper introduces a new method to create ultra-thin copper foils with enhanced strength and ductility using a layered gradient grain structure.

## Contribution

A scalable electrodeposition method to fabricate ultra-thin copper foils with a controllable 'fine–coarse–fine' gradient grain structure.

## Key findings

- The 3L-ABA copper foil achieves a tensile strength of 604 ± 18 MPa and elongation of 3.6 ± 0.25%.
- Gradient grain structure enables synergistic strengthening and toughening through fine-grain and dislocation effects.
- Additive A promotes grain refinement, while Additive B supports grain growth through electrochemical polarization.

## Abstract

Traditional homogeneous copper foils suffer from a trade-off between strength and ductility, while gradient or heterogeneous structures are mostly based on deformation processing, making it difficult to achieve controllable construction within a thickness of ≤10 μm. This study aims to directly construct a layered structure with a “fine–coarse–fine” (A-B-A) gradient grain distribution, denoted as 3L-ABA in an 8 μm copper foil via direct current electrodeposition, which utilizes composite additives to regulate electrochemical polarization and nucleation modes. Through systematic characterization and mechanical testing, it was found that the 3L-ABA copper foil exhibits a tensile strength of 604 ± 18 MPa, an elongation of 3.6 ± 0.25%, and low surface roughness Rz of 0.46 μm. Microscopic mechanism analysis demonstrates that the gradient structure achieves synergistic strengthening and toughening through surface fine-grain strengthening, intermediate coarse-grain coordinated plastic deformation, combined with dislocation density and twin strengthening. Electrochemical tests confirm that Additive A (containing collagen, bis-(3-sulfopropyl)-disulfide (SPS), thiourea and 2-mercapto-5-benzimidazolesulfonic acid sodium salt (2M5S)) induces strong cathodic polarization, promoting instantaneous nucleation and grain refinement, whereas Additive B (containing collagen and bis-(3-sulfopropyl)-disulfide (SPS) shows weaker polarization and promotes grain growth. This research provides a scalable electrodeposition solution for the microstructural design and performance regulation of ultra-thin copper foils.