# Quantifying synergistic interactions of ternary additives for microstructural control in ultrathin Li-ion battery copper foils

## TL;DR

This study explores how combining three additives improves the strength and quality of ultrathin copper foils used in lithium-ion batteries.

## Contribution

A low-cost ternary additive system is proposed to enhance mechanical and microstructural properties of ultrathin copper foils.

## Key findings

- SPS promotes densification and grain morphology transition in copper foils.
- Collagen enables ultra-grain refinement, while HEC improves surface flattening and mechanical strength.
- Synergistic optimization increases elongation by 229.0%, tensile strength by 188.2%, and reduces surface roughness by 73.8%.

## Abstract

The advancement of lithium-ion batteries toward higher energy density and safety necessitates the development of ultrathin copper foil current collectors (such as 6 µm or thinner) with enhanced mechanical properties. However, reducing the foil thickness invariably compromises its mechanical integrity, posing safety risks. This study investigates a low-cost ternary additive system comprising 3,3′-dithiodipropanesulfonic acid disodium salt (SPS), collagen, and hydroxyethyl cellulose (HEC) for the direct current (DC) electrodeposition of 6 µm copper foils and the effects of individual and composite additives on microstructural evolution (morphology, grain orientation, and roughness) and mechanical performance (tensile strength and elongation). A quantitative correlation between the additives and the microstructure–mechanical properties of the copper foil is established. The results reveal complementary mechanisms, where SPS promotes deposit densification and grain morphology transition from conical to hill-like; collagen facilitates ultra-grain refinement; and HEC dominates surface flattening and mechanical enhancement. Through synergistic optimization, exceptional properties are achieved, where compared to the additive-free foils, the elongation increases by 229.0% (to 4.58%), the tensile strength increases by 188.2% (to 661.4 MPa), and the surface roughness decreases by 73.8% (Rz to 0.84 µm). This work not only elucidates the microscopic synergy of SPS/collagen/HEC but also proposes a scalable, cost-effective strategy for the industrial production of high-performance ultrathin copper foils.

The advancement of lithium-ion batteries toward higher energy density and safety necessitates the development of ultrathin copper foil current collectors (such as 6 µm or thinner) with enhanced mechanical properties.