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

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% (R_z 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.