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

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.