Rhombic Dilithium Coordination Amplifies Folding-Resistance of Polyamide-Imide Films under Ultra-Large Curvatures

As flexible displays progress toward greater foldability and lightweight designs, addressing the issue of creasing in colorless substrates during ultralarge curvature folding becomes an urgent challenge in flexible electronics. In this paper, we propose a strategy to fabricate rhombic dilithium coordination to amplify the folding resistance of polyamide-imide (CPAI) during ultralarge curvature folding (folded 400,000 cycles at 0.5 mm radius without fracture). Due to the steric hindrance from the CPAI macromolecule and the inherent packing driving force of lithium chloride, dilithium coordination emerges within the concentration range of 0.2–0.8 wt %. The dilithium coordination not only effectively occupies the dangling amide bonds of the CPAI macromolecule but also exhibits stretchable/compressible deformation properties, extending the sliding space with a distance of 5.45–7.45 Å. The obtained CPAI-Li films could endure 400,000 folding cycles without fracture at a folding radius of 0.5 mm, far exceeding the 3.0 mm. Additionally, the dilithium coordination structure in the CPAI-Li films enhances transparency (T₅₅₀ = 89.71%) and exhibits excellent thermal properties (T_g = 407.69 °C; CTE = 8.4 ppm K^–1) and mechanical performance (σ_max = 165.52 MPa, E = 5.10 GPa, ε_b = 19.21%), making them promising candidates for flexible display applications.