Zn ²⁺ ‐Driven Crystalline Reorganization Enables Strong and Thermally Stable Tunicate Cellulose Filaments

ABSTRACT Tunicate‐derived cellulose nanofibers (CNFs) exhibit exceptional intrinsic mechanical properties; however, translating these attributes to macroscale fibers remains challenging because dense packing and long‐range alignment are difficult to achieve. Here, we report a coordination‐driven assembly of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl‐oxidized CNFs using multivalent metal cations (Ca 2+ , Cu 2+ , Zn 2+ , Fe 3+ , and Al 3+ ) to direct microstructural evolution. We find that specific coordination chemistries trigger pronounced structural reorganization; notably, Zn 2+ induces a phase transformation in which native cellulose Iβ crystallinity is replaced by a robust metal‐coordinated network. This transformation produces filaments with outstanding mechanical performance, achieving a tensile strength of 581.8 ± 29.4 MPa and a Young's modulus of 18.8 ± 3.3 GPa, exceeding previously reported metal‐ion‐crosslinked cellulose composites. Coordination further enhances thermal stability, increasing the onset degradation temperature by up to 48.6°C. These results establish a direct link between ionic coordination and crystalline restructuring, providing a scalable route to strong, heat‐resistant, and sustainable structural fibers.