Crosslinked Sodium Alginate Binder for Mechanically Robust and Long‐Life Hard Carbon Anodes in Sodium‐Ion Batteries

Hard carbon is widely regarded as one of the most promising anode materials for sodium‐ion batteries owing to its low cost, intrinsic structural stability, and suitable interlayer spacing for sodium‐ion accommodation. Nevertheless, its practical deployment remains constrained by mechanical degradation and limited electrochemical durability. As a key component governing electrode integrity, the binder critically influences cycling stability and mechanical robustness. Sodium alginate, a water‐soluble and environmentally benign biopolymer, has attracted increasing attention as an alternative binder for strong adhesion and low irreversible capacity. In this work, a 3D gel binder is developed via cross‐linking, forming a “rigid‐flexible” interpenetrating network that enhances mechanical cohesion and stress dissipation. Comprehensive structural characterization verifies successful PMA incorporation and robust hydrogen‐bonding network formation. Benefiting from this engineered architecture, the binder substantially enhances both the mechanical properties and electrochemical performance of hard carbon anodes, delivering a high initial coulombic efficiency and excellent cycling stability with 94.0% capacity retention after 500 cycles, and remarkable rate capability of 249.9 mAh g −1 at 1 A g −1 . Furthermore, full cells with a low N/P ratio of 1.12 retain 78.3% capacity after 800 cycles, demonstrating strong potential for practical applications.