Hydrophobic–Hydrophilic Balance in Copolymer Binder Enables High-Loading, High-Rate, and Stable Flexible Aqueous Zinc-Ion Batteries

Flexible aqueous zinc-ion batteries are promising for wearable electronics, yet their energy density is limited by conventional hydrophobic binders that impede ion transport in thick cathodes. Here, we propose ethylene vinyl alcohol (EVOH) as an excellent aqueous binder to address this challenge. The unique copolymer structure, combining hydrophilic vinyl alcohol and hydrophobic ethylene units, achieves an optimal hydrophilicity/hydrophobicity balance. This design enables excellent electrolyte affinity and robust structural stability without crosslinking. Compared to polyvinylidene fluoride (PVDF), EVOH facilitates uniform dispersion of active materials and carbon nanotubes, forming a robust, free-standing electrode with high mechanical strength. The abundant hydroxyl groups in EVOH significantly enhance ion transport kinetics, enabling a high-rate capability up to 40C and a remarkable areal capacity of 3.9 mAh cm−2 at a mass loading of 33.1 mg cm−2. Furthermore, EVOH effectively encapsulates the cathode particles, suppressing Jahn–Teller distortion and ensuring long-term cyclability with 94.5% capacity retention after 1000 cycles at 10C. A flexible battery prototype demonstrates stable operation under repeated bending, powering LEDs consistently. This work provides an effective binder design strategy for developing high-performance, flexible aqueous batteries.