<p>Long-duration energy storage is critical for integrating renewable energy, yet few technologies simultaneously achieve low cost, long cycle life and high safety. Here we report a flowing Zn slurry (FZS) battery in which nanoscale Zn particles dispersed within a conductive network undergo reversible Zn/Zn²⁺ redox conversion with enhanced stability. Ligand-assisted confinement coordinates onto Zn nanoparticle surfaces, suppressing excessive Zn growth and parasitic reactions, enabling uniform, monodisperse Zn nanocrystal deposition throughout the slurry. In FZS | |Cu asymmetric cells at 8 mA cm⁻², it achieves a Coulombic efficiency of 99.94%, whereas symmetric cells operate reversibly for 5,128 h at 22.5 mA cm⁻², 135 mAh cm⁻² under continuous flow. Extending from these results, FZS | |MnO₂ full cells retain 81.1% capacity after 5,500 cycles at 10 A g⁻¹, and FZS | | O₂ full cells deliver 1.65 Ah over 100 h at 1.35 mA cm⁻². This work demonstrates the viability of FZS for long-duration energy storage, providing a framework for scalable, stable metal-slurry-based flow battery systems.</p>