Failure-Guided Coupling of Interphase and Solvation Chemistry for Durable Aqueous Zn-S Batteries

Aqueous Zn-S batteries offer high theoretical energy density and intrinsic safety, but iodine-mediated sulfur conversion introduces severe chemical and electrochemical instability at Zn anode. Herein, life-cycle-resolved three-electrode measurements reveal that, although sulfur utilization gradually decreases, the rapid growth of Zn anode polarization and eventual internal short circuit dominate terminal cell failure. Guided by this failure diagnosis, iodoacetamide (IAM) is introduced as a multifunctional additive to couple interphase and solvation regulation. Interfacial spectroscopy and depth profiling indicate the in situ formation of a gradient interphase comprising an N-containing organic-rich outer region and a ZnS/ZnSO₃-rich inorganic inner region, which suppresses polyiodide corrosion and homogenizes Zn²⁺ flux. Spectroscopic analyses and molecular simulations further show that IAM modulates the local Zn²⁺ coordination environment and water hydrogen-bond network, accompanied by enhanced ion pairing and a lower apparent interfacial activation barrier for Zn deposition. As a result, Zn||Cu cells achieve over 2000 cycles with an average coulombic efficiency of 99.91%, while full Zn-S cells retain 484.5 mAh g^-1 after 550 cycles at 20 A g^-1, corresponding to 75.95% capacity retention (27.26% without IAM). This work highlights the need to balance iodine-enabled sulfur conversion with Zn stability, offering a practical design principle for durable aqueous Zn-S batteries.