Separator-Free Aqueous Zinc-Iodine Batteries: An In-Situ Dual-Confinement Interface Enabling Electrostatic Reversible Polyiodide Capture/Release

Abstract

Separator-free battery design can maximize volumetric energy density by eliminating the separator-induced ion-transport barrier. However, its application in aqueous zinc-iodide (Zn-I2) batteries has been hampered by severe polyiodide shuttling and uncontrolled Zn dendrite growth, which limits cycle life and stability. Herein, we demonstrate a high-performance separator-free Zn-I2 battery enabled by an in-situ engineered interface layer (NCNF, ~24 μm) on the Zn anode via a simple one-step casting of Ni-MOF modified cellulose nanofibers (CNF). The complementary interactions between CNF’s polyiodide adsorption and Ni-MOF anions’ electrostatic repulsion establish a dual-confinement barrier with synergistically strengthened adsorption capability, enabling electric-field-driven reversible capture/release of polyiodides to promote cathode conversion. Meanwhile, interactions between polar groups within NCNF and water molecules effectively reduce the free-water activity and weaken Zn2+ desolvation, thereby suppressing corrosion and dendrite formation. Benefiting from these coupled effects, the separator-free Zn-I2 batteries achieved an ultra-long cycle life of 43,000 cycles at 5.0 A g-1 with 80.7% capacity retention, and delivered 4.06 mAh cm-2 with a high iodine loading of 21.8 mg cm-2. Additionally, a flexible pouch cell delivered 122.1 mAh g-1 after 2000 cycles at 1.0 A g-1, highlighting the potential of interface-engineered, separator-free configurations for practical, high-energy aqueous batteries.