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.