Bifunctional Electrolyte Additive Enables In Situ Solid Electrolyte Interphase Formation and Restrains Polyiodides Shuttling for High-Efficiency Zinc–Iodine Batteries

Aqueous zinc–iodine batteries (ZIBs) exhibit great promise for energy storage applications owing to their high safety, cost-effectiveness, and environmental friendliness. However, the side reactions of the Zn anode and the shuttle effect of polyiodides hinder the practical application of ZIBs. Herein, we propose sodium alginate (SA) as a bifunctional electrolyte additive to address both issues simultaneously. At the Zn anode, SA reconstructed the Zn2+ solvation sheath and preferentially adsorbed on the anode surface, inducing the in situ formation of a stable composite solid electrolyte interphase (SEI) that effectively suppressed hydrogen evolution and dendrite growth. At the I2 cathode, the oxygen-containing polar groups of SA coordinated with polyiodides, anchoring them in the cathode region and substantially inhibiting the shuttle effect. In this work, the Zn//Zn symmetric cell with 5 mM SA achieved a long cycling lifespan of 2400 h at 1 mA cm–2 and 1 mAh cm–2. Meanwhile, the Zn//I2 full cell maintained a high discharge specific capacity of 141.1 mAh g–1 after 8000 cycles at 2 A g–1. This study simultaneously improved the electrochemical behavior of both the anode and cathode via a facile modification strategy, providing new insights for the design of ZIBs electrolytes.