Key Takeaways & Executive Findings
- •• • Iodine cathodes offer a theoretical specific capacity of 211 mAh g−1 and a moderate redox potential of ~1.3 V, but suffer from poor conductivity and polyiodide shuttle, limiting cycle life and rate performance. • • Physical confinement in porous carbons with high specific surface area (e.g., >1000 m2 g−1) effectively traps iodine species, but alone is insufficient to suppress shuttling; chemical adsorption via nitrogen doping enhances binding of I3− and I2, improving capacity retention. • • Electrocatalytic hosts, particularly single-atom catalysts (e.g., Co, Zn) anchored on nitrogen-doped porous carbon, significantly accelerate iodine redox kinetics, enabling high-rate performance and long-term cycling stability (e.g., >10,000 cycles at high current densities). • • Multi-electron iodine redox pathways (e.g., four-electron I−/I2/I+ and twelve-electron I−/I2/I+/IO3−) can theoretically boost energy density, but require stabilization of I+ via halogen coordination (e.g., Cl−, Br−) or other strategies to overcome high activation barriers.
Abstract
Aqueous zinc-iodine (Zn-I2) batteries are promising for large-scale energy storage due to their intrinsic safety, low cost, and high theoretical capacity (211 mAh g−1 for iodine). However, their practical application is hindered by the poor electronic conductivity of iodine, sluggish redox kinetics, and the shuttle effect of polyiodides. This review systematically analyzes the reaction mechanisms of iodine cathodes, including two-electron (I−/I2) and multi-electron (I−/I2/I+ and I−/I2/I+/IO3−) pathways, and identifies key bottlenecks. It then comprehensively summarizes recent advances in iodine host materials, categorized into three strategies: physical confinement, chemical adsorption, and electrocatalysis. Representative host materials such as porous carbons, covalent organic frameworks (COFs), porous aromatic frameworks (PAFs), polymers, MXenes, and Prussian blue analogs (PBAs) are discussed, with emphasis on the structure–performance relationships. The review highlights that heteroatom doping (e.g., nitrogen) enhances chemical adsorption of iodine species, while single-atom catalysts (e.g., Co, Zn) provide electrocatalytic sites that accelerate conversion kinetics. Finally, future research directions are proposed, including exploration of multi-electron systems, mechanistic elucidation of iodine conversion, development of advanced host materials, and optimization of zinc anodes, to accelerate the commercialization of Zn-I2 batteries.
1. Introduction
Aqueous zinc-iodine (Zn-I2) batteries have emerged as a leading candidate for next-generation energy storage due to their intrinsic safety, low cost, and high theoretical energy density. However, their commercial viability is constrained by three critical challenges: the poor electronic conductivity of iodine, sluggish redox kinetics, and the severe shuttle effect of soluble polyiodide intermediates (e.g., I3−, I5−). These issues lead to low active material utilization, rapid capacity decay, and poor coulombic efficiency, hindering practical deployment.
To address these bottlenecks, extensive research has focused on designing advanced iodine host materials that can effectively confine iodine species, enhance adsorption, and catalyze redox reactions. This review systematically categorizes host materials into three functional strategies: physical confinement (e.g., porous carbons), chemical adsorption (e.g., heteroatom-doped carbons), and electrocatalysis (e.g., single-atom catalysts). By correlating material structure with electrochemical performance, this work provides a comprehensive framework for rational design of high-performance iodine hosts, aiming to accelerate the industrialization of Zn-I2 batteries.
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DONG Tianyi, SHI Huifa, ZHANG Fan, DONG Chunwei, ZHU Xiaoyang, LAN Hongbo, HUANG Zhenghong (2026). Progress in Iodine Host Materials for Aqueous Zinc-Iodine Batteries: From Physical Confinement, Chemical Adsorption to Electrocatalysis. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61094-3
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Frequently Asked Questions
What are the primary failure mechanisms of Zn-I2 batteries under high-rate cycling, and how do host materials mitigate them?
Under high-rate cycling, the primary failure mechanisms include dissolution of polyiodides (I3−, I5−) leading to active material loss and zinc anode corrosion, as well as sluggish iodine redox kinetics causing polarization and capacity fade. Host materials mitigate these by physical confinement in porous structures (e.g., high-surface-area carbons) to trap iodine species, chemical adsorption via heteroatom doping (e.g., N) to bind polyiodides, and electrocatalytic sites (e.g., single-atom Co or Zn) that accelerate conversion reactions, thereby improving rate capability and cycling stability.
How does the choice of iodine source (solid I2 vs. dissolved I−/I3−) affect the design of the cathode and overall battery performance?
Using solid I2 as the active material requires loading into a conductive host (e.g., porous carbon) via melt infiltration or electrodeposition, but the low conductivity of I2 limits active material content (<50%) and causes volatilization issues. In contrast, using dissolved I−/I3− in the electrolyte avoids electrode drying losses but restricts reactions to the electrode surface, underutilizing the internal host volume. The choice influences host design: for solid I2, high pore volume and conductivity are critical; for dissolved species, surface area and adsorption sites are more important.
What are the scalability challenges for producing high-performance iodine host materials, and what cost-effective alternatives exist?
Scalability challenges include the high cost of single-atom catalysts (e.g., noble metals) and complex synthesis routes for advanced porous materials (e.g., MOF-derived carbons). Cost-effective alternatives include using biomass-derived carbons with heteroatom doping (e.g., N, S) that provide adsorption and catalytic activity at lower cost. Additionally, optimizing the electrolyte composition (e.g., adding halide ions) can stabilize I+ and enable multi-electron reactions, potentially increasing energy density without expensive host modifications.
How do multi-electron iodine redox pathways (e.g., four-electron or twelve-electron) impact the energy density and cycling stability of Zn-I2 batteries?
Multi-electron pathways can theoretically increase specific capacity beyond the conventional two-electron (I−/I2) reaction, potentially boosting energy density. For example, the four-electron pathway (I−/I2/I+) and twelve-electron pathway (I−/I2/I+/IO3−) involve higher oxidation states, but they require high overpotentials and stabilization of reactive intermediates (e.g., I+). Strategies such as halogen coordination (e.g., Cl−, Br−) can stabilize I+ and enable these reactions, but they may introduce side reactions and reduce cycling stability. Current research focuses on balancing energy density gains with long-term durability.
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