Catalysis in Aqueous Zinc–Iodine Batteries: Materials and Mechanisms

Abstract

Aqueous zinc–iodine (Zn–I₂) batteries are promising for safe and sustainable energy storage, yet their application is constrained by sluggish iodine conversion kinetics and severe polyiodide shuttling. Recent advances have established cathode catalysis as a pivotal strategy to tackle these challenges. This review systematically analyzes the catalytic mechanisms and structure–performance relationships in catalyst design for Zn–I₂ battery cathodes and categorizes the catalytic functions into three synergistic mechanisms, namely, chemisorption of polyiodides, acceleration of interfacial electron transfer via electronic structure engineering, and regulation of reaction pathways to circumvent intermediate accumulation. Our statistical analysis identifies the prevailing catalyst categories and metal elements in this field, and reveals a clear correlation between catalytic performance and the d-band center descriptor of metal active sites, providing a theoretical basis for understanding the observed structure–performance trends. The review further discusses design strategies including pore engineering, heterojunction construction, and coordination environment modulation. This review aims to provide theoretical guidance for the rational design of high-performance cathodes for Zn–I₂ batteries and related conversion-type systems.

Graphical abstract

Keywords

Aqueous zinc–iodine batteries / Cathode catalysis / Structure–performance relationship / Reaction mechanism

Highlight

	● A mechanism-centric framework categorizing catalytic functions into chemisorption of polyiodides, interfacial electron transfer and regulation of reaction pathways.
	● Statistical analysis maps catalyst development trends and establishes the d -band center as a universal descriptor for activity trends across metal sites.
	● Identification of key practical bottlenecks toward high-loading, wide-temperature, and high-energy Zn–I2 batteries.

1 Introduction

Aqueous zinc–iodine (Zn–I₂) batteries are regarded as one of the most promising candidates for next-generation safe, cost-effective energy storage systems, yet their practical viability remains an open question [1-3]. Despite the high theoretical capacity of iodine and the inherent safety of aqueous electrolytes, a persistent gap remains between laboratory achievements and practical application. At the heart of this gap lie two long-standing obstacles, namely, the sluggish redox kinetics of iodine species and the relentless polyiodide shuttle effect. These intertwined issues not only undermine rate capability and Coulombic efficiency but also provoke parasitic anode corrosion, rendering the overall system far less durable than its electrochemistry would suggest [4-6].

Confining iodine within porous carbon hosts, such as microporous or hierarchically porous frameworks, has typically been the most straightforward countermeasure, yet it ultimately proves insufficient. Physical entrapment cannot resolve the fundamental instability of soluble polyiodides but can only postpone their escape [7]. This limitation has shifted the focus toward catalytic cathode architectures that go beyond merely trapping iodine species and actively facilitate their conversion. Over the past five years, a rapidly growing body of research has demonstrated that judiciously engineered catalytic sites, spanning single-atom catalysts, metal-organic frameworks, molecular catalysts, and heterojunctions, can accelerate interfacial electron transfer, lower activation barriers, redirect reaction pathways, and, in some cases, unlock multi-electron iodine redox chemistry [8-10]. While the field has advanced beyond early proof-of-concept demonstrations toward systematic mechanistic investigations, the growing diversity of reported catalyst systems and competing proposed mechanisms have made the literature increasingly difficult to synthesize and navigate [11].

Existing reviews typically organize the field by material class, such as carbon hosts, metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), single–atom catalysts (SACs), and the like, and offer useful summaries of synthesis, structure, and performance for each category. While these serve as valuable entry points, a review that synthesizes mechanistic principles across material boundaries and distills structure–performance relationships from the accumulated data is still missing [12-14]. We present such a mechanism-centric review, motivated by the growing repository of mechanistic insights and the pressing need to transcend empirical, case-by-case material screening.

Through literature retrieval, we obtained a total of 113 research articles related to zinc–iodine battery cathode catalysts. Mechanism-related keywords were extracted from the abstracts of each paper for frequency analysis. In addition, information on publication year, catalyst name, catalyst type, main metal element, iodine loading, cycling capacity, and cycle number were systematically collected for statistical analysis. If a data point was not reported, it was excluded from the corresponding results. In this review, we move beyond a materials-based classification and organize the discussion directly around catalytic function. According to statistical analysis, we first map out catalyst development trends and establish performance benchmarks. We then categorize the reported catalytic functions into three mechanistic types: chemisorption of polyiodides [22-25], interfacial electron transfer [26-31], and regulation of reaction pathways (Fig. 1) [32-38]. For each mechanism, we examine how specific structural features, including coordination environment, electronic configuration, and pore architecture, translate into electrochemical function, with particular emphasis on electronic structure descriptors such as the d-band center that can rationalize performance trends across different metal centers. Our objective is to provide a coherent framework that connects catalyst design to electrochemical function, offering practical guidance for the development of Zn–I₂ cathodes and, more broadly, for other conversion-type battery systems facing analogous catalytic challenges.

2 Catalytic Materials for Zn–I₂ Batteries

Based on a statistical analysis of the published literature, the annual publication output of different catalyst categories for Zn–I₂ batteries show a clear diverging trend. Among them, metal-based materials and SACs exhibit the most prominent increase, reflecting the growing research interest in regulating iodine redox reaction pathways and enhancing active site utilization at the atomic level (Fig. 2A) [39-45]. Keyword frequency analysis of 113 article abstracts shows that “adsorption”, “redox kinetics”, and “electronic structure” appear most frequently, corresponding to polyiodide adsorption, interfacial redox kinetics, and electronic structure modulation [46-52]. The high frequencies of “active sites”, “coordination”, and “single atom” suggest that designing atomic-level active sites and their coordination environments is a key strategy. Meanwhile, the frequencies of “energy barrier”, “pathway”, and “shuttle effect” indicate that catalysts are widely studied for their ability to suppress the shuttle effect by lowering energy barriers and altering conversion pathways. Based on this analysis, this review discusses the catalytic mechanisms of Zn–I₂ battery catalysts from three perspectives: chemisorption of polyiodides, interfacial electron transfer, and regulation of reaction pathways (Fig. 2B) [53-57].

Statistical analysis of metal element frequencies shows that Fe, Co, Ni, and Zn are the most frequently reported elements. These elements are mainly incorporated into metal-based composites and SACs (Fig. 2C) [58-61]. The Sankey diagram illustrates the relationships among metal elements, catalyst structures, catalytic mechanisms, and iodine loading [62-67]. Among metal elements, 66.6% of Fe-based catalysts are SACs, while 64.3% of Co-based catalysts are metal-based materials. Regarding catalytic mechanisms, 61.1% of SACs and 56.5% of metal-based materials are dominated by interfacial electron transfer. Notably, this mechanism shows the highest proportion of high iodine loading cases (> 2 mg·cm^–2), reaching 41.1%, suggesting that interfacial electron transfer, especially on atomic-scale active sites, favors iodine redox catalysis and high areal capacity (Fig. 2E). Based on these statistics, SACs generally maintain high specific capacities under high cycle numbers in coin cells at room temperature with relatively low iodine loadings (< 2 mg·cm^–2) [68-71]. By contrast, metal-based composites are sometimes tested under higher loading conditions, where capacity may be limited by mass transport. Overall, SACs exhibit a favorable balance between specific capacity and cycle life, though their performance advantages should be interpreted within the specific testing conditions of each study (Fig. 2D) [72-75].

3 Catalytic Mechanisms of Iodine Cathode Catalysts

Although the three catalytic functions of chemisorption, interfacial electron transfer, and reaction pathway regulation are conceptually different, many catalyst systems inherently exhibit multiple effects at the same time. In the following subsections, we classify each catalyst according to the primary function emphasized in the original reports, combined with the results of our text mining analysis. These mechanisms typically work in synergy rather than in isolation.

3.1 Chemisorption of polyiodides

During the charge–discharge process of Zn–I₂ batteries, the iodine cathode undergoes reversible transformations among I⁻, I₃⁻, I₂, and higher order polyiodides. Among these species, soluble polyiodide intermediates tend to diffuse from the cathode side into the electrolyte and further migrate to the surface of the zinc anode, inducing the shuttle effect, self-discharge, and anode corrosion, which severely compromise the capacity retention and cycling stability of the battery [76,77]. To address this issue, constructing host structures with spatial confinement capability and interfacial barrier function is considered an effective strategy to suppress polyiodide migration.

Unlike physical confinement, which merely delays the escape of polyiodides from porous hosts, chemical anchoring via dipole interactions offers a more robust approach by establishing directional electrostatic attractions between polar surface sites and charged polyiodide anions. In this context, polar functional groups and heteroatom-doped sites on host surfaces can serve as electron-rich centers, interacting with I₃⁻ and I₅⁻species through ion–dipole interactions and thereby stabilizing them within the cathode region [78,79].

Chen et al. prepared a porous oxygen-rich salt-templated carbon (OSTC) using H₂O as an oxidant and ZnCl₂ as a salt template (Fig. 3A) [15]. The abundant oxygen-containing functional groups (e.g., C=O, –OH) on the OSTC surface create localized dipole moments that act as polar adsorption sites for iodine species. These electronegative oxygen centers engage in strong ion–dipole interactions with polyiodide anions, effectively immobilizing soluble intermediates and stabilizing reaction species. Benefiting from this polar chemisorption effect, the resulting Zn–I₂ battery achieves a good rate performance (Fig. 3B). Beyond oxygen functionalities, nitrogen-containing polar sites offer complementary chemisorption capability for polyiodides. Ma et al. [80] demonstrated that a MXene composite cathode enriched with triazine-N and secondary amine-N anchors exhibits strong affinity toward iodine species (Fig. 3C). The electronegative nitrogen sites, particularly the triazine-N and secondary amine-N configurations, generate pronounced local dipoles that interact with polyiodides via ion–dipole interactions, effectively suppressing the shuttle effect. Through calculation of the adsorption energies of iodine species on MF structures with differenttriazine-N (0.84 eV) are nearly identical, with secondary amine-N being slightly higher. For I₂ and I₃^–, however, the adsorption energies on triazine-N are significantly higher than those on secondary amine-N. As a result, the assembled Zn–I₂ battery delivers stable cycling performance with negligible capacity decay over extended operation (Fig. 3D). Conjugated polar polymers represent another class of metal free, carbon free hosts capable of dipole-driven polyiodide chemisorption. For example, a protonated polyaniline (C-PANI) catalyst has been reported to regulate iodine conversion chemistry through a “proton–iodine” synergy (Fig. 3E) [81]. The protonated amine and imine nitrogen sites within the PANI backbone create polar surface microenvironments that chemically anchor polyiodides via dipole interactions, while simultaneously facilitating iodine redox kinetics. Density functional theory (DFT) calculations show that C-PANI exhibits lower binding energies toward all iodine species compared to bare PANI, with the most pronounced difference observed for I₃^–. This indicates that C-PANI has a strong capability for chemicoutstanding cycling stability with high capacity retention over prolonged cycling (Fig. 3F).

Distinct from dipole-driven chemisorption, coordination interaction offers another powerful pathway for polyiodide anchoring through Lewis acid-based chemistry [82]. MOFs and COFs, owing to their regular pore structures, accessible metal centers, and tunable interfacial chemistry, demonstrate unique advantages in chemical adsorption. For MOFs, the ordered pore channels not only help restrict the migration of iodine species but also enable selective capture of polyiodides through metal nodes and charged backbones. For instance, Kim et al. [83] designed a MOF-808/glass fiber (GF) composite separator (Fig. 4A). The MOF-808 framework contains open Zr clusters that act as Lewis acidic sites, while the nanometer pores provide size exclusion effects. These features synergistically suppress polyiodide crossover, as confirmed by diffusion tests (Fig. 4B), thereby stabilizing multi electron Zn–I₂ chemistry. Furthermore, the MOF host IL-ZIF-90 containing Zn²⁺ metal sites (Fig. 4C) [84], designed synergistically based on Lewis acid-based interactions and coulombic interactions, exhibits stronger adsorption capability toward soluble intermediates such as I₃⁻, indicating that open metal sites and modulation of local charge distribution can effectively enhance the chemical anchoring of higher order polyiodides. MCOF (Fig. 4D) [17], a Co-induced COF material, features Co centers that act as Lewis acid sites to form stable coordination with polyiodide anions. DFT calculations show that the adsorption energies of different iodine species (I₂, I^–, I₃^–, and I₅^–) on Co-TAPT-Tp-COF are −2.66, −1.31, −0.98, and −0.81 eV, respectively, which are considerably more negative than the corresponding values on TAPT-Tp-COF (−0.07, 0.35, 0.81, and 0.51 eV). Meanwhile, the hybridization between the Co d-orbitals and the framework p-orbitals enhances interfacial electronic coupling, endowing the material with dual functions of adsorption and catalysis. Similarly, Feng et al. [85] synthesized a COF incorporating Ru active centers (Fig. 4E). The Ru sites engage in coordination adsorption with iodine species, resulting in strong polyiodide capture capability (Fig. 4F). The ordered conjugated framework further stabilizes the adsorbed state and prevents premature desorption of intermediates. Quantitative analysis revealed that the adsorption capacity toward I₃⁻ reached 0.43 g·g⁻¹, substantially higher than that toward I⁻ (0.07 g·g⁻¹), confirming the material’s pronounced selectivity for higher order polyiodides (Fig. 4G).

Chemical anchoring strategies based on MOFs and COFs offer a powerful approach to suppress polyiodide shuttling by combining physical confinement with specific chemical interactions. Through the rational selection of metal nodes (e.g., Zr, Zn, Co, Ru), functionalization of framework backbones, and modulation of pore chemistry, the adsorption selectivity and capacity for polyiodides can be tuned. Effective chemisorption can also stabilize the zinc anode. Soluble polyiodides escaping from the cathode can migrate to the anode and trigger side reactions, which consume active zinc, promote non-uniform deposition, accelerate dendrite formation, and induce self-discharge, compromising battery durability. By strongly anchoring polyiodides within the cathode via dipole or coordination interactions, the shuttle effect is effectively suppressed, thereby extending the overall battery lifespan.

3.2 Interfacial electron transfer

Once polyiodides are effectively adsorbed and immobilized within the cathode region, the efficiency of interfacial electron transfer between iodine species (I₂, I⁻, I₃⁻) and the electrode becomes another critical factor determining the rate performance and overpotential. The redox reactions of iodine involve multi-step electron transfer processes (e.g., 2I⁻ \overrightarrow{\leftarrow \phantom{{}_{\text{vbox}to.5ex\text{vss}}}} I₂+2e⁻) [86]. These electrons must traverse the electrode electrolyte interface; if the charge transfer impedance at the interface is high, the reaction overpotential will be significantly increased. Therefore, the kinetic behavior largely depends on the strength of electronic coupling between the active sites and iodine species, the charge transfer impedance, and the carrier transport capability at the interface [87-89].

Single atom M–N–C (M = Fe, Co, Ni, Cu, etc.) composites offer unique advantages in promoting interfacial electron transfer due to their high atomic utilization, well defined coordination structures, and tunable local electronic environments [93-95]. Among these, Fe-based M–N–C structures have attracted extensive attention in Zn–I₂ batteries owing to their strong electron regulation capability, rich coordination chemistry, and favorable conductive support frameworks [96]. Taking Fe single atoms as an example, physical confinement alone can merely restrict polyiodides within the cathode region but fails to accelerate their redox conversion (Fig. 5A). In contrast, the introduction of Fe SACs enhances the electronic coupling between iodine species and the electrode interface through Fe–N₄ active sites [90], achieving a “confinement–catalysis” synergistic effect that significantly reduces the charge transfer impedance (Fig. 5B) and maintains high capacity even at high rates (Fig. 5C). Following a similar design principle, Shi et al. [91] designed a series of Fe doped porous carbons (Fe–N–C, denoted as M9) derived from Zn-MOF precursors via a facile self-assembly method (Figs. 5D and 5E). The resulting Fe–N₄ sites served as electron exchange centers, facilitating interfacial charge transfer between the electrode and iodine species. Electrochemical impedance spectroscopy confirmed a marked reduction in charge transfer resistance for the M9-based electrode (Fig. 5F), which translated into enhanced rate capability and cycling stability. While Fe-based SACs have been extensively studied, Ni single atoms offer complementary advantages. Huang et al. [92] dispersed Ni single atoms (Ni SAs) onto a stacked porous carbon framework to construct NiSAs-HPC (Fig. 5G). Their results indicate that the introduction of Ni single atoms effectively reduces charge transfer impedance (Fig. 5H), thereby accelerating the interfacial charge exchange of soluble polyiodides and leading to superior cycling performance (Fig. 5I).

Beyond single atom catalysts, molecular catalysts offer a distinct paradigm for regulating electron transfer at the molecular scale. Chloro(protoporphyrinato)iron(III) (Hemin) [97], a molecular catalyst containing a five-coordinate electrophilic Fe atom (Fig. 6A), accelerates the conversion of iodide ions through directional electron transfer at the molecular scale and suppresses the shuttle effect by reducing the concentration of triiodide ions. Benefiting from molecular catalysis, the Zn–I₂ batteries achieve stable cycling for over 62,000 cycles with a capacity decay of only 0.00052% per cycle (Fig. 6B). Inspired by click chemistry, Wang et al. [98] designed a Cp(Fe(CO)₂)₂-derived molecular catalyst (Fe–Cp) (Fig. 6C). Unlike conventional physical adsorption, Fe–Cp forms directional and stable Fe–I coordination bonds, effectively locking iodine species within the Fe–Cp complex. Furthermore, Fe–Cp enables axial electron transfer, promoting reversible charge redistribution and dynamic iodine redox conversion. As a result, the Zn–I₂ batteries maintained approximately 100% Coulombic efficiency after more than 63,000 cycles at an ultrahigh current density of 20 A·g⁻¹ (Fig. 6D).

Heterojunction interfaces, through the work function difference between two materials and the consequent band bending at the interface, generate a built-in electric field that accelerates electron transport from the carbon substrate to iodine species. Fu et al. [99] designed a PNC@TiN heterojunction, where nitrogen-doped porous carbon (PNC) was coupled with TiN (Fig. 6E). The work function difference between TiN and PNC induces a built-in electric field at the interface, driving directional electron migration from the carbon framework to iodine species. Moreover, the coupling between Ti 3d, C/N 2p, and I 5p orbitals enhances Ti–I interactions (Fig. 6F). Compared with PNC_I₂, PNC@TiN_I₂ exhibits a higher theoretical specific capacity and a smaller polarization overpotential at 0.2 A·g⁻¹ (Fig. 6G).

From a deeper perspective, both rapid charge transfer at single atom sites and directional electron transfer in molecular catalysts are related to the optimization of the electronic structure of active centers. These optimizations include the d-band position, spin state, and local charge distribution of the metal centers. These electronic characteristics determine the strength of orbital hybridization between the catalyst and iodine species, the adsorption energy, and the binding mode of reaction intermediates [100].

Qu et al. [101] demonstrated the power of spin state engineering by introducing monodisperse Zn atoms adjacent to individual Ni atoms. This atomic scale modification changed the spin state of Ni from high spin to low spin (Fig. 7A). The low spin configuration optimized d−p orbital hybridization between Ni and I atoms and reduced the occupancy of antibonding electrons. UV-vis measurements confirmed that the low spin NiZnN₄ catalyst showed higher selective adsorption toward polyiodide ions than its high spin counterpart (Fig. 7B). Yang et al. [19] used DFT calculations to screen a series of SACs with different d-block transition metal sites. Their results suggested that Nb–NC, owing to its many unoccupied antibonding orbitals, allows effective d−p hybridization between Nb–d and I–p orbitals (Fig. 7C). As a result, Nb–NC with a d-band center of 0.271 eV showed the highest polyiodide binding energy and the lowest reaction barrier for the rate determining step (I₃⁻ → I⁻), which may help suppress the shuttle effect and enhance iodine redox conversion. A Zn‖Nb–NC/I₂ full cell delivered a capacity of 140 mAh·g⁻¹ over 50,000 cycles at 10 A·g⁻¹, with a capacity decay of only 0.00008% per cycle, and exhibited better rate performance compared with NC (Fig. 7D). The pouch cell with a loading mass of 9 mg·cm⁻² could also perform up to 1500 cycles, maintaining a capacity of 1.12 mAh·cm⁻² at 2 mA·cm⁻². By analyzing 41 atomic scale electronic and geometric features, Chen et al. [102] identified two key descriptors: d/f orbital electron occupancy and net charge polarization. Calculations reveal that Ce 5d orbitals stabilize iodine species through strong bonding, while Ce 4f orbitals near the Fermi level introduce antibonding interactions that controllably weaken I–I bonds (Figs. 7E and 7F). The Ce SACs/I₂ cathode achieves an areal capacity of 10.2 mAh·cm⁻² under a high loading of 44.7 mg·cm⁻², and a pouch cell maintains a capacity output of 25.6 mAh·cm⁻² even under an ultrahigh loading of 115 mg·cm⁻², realizing a synergistic catalytic mechanism involving 4f–5d orbitals.

Enhancing interfacial electron transfer is important for improving the kinetics of iodine redox reactions. The performance of a catalyst is closely related to the electronic structure of its active centers, such as d-band position, spin state, and orbital hybridization. By adjusting these electronic properties through strategies like spin state engineering, orbital modulation, and descriptor guided screening, it is possible to lower charge transfer impedance and achieve better rate capability. These findings help to build a clearer picture of how electron transfer works at the catalytic interface and offer useful guidance for designing more efficient cathode materials for Zn–I₂ batteries.

3.3 Regulation of reaction pathways

Distinct from the electron transfer mechanism, which focuses on the rate of interfacial charge transport, the reaction pathways regulation mechanism concerns the specific conversion process of iodine species at the catalytic interface. It gradually steers the reaction pathways from a solution–phase–dominated route, which is accompanied by polyiodide diffusion and intermediate accumulation, toward a reversible conversion pathway characterized by interfacial confinement and controlled evolution of key intermediates. In conventional iodine cathodes, the I₂/I⁻ reaction is often accompanied by the formation and diffusion of soluble intermediates such as I₃⁻, exhibiting a solution phase dominated pattern of “iodine species dissolution–diffusion–reconversion” [103,104]. Although the pathway enables redox reactions, it tends to cause the accumulation of higher order polyiodides in the electrolyte, inducing the shuttle effect, self-discharge, and side reactions. By constructing catalytically active interfacial sites, researchers aim to transform this process into a confinement catalysis pathway that adsorbs and rapidly converts intermediates in situ at the electrode interface, preventing their uncontrolled diffusion into the bulk electrolyte [105].

The Fe–N₄ sites and porous structure in the Al–TCPP(Fe) metal-organic framework (Fig. 8A) function synergistically [18]. In situ Raman spectroscopy confirms that this structure stabilizes the key intermediates during the I₃⁻ ↔ I⁻conversion process (Fig. 8B), steering the reaction from a disordered surface/solution mixed conversion pathway toward an ordered interfacial conversion pathway within confined pores, thereby significantly reducing the free energy barrier of the rate determining step. The Zn–I₂ battery based on this catalyst maintains stability for over 50,000 cycles at a high rate of 50 C (Fig. 8C). The pouch cell assembled with the I₂@Al-TCPP(Fe) cathode exhibits well-defined charge–discharge characteristics, delivering a practical-level capacity of 1.5 Ah at 0.2 C with a mass loading of 286 mg·cm^–2. Similarly, SAMn–N₃–C generated on a MET-6 substrate also possesses abundant pore structures (Fig. 8D) [106]. By optimizing the adsorption strength of iodine species and the local electronic structure, it suppresses the accumulation of higher order polyiodides in the electrolyte, shifting the reaction from a pathway prone to intermediate diffusion toward a surface catalyzed pathway featuring more stable intermediates and more continuous conversion (Fig. 8E). Compared with NC, SAMn–N₃–C exhibits superior cycling stability (Fig. 8F). Qu et al. [107] utilized the Ni–N₄ structure to uniformly disperse Ni single atoms on carbon nanofibers(Fig. 8G), providing abundant catalytic active sites. In situ Raman spectroscopy revealed that the catalytic interface promoted the conversion of polyiodides (I₃⁻ and I₅⁻) and I₂ to I⁻ (Fig. 8H), thereby suppressing the uncontrolled diffusion of polyiodides into the electrolyte. Benefiting from this reaction pathway regulation mechanism, the Zn–I₂ battery with the Ni–N₄CNF/I₂ cathode exhibited good cycling stability and a twofold higher capacity compared to the NCF/I₂ cathode in long cycling tests (Fig. 8I), further demonstrating the key role of reaction pathways optimization in enhancing the performance of Zn–I₂ batteries.

Besides directly regulating polyiodide formation along the I₂ ↔ I₃⁻ ↔ I₅⁻ ↔ I⁻ pathways, certain catalysts can also modify the reaction route and inhibit the shuttle effect through the generation of intermediates, including complexes or insoluble compounds. Guo et al. [108] proposed a multifunctional catalytic strategy based on 1,3-dioxolane regulation, which achieves synergistic improvement in electrode interface stability and iodine redox kinetics through π_sp-p orbital coupling (Fig. 9A). As a p-orbital donor, 1,3-DX modulates the cleavage and reconstruction of sp-hybridized Zn–I bonds via π_sp-p interactions, redirecting the conventional iodine conversion pathway to I₂ ↔ ZnI₂ and suppressing polyiodide shuttling (Fig. 9B). Meanwhile, this catalyst promotes Zn²⁺ desolvation, accelerates ion transport, and induces the formation of a dense and uniform zinc deposit on the anode surface, reducing the interference of seawater ions and hydrogen evolution side reactions. Experimental results show that a flexible Zn–I₂ battery modified with 1,3-DX exhibits stable electrochemical performance, delivering a specific capacity of 130 mAh·g⁻¹ after 25,000 cycles. Furthermore, under a high mass loading of 40 mg·cm⁻², a pouch cell achieves a capacity of 1.01 Ah and retains approximately 90% of its capacity after 100 cycles (Fig. 9C).

Chen et al. [21] reported that the shuttle effect in Zn–I₂ batteries originates from a coupled stepwise iodine reduction pathway (I₂ → I₅⁻ → I₃⁻ → I⁻). Conventional single site catalysts accelerate I₃⁻ reduction but inevitably stabilize the long chain I₅⁻ intermediate. To address this issue, the team introduced ACCs with tailored atomic geometries that can decouple the adsorption energies of key intermediates (Fig. 9D). During the reaction, ACCs disrupt the formation of the I₅⁻ chain and modify the reduction kinetics of I₃⁻, thereby redirecting the reaction toward a low barrier I₂ → I₃⁻ → I⁻ pathway and suppressing the formation of soluble I₅⁻ at its source (Fig. 9E). The Zn₁Co ACCs/I₂ cathode delivered a specific capacity of nearly 200 mAh·g⁻¹ at 1 A·g⁻¹ under a high iodine loading of 8.7 mg·cm⁻² (Fig. 9F). Ferrocene (Fc) is an organometallic compound capable of undergoing reversible Fc/Fc⁺ conversion through changes in the valence state of the Fe atom (Fig. 9G). Zhang et al. [20] found that Fc can bind with iodine to form FcI_x complexes, thereby altering the reaction pathway while simultaneously suppressing the shuttling of Fc⁺ and I₃⁻. When the voltage exceeds 1.2 V, these FcI_x species undergo mutual reversible conversion (Fig. 9H). The 0.5 Ah pouch cell shows excellent cycling stability and Coulombic efficiency (99.9%) at both 10% and 50% zinc utilization rates (ZUR). The fabricated 1.2 Ah pouch cell delivers a high areal capacity of 8.4 mAh·cm⁻², a ZUR of 76.3%, and it retains 90.0% of its initial capacity after 600 cycles at 0.5 C (Fig. 9I).

Reaction pathways regulation offers a fundamental approach to address the polyiodide shuttle problem by redirecting iodine conversion from a solution phase dominated route to an interfacial confined catalytic route. Furthermore, theory guided descriptors such as the d-band center provide a rational basis for predicting and optimizing catalyst performance. Through these approaches, the reaction pathways can be systematically engineered to achieve fast, reversible, and stable iodine redox chemistry.

4 Structure–Performance Relationship Analysis

Statistical comparison among different catalyst categories shows that single atom catalysts tend to achieve higher specific capacity and longer cycle life (Figs. 10A and 10B). SACs have well-defined and uniform coordination structures, allowing each metal atom to serve as an accessible active site. The median specific capacity of SACs (212 mAh·g^–1) is higher than that of metal-based composites (179 mAh·g^–1). SACs also exhibit superior cycling stability. Their median cycle number is > 14,000, which is noticeably higher than that of metal-based composites (< 8000). The median was chosen over the mean for this comparison, because extreme values exist in both catalyst categories and could skew the average. According to the data, SACs show higher specific capacity and better capacity retention during long-term cycling.

Beyond the comparison between catalyst categories, the choice of metal element within SACs also plays a critical role in determining overall performance. Fe-based catalysts tend to show the most stable cycling performance, followed by cobalt, nickel, and zinc (Fig. 10C). Compared with metal particles in conventional catalysts, SACs exhibit notable changes in electron cloud distribution and orbital energy levels. According to d-band theory, a d-band center closer to the Fermi level (less negative or higher in energy) leads to stronger adsorption of iodine species, as it promotes greater occupancy of antibonding states (Fig. 10D) [109,110]. It should be noted that the data points are compiled from three independent DFT studies with different computational settings [19,41,109]. Despite the methodological variations, a consistent qualitative trend is observed. The d-band center is a robust descriptor for interfacial electron transfer, where it directly governs orbital hybridization and charge transfer. It also provides a useful reference for understanding chemisorption through its correlation with binding energy. However, its relevance to reaction pathway regulation is more indirect, as pathway selection is also influenced by geometric and solvation effects. In addition, this descriptor is applicable primarily to transition metal sites with localized d orbitals and does not extend to non-metal catalysts. Despite these limitations, the d-band center remains a valuable starting point for rationalizing activity trends across transition metal sites in Zn–I₂ battery cathodes.

5 Summary and Outlook

This review systematically summarizes the catalytic design of cathode materials for aqueous Zn–I₂ batteries. Leveraging statistical analysis of published studies, we map the overall development landscape of iodine cathode catalysts and categorize the underlying catalytic mechanisms into three types: chemisorption of polyiodides, interfacial electron transfer, and regulation of reaction pathways. For each mechanism, we elaborate the fundamental physicochemical principles and dissect the structure–performance relationships across a broad spectrum of representative material systems, including porous carbon hosts, MOFs, COFs, SACs, molecular catalysts, heterojunctions, and ACCs. Notably, we identify the d-band center as a universal electronic structure descriptor that can well rationalize the activity differences of metal-based active sites, laying a solid theoretical foundation for the predictive design of high-efficiency iodine cathode catalysts. The three-mechanism analytical framework established in this work offers coherent theoretical guidance for the rational development of high-performance Zn–I₂ battery cathodes and also serves as a general reference for catalytic material design in other conversion-type energy storage systems facing analogous kinetic limitations and intermediate shuttle challenges.

Despite remarkable advances in catalytic cathode design in recent years, the practical deployment of Zn–I₂ batteries still faces multifaceted challenges, including mass transport limitations in practical electrodes, extreme environment adaptability, and energy density ceilings. Moving forward, we identify three critical research directions that merit in-depth investigation, all of which demand the synergistic coupling of the three catalytic mechanisms discussed above.

(1) High-loading cathodes under lean electrolyte conditions: In high loading cathodes, especially under lean electrolyte conditions, the key issue extends beyond intrinsic catalytic activity to the spatial control of soluble iodine species and uniform reaction kinetics across thick electrodes. Excessive polyiodide dissolution and shuttle under high loading will rapidly offset the benefits of high active material content, leading to low utilization efficiency, steep concentration gradients, and accelerated capacity fading. Mere physical confinement or chemical adsorption is insufficient to address this issue. Future cathode design should integrate hierarchical pore architecture, iodine-affinitive interfaces, and highly dispersed catalytic centers to realize the synergistic regulation of polyiodide adsorption, interfacial electron transfer, and reaction pathways. Such integrated architectures can promote homogeneous iodine conversion throughout the full depth of the electrode, rather than only on the surface, thereby enabling stable operation with high areal capacity [111].

(2) Wide-temperature catalytic systems for adaptable operation: The electrochemical performance of Zn–I₂ batteries is temperature-sensitive, imposing strict requirements on both catalytic activity and stability. At low temperatures, sluggish interfacial charge transfer amplifies reaction polarization and slows iodine redox conversion; at elevated temperatures, accelerated reaction kinetics exacerbate iodine dissolution and parasitic side reactions [112]. Accordingly, efficient interfacial electron transfer remains the design priority for wide-temperature operation to mitigate low-temperature polarization. Meanwhile, selective chemical anchoring is indispensable to retain mobile iodine species at high temperatures, and reaction pathway regulation can further suppress the generation of thermally unstable intermediates and side reactions. Rational electronic structure engineering of active sites is a promising avenue to achieve balanced kinetics and adsorption strength across a broad temperature range.

(3) Catalytic regulation of high-valence iodine chemistry for higher energy density: High-valence iodine chemistry has emerged as a promising strategy to boost the energy density of Zn–I₂ batteries, yet it suffers from intrinsically sluggish multi-electron transfer kinetics and unstable intermediate species. Accelerating interfacial charge transfer alone is insufficient to achieve stable multi-electron storage, since fast electron transfer cannot guarantee long-cycle stability if the reaction proceeds through unstable or poorly reversible intermediates. Instead, the coupled regulation of electron transfer and reaction pathway is essential. Catalysts should be designed to lower the energy barriers of stepwise multi-electron redox and steer intermediate evolution toward highly reversible channels. Appropriate adsorption strength can further stabilize transient high-valence iodine species within a suitable binding energy window, complementing kinetic and pathway regulation.

(4) Practical considerations beyond catalytic performance: Moving Zn–I₂ batteries from the lab to industry requires attention to scalable synthesis, catalyst cost, and environmental impact. Scalable synthesis should prefer earth-abundant and low-cost metals (e.g., Fe, Zn and Mn) and use solution-based or roll-to-roll compatible methods. The environmental footprint over the full life cycle of catalysts should be minimized by using non-toxic and recyclable materials. Long-term safety under real operating conditions must be carefully checked, because catalytic materials may bring risks such as metal dissolution or gas release. In addition, the wide variety of testing conditions in current studies makes it difficult to compare different catalyst systems on a fair basis. Finding practical solutions to these issues is essential for connecting academic research with industrial applications.

Looking ahead, research on Zn–I₂ battery catalysis should shift from mechanism verification under idealized laboratory conditions to the synergistic modulation of multiple catalytic mechanisms under realistic operating scenarios. In this context, bifunctional or symmetric design strategies that work on both the cathode and the anode represent a promising new direction. By creating a shared active interface, these strategies can speed up iodine conversion and promote even zinc deposition at the same time. This helps to reduce anode corrosion caused by the shuttle effect, making it a worthwhile approach for improving full-cell performance [113]. The three-mechanism analytical framework established in this work is not limited to Zn–I₂ systems. It can also serve as a general reference for catalytic design in other conversion-type battery systems such as lithium–sulfur and zinc–bromine batteries, providing guiding principles for addressing analogous challenges in electrochemical energy storage.

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