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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Review

Carbon Materials for Zinc-Iodine Battery Cathodes

  • Yinyan Guan 1 ,
  • Xiaorui Hao , 2, * ,
  • Rui Xu 3 ,
  • Hongfei Li 2 ,
  • Yuhan Wu , 1, 4, 5, * ,
  • Jiyan Liang 1
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  • 1 School of Environmental and Chemical Engineering,Shenyang University of Technology,Shenyang 110870,China
  • 2 School of System Design and Intelligent Manufacturing,Southern University of Science and Technology,Shenzhen 518055,China
  • 3 Helmholtz Institute of Ion Beam Physics and Materials,Dresden 01328,Germany
  • 4 Key Laboratory of Chemistry of Advanced Energy Materials,Ministry of Education,Nankai University,Tianjin 300071,China
  • 5 School of Chemistry and Chemical Engineering/State Key Laboratory Incubation Base for Green Processing of Chemical Engineering,Shihezi University,Shihezi 832003,China
* (Xiaorui Hao);
(Yuhan Wu)

These authors contributed equally to this work

Received date: 2024-06-21

  Revised date: 2024-09-30

  Online published: 2025-02-25

Supported by

Education Department of Liaoning Province(JYTQN2023285)

Education Department of Liaoning Province(LJKMZ20220499)

Shenyang University of Technology(QNPY202209-4)

China Scholarship Council(202408320117)

Key Laboratory of Functional Inorganic Material Chemistry(Heilongjiang University)Ministry of Education,and the Science and Technology Department of Liaoning Province(2024-BSLH-172)

Key Laboratory of Functional Inorganic Material Chemistry(Heilongjiang University)Ministry of Education,and the Science and Technology Department of Liaoning Province(2023-MSLH-257)

Abstract

Zinc-iodine batteries have attracted widespread attention as a novel green,low-cost,and highly safe electrochemical energy storage technology. Its basic principle is to use the electrochemical reaction between zinc and iodine to store and release energy. However,the low electronic conductivity,shuttle effect,and high solubility of iodine limit the practical application of zinc-iodine batteries. This work provides a systematic review of the research progress on carbon materials used in the cathode of zinc-iodine batteries,with a focus on several commonly used carbon materials,such as carbon nanotubes,graphene,activated carbon,biomass-derived carbon,and other porous carbon materials. Owing to their excellent conductivity,high specific surface area,and good chemical stability,these carbon materials can not only effectively adsorb and immobilize iodine molecules,preventing iodine loss and the shuttle effect,but also promote iodine redox reactions by regulating the pore structure and surface chemical properties,thereby improving the specific capacity and cycling stability of the battery. Additionally,we put forward the challenges and issues faced by carbon materials in the practical application of zinc-iodine batteries,including how to further enhance iodine adsorption capability and improve the structural stability of the electrode. Accordingly,several potential future research directions are proposed with a view to further improving the electrochemical performance and reducing the manufacturing cost,thus laying the foundation for advancing the development and application of this emerging battery technology.

Contents

1 Introduction

1.1 Research background and significance of zinc-iodine batteries

1.2 The importance of carbon materials in zinc-iodine batteries

2 Overview of zinc-iodine batteries

2.1 Reaction mechanism of zinc-iodine batteries

2.2 Advantages and problems of zinc-iodine batteries

3 The application of carbon materials in the cathode of zinc-iodine batteries

3.1 Carbon nanotube-based cathodes

3.2 Graphene-based cathodes

3.3 Activated carbon-based cathodes

3.4 Biomass-derived carbon-based cathodes

3.5 Other porous carbon material-based cathodes

4 Conclusions and outlook

Cite this article

Yinyan Guan , Xiaorui Hao , Rui Xu , Hongfei Li , Yuhan Wu , Jiyan Liang . Carbon Materials for Zinc-Iodine Battery Cathodes[J]. Progress in Chemistry, 2025 , 37(5) : 775 -787 . DOI: 10.7536/PC240610

1 Introduction

1.1 Research Background and Significance of Zinc-Iodine Batteries

With the rapid development of electric vehicles, the demand for lithium-ion batteries in the electrochemical energy storage market has significantly increased[1-3]. However, the relatively scarce reserves and uneven distribution of lithium resources in the Earth's crust have affected the supply stability and price consistency of lithium-ion batteries. In recent years, frequent safety issues exposed by commercialized lithium-ion batteries have raised public concerns[4-6]. Therefore, developing new electrochemical energy storage technologies with low cost and high safety has become a key focus for researchers[7-8].
Aqueous zinc-based batteries are considered a promising electrochemical energy storage technology due to their high theoretical capacity (820 mAh·g-1 and 5855 mAh·cm-3), abundant zinc resources, low electrode potential (-0.76 V vs SHE), and good stability in air[9-10]. Unlike conventional zinc-ion batteries that rely on the intercalation/deintercalation of Zn2+, zinc-iodine batteries represent a new type of energy storage battery using zinc metal as the anode and iodine as the cathode, storing energy through redox reactions of cathodic iodine and deposition/stripping of anodic zinc. This type of battery offers advantages such as high energy density, low cost, good safety, and environmental friendliness[11], making it one of the most competitive candidates for future large-scale energy storage devices. In zinc-based aqueous batteries, the redox electrochemical reaction of iodine can provide considerable potential and theoretical capacity (211 mAh·g-1), thus attracting widespread attention[12]. Additionally, iodine's multiple valence states (e.g., -1, 0, +1, +3, +5, and +7) theoretically endow it with multi-electron transfer capability[13]. Despite this, as an emerging energy storage technology, zinc-iodine batteries remain at an early stage of research and face numerous challenges in practical applications, including poor cycling stability and low reaction rates[14]. In zinc-iodine batteries, energy is stored via electron transfer between elemental iodine and zinc ions. During charging and discharging processes, elemental iodine converts into water-soluble polyiodide ion intermediates (such as I3-, I5-, etc.)[14], which migrate through the electrolyte to the zinc anode and react with zinc (Reaction 6), triggering a series of side reactions that shorten the battery's cycle life. This phenomenon is known as the "shuttle effect" of iodine[15-16]. Moreover, zinc corrosion caused by polyiodides exacerbates the growth of zinc dendrites, potentially leading to short circuits and battery failure[17-18].
To suppress the polysulfide shuttle effect and address the poor conductivity and low utilization of iodine, a common approach is to composite iodine with carrier materials (such as polyaniline[19], starch[20-21], porous carbon (I2/OSTC)[22], etc.). By utilizing the physical/chemical adsorption capacity and dense pore structure of the carrier materials to immobilize iodine active substances, the escape and shuttling of polysulfides can be effectively prevented[23]. Among various carrier materials, carbon materials (such as carbon nanotubes[24-25], graphene[26], porous carbon[27-28], etc.) have been extensively studied due to their excellent physicochemical properties, such as high electrical conductivity, high chemical stability, and tunable pore structures. Meanwhile, the inherent good electronic conductivity of carbon materials can effectively alleviate the sluggish reaction kinetics caused by the low conductivity of iodine[15].
Carbon materials, as the most extensively studied iodine hosts in zinc-iodine batteries, have shown promising application prospects; however, a review summarizing their research progress is currently lacking. In response to this gap, this paper systematically summarizes the research advancements regarding carbon materials used as cathodes in zinc-iodine batteries for the first time. Firstly, the fundamental principles and reaction mechanisms of zinc-iodine batteries are introduced, along with the problems and challenges they face. Secondly, the current research status of different types and structures of carbon materials applied in iodine cathodes is summarized. Finally, the effects of carbon materials on enhancing the electrochemical performance of zinc-iodine batteries are outlined, and potential future research directions are proposed. This review aims to provide a valuable reference for further studies on carbon materials as cathodes in zinc-iodine batteries, encouraging more researchers to engage in this field and laying a foundation for the practical application of zinc-iodine batteries.

1.2 The Importance of Carbon Materials in Zinc-Iodine Batteries

To address the issues of low energy density, slow iodine conversion kinetics, polysulfide shuttle effects, and instability of zinc metal anodes in zinc-iodine batteries[29], researchers have explored various strategies to enhance the performance of zinc-iodine batteries. Among these, one of the most important approaches is utilizing different types of carbon materials as carriers for iodine[30].
By utilizing the high specific surface area, tunable pore structure, and iodine adsorption capability of carbon materials, iodine can be effectively captured and stabilized, thereby suppressing the shuttle effect during battery charging and discharging processes, which improves the cycling stability and energy efficiency of the battery[31]. In zinc-iodine batteries, the reduction and oxidation processes of iodine are critical, and these processes can be accelerated by introducing electrocatalysts[32]. Carbon materials play a key role in this regard, particularly those with special surface structures and physicochemical properties. Studies have shown that these carbon materials can serve as effective electrocatalysts, significantly enhancing the reaction rates of iodine reduction and oxidation, thereby improving the overall battery performance. Additionally, carbon materials provide excellent structural stability to battery components (such as electrodes), extending the battery's lifespan. During charge-discharge cycles, carbon materials demonstrate unique advantages, better accommodating volume and structural changes within the battery, effectively reducing the risk of battery damage or performance degradation. Carbon materials typically possess large specific surface areas, allowing them to load more active substances and thus increasing the energy density of the battery[33]. More importantly, carbon materials exhibit good regenerability and recyclability, aligning with societal demands for environmental friendliness and sustainability[34]. Another crucial advantage is that the incorporation of carbon materials improves the electrical conductivity of elemental iodine, thereby enhancing electron transport rates and rate capability.
Based on these advantages, researchers are actively developing novel carbon materials for use in zinc-iodine batteries. These efforts will bring significant breakthroughs toward achieving high-performance zinc-iodine batteries and promote the development and application of battery technology.

2 Overview of Zinc-Iodine Batteries

2.1 Electrochemical Reaction Mechanism of Zinc-Iodine Batteries

As a highly anticipated secondary battery system, the core of zinc-iodine battery energy storage lies in the redox reaction between zinc and iodine[30,35]. This type of battery system consists primarily of a zinc metal anode, an iodine-containing cathode, and an electrolyte (see Figure 1). During charging, zinc ions accept electrons at the zinc metal anode and deposit to form metallic zinc (Equation 4), while iodide ions at the iodine-containing cathode lose electrons to form slightly soluble I2 (Equation 1). The presence of free I- can lead to spontaneous formation of polyiodides, especially I3- and I5- (Equations 2 and 8)[36-37]. During discharge, the reverse processes occur. I3- undergoes a reduction reaction at the cathode-electrolyte interface, being reduced to I- (Equation 3), while metallic zinc at the anode surface dissolves simultaneously (Equation 6)[38]. It is worth noting that some researchers have proposed more complex redox reaction pathways, particularly in battery configurations using solutions containing I3-/I- as the electrolyte[30]. According to this theory, iodine molecules may undergo a two-step redox process involving I2/I3- and I3-/I-[18]. This theoretical framework contributes to a deeper understanding of the working mechanism of zinc-iodine batteries and provides new perspectives for optimizing battery performance. The corresponding chemical equations are as follows:
Positive electrode:
2 I - - 2 e - I 2
I - + I 2 I 3 -
I 3 - + 2 e - 3 I -
negative electrode:
Z n 2 + + 2 e - Z n
overall reaction:
Z n + I 2 Z n 2 + + 2 I -
Z n + I 3 - Z n 2 + + 3 I -
Z n + 3 I 2 Z n 2 + + 2 I 3 -
图1 锌-碘电池机理示意图

Fig.1 Schematic illustration of the mechanism of zinc-iodine batteries

In summary, although the electrochemical reaction mechanism of zinc-iodine batteries is relatively simple, research on the reaction mechanism plays an important role in understanding the decay mechanisms, analyzing the specific electrochemical reaction processes, and exploring ways to enhance battery performance.

2.2 Advantages and Existing Issues of Zinc-Iodine Batteries

Zinc-iodine batteries possess numerous appealing advantages. Firstly, their main components, zinc and iodine, are both abundant in the Earth's crust and relatively low-cost elements, making the material cost of this battery system comparatively low. Secondly, by enabling iodine redox electrochemical reactions in zinc-based aqueous batteries, zinc-iodine batteries can deliver a high voltage and a theoretical capacity as high as 211 mAh·g-1[12], indicating that they offer higher energy density than many existing battery technologies. Furthermore, the environmental friendliness of zinc and iodine makes zinc-iodine batteries a "green" energy storage technology, aligning with society's promotion of "sustainable development."
Although zinc-iodine batteries have the above advantages, they still face some challenges in practical applications. First, the generation of I3- ions occurs, and the migration and transformation of I3- ions can lead to a series of adverse reactions[39-40]. When excess I2 is generated under high charge states, higher-order polyiodide ions (I5-, I7-, I9-, etc.)[12,20] (Equation 8) may also form, further increasing reaction complexity and instability.
I 3 - + I 2 I 5 -
in zinc-iodine batteries, these polyiodide ions (such as I3- and I5-) may gradually accumulate on the electrode surface. Due to the presence of a concentration gradient, they can cross the separator and reach the zinc anode. However, since the standard redox potential of I3-/I- is higher than that of Zn2+/Zn[41], metallic zinc may be oxidized by I3- ions, leading to corrosion of the zinc anode (Equation 6). Moreover, the electron transfer between zinc and polyiodide ions occurring inside the battery can cause severe self-discharge, thereby reducing the specific capacity and coulombic efficiency of the battery[42]. Additionally, non-uniform corrosion on the surface of the zinc anode not only affects the uniform deposition of metallic zinc but may also trigger hydrogen evolution and passivation reactions, which can impact the long-term cycling stability and capacity retention of the battery.
Compared with the conversion between I- and I3-, the transformation rate between polyiodide ions and I2 is slower and irreversible, leading to a slowdown in reaction kinetics, which ultimately results in a battery output voltage lower than the theoretical value. Under high-concentration and low-temperature conditions, polyiodide ions tend to release I2, and the generated solid I2 precipitates due to aggregation under insufficient coordination and stabilization conditions provided by excess I- ions[43]. This aggregation can block electrode pores and pore structures, thereby reducing the energy density and electrochemical performance of the battery[44].
To address these challenges faced by zinc-iodine batteries, the surface properties of carbon materials can be effectively tailored to maximize the immobilization and catalysis of solid I2. During battery discharge, the solid I2 deposited on the electrode surface can be reversibly converted back to I-, thereby enhancing the energy density[45]. However, despite the great potential of zinc-iodine batteries as a novel battery technology, critical issues such as the polysulfide-shuttle-like effect, sluggish reaction kinetics, reduced energy density, and instability of the zinc anode still severely limit their practical applications. Therefore, research on zinc-iodine batteries, especially the application of carbon materials within them, holds significant importance.

3 Application of Carbon Materials in the Cathode of Zinc-Iodine Batteries

Carbon materials have been widely used as host materials in the past due to their excellent electrical conductivity, high specific surface area, tunable pore structure, outstanding chemical inertness and thermal stability, as well as strong chemical/physical adsorption properties toward iodine. The specific surface area and pore structure of carbon materials usually have a significant impact on the loading of active I2. Confining I2 within carbon pores can effectively suppress the formation of unwanted intermediates such as I3-. Additionally, tailoring the nanoporous structure (e.g., micropores, mesopores, and macropores) of carbon electrodes can enhance the physical interactions between iodine and the carbon electrode. Studies have shown that micropores are beneficial for iodine adsorption, whereas mesopores and macropores facilitate ion diffusion. However, due to weak van der Waals forces, iodine may escape into the electrolyte during cycling. Therefore, by rationally tuning the pore structure and chemical properties of carbon materials, the performance of zinc-iodine batteries can be regulated.

3.1 Carbon Nanotube-Based Cathode

Carbon nanotubes (CNTs) have attracted significant attention due to their unique properties. Their cylindrical structure composed of hexagonally arranged carbon atoms makes them an ideal choice as micro reaction vessels, which can confine the movement of small molecular active species during electrochemical reactions, thereby significantly enhancing the electrochemical performance of electrode active materials. In the study by Chai et al.[24], MOF-derived hierarchical hollow carbon nanotubes were utilized as carriers for small molecular iodine (Figure 2a). Compared with commercial carbon nanotubes, this structure not only possesses a hollow tubular configuration but also retains microporous structures and formed mesopores/macropores, enabling the storage of more charge-active sites and greatly shortening the transport paths for ions and electrons. When assembled into a full battery using zinc as the anode, it exhibited higher specific capacity and excellent cycling stability compared to commercial carbon nanotubes (Figure 2b). This superior performance is attributed to the following factors: the abundant porous structure effectively captures and adsorbs iodine; the unique hollow structure provides convenient transport channels for electrons and reactants; and the formed carbon framework enhances electronic conductivity, effectively improving the charge transfer rate and utilization efficiency of the electrode material. Additionally, Jin et al.[49] fabricated a hydrophilic carbon nanotube cathode containing abundant oxygen functional groups using laser direct writing technology. Its tubular structure and rich oxygen functionalities facilitated the electrochemical deposition of I-/I3- on the cathode surface, effectively mitigating the shuttle effect of I3-. Consequently, the zinc-iodine battery demonstrated a high volumetric energy density of up to 1647.3 mWh·cm-3 and an areal energy density of 2339.1 μWh·cm-2. He et al.[25] assembled a zinc-iodine battery by designing a composite cathode with a porous architecture consisting of carboxylated multi-walled carbon nanotubes (c-MCNTs) and microporous carbon (MPC) (Figure 2c). The internal c-MCNTs act as a structurally stable conductive framework, providing sufficient electrons for the redox reactions between Zn2+ and I- while firmly anchoring iodine. Meanwhile, the external MPC layer functions as an effective barrier to restrict iodide diffusion. Utilizing CNT@MPC12 as a host material to adsorb iodide ions yielded the CNT@MPC12-I- electrode. Results showed that this electrode structure exhibited excellent cycling performance, maintaining a capacity of 0.38 mAh·cm-2 and a capacity retention rate of 91% even after 12,000 cycles (with a very low capacity decay rate of only 0.0076% per cycle). This is attributed to the substitution of carboxyl groups on c-MCNTs by I- ions, forming strong interactions that effectively suppress the shuttle effect. Furthermore, the microporous carbon within MPC serves as a dual constraint for iodine species, ensuring the diffusion of Zn2+ ions. Moreover, trace amounts of iodine in the system help inhibit the formation of side products, offering an effective strategy for enhancing the performance of zinc-iodine batteries.
图2 (a)不同温度下获得的InOF-1衍生的分级多孔碳材料(HCNS);(b)在1 A·g-1时的循环稳定性(内嵌图是由组装的电池器件驱动的风扇电机)[24];(c)CNT@MPC对I-离子(ZnI2)的吸附示意图[25]

Fig. 2 (a)Schematic illustration to obtain InOF-1 derived hierarchically porous HCNS with their differentiated carbonized stages at different temperatures.(b)Cycling stability at 1 A·g-1(the insert diagram is a fan motor driven by an assembled device)[24].(c)Schematic illustration of the adsorption of I- ions(ZnI2)by CNT@MPC[25]

Carbon nanotubes, as iodine carriers, can not only help stabilize the electrode material structure but also provide more active sites to catalyze electrochemical reactions due to their large specific surface area, thereby enhancing electrochemical performance. In addition, carbon nanotubes can increase the contact area between the electrode and the electrolyte, promoting the adsorption and release of iodide ions, thus improving the battery's energy density and cycling stability. At the same time, the surfaces of certain functionalized carbon nanotubes may exhibit electrocatalytic properties, facilitating the reduction and oxidation reactions of the iodine redox couples (I₂/I-, I₃-/I-) and accelerating reaction kinetics.

3.2 Graphene-Based Cathodes

Graphene-based materials are widely applied in the field of energy storage batteries due to their excellent material and structural properties. First, they possess high electrical conductivity reaching up to 104~105 S·m-1 and a specific surface area of 2630 m2·g-1[50], effectively enhancing the energy conversion efficiency and energy density of batteries. Secondly, graphene exhibits good mechanical properties, with a tensile strength of 125 GPa and an elastic modulus of 1.1 TPa[52], while also maintaining excellent chemical stability[53], ensuring long-term stable operation within electrochemical systems. Additionally, the microscale flexibility of graphene enables adaptation to various battery designs with different shapes and sizes, improving design flexibility and customization capabilities[54], thus promoting the development and application of energy storage technologies.
To address the issues of poor electronic conductivity and low solubility of iodine, Park et al.[55] synthesized a three-dimensional (3D) graphene-like carbon material with large specific surface area and high conductivity as a host material (see Fig. 3a). This structure enables full utilization of iodine and can trap I2 within the carbon micropores, enabling a reversible redox reaction between I2 and 2I-. The 3D graphene-like architecture provides excellent electron transport pathways while offering sufficient space for ion diffusion and storage. The electrode delivers a capacity of 342 mAh·g-1 and a gravimetric energy density of 434 Wh·kg-1 at a power density of 85 W·kg-1, significantly outperforming conventional mesoporous carbon and activated carbon-based iodine cathodes (see Fig. 3b). Ji et al.[26] employed a hydrothermal reduction method to prepare reduced graphene oxide (rGO) with a 3D microporous structure, large specific surface area, high mechanical strength, and excellent electrical conductivity, which was used as an I2 host. The porous structure of rGO allows rapid diffusion of iodide ions and enhances the thermal stability of adsorbed iodine. In addition to enabling van der Waals adsorption, the vacancies in the graphene structure are also considered potential binding sites for iodide ions. Meanwhile, residual oxygen-containing functional groups enhance iodine adsorption. During fast charge-discharge processes, an efficient plating approach loads I2 onto the rGO, preventing loss of active materials caused by thermal instability. The 3D porous structure and superior conductivity of rGO enable high I2 loading (25.33 mg·cm-2) and fast kinetics, delivering a high reversible capacity of 6.5 mAh·cm-2 at a current density of 2 mA·cm-2. Even at a high current density of 80 mA·cm-2, a capacity of 1 mAh·cm-2 is maintained. Due to the strong adsorption capability of rGO, I2 and I3- can be firmly confined within the electrode, effectively reducing the loss of iodine active materials. Although sophisticated structural designs of carbon-based materials have been adopted for iodine adsorption, pure carbon materials are typically nonpolar and often fail to meet ideal adsorption requirements. Heteroatom doping represents an effective strategy to control the surface chemistry of carbon materials and improve chemical interactions between carbon and iodine[31,56]. Liu et al.[57] utilized nitrogen-doped graphene carbon to catalyze iodine and suppress the formation of triiodide, achieving a high energy density of 320 Wh⋅kg-1 and an extended cycle life of 10,000 cycles in a zinc-iodine battery (see Fig. 3c). Results indicate that the strong catalytic activity of graphitic nitrogen facilitates the direct conversion of I2/I-, kinetically suppressing triiodide formation by lowering the dissociation energy barrier, thereby achieving high utilization of active materials. The reduced energy barrier for solid iodine nucleation/electrodeposition also favors this direct conversion. Dang et al.[58] designed a functionalized graphene electrode with higher surface area, enhanced conductivity, and optimized nitrogen doping, significantly promoting the redox reactions of iodides while restricting the formation of polyiodides, thus achieving high power and energy densities (see Fig. 3d).
图3 (a)I2/3DGC的制备示意图;(b)I2/3DGC电极的循环性能[55];(c)PNC-1000-I2在电流密度为1 A·g-1、碘的面积负载量为6 mg·cm-2时的循环性能[57];(d)传统锌-碘电池的示意图[58]

Fig.3 (a)Illustration of the formation of I2/3DGC.(b)Cyclic performance of I2/3DGC electrode[55].(c)Cycling performance of the PNC-1000-I2 at a current density of 1 A·g-1,the iodine areal loading is 6 mg·cm-2[57].(d)Schematic representation of a conventional Zn-I2 battery[58]

Zhang et al.[59] reported that the heterogeneous structure of graphene/polyvinylpyrrolidone (G/PVP) can effectively suppress the shuttle effect. On one hand, the strong π-π interactions between G and PVP contribute to the uniform stability of the G/PVP structure; on the other hand, the robust electrostatic interactions between PVP and iodine effectively inhibit the shuttle effect. The unique energy storage mechanism involves a conversion reaction, which provides sufficient Zn2+ for the anode, thereby significantly enhancing the energy density (162 Wh·kg-1) and delivering good cycling stability (63.8% capacity retention after 200 cycles). Additionally, Zhang et al.[60] prepared porous graphene with high density (1.4 g·cm-3), ordered pore structure, and abundant oxygen-containing functional groups (HOPG) as an I2 host cathode material. After KOH etching, HOPG exhibited more microporous structures and a larger specific surface area, enabling higher loading of active materials. This composite electrode retained up to 96.4% after 20,000 cycles at a current density of 5 A·g-1. The ordered structure and rich C̿    O functional groups in HOPG help suppress polysulfide shuttling while facilitating rapid ion transfer in the electrolyte and alleviating electrode structural collapse, thus improving cycling stability.
Graphene's excellent electrical conductivity and the ultra-large specific surface area provided by its two-dimensional structure can significantly accelerate electron transfer, reduce internal resistance, and offer more active sites to effectively adsorb iodine and iodide ions (such as I₂, I₃-), preventing iodine dissolution and loss, thus reducing the consumption of active materials. In addition, graphene's structural characteristics can enhance the mechanical stability of electrode materials, alleviate volume changes during cycling, and significantly improve the battery's cycle life.

3.3 Activated Carbon-Based Cathode

Activated carbon is also extensively applied in zinc-iodine batteries. Firstly, its well-developed porous structure and high specific surface area provide a stable carrier framework, which helps immobilize and embed iodides within it, thereby maintaining the stability of the electrode structure. Secondly, the high adsorption capacity and large specific surface area of activated carbon significantly enhance its ability to adsorb iodine, thus improving the energy density of the battery. Meanwhile, its good electrical conductivity facilitates the reduction reaction of iodine, thereby enhancing the discharge efficiency and stability of the battery. Finally, the strong adsorption capability of activated carbon also helps reduce the shuttle effect of iodine during charging and discharging processes, extending the cycle life of the battery.
Li et al.[61] reported a long-life zinc-iodine battery based on an activated carbon encapsulated I2 (I2@C-50) composite material (Fig. 4a, b). This composite material exhibited a high density of up to 1.4 g·cm-3, with a capacity of 210 mAh·g-1 and an energy density of 237 Wh·kg-1. After 10,000 cycles at a current density of 100 mA·g-1, it retained a capacity retention rate of 66% (Fig. 4c). The I2@C composite material achieved high capacity and excellent stability under both low and high loading conditions. The porous activated carbon not only stored and adsorbed I2, suppressing its dissolution into the electrolyte, but also acted as a conductive matrix that facilitated the reversible conversion reaction between I2 and ZnI2. Zhang et al.[62] successfully synthesized high-density microporous activated carbon (AC) using 3,4,9,10-perylenetetracarboxylic dianhydride as a precursor through an alkali activation strategy. This porous activated carbon possessed a unique pore structure capable of achieving a high iodine loading of 59.5%, while effectively confining iodine or polyiodides within the micropores/ mesopores during cycling, thereby significantly mitigating the shuttle effect and improving the self-discharge issue. Its excellent electronic conduction pathways and porous structure favored ion diffusion and storage, shortening the ion transport path and enabling rapid ion diffusion and electron transfer. Pan et al.[63] investigated the influence of microporous structures in activated carbon fiber cloth on the performance of zinc-iodine batteries (Fig. 4d, e). Superior adsorption capability resulted in higher surface iodine loading, stable capacity, and coulombic efficiency.
图4 (a)I2@C-50复合材料合成示意图;(b)I2@C-50复合材料的元素分布图;(c)在电流密度为1和5 A·g-1时,不同I2碘负载量的I2@C复合材料的循环性能[61];(d)锌-碘电池的反应示意图;(e)不同碘负载量的I2/ACF电极的放电倍率性能[63]

Fig.4 (a)Illustration of the synthesis of I2@C-50 composites.(b)Elemental mapping of the I2@C-50 composite.(c)Cycling performance of the I2@C composite with different I2 mass loading values at current densities of 1 and 5 A·g-1[61].(d)Reaction diagram of Zn-I2 batteries.(e)Discharge rate capability of I2/ACF electrodes with different iodine mass loading[63]

The porous structure of activated carbon enhances the electrode's wettability to the electrolyte and promotes ion transport, thereby improving the electrochemical performance of the battery. In addition, its preparation cost is relatively low, and the pore structure and surface chemical properties can be optimized by adjusting the carbonization temperature and activation conditions, thus providing multiple possibilities for designing iodine-based cathode materials with well-integrated performance.

3.4 Biomass-Derived Carbon-Based Cathodes

Biochar has become one of the ideal material choices for large-scale energy storage due to its renewability and environmental friendliness, while also helping to reduce excessive dependence on limited resources. Secondly, its abundant pore structure and high specific surface area provide numerous adsorption sites, thereby enhancing energy density. Additionally, its good electrical conductivity enables rapid electron transfer, promoting electrochemical reactions, improving energy conversion efficiency, and enhancing the performance stability of energy storage systems. Finally, by adjusting its pore structure and surface chemical properties through different preparation methods and processing techniques, it can meet diverse energy storage requirements and application scenarios, offering a sustainable and efficient solution for the development of energy storage technologies.
Xu et al.[64] successfully prepared biomass-derived hierarchical porous carbon composites (BCHP) using ginkgo as the raw material through an alkali activation strategy (shown in Figure 5a). The hierarchical porous structure of BCHP provides a large specific surface area (1176 m2·g-1), abundant electrolyte penetration channels, and numerous pores for iodine adsorption/inhibition. It plays a role in physically capturing iodine during electrolyte infiltration and electrochemical cycling. These features enhance iodine utilization, accelerate reaction kinetics, and achieve excellent cycling stability and high Coulombic efficiency. Yue et al.[65] synthesized a nitrogen-doped lychee shell-derived porous carbon (N-LPC) cathode host material via an activation calcination method (shown in Figure 5b). The porous structure of N-LPC facilitates physical adsorption of iodine and offers abundant anchoring sites by introducing nitrogen heteroatoms, promoting reversible conversion between I2 and I-. The porous structure and nitrogen doping of N-LPC provide rich and robust active sites for iodine loading and electron/ion transfer, significantly restricting the shuttle behavior of iodine, enhancing the utilization of active iodine, and accelerating reaction kinetics. Therefore, the N-LPC/I2 composite exhibits a specific capacity of 127 mAh·g-1 at 100 mA·g-1, along with excellent rate capability and cycling stability. Han et al.[66] developed activated porous corncob carbon (APCC) with high specific surface area, high conductivity, and strong adsorption capacity, which was utilized as an iodine carrier (shown in Figure 5c), enhancing the confinement effect and suppressing the shuttle effect. Moreover, by optimizing redox kinetics using a water-in-salt ZnCl2 electrolyte, a higher voltage reaction platform corresponding to the four-electron transfer mechanism of I-/I0/I+ was achieved in the battery (shown in Figure 5d), clearly distinguishing it from conventional zinc-iodine batteries (I-/I0). Ultimately, this battery delivers a specific capacity of 1118.6 mAh·g-1 and an energy density of 1302.2 Wh·kg-1.
图5 (a)I-BCHP和I-BCNP的合成过程示意图[64];(b)N-LPC/I2和LPC/I2的合成过程示意图[65];(c)以玉米芯衍生的多孔活性碳为正极的锌-碘电池示意图;(d)I2@APCC//WiS ZnCl2 5.9 + 0.20 M KI//Zn电池在电流密度为60~1200 mA·g-1时的GCD曲线[66]

Fig.5 (a)Illustration of I-BCHP and I-BCNP synthesis process[64].(b)Schematic illustration of the synthesis process of N-LPC/I2 and LPC/I2[65].(c)Schematic diagram of zinc-iodine batteries with active porous corncob carbon as positive electrode.(d)The GCD curves at the current density of 60~1200 mA·g-1 for the I2@APCC//WiS ZnCl2 5.9 + 0.20 M KI//Zn battery[66]

Biomass-derived carbon materials are highly competitive due to the renewability and environmentally friendly characteristics of their precursors. Their pore structures can be flexibly regulated through different heat treatment or chemical activation methods, thereby optimizing the adsorption capacity for iodine and iodide ions. In addition, the abundant oxygen-containing functional groups on the material surface (such as hydroxyl and carbonyl groups) can interact with iodine, thus stabilizing iodine and reducing its dissolution loss.

3.5 Other Porous Carbon Materials-Based Cathodes

In addition to the commonly used carbon materials mentioned above, other porous carbon materials have also been developed as iodine cathodes, such as mesoporous and microporous carbon materials. These materials also possess excellent electrical conductivity, chemical stability, and abundant pore structures, making them suitable for various energy storage systems. Moreover, the pore characteristics of porous carbon are closely related to the adsorption behavior of I- and I3-, further enhancing their performance as iodine cathodes. Yan et al.[28,67] selected a zinc citrate-derived porous carbon with an ultra-high specific surface area (2966.3 m2·g-1) as an iodine-adsorbing host material (Fig. 6a), and investigated its different functions in both the anode and cathode of a zinc-iodine battery. By utilizing the porous carbon to modify the zinc anode while simultaneously serving as the host for the iodine cathode, a "two birds with one stone" effect was achieved. This multifunctional porous carbon not only facilitates uniform zinc deposition but also immobilizes iodine within the electrode. At a current density of 12 C, after 3000 cycles, the capacity retention rate reached 88.1% (Fig. 6b). Liu et al.[69] synthesized a nitrogen-doped porous carbon nanocage (NCCs) as an iodine host. In addition to providing cages for immobilizing iodides, this material can also serve as an efficient electrocatalyst for catalyzing electrochemical reactions. It offers abundant and stable polysulfide anchoring sites through electrostatic interactions between the nitrogen-doped sites of NCCs and the iodides. Furthermore, NCCs exhibit excellent catalytic capability in the reversible conversion between iodine and iodides, thereby suppressing the accumulation of polysulfides in the electrolyte and further preventing the shuttle effect of polysulfides. Meanwhile, the nanoscale porous structure of NCCs provides more adsorption and catalytic sites for iodides and reduces diffusion barriers. The strategy of anchoring iodides and accelerating conversion kinetics helps improve the electrochemical performance and cycling stability of zinc-iodine batteries. Therefore, when NCC/I2 is used as the electrode material for a zinc-iodine battery, it delivers a high specific capacity of 259 mAh·g-1 at 0.1 A·g-1, along with excellent cycling stability and good rate capability. Xu et al.[70] synthesized hierarchical micro-mesoporous carbon nanospheres (HMMC NSs) with uniformly distributed pores (Fig. 6c). The hierarchical micro-mesoporous structure can fully optimize the material's ability to immobilize nanoscale iodine molecules, effectively suppressing the shuttle phenomenon. The ordered pore structure ensures uniform iodine loading, significantly reducing iodine aggregation during the reaction process. With the synergistic effects of the hierarchical micro-mesoporous structure and ordered pore distribution, the Zn-I2 battery exhibits favorable cycling performance (Fig. 6d).
图6 (a)Zn/C-I2电池及其充电过程示意图;(b)Zn/C-I2电池的长循环性能[68];(c)HMMC-I2 NSs的制备示意图;(d)HMMC-I2 NSs在8.0 A·g-1下的长期循环性能[70]

Fig.6 (a)Schematic illustration of the Zn/C-I2 battery and its charging process.(b)Long cycling performance of yhe Zn/C-I2 battery[68].(c)Schematic illustration of the preparation of HMMC-I2 NSs.(d)Long-term cycling performance of HMMC-I2 NSs at 8.0 A·g-1[70]

Guo et al.[71] utilized mesoporous carbon with an aperture size of 50 nm to adsorb iodine (I2/CMKs), effectively enhancing the electrochemical performance. The synthesized I2/CMK composite confines the I3-/I- conversion reaction within the pores, eliminating the shuttle effect caused by polyiodides and significantly improving cycling stability. At a high current density of 5 A·g-1, this electrode material exhibited a specific capacity of 90 mAh·g-1 and achieved an ultralong cycle life of 39,000 cycles at 10 A·g-1 with a capacity retention rate of 80.6%. Furthermore, the spatial structure of microporous carbon materials with pore sizes of 1-3 nm closely matches the spatial arrangement of iodine, making high iodine loading feasible. Hou et al.[72] developed a uniform precursor with hydrogen-bonding characteristics and precisely controlled different pore-forming agents during synthesis, demonstrating that a pore structure of approximately 2.5 nm in nanocarbon significantly contributes to iodine adsorption and rapid polyiodide conversion. The presence of 2.5 nm pore structures in the carbon material enhanced iodine adsorption efficiency, facilitated rapid conversion between I-/I2, and suppressed iodide dissolution. Nitrogen-doped nanocarbon can achieve an iodine loading capacity of up to 60.8 wt%, delivering a large capacity of 178.8 mAh·g-1 at 5 C with long-term cycling stability exceeding 4000 hours.
The rich pore structure of porous carbon materials can enhance the permeability of the electrolyte, shorten the ion diffusion path, and increase the contact area between zinc ions and iodine, thereby improving the utilization efficiency of iodine. In addition, they can effectively adsorb iodine, reducing its dissolution loss, thus enhancing the battery's Coulombic efficiency and cycle life.
In summary, the key roles of carbon materials in zinc-iodine batteries include facilitating electron conduction and reducing internal resistance; stabilizing iodine and reducing its dissolution loss through physical and chemical adsorption; certain functionalized carbon materials possess electrocatalytic activity, which can accelerate the redox reactions of I2/I-; promoting ion transport and enhancing electrochemical reaction kinetics. These characteristics endow iodine/carbon composite materials with great potential for improving the electrochemical performance, structural stability, and cycling life of zinc-iodine batteries. It should be noted that different carbon materials play varying roles as iodine carriers due to differences in their intrinsic material properties, resulting in different electrochemical behaviors of the iodine cathode, as shown in Table 1. Additionally, for new battery technologies aimed at large-scale pure energy storage applications, attention should be paid to the complexity of preparation processes and cost issues when developing composite electrodes.
表1 基于不同碳材料的锌-碘电池的电化学性能对比

Table 1 Comparison of electrochemical properties of zinc-iodine batteries with different carbon materials

type cathode materials anode materials electrolyte cyclic stability retention Ref
Carbon nanotube-based cathodes HCNS Zn flake 0.5 M ZnSO4 + 0.5 M H2SO4 94 mAh·g-1@1 A·g-1/1500 cycles 87% 24
HCNT-O Zn foil hydrogel electrolyte(CPAM-Zn-I-0.5) 352.5 μAh·cm-2@10 mA·cm-2/2600 cycles 89.20% 48
CNT@MPC12 Zn foil 1 M ZnSO4 0.38 mAh·cm-2@10 mA·cm-2/12000 cycles 91% 25
Graphene-based cathodes I2/3DGC Zn foil 1 M ZnSO4 205 mAh·g-1@3.2 C/400 cycles 99% 54
rGO-I2 Zn foil 20 M ZnCl + 5 M KI 2 mAh·cm-2@50 mA·cm-2/2000 cycles 100% 26
PNC-1000-I2 Zn foil 1 M ZnSO4 200mAh·g-1@1 A·g-1/1000 cycles 89% 56
N-rGO Zn@rGO 2 M KI + 0.5 M NaSO4 150 mAh·g-1@5 A·g-1/2000 cycles 96.70% 57
G/PVP@ZnI2 CuNC@Cu/Zn 2 M ZnSO4 125.7mAh·g-1@1 A·g-1/200 cycles 63.80% 58
HOPG Zn foil ZnI2 + ZnSO4/H2O + EG 144.1 mAh·cm-3@5 A·g-1/15000 cycles 97.60% 59
Activated carbon-based cathodes I2@C-50 Zn foil 2 M Zn(CF3SO32 151 mAh·g-1@5 A·g-1/10000 cycles 75% 60
I2-AC-6 Zn foil 1 M ZnSO4 104 mAh·g-1@5 A·g-1/6000 cycles 90% 62
I2@APCC Zn foil 5.9 M ZnCl2 + 0.2 M KI 523.69 mAh·g-1@500 mA·g-1/600 cycles 92.20% 61
Biomass-derived carbon-based cathodes I-BCHP Zn foil 1 M ZnSO4 70 mAh·g-1@300 mA·g-1/800 cycles / 62
N-LPC/I2 Zn foil 1 M ZnSO4 114 mAh·g-1@300 mA·g-1/400 cycles 99% 64
I2@APCC Zn foil 5.9 M ZnCl2 + 0.2 M KI 523.69 mAh·g-1@500 mA·g-1/600 cycles 92.20% 65
Other porous carbon material-based cathodes I2@C Zn/C 2 M ZnSO4 122.3 mAh·g-1@12 C/3000 cycles 88.10% 67
NCCs/I2 Zn foil 2 M ZnSO4 131 mAh·g-1@5 A·g-1/3500 cycles / 68
HMMC-I2 NSs Zn foil 1 M ZnSO4 137.5mAh·g-1@8 A·g-1/9200 cycles 92.10% 69
CMK-3@I2 Zn foil 2 M ZnSO4 10 A·g-1/39000 cycles 80.60% 70
NGA/I2 Zn foil 1 M ZnSO4 178.8 mAh·g-1@5 C/10000 cycles / 71

4 Conclusion and Prospect

Zinc-iodine batteries have attracted significant attention as an emerging electrochemical energy storage technology due to their advantages of high energy density, low cost, and environmental friendliness. However, the intrinsic properties of the iodine cathode and the characteristics of products during the electrochemical reaction process severely degrade the electrochemical performance, thereby limiting further development. Carbon materials possess excellent material properties, such as porosity, large specific surface area, and high conductivity, making them ideal support materials for iodine and playing a crucial role in addressing the aforementioned issues. Graphene, carbon nanotubes, activated carbon, and biomass carbon have all demonstrated tremendous application potential and have been extensively studied in zinc-iodine batteries. The high conductivity and large surface area of graphene can effectively enhance electrode reaction activity and charge transport rates, thereby improving the battery's energy density and power density. Meanwhile, activated carbon and biomass carbon provide sufficient space for iodine adsorption and storage through their abundant pore structures and large surface areas, effectively increasing the electrode's iodine loading capacity and subsequently enhancing the battery's energy density.
As a carrier for iodine cathodes, attention should be paid to the ability of carbon materials to anchor polyiodide ions, suppress polysulfide shuttling, and catalyze electrochemical reactions. Rapid conversion of I- and I3- is more beneficial than simply immobilizing iodine on the material surface. Considering the slow kinetics of iodine conversion reactions, carbon materials with catalytic properties can accelerate the transformation of I2 species. Although certain carbon materials have achieved significant progress in the I-/I3- catalytic conversion in zinc-iodine batteries, it is still necessary to develop low-cost, highly catalytic, and scalable carbon materials.
Based on previously reported research findings and our own research experience, future studies on zinc-iodine batteries can focus on the following aspects: (1) Pore structure and surface property regulation of carbon materials (e.g., introduction of functional groups, heteroatom doping, etc.). A favorable pore structure provides sufficient space for iodine loading and offers effective physical confinement, enabling simultaneous enhancement of capacity and stability. The surface properties directly influence the interactions between iodine and the carbon material; by regulating these properties, the stability of iodine on the carbon surface can be enhanced, while suppressing iodine precipitation and the shuttle effect. Moreover, certain active sites can catalyze iodine conversion reactions, improving electrochemical reaction kinetics. Therefore, rational design and regulation of the multi-scale structures of carbon materials are crucial for enhancing electrochemical performance and addressing practical challenges in zinc-iodine battery applications. (2) Anchoring mechanisms of polyiodide ions and catalytic conversion mechanisms of iodine. Currently, related research is still at an early stage, with many issues remaining unclear. Advanced in situ characterization techniques and theoretical calculation methods are feasible strategies to help understand these mechanisms. For example, in situ liquid-phase TOF-SIMS technology can reveal reaction pathways and mechanisms of polyiodide ions by detecting adducts and intermediates formed during the reaction process; in situ infrared spectroscopy can monitor molecular vibration modes in electrodes or electrolytes, revealing interface formation, molecular decomposition, gas evolution, and interactions between the electrode and electrolyte during electrochemical reactions; in situ Raman spectroscopy enables real-time monitoring of phase transitions, structural changes, and reaction products of electrode materials during electrochemical reactions; in situ UV-Vis spectroscopy can detect optical absorption characteristics of electrode materials, monitor changes in electronic structure, and is particularly suitable for analyzing redox behavior and valence state variations of electrode materials during electrochemical reactions. Theoretical calculations simulate and predict the interaction capabilities and reaction pathways between polyiodide ions and carbon materials, offering theoretical guidance for material design and optimization. (3) Preparation of electrode materials and electrochemical testing with a focus on practical applications. During research, considerations should be given to increasing the iodine loading and conducting electrochemical tests under high-loading conditions. This helps improve the energy density of the battery, analyze the feasibility of material application, explore potential application scenarios of zinc-iodine batteries, and further promote the development of this energy storage technology. (4) Zinc dendrite formation and the shuttle effect of iodide ions are key factors contributing to poor performance in zinc-iodine batteries. To address this issue, constructing a three-dimensional structure with shrinkable features on the zinc anode represents an effective strategy. Such a structure can provide a stable supporting framework and effectively suppress the shuttle effect of iodide ions during charging and discharging processes, thereby reducing iodine reduction reactions on the zinc anode surface, decreasing the corrosion rate of zinc, and extending the battery's cycle life. (5) Furthermore, optimizing the electrolyte is also critical for enhancing the performance of zinc-iodine batteries. By regulating the preferential coordination between anions in the electrolyte and iodine, the shuttle problem of polyiodides can be suppressed, and the redox kinetics of I·/I₂ conversion can be improved. At the same time, to further increase the battery's energy density, optimizing the electrolyte to activate multi-electron transfer redox mechanisms is an effective approach.
Zinc-iodine batteries are a promising electrochemical energy storage system with high performance, scalability, and environmental friendliness. Carbon materials play a crucial role in the cathode of zinc-iodine batteries and represent a key component for their practical application. Although the current theoretical framework is not yet complete and many challenges remain to be addressed, the breakthrough research achievements already attained inspire confidence in this field.
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