Organic small-molecule cathodes for aqueous zinc-ion batteries: design strategy, application and mechanism

Abstract

Owing to its high safety, environmental friendliness and low cost, aqueous zinc-ion batteries (AZIBs) have been considered an effective alternative to other energy-storage batteries. Among the adopted cathode materials for AZIBs, organic small molecules (OSMs) show potential due to their renewable resources, diverse structures, easy preparation and fast kinetics. In this review, recent progress on OSMs as cathodes for AZIBs has been summarized, and various types of OSMs (including quinone-based, amide-based, nitrogen-containing, and related composites) have been discussed and compared in detail. The energy storage mechanism for these OSM cathodes and the relationship between their structure and performance have been deeply explored. Furthermore, the key problems of OSM cathodes in practical applications and the corresponding strategies for improving their application performance, in terms of molecular structure design and composite material modification, have also been discussed and illuminated. Finally, the future development and prospect of OSM cathodes for AZIBs are envisioned, aiming to provide a reference for promoting further research and application.

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Lingyan Long

Lingyan Long is currently pursuing her master's degree at the School of Environmental and Chemical Engineering, Shanghai University, China. Her research primarily includes the structural design of organic framework materials and their application for high-performance energy storage systems, especially with a focus on organic electrodes for aqueous zinc-ion batteries.

Kailing Mei

Mei Kailin is currently pursuing her master's degree at the School of Environmental and Chemical Engineering, Shanghai University, China. Her research primarily focuses on the structural design and performance enhancement of advanced energy-storage systems, particularly with an emphasis on electrodes for aqueous zinc-ion batteries.

Zheyun Hou

Zheyun Hou is pursuing his bachelor's degree in the School of Environmental and Chemical Engineering, Shanghai University, China. His research interest mainly focuses on electrode materials for various kinds of energy-storage batteries.

Yong Wang

Yong Wang completed his PhD in Chemical Engineering at the National University of Singapore in 2004. Following this, he served as a research fellow at the Singapore-MIT Alliance from 2004 to 2006. In 2007, he joined Shanghai University as a professor. Presently, he holds the position of Executive Dean at Sino-European School of Technology at Shanghai University, where his research primarily centers on materials for energy storage and environmental applications.

Haijiao Zhang

Haijiao Zhang is a full-time professor in the Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University. She received her PhD degree in chemistry from East China Normal University and was supervised by Professor Mingyuan He. From 2015 to 2016, she was engaged as a senior visiting scholar in The University of Queensland, Australia. Currently, her main research interests include the functional design of porous nanomaterials and their applications in energy storage and conversion.

Weiwei Sun

Weiwei Sun received her PhD (2009) in chemistry from East China Normal University and was a postdoctoral fellow at Fudan University, China, and Wollongong University, Australia, before joining Shanghai University. Currently, Dr. Sun is an associate professor at the School of Environmental and Chemical Engineering at Shanghai University, and her research focuses on porous framework materials for carbon dioxide capture and energy storage/conversion.

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1. Introduction

To solve the problem associated with the massive consumption of fossil energy, in terms of the unprecedented energy crisis¹ and serious environmental pollution, the development of renewable energy has been considered an effective strategy.² Aqueous zinc-ion batteries (AZIBs), endowed with the significant superiority of high safety and environmental friendliness, have become a highly promising system for next-generation energy storage.³ Zinc metal electrodes have been recognized for their high theoretical capacity of 820 mAh g⁻¹, abundant reserves, superior environmental compatibility and electrochemical performance.^4,5 Besides, enhanced safety and reduced cost can be achieved for AZIBs due to the use of aqueous electrolytes. Notably, the high ionic conductivity of the aqueous electrolyte of AZIBs also contributes to the excellent rate performance of the battery system.⁶

Despite these advantages, AZIBs face a number of challenges, in terms of their unsatisfactory cycle life and safety, which are severely impacted by the zinc anode-related issues of dendrite formation and corrosion.^7,8 Besides, the capacity and stability of cathode materials are not sufficient for practical applications. With further research into energy storage systems, organic small molecules (OSMs) as electrode materials have gradually gained attention in recent years. When applied in AZIBs, the OSM materials can be adopted as anode protective coatings to regulate the zinc deposition behavior.⁹ The OSM materials with high surface area, porous characteristics and good conductivity has been considered for enhancing the properties of OSM-based anodes. Meanwhile, OSMs can also be used as cathodes in AZIBs. Their advantages of designability, structural diversity, environmental friendliness, and abundant resources endow the OSM cathodes with satisfactory application prospects.^10–14 Traditional inorganic cathode materials, such as MnO₂ and VO_x, which have been extensively studied, often suffer from dissolution and limited cycling stability.^15–17 In comparison, the electrochemical properties, including voltage, specific capacity, conductivity and redox kinetics, of OSM-based cathode materials can be adjusted by changing/designing their molecular structures. OSMs are remarkable for their structural diversity and designability. The redox potential, charge storage mechanism and compatibility with electrolyte can be precisely regulated through rational molecular design.^18–20 The introduction of more functional groups, which can irreversibly react with Zn ions, can promote the specific capacity and conductivity of OSM-based ZAIBs, while the redox reactions of OSMs usually have fast kinetics and can support high-rate charging and discharging, leading to their improved cycling and rate properties.^21–23 Various functional groups in the structure of organic molecules, including carbonyl groups (C=O) and imino group (C=N), can be actively applied to take part in the storage process of Zn²⁺ ions.¹⁸ In the case of the carbonyl group, for example, it can achieve charge storage through coordination with Zn²⁺ ions.^19,24,25 This flexibility based on molecular design provides a broad space for exploration of high-performance cathode materials for AZIBs.^26,27 From the perspective of environmental friendliness, OSMs usually consist of common elements such as C, H, O, and N. After the battery is discarded, there is far less pollution to the environment in comparison to that from inorganic materials containing heavy metals.^28–30 It is well known that MOF, COF or some polymer materials have also been adopted as cathodes of AZIBs. However, the complicated synthesis processes with harsh reaction conditions and high-cost ligands, as well as poor structural stability during electrochemical interactions and low ion diffusion efficiency also limit further exploration of these cathode materials. Comparably, OSM materials can achieve the simple and controllable synthesis pathways under mild conditions with inexpensive raw materials and easy modular modification, and the tunable redox-active sites and fast ion-storage kinetics can also endow OSM cathode materials with enhanced properties.³¹ Of course, OSMs also face many challenges as cathode materials for AZIBs. The unsatisfactory surface area of OSM materials, compared to MOFs or COFs, and their poor conductivity result in poor cycling stability, restricted ion transport under high loading conditions, and limit the further applications of OSM materials.^32–35 The solubility of OSMs in aqueous electrolytes should also not be ignored, as it leads to the loss of active substances, and further the decay of the battery capacity and reduced cycling stability.^36–39

Recent studies indicate that the electrochemical performance of OSM cathodes can be enhanced by the rational design of the molecular structure or post-modification process, including doping, functionalization or composite formation.⁴⁰ According to reports in recent years, the molecular design aspect includes the introduction of quinone/carbonyl active centers to achieve the reversible storage of Zn²⁺, extending the conjugated skeleton or N/S doping to enhance the electrical conductivity, and modulating the layer spacing or constructing porous frameworks to promote ion transport. Post-modification strategies include chemical doping to regulate electrical properties, surface functionalization (sulfonate groups) to inhibit solubility, combination with carbon materials (CNTs) or conductive polymers to enhance stability, and nano-sized morphology to shorten ionic paths.⁴¹

This review comprehensively and systematically summarizes the classification of OSMs according to their functional groups and the current status of their research as cathodes for AZIBs. Although there are a few reported reviews on the AZIBs with OSM cathodes, most of them focus on the application of pristine OSM cathodes. In this review, besides the pristine OSM with quinone, imide, nitrogen-containing and other functional units, the OSM-involved composites with CNTs, graphene and MXene addictive have also been summarized in detail. It is worth noting that the combination of OSM materials with carbonaceous materials of different dimensions would effectively alleviate the solubility of OSM materials in organic electrolytes. Thus, the summary on OSM-involving composite cathodes would extend the further exploration on combining OSMs with functional additives. In addition, the corresponding strategies for improving the application performance of OSM-based cathodes, in terms of molecular structure design and composite material modification, have also been illuminated in this review, providing reference for rational design, extended research, and further application of OSM and its related materials.

2. Storage mechanisms of OSMs

AZIBs are assembled from several key components, including a zinc negative electrode, a neutral or slightly acidic zinc ion-containing electrolyte solution, a diaphragm, and a zinc storage cathode material.^41–43 For organic AZIBs, metallic zinc has been widely used as the negative electrode, while the OSMs serve as the positive electrode, and Zn²⁺ and Mn²⁺ ions in neutral or weakly-acidic aqueous solutions are selected as the electrolyte.^9,37 Based on the electron transfer behavior of the active sites during redox reactions, organic electrodes for AZIBs can be categorized as n-type, p-type and bipolar organic compounds.^12,37 For n-type materials, the carbonyl and imino groups can gain electrons and then be reduced to anions, which can further coordinate with Zn²⁺ or H⁺ in the electrolyte. In contrast, p-type organics (including nitroxides and organosulfur polymers) can usually lose electrons during oxidation to form cations, which can further react with anions of SO₄²⁻, Cl⁻ and CF₃SO₃⁻ from the electrolyte. In addition to the uni-polar electrodes mentioned above, bipolar organic electrodes can combine the advantages of n-type and p-type materials and undergo either oxidation or reduction reactions. In detail, the bipolar electrode can be reduced and connected with cations during the discharge process but can be oxidized upon connecting with anions during the charging process. Thus, the bipolar electrode materials can integrate the advantages of n-type groups with high-capacity and p-type groups with high-voltage, further opening up new avenues for AZIBs.^44,45 It is worth noting that four ion-storage mechanisms can be detected for the OSM cathodes of AZIBs (Fig. 1), including Zn²⁺ storage, H⁺ storage, Zn²⁺/H⁺ co-storage and anion storage.^46–48

Fig. 1 Schematic presentation on the ion-storage mechanisms for AZIBs: (a) Zn²⁺ storage, (b) H⁺ storage, (c) Zn²⁺/H⁺ co-storage, (d) anion storage.

2.1. Zn²⁺ storage

Most Zn²⁺ storage mechanisms have been detected for n-type organic materials that achieve energy storage through reversible coordination with Zn²⁺. The storage mechanism is quite similar to that of conventional LIBs; that is, Zn²⁺ ions can insert into the lattice of the cathode structure during discharge and then extract during the subsequent charge process.⁴⁹ Reversible Zn²⁺ insertion/removal in n-type cathodes typically occurs via the coordination with neighboring functional C=N or C=O groups within a molecule, or inter-molecular neighbouring functional groups. Meanwhile, the organic electrode materials adopted for storing Zn²⁺ should typically contain one or more pairs of redox functional units in the structure.⁵⁰ Taking the PTO (pyrene-4,5,9,10-tetraone) as an example,⁵¹ Zn²⁺ ions are embedded in the PTO molecular structure from the electrolyte during the discharge process with carbonyl groups in the PTO molecule undergoing a coordination interaction with Zn²⁺ ions to form a stable coordination compound. Then, the Zn²⁺ ions are extracted from the PTO molecule and returned to the electrolyte during the charging process.⁵¹ Specific functional groups, including C=O and C=N, can be detected for most OSM cathode materials, and can be considered as the active sites for Zn²⁺ storage,^52,53 during which Zn²⁺ ions can react with O atoms from the carbonyl groups or the N atoms from imino groups to achieve the insertion of Zn²⁺ and storage of charge.⁵⁴ This mechanism based on coordination is prevalent in a variety of organic small-molecule cathode materials, and the reversibility of the reaction is critical to the cycling performance of AZIBs. If the coordination interaction is too strong, it may lead to difficulty in dislodging Zn²⁺ ions, affecting the discharge performance of the battery. Meanwhile, if the coordination interaction is too weak, it will be difficult for Zn²⁺ ions to be efficiently inserted during charging, reducing the battery capacity.^48,49

2.2. H⁺ storage

Beside Zn²⁺ storage, the H⁺ ion insertion and extraction mechanism can also be observed for several OSM cathode materials. Due to the smaller radius, it is easier to insert the H⁺ ion into or extract from the OSM structure with coordination on active functional units. Compared to Zn²⁺ ions, H⁺ ions have a weaker electrostatic force and therefore usually exhibit faster kinetic behaviour.^55,56 Adopting the nitrogen-containing heterocyclic organic small-molecule HATN (diquinoxalino [2,3-a:2′,3′-c] phenazin) as a cathode material, the water in the electrolyte undergoes weak ionization to produce H⁺ and OH⁻ ions.⁵⁷ During the charging process, H⁺ ions will be embedded into the molecular structure of HATN, while each HATN molecule with multiple sites binding to H⁺ can achieve the embedding and further reduction of multiple protons. During the discharge process, the embedded H⁺ is extracted from the HATN molecules into the electrolyte, while the HATN molecules lose electrons to be oxidized. Meanwhile, in order to maintain the pH equilibrium of the electrolyte system, OH⁻ ions can react with the Zn²⁺ or other cations from the electrolyte to form Zn₄SO₄(OH)₆·5H₂O or other substances. The insertion/extraction mechanism of H⁺ ions not only affects the multiplicity performance of the batteries, but may also affect their cycling stability. For example, in some organic materials, the insertion of H⁺ ions may lead to dissolution or structural degradation of the material, thus affecting the cycle life of the battery.^58,59 Therefore, an optimized structure design with the aim of improving the storage stability and kinetic properties of H⁺ is important for further exploration. Meanwhile, the storage mechanism based on H⁺ ions for AZIBs with OSM cathodes is a complex electrochemical process, and an in-depth study of this mechanism is important for further exploration of high-performance organic electrodes.^46,60

2.3. Zn²⁺/H⁺ co-storage

The co-storage of H⁺ and the Zn²⁺ mechanism indicates that the storage process of Zn²⁺ ions is transferred between the anode and cathode, while the H⁺ ions are transported between the electrodes and electrolyte, accompanied by the formation or decomposition of Zn₄SO₄(OH)₆ in an aqueous solution. However, the order of storage and the proportion of contribution of H⁺ and Zn²⁺ are still controversial for the co-storage of H⁺ and Zn²⁺ mechanism.⁵⁶ Some functional groups in OSMs, including carbonyl and imino groups, usually have strong electronegativity, which will give the molecules a local negative charge. As a kind of fully conjugated structure, enhanced π-electron delocalization and intermolecular interactions endow the HATAN (anthraquinone-quinoxaline) material with improved electronic conductivity and excellent stability for Zn²⁺/H⁺ co-insertion/co-extraction.⁶¹ Although the Zn²⁺/H⁺ co-storage mechanism shows great potential for enhancing the battery performance, it still faces some challenges, such as the irreversible detachment of H⁺ that may lead to electrolyte pH fluctuations and deterioration of the electrode structures. Therefore, future research needs to further optimize the structure of the electrode materials and stability of the electrolyte to achieve more efficient ion storage and more stable battery performance. In summary, the Zn²⁺/H⁺ co-storage delivers significant performance advantages for organic small molecule based AZIBs, but its optimization and application still need to be further studied.⁶²

2.4. Anion storage

It is well known that the p-type organic materials lose electrons during oxidation to form cations, and subsequently bind to anions from the electrolyte to retain the charge neutrality. Thus, the storage mechanism involving an adsorption/desorption process of the anion has also been detected for several p-type organic materials.⁴⁹ During the discharging process, besides cations in OSM electrodes, the uptake/removal of anions (e.g., SO₄²⁻, CF₃SO₃⁻, Cl⁻ or ClO₄⁻) is also involved for the above-mentioned p-type organic electrodes.^22,56,63 The ion-storage mechanism is an important electrochemical process for AZIBs, which has received extensive attention, especially in organic cathode materials. Unlike the conventional cation (Zn²⁺ or H⁺) storage mechanism, the anion-storage mechanism indicates the insertion/extraction of anions (TFSI⁻, OTf⁻, or other anions) during the cycling process. During the charging process, the cathode material loses electrons and a positive charge is formed, while the anions transfer from the electrolyte into the cathode material to balance the charge. Meanwhile, during the discharge process, the positive material gains electrons and the anions are extracted from the positive material and returned to the electrolyte.⁵⁸ This mechanism exhibits unique advantages in certain organic cathode materials, such as high-voltage plateaus, fast kinetics, and good cycling stability. For example, the BDB (1,4-bis-diphenylamino benzene) molecule, which contains two tertiary N atoms, undergoes reversible redox reactions, accompanied with insertion/extraction of TFSI⁻ or OTf⁻ ions during the redox process.⁶⁴ During the charging process, the nitrogen atoms in BDB are progressively oxidized to form BDB⁺-A⁻ and BDB²⁺-2A⁻ intermediates (A⁻ present the anions). At this point, H⁺ and Zn²⁺ are inserted into the electrode material from the electrolyte and undergo coordination with the nitrogen atoms to achieve charge equilibrium. Under the following discharging process, BDB²⁺-2A⁻ is gradually reduced to BDB⁺-A⁻ and eventually back to neutral BDB, during which H⁺ and Zn²⁺ are removed from the electrode material and return to the electrolyte. Accompanied by the redox process, the storage of anions can balance the charge, ensure the smooth reaction process, and promote the electrochemical performance of AZIBs. Furthermore, anion species during the anion-storage mechanism can affect the operating voltage and cycling stability of AZIBs with organic cathodes.

3. OSMs for AZIBs

In contrast to inorganic electrodes, organic electrodes containing adjustable redox functional units can deliver enhanced electrochemical performance and abundant storage mechanism. Organic small molecules (OSMs) and related electrode materials have been widely explored with aqueous electrolytes, and have the advantages of an easy synthesis, mass production and low cost. Moreover, the weak intermolecular interactions between the OSMs allow for the large size of Zn²⁺, as well as the rapid diffusion and storage of multivalent ions, which makes the organic small-molecule materials a promising alternative as the electrodes for AZIBs. The application of OSM and related materials as cathodes for AZIBs has been summarized and compared according to their active sites, as shown in Table 1.

Table 1 Summary and comparison of the electrochemical properties of recently-reported OSMs and composite cathodes for AZIBs

Cathode	Charge carrier	Electrolyte	Initial capacity/mAh g^−1 at current density/A g^−1	Capacity retention/cycle number@current density/A g^−1	Ref.
Quinone-based OSMs
PTO	Zn^2+	2 M ZnSO4	337 at 0.04	70%/1000@3.00	51
C4Q	Zn^2+	3 M Zn(CF3SO3)2	335 at 0.02	87%/1000@0.50	65
PQ-Δ	Zn^2+	3 M Zn(CF3SO3)2	215 at 0.15	∼100%/500@0.15	66
DTT	Zn^2+ + H^+	2 M ZnSO4	211 at 0.05	∼84%/23 000@0.05	67
4S6Q	H^+	3.5 M Zn(ClO4)2	240 at 0.15	∼100%/20 000@3.00	68
TDT	Zn^2+ + H^+	1 M ZnSO4	369 at 0.20	∼96%/3000@10.00	69
BDTD	Zn^2+ + H^+	2 M ZnSO4	206 at 0.05	∼82%/12 000@5.00	70
C8Q	Zn^2+	3 M Zn(OTf)2	∼207 at 1.00	∼47%/10 000@10.00	71
DBPTO	Zn^2+ + H^+	2 M ZnSO4 + 0.05 M H2SO4	382 at 0.05	62%/60 000@5.00	72
DEBQ	Zn^2+ + H^+	1 M ZnSO4 + 2 M Na2SO4	372 at 0.10	∼60%/120 000@10.00	73
TAHQ	Zn^2+ + H^+	2 M ZnSO4	254 at 0.50	71%/1000@5.00	11
P-PNQ	Zn^2+ + H^+	2 M Zn(CF3SO3)2	279 at 1.00	∼95%%/270@0.05	8
m-DMBQ	Zn^2+ + H^+	2 M ZnSO4	312 at 0.16	∼75%/4000@6.38	74
IDT	Zn^2+ + CF3SO3^−	2 M Zn(OTF)2	238 at 0.20	∼89%/3000@10.00	75
NI-DAQ	Zn^2+ + H^+	2 M ZnSO4	192 at 0.05	∼74%/3000@0.50	60
Me-NQ	Zn^2+ + H^+	2 M ZnSO4	316 at 0.10	∼73%/3500@5.00	76
TAPQ	Zn^2+ + H^+	1 M ZnSO4	443 at 0.05	92%/250@0.05	77
Amide-based OSMs
NDI	Zn^2+	2 M ZnSO4 + 0.5 M Na2SO4	200 at 0.05	76%/5000@3.00	78
RF	Zn^2+	3 M Zn(OTF)2	∼146 at 0.01	∼93%/3000@5.00	79
Nitrogen-active OSMs
HATN	H^+	2 M ZnSO4	405 at 0.10	∼93%/5000@5.00	57
BDB	Zn^2+ + OTf^−	1 M Zn(OTf)2 + 2 M Zn(OTF)2	112 at 0.39	75%/1000@0.78	64
DC-PDESA	Zn^2+	3 M Zn(CF3SO3)2 + 40% HBCD + 60% H2O	121 at 0.29	∼98%/2000@14.80	80
DHTAT	Zn^2+ + ClO4^−	3 M Zn(ClO4)2	224 at 0.05	73%/5000@5.00	81
PNZ	Zn^2+	2 M ZnSO4	232 at 0.02	79%/1000@1.00	82
HATN-3CN	Zn^2+ + H^+	2 M ZnSO4	320 at 0.05	∼91%/5800@5.00	83
HATN-PNZ	Zn^2+ + H^+	2 M ZnSO4	257 at 5.00	∼80%/45 000@50.00	84
TRT	Zn^2+	2 M ZnSO4	220 at 0.10	∼99%/1000/0.50	85
HATNQ	Zn^2+ + H^+	3 M ZnSO4	∼483 at 0.20	∼95%/11 000@5.00	86
TABQ-PQ	H^+	3.5 M Zn(ClO4)2	∼241 at 0.10	∼91%/30 000@5.00	87
TAT-HATTI	Zn^2+ + H^+	2 M ZnSO4	531 at 0.05	78%/1000@2.00	88
DQP	Zn^2+	1 M ZnSO4	413 at 0.05	86%/1000@5.00	89
MB	Zn^2+ + CF3SO3^−	3 M Zn(CF3SO3)2	160 at 1.67	86%/20 000@16.70	90
TCNAQ	Zn^2+	2 M ZnSO4	165 at 0.05	81%/1000@0.50	91
DMPZ	Zn^2+ + ClO4^−	17 M NaClO4 + 0.5 M Zn(CF3SO3)2	231 at 0.05	81%/1000@0.23	92
HATAQ	Zn^2+ + H^+	1 M ZnSO4	492 at 0.05	99%/1000@/20.00	93
HATTA	Zn^2+	1 M Zn(CF3SO3)2	∼226 at 0.05	∼84%/10 000@25.00	59
HFHATN	Zn^2+ + H^+	2 M ZnSO4	476 at 0.04	172/1000@20.00	94
BBQPH	Zn^2+ + H^+	3 M ZnSO4	∼499 at 0.20	95%/1000@5.00	95
DQDPD	Zn^2+ + H^+	1 M ZnSO4 + 2M Na2SO4	509 at 0.10	92%/7500@10.00	96
TPHATP	Zn^2+ + H^+	2 M ZnSO4	∼318 at 0.10	∼97%/5000@10.00	97
TQD	Zn^2+ + H^+	4 M ZnSO4	503 at 0.10	∼71%/400/1.00	98
BBPD	Zn^2+ + H^+	2 M ZnSO4	127 at 0.10	∼91%/10 000@20.00	99
TCLHATN	Zn^2+ + H^+	2 M ZnSO4 + H2SO4	278 at 0.10	∼92%/1000@5.00	100
HCLHATN	Zn^2+ + H^+	2 M ZnSO4	226 at 0.50	98%/1000@2.00	101
OSMs with other functional groups
PMC	Zn^2+	2 M ZnCl2	∼123 at 0.20	∼81%/500@8.00	102
dNPC	TFSI^−	20 M LiTFSI + 1 M Zn(TFSI)2	90 at 0.25	∼96%/1000@0.50	103
TTF	ClO4^−	3 M Zn(ClO4)2	220 at 2.00C	∼69%/10 000@20.00C	104
CP+	OTF^− + Zn^2+	1 M Zn(OTf)2	∼158 at 2.00	95%/10 000@2.00	105
PDI	Zn^2+ + H^+	2 M ZnSO4	72.8 at 0.10	∼99%/50 000@3.00	106
OSM-involved composites
APh-NQ@CNT	Zn^2+	2 M Zn(CF3SO3)2	202 at 0.10	68%/1000@0.1	107
p-DB@ carbon nanoflower	Zn(OTF)^+	3 M Zn(OTF)2	402 at 0.10	∼94%/25 000@5.00	108
DNPT/rGO	Zn^2+ + H^+	3 M Zn(CF3SO3)2	150 at 0.24	94%/2000@3.00	109
TCNQ/CCP	Zn^2+	1 M ZnSO4	123 at 0.01	∼79%/100@1.00	110
TAP/Ti3C2Tx	Zn^2+ + H^+	2 M ZnSO4	303 at 0.04	∼82%/10 000@0.04	111
PNZ/KB	Zn^2+	2 M ZnSO4	179 at 0.05	∼71%/1000@5.00	112
IV@rGO	Zn^2+ + H^+	3 M Zn(CF3SO3)2	∼327 at 0.10	∼46%/2000@2.00	113
TMBQ@KMCN	Zn^2+	2 M ZnSO4	315 at 0.16	∼91%/15 000@6.52	114
SAS@RGO	Zn^2+ + H^+	3 M Zn(OTf)2	376 at 0.20	∼100%/50 000@20	63
PNO@KB	(OTF)^−	3 M Zn(OTf)2	215 at 2.00	∼100%/3500@2.00	115
6CN-HAT@MXene	Zn^2+ + H^+	2 M Zn(CF3SO3)2	413 at 0.05	91%/5000@5.00	39
PDSe/CMK-3	Zn^2+ + OTF^−	3 M Zn(OTF)2	621 at 10.00	∼97%/12 000@10.00	116

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3.1. Pristine OSM materials

Up to now, there have been several reports on pristine OSM materials as cathodes of AZIBs, which can be divided into four kinds based on their functional groups participating in the Zn²⁺ ion storage and reaction, in terms of quinone-base, amide-base, N-active containing or other functional groups containing OSM materials. Considering the reaction mechanism, the OSM materials with different functional groups refer to different ion-storage mechanisms. Generally speaking, the direct Zn²⁺ storage and H⁺ can also be detected for the OSM with C=N and C=O units, while the OSM with amide-based units always deliver the Zn²⁺ ion-storage mechanism.

3.1.1. Quinone-based OSMs. Quinone is a representative small-molecule organic electrode material containing two adjacent/separated carbonyl groups for each unsaturated six-membered ring.¹¹⁷ Originating from the molecular diversity and structural adaptability, quinone-based materials have been widely used for AZIBs with high specific capacity and excellent electrochemical reversibility. More importantly, quinone-based electrodes are generally not subject to counterion selection. This results in them being attractive for Zn²⁺, H⁺ storage, making quinone and related materials promising candidates for superior electrochemical storage.¹¹⁸ The quinone material was first adopted in 1972 by Sandstede and coworkers as the electrode for AZIBs due to its reduction potential of 0.7 V in H₂SO₄ solution.¹¹⁹ Recently, quinone-based materials have been widely applied for energy storage, demonstrating their stability under variable pH conditions, with different current-carrying ions, over a wider range of temperatures and in different atmospheres.¹¹⁹

As a kind of OSMs, the calix[4]quinone (C4Q) cathode was first proposed for AZIBs by Chen with coworkers,⁶⁵ and it delivers an open-bowl structure containing eight carbonyl units in each molecule. It demonstrated that C4Q can form a reversible coordination with Zn²⁺ ions through its carbonyl group as the active center of the electrochemical reactions. During the charge/discharge process, Zn²⁺ ions are connected to carbonyl groups in C4Q, in which each C4Q molecule can connect with three Zn²⁺ ions. Thus, the C4Q cathode for rechargeable AZIBs, along with the aqueous electrolyte and cation-selective membrane, exhibits an initial capacity of 335 mAh g⁻¹ at 20 mA g⁻¹, as well as a high-capacity retention of 87% after 1000 cycles at 500 mA g⁻¹. Its outstanding performance primarily stems from the molecular structure featuring abundant multi-carbonyl active sites, which can endow the C4Q cathode with high reversible capacity, and the distinctive open-bowl configuration that effectively ensures structural stability during charge/discharge processes.

However, due to the gradual dissolution of quinone-based OSMs into the electrolyte, the cycling stability of several quinone-based electrodes is unsatisfactory. So, rational design strategies on the OSM structure with rigid geometry have been proven to be effective for enhancing electrochemical performance of quinone-based cathodes. A kind of quinone-based, triangular phenanthrenequinone-based macrocycle, named PQ-Δ, has been reported by Stoddart and coworkers,⁶⁶ which exhibits a layered superstructure consisted of robust triangular molecules (Fig. 2a). During the storage process, Zn²⁺ ions and H₂O molecules can be inserted into the structure of PQ-Δ, and the cathode/electrolyte interfacial resistance can also be reduced. The synergistic effects of the rigid triangular structure of PQ-Δ and the hydrated Zn²⁺ ion-storage mechanism can achieve a large reversible capacity of 210 mAh g⁻¹ at 150 mA g⁻¹. Moreover, an improved cycling performance of ∼99.9% retained after 500 cycles can also been obtained for this PQ-Δ cathode (Fig. 2b). It is proven that effective applications of electron-active organic materials and obviously-reduced interfacial resistance can promote the metal-ion battery performance with OSM-based cathodes.

Fig. 2 PQ-Δ cathode for AZIBs: (a) electrochemical redox chemistry; (b) cycling performance at 150 mA g⁻¹. Reproduced with permission,⁶⁶ copyright 2020, American Chemical Society. 4S4Q and 4S6Q materials for AZIBs: (c) synthesis process; (d) cycling performance at 3 A g⁻¹. Reproduced with permission,⁶⁸ Copyright 2023, Springer Nature.

Besides the dissolution problem, the low conductivity of the organic small molecules has also attracted much attention from researchers.¹²⁰ Novel sulfur heterocyclic quinones 6a,16adihydrobenzo[b]naphtho[2′,3′: 5,6][1,4]dithiino[2,3i]thianthrene-5,7,9,14,16,18-hexaone (4S6Q) and benzo[b]naphtho[2′,3′ : 5,6] [1,4]dithiino[2,3-i] thianthrene-5,9,14,18-tetraone (4S4Q) compounds have been fabricated via molecule structural design by Tao and his coworkers in 2022.⁶⁸ As shown in Fig. 2c, the flexible molecular skeleton with the extended π-conjugated -S-connection knots of 4S6Q and 4S4Q can effectively improve their conductivity and further inhibit their dissolution into the electrolyte. It is observed that improved electrochemical properties can be detected for the Zn//4S6Q battery with 3.5 M Zn(ClO₄)₂ electrolyte, in terms of the charge capacities of 240 and ∼209 mAh g⁻¹ under 0.15 and 30 A g⁻¹, respectively, the super-long lifetime of up to 20 000 cycles at 3.0 A g⁻¹ with no capacity fading, and the excellent rate capability associated with fast reaction kinetics (Fig. 2d). It is noteworthy that based on the TEM mappings results, H⁺ as a carrier ion participates in the reaction of the 4S6Q electrode in 3.5 M Zn(ClO₄)₂ electrolyte with no involvement of the Zn²⁺ ions. Moreover, Sun and co-workers explored the piperazine-linked quinone-based 2,3,7,8-tetraamino-5,10-dihydrophenazine-1,4,6,9-tetraone (TDT) with high conductivity.⁶⁹ Theoretical calculations indicated the extended conjugation structure between the p electrons of the quinone units and N atoms, which can effectively delocalize the electrons throughout the molecule. EPR analysis can trace the formation of cationic radicals on the N atoms of piperazine rings, which can stabilize the structure and further enhance the electrical conductivity for TDT. Thus, the TDT cathode delivers a high capacity of 369 mAh g⁻¹ at 0.2 A g⁻¹ and 182 mAh g⁻¹ under 10 A g⁻¹. A stable cyclability of up to 3000 cycles can be achieved for the TDT cathode, originating from its improved insolubility with the extended conjugation structure.

In addition to the planar structural design of quinone-based OSM materials, the enhanced electrochemical properties of OSM-based cathodes originating from the spatial structure have received increasing attention. Thioether-linked naphthoquinone-derived isomers 2,2′-(1,4-phenylenedithio) bis(1,4-naphthoquinone) (p-PNQ) materials with tunable spatial structures have been designed and employed as cathodes for AZIBs (Fig. 3a).⁸ In the structure of these p-PNQ isomers, the incomplete conjugated structure within the molecule ensures the redox reactions of the active groups, which further facilitates the full utilization of active sites and endows the materials with high redox reversibility. Moreover, the positional isomerization of naphthoquinones on the benzene rings modifies the zinc-philic activity and redox kinetics of the isomers, highlighting the significance of the spatial structure for determining the energy-storage properties. Consequently, p-PNQ, which features the reduced hindrance and enhanced redox behavior of the active sites, demonstrates remarkable electrochemical performance. It exhibits an outstanding rate capability of 167 mAh g⁻¹ at 100 A g⁻¹ and a long-term cycling stability, lasting over 2800 h at 0.05 A g⁻¹ (Fig. 3b). Distinct from previous studies, the ion-storage mechanism for this p-PNQ cathode primarily involves Zn²⁺ storage with the partial participation of H⁺. Moreover, the polarized isomer of 2,6-dimethoxy-1,4-benzoquinone (m-DMBQ), with asymmetric charge distribution, has also functioned as an advanced cathode for AZIBs (Fig. 3c), demonstrating unexpectedly superior performance compared to the non-polar isomer of p-DMBQ.⁷⁴ The asymmetric charge distribution for the p–π conjugated backbone of m-DMBQ leads to its enhanced electronic conductivity and redox activity. It achieves a large reversible capacity of 312 mAh g⁻¹, which is close to the theoretical value, and a large energy density of 275 Wh kg⁻¹ with a high energy efficiency of 91%. Furthermore, the co-insertion of Zn²⁺ and H⁺, and the significantly enhanced charge transfer kinetics and reversibility have been demonstrated by this compound. Outstanding rate performance and long-term cycling performance have also been obtained for the m-DMBQ cathode (Fig. 3d). This work offers molecular engineering methods by adjusting the charge distribution symmetry to enhance the charge storage in organic cathodes. Moreover, the TAHQ material with a novel quinone/pyrazine alternately conjugated molecular structure, has been easily synthesized and adopted as the cathodes for AZIBs.¹¹ It shows that the highly-reversible redox reactions originated from its multi-active structures with the Zn²⁺/H⁺ insertion/extraction mechanism. Thus, the assembled Zn//TAHQ battery delivers an initial discharge capacity of ∼254 mAh g⁻¹ at 0.5 A g⁻¹, along with enhanced cyclability of 71% capacity retained after 1000 cycles at 5 A g⁻¹).

Fig. 3 The p-PNQ cathode for AZIBs: (a) schematic synthesis process; (b) cycling performance at 0.05 A g⁻¹. Reproduced with permission,⁸ Copyright 2024, Wiley. The m-DMBQ material for AZIBs: (c) frontier molecular orbital energy levels; (d) long-term cycling stability at 20C. Reproduced with permission,⁷⁴ Copyright 2024, Wiley.

Quinone-based OSM materials possess several advantages as cathodes for AZIBs, and have been considered as effective cathode candidates. The abundant carbonyl functional groups in quinone-based materials can be adopted as active redox sites, which endow them with large reversible Zn²⁺ ion-storage capacities and high energy densities. Furthermore, the stable structures of quinone-based OSM materials can effectively suppress their dissolution in aqueous electrolytes, contributing to the improved cycling stability of batteries. Additionally, quinone-based OSM materials are almost environmentally friendly and cost-effective, and their favorable voltage platform and safety features also render them competitive as promising cathodes for aqueous ZIBs.

3.1.2. Amide-based OSMs. Amide-based OSM materials have also attracted increasing attention as cathodes for AZIBs due to their enhanced redox activity and hydrolysis resistance, which originated from the lone pair of electrons in the N atom in the structure. Introduction of amide groups can reduce the solubility of OSMs in certain solvents, and further increase the stability of OSM molecular structures. More importantly, the amide groups can act as a mediator of electron transfer, which can further improve the electronic conductivity and resultant electrochemical properties of the OSM cathodes.

In 2021, a series of amide materials, including 1,4,5,8-naphthalene diimide (NDI), pyrene-based 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), and N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic bisimide (DANTCBI), were obtained and adopted as the cathodes for AZIBs (Fig. 4a). Among them, the nano-structured NDI, which contains the amide units (–CONH–) instead of carbonyl groups, holds great promise as an organic cathode for AZIBs. It does not suffer from severe dissolution issues, nor does it experience deactivation of its active species in contrast to other prevalent organic materials.⁷⁸ It should be noted that C=O groups remaining in the NDI skeleton can also serve as active redox sites for sequential insertion of Zn²⁺ ions, leading to a high reversible capacity of up to 200 mAh g⁻¹. The electrolyte formulation, achieved by blending with the Na₂SO₄ additive, brings about a remarkable enhancement in both cycling and rate properties. As a consequence, an ultra-long lifespan of 5000 cycles can be attained at a high current density of 3.0 A g⁻¹ for this NDI cathode. The strong structural rigidity and low solubility of the NDI cathode can reduce its deactivation and enhance its long-term cycling stability, and the reversible redox on the carbonyl functional groups leads to significantly enhanced electron and ion transportation, as well as further improved electrochemical properties for NDI-based AZIBs (Fig. 4b and c). In addition, through electrolyte optimization, such as the addition of Na₂SO₄, the growth of Zn dendrites can be restrained and the rate performance of NDI-based AZIBs is further improved. Moreover, a kind of biomimetic AZIBs using riboflavin (RF, vitamin B₂), an organic molecule containing an isoalloxazine moiety, as the cathode materials has been reported.⁷⁹ Owing to the reversible redox reaction with the two-electron transfer mechanism on the isoalloxazine moiety of the RF structure, this riboflavin-based cathode can achieve improved battery performance, in terms of its large capacity of 145.5 mAh g⁻¹ under 0.01 A g⁻¹, excellent rate performance, and high cycle stability.

Fig. 4 NDI cathode for AZIBs: (a) synthesis route; cycling performance at (b) 50 mA g⁻¹ and (c) 3000 mA g⁻¹ in the electrolytes with different Na₂SO₄ concentrations. Reproduced with permission,⁷⁸ Copyright 2021, Elsevier.

As the cathodes for AZIBs, the amide-based OSM materials contain the amide groups (–CONH–), which can function as redox-active centers for Zn²⁺ or H⁺ ion storage. Compared with the quinone-based OSM materials, amide-based OSM materials can provide improved structural stability, lower solubility in aqueous electrolytes, and a higher voltage plateau. However, further studies are necessary to enhance the battery performance and reduce production costs for the amide-based OSM materials, which will facilitate their better applications for AZIBs toward sustainable development with greater safety.

3.1.3. Nitrogen-active OSMs. Beside the amide groups (–CONH–), other nitrogen-containing units can also endow the OSM materials with activated and enhanced electrochemical properties. By rational design on the molecular structure, it is possible to synthesize OSMs containing multiple redox-active groups, which would greatly improve their energy density and power density. In addition to quinone- and amide-containing compounds, nitrogen-containing active compounds show increasingly important application prospects for AZIBs.

Tertiary ammonium chemistry, considered as the system with fast kinetics, has been widely used for non-hydrated energy-storage battery system, but it has been rarely explored for aqueous batteries.¹²¹ In 2019, Kund and coworkers introduced a kind of 1,4-bis(diphenylamino)benzene (BDB) material for applications in AZIBs.⁶⁴ A capacity of nearly 125 mAh g⁻¹ at an average voltage of 1.25 V can be achieved for the BDB-based AZIBs originating from the two-electron redox mechanism, in which two tertiary nitrogen centers are reversibly oxidized/reduced stepwise during cycling (Fig. 5a). Furthermore, the anion (OTf⁻/TFSI⁻) insertion into or extraction from the aqueous electrolyte with high oxidative stability also promotes the electrochemical properties of the BDB-based AZIBs. Thus, the BDB cathode exhibits a capacity of 112 mAh g⁻¹ with 82% retention after 500 cycles at a 390 mA g⁻¹ (3C) and 75% retention after 1000 cycles at 780 mA g⁻¹ (6C) with CE of ∼100% (Fig. 5b and c).

Fig. 5 BDB cathode for AZIBs: (a) two-step oxidation/reduction process. Cycling performance at a (b) 3C and (c) 6C. Reproduced with permission,⁶⁴ Copyright 2019, American Chemical Society.

In contrast, phenazine (PNZ) and its analogues, which contain redox-active imino groups (C=N) in the structure, can also be adopted as cathodes for AZIBs.⁸² Thus, PNZ-based AZIB can deliver a specific capacity of up to 232 mAh g⁻¹ under 20 mA g⁻¹ and enhanced cycling performance (capacity retention of 79% after 2000 cycles at 1 A g⁻¹), originating from the water insolubility of PNZ with fewer peripheral hydrophilic groups. It is worth noting that the redox-active imino groups (C=N) with electron-rich nitrogen atoms can effectively adsorb and react with Zn²⁺ ions, promoting the enhanced insertion/extraction and fast diffusion of Zn²⁺ ions into the PNZ cathode. Meanwhile, the three-tooth PNZ structure with multiple active centers and extended π-conjugated systems have also improved the performance of PNZ-based AZIBs.

Moreover, a π-conjugated N-heteroaromatic hexaazatrinaphthalene-phenazine (HATN-PNZ) material has been designed and synthesized by Cheng and coworkers.⁸⁴ Compared with the original phenazine monomers, the HATN-PNZ material delivers abundant charge delocalization around the N-heteroaromatic hydrocarbons (Fig. 6a), which can efficiently deliver electrons inside molecules, resulting in the improved conductivity and fast-kinetics reaction. The enlarged π-conjugated aromatic structure significantly inhibits its dissolution under aqueous electrolytes, resulting in enhanced stability. As a result, a large reversible capacity of 257 mAh g⁻¹ at 5 A g⁻¹, improved rate capability of 144 mAh g⁻¹ at 100 A g⁻¹ and cycling properties of 45 000 cycles at 50 A g⁻¹ can be achieved for this HATN-PNZ cathode of AZIBs (Fig. 6b and c). It demonstrates the mechanism of the co-coordination of Zn²⁺ and H⁺ ions on the phenanthroline groups in the structure of HATN-PNZ.

Fig. 6 HATN-PNZ cathode for AZIBs: (a) synthesis process; (b) rate properties; (c) cycling performance at 50 A g⁻¹. Reproduced with permission,⁸⁴ Copyright 2023, Wiley.

In addition, the existence of electron-withdrawing cyano (–CN) units in the structure of OSM materials, which can reduce the LUMO energy level, can be an innovative and efficient strategy for enhancing the operating voltage of OSM-based AZIBs. A kind of cyano-containing OSM material, phenazine-2,8,14-tricarbonitrile (HATN-3CN), has been successfully fabricated by introducing the strong electron-absorbing conjugate group (–CN) into the structure of HATN.⁸³ When adopted as the cathode for AZIBs, the HATN-3CN electrode demonstrates excellent performance, retaining 60.7% of its capacity after 6000 cycles at 20 A g⁻¹ and 90.7% after 5800 cycles at 5 A g⁻¹. Furthermore, the mechanism of the active H⁺/Zn²⁺ storage on the C=N groups has been confirmed by combining experimental studies with DFT calculations.

It is worth noting that the OSM materials with a combination of N-active and C=O units can deliver enhanced ion-storage mechanism, based on the synergistic effect between the two kinds of active units. Zhang and coworkers introduced a kind of organic small-molecule compound, triresazurin-triazine (TRT) (Fig. 7a), which was constructed from the resazurin sodium salt (RSS) and 2,4,6-trichloro-1,3,5-triazine (TCT) via the desalinization method.⁸⁵ In the structure of TRT, the existence of active carbonyl and nitro oxygen groups can generate a highly-conjugated network, leading to its inhibited dissolution in electrolytes and improved conductivity. These TRT-based AZIBs exhibit superior capacity (220 mAh g⁻¹ at 0.1 A g⁻¹), extended cycling performance (99.8% capacity retained after 1000 cycles at 0.5 A g⁻¹), and enhanced rate properties (78 mAh g⁻¹ at 2 A g⁻¹) (Fig. 7b and c). It is proved that the N–O* and C=O active site units in the structure of TRT can bind with Zn²⁺ and H⁺ ions (Fig. 7d). Another OSM material, hexaazatrinaphthalenequione (HATNQ), containing the layered supramolecular structure with abundant hydrogen bonds and out-of-plane π–π network based on C=O and C=N units, has been fabricated and adopted as a cathode of AZIBs (Fig. 8a).⁸⁶ Simultaneous Zn²⁺/H⁺ ion storage can be promoted for the HATNQ-based AZIBs, which delivers a capacity of up to 482.5 mAh g⁻¹ at 0.2 A g⁻¹ with a high-capacity retention of 93.7% after 100 cycles at 0.5 A g⁻¹ (Fig. 8b) and excellent cycling performance (95% capacity retained after 11 000 cycles at 5 A g⁻¹). Tao and coworker also reported a dibenzo-[a,c]-dibenzo-[5,6 : 7,8]-quinoxalino-[2,3-i]-phenazine-10,21-dione (TABQ-PQ) materials containing C=O and C=N units, which delivers an extended π-conjugation plane with enhanced π-conjugation effect and π–π packing interaction.³² Thus, the TABQ-PQ cathode with enhanced structural stability exhibits a mechanism with high-capacitance control behavior of the H⁺ ions, which provides the possibility of fast reaction kinetics. A high discharge capacity of 140.7 mAh g⁻¹ at 20 A g⁻¹ and an ultra-long cycling performance of up to 30 000 cycles can be achieved for this TABQ-PQ cathode. The abundant carbonyl and C=N groups are discharged through multiple hydrogen bonds between the adjacent molecules within the fast ion transport channels that enhance the charge transfer, insolubility, and fast charging of the battery.

Fig. 7 TRT cathode for AZIBs: (a) structural formula; (b) cycling performance at 0.1 A g⁻¹; (c) long-term cyclability at 500 mA g⁻¹; (d) Zn storage mechanism. Reproduced with permission,⁸⁵ Copyright 2023, Royal Society of Chemistry.

Fig. 8 HATNQ cathode for AZIBs: (a) schematic synthetic process; (b) cycle performance at 0.5 A g⁻¹. Reproduced with permission,⁸⁶ copyright 2022, Wiley. DHTAT material for AZIBs: (c) synthesis process; (d) cycle performance at 5.0 A g⁻¹. Reproduced with permission,⁸¹ copyright 2024, Wiley.

Of course, the synergistic effect between different kinds of N-active units in the structure of OSM materials can also promote their electrochemical application for AZIBs. Tris(3-amino-1,2,4-triazolyl) hexaazatriphenylenehexacarboxy triimide with its π-conjugated structure (TAT-HATTI), containing the active units of C=N functional groups and C=O functional groups, was reported as a cathode material for high-rate AZIBs.⁸⁸ The multiple active sites of the TAT-HATTI cathode endow it with a high specific discharge ratio capacity (531 mAh g⁻¹ at 0.05 A g⁻¹), excellent rate performance (83 mAh g⁻¹ at 20 A g⁻¹), as well as good cycling stability (78% capacity retained after 1000 cycles at 2 A g⁻¹). The assembled Zn//TAT-HATTI battery delivers a large energy density of 4.4 mWh cm⁻³. The charge storage mechanism of TAT-HATTI has been systematically demonstrated to be the synergistic effect of the C=N/C=O groups with H⁺/Zn²⁺ during cycling. Moreover, the intrinsic insolubility and π-conjugated structure for the TAT-HATTI material also contribute to the enhanced properties of the TAT-HATTI-based AZIBs. In addition, a simple one-step solid-phase reaction has been adopted to synthesize the 5,12-dihydro-5,6,11,12-tetraazatetracene (DHTAT) material,⁸¹ which delivers a charge storage mechanism with the two-electron transfer reaction based on the oxidation–reduction of –NH– units (Fig. 8c). It is worth noting that the charge storage on DHTAT depends on the adsorption/desorption of ClO₄⁻ on the –NH– groups, which is confirmed by both the experimental investigations and computational analysis. When used as a cathode for AZIBs, the DHTAT cathode delivers a stable high voltage of 1.12 V and high capacity of 224 mAh g⁻¹ at 50 mA g⁻¹ with a capacity retention rate of 73% even after 5000 cycles at 5 A g⁻¹ (Fig. 8d). These investigations prove that the charge storage mechanism mainly originates from the double electron redox reaction on the –NH– groups in the DHTAT structure, originating from its anion-attractive properties.

N-active OSMs have attracted much attention as cathodes for AZIBs based on their abundant redox-active sites and tunable molecular structures, in which highly efficient energy storage can be achieved via the reversible coordination of Zn²⁺ and H⁺ on the lone pair electrons of the N atoms. Thus, the N-active OSMs usually exhibit higher redox potentials, better structural stability, and lower water solubility, thus enhancing the voltage plateau and cycle life of the batteries. However, the unsatisfactory electronic conductivity and irreversible reactions of some active groups also limit the practical applications of N-active OSMs. Future studies are needed to further optimize their electrochemical properties through molecular engineering (conjugate extension, heteroatom doping) and composite strategies (carbon material composites) to promote the practical application of N-active organic cathodes for high safety and low-cost AZIBs.

3.1.4. OSMs with other functional groups. Other functional groups (e.g., anhydride, carbazole and S-involved units) can also be regarded as active sites which are able to undergo a reversible proton/zinc-ion storage reaction during the charging and discharging process, and thus improve the electrochemical performance of OSM-based AZIBs.

Lu and coworkers reported a kind of aromatic organic molecular material of 3,4,9,10-perylenetetracarboxylic dianhydride (π-PMCs), which promotes its ion transport kinetics by enhancing the π–π stacking interactions.¹⁰² When used as the cathode for AZIBs, this π-PMCs cathode delivers enhanced rate properties of 62.6% capacity retention, along with the current rate increasing from 1.6 to 260.4C, and excellent storage capacity retention (81% capacity retention at 8 A g⁻¹). The co-storage of H⁺ and Zn²⁺ ions through the enhanced π–π stacking interactions has been highlighted for this π-PMCs cathode. However, the anhydride-involved OSM materials exhibit side reactions with the electrolyte, resulting in a loss of active substances, and thus affecting the long-term performance. In order to improve the structural stability and energy-storage properties, further explorations on the design and structure optimization of anhydride-based OSM materials are necessary.

Carbazole-involving OSM materials can achieve a large conjugated structure, further promoting the good electron transport properties and improving the structural stability of the cathodes. Kundu and coworkers presented the in situ fabrication of a dicarbazole material via the solid-state electro-oxidation strategy based on the electro-oxidative coupling of N-phenyl carbazole (NPC).¹⁰³ The favourable electrode kinetics and in situ formed film-like morphology endow this conductive carbazole cathode with a large reversible capacity of ∼100 mAh g⁻¹ at 1.3 V with > 95% of the capacity retained (CE: ∼100%) after more than 1000 cycles. It should be ascribed to the charge storage mechanism based on the oxidation/reduction for this NPC cathode. The NPC cathode material prepared by the solid-state electro-oxidation strategy also delivers excellent co-insertion behaviour of Zn²⁺ and H⁺ ion storage during electrochemical processes in AZIBs.

By virtue of the redox activity of the polysulfide bonds, the tetrathionic functional groups-involving OSM materials have also attracted much attention for application on AZIBs. Wang and coworkers investigated two kinds of tetrathionic-based OSM materials, tetrathiaful-valene (TTF) and tetrathianaphthalene (TTN), and adopted them as the cathodes for AZIBS.¹⁰⁴ As shown in Fig. 9a, TTF and TTN have similar molecular formula, and are constructed from tetrathia-substituted ethene from ethene connected by sulfur centers with end-on and side-on styles for TTF and TTN, respectively. Two monovalent anion storage can be detected for the TTF cathode, leading to an average discharge voltage of 1.05 V and capacity of 220 mAh g⁻¹ at 2C (Fig. 9b). In contrast, the TTN cathode stores only one monovalent anion and undergoes irreversible molecular rearrangement under further oxidation. Originating from the enhanced electrical conductivity and ion diffusion coefficient, as well as the reduced charge transfer resistance, the TTF cathode also delivers an improved cycling performance of 104 mAh g⁻¹ after 10 000 cycles (capacity retention: 68.8%) at 20C (Fig. 9c). Based on the charge storage mechanism, electron loss on the S atom along with anion storage can be confirmed for the TTF cathode (Fig. 9d–f), leading to its improved electrochemical storage capacities.

Fig. 9 TTF cathode for AZIBs: (a) schematic of structures; (b) cycling performance at 2C; (c) cycling stability at 20C. (d) FTIR tests during cycling; (e) in situ ATR-FTIR test during cycling; (f) ex situ EPR spectra under different states. Reproduced with permission,¹⁰⁴ Copyright 2024, Springer Nature.

The cyclopropenyl ring, which is the smallest aromatic unit containing two π-electrons, can help to enhance the molecular rigidity and improve the structural stability, further promoting the battery performance of OSM materials. Alex and coworkers designed a novel N-heteroaromatic organic cathode material (CP⁺) through molecular engineering, and applied it to the high-voltage and high-stability AZIBs.¹⁰⁵ The research team successfully prepared CP⁺ by replacing the benzene ring in the conventional triphenylamine structure with the cyclopropenyl cation (Fig. 10a). This cyclopropenium-based cathode material delivers a high voltage of 1.7 V and a specific capacity of 157.5 mAh g⁻¹ (based on cation mass) during discharge, and 95% of the capacity was retained after 10 000 cycles at 2 A g⁻¹ (Fig. 10b). Besides, a mechanism has been confirmed for this CP⁺ cathode, wherein the counterion balance originates from the loss of one electron from the N atoms to fabricate the cation radical, achieving the coordination of the OTf⁻ anion for the charge balance (Fig. 10c).

Fig. 10 CP⁺ cathode for AZIBs: (a) synthesis process; (b) cycling performance at 2 A g⁻¹; (c) molecular structure transformations during the cycling process. Reproduced with permission,¹⁰⁵ Copyright 2024, Wiley.

Although these OSM cathodes with different functional groups show many advantages, they still face some challenges in practical applications. The high solubility of some OSMs in aqueous electrolytes may lead to the loss of active substances, affecting the cycling performance of batteries. Moreover, the electronic conductivity of OSM materials is generally unsatisfactory, which limits their charging and discharging rate properties. Therefore, further in-depth research is needed in the future to overcome these problems through molecular structure design and material composites, with the aim to promote the organic small-molecule cathodes for AZIBs from the laboratory to practical applications.

3.2. OSM-involved composites

OSM materials containing different functional groups have attracted much attention for AZIBs applications. To further avoid the dissolution in organic electrolyte and enhance the energy-storage properties with improved conductivity, the effective combination of OSM materials with other organic/inorganic materials, including the carbonaceous materials and conductive polymers, has also been extensively studied. Altering OSM materials via the addition of functional groups or by encapsulating/compositing OSM materials with a carbon framework can deliver advantages for improving the energy-storage performance.¹²²

3.2.1. OSMs with CNT or CNF. Carbonaceous materials, including CNT and CNF, deliver good chemical stability and high conductivity. The combination of OSMs with carbonaceous materials can enhance the electrical conductivity of OSMs, accelerate electron transport, and enable batteries to have higher charging and discharging efficiencies. Moreover, the carbonaceous materials can provide structural support for OSMs, buffer the volume changes during the charging and discharging process, inhibit their dissolution and loss into the organic electrolyte, and enhance the cycling stability.¹²³ In 2021, Fu and his coworkers combined naphthoquinone (NQ) with carbon nanotubes to prepare the binder-free composite OSM-based cathode,¹⁰⁷ which takes advantage of the high conductivity and nanoporous structure of CNTs to effectively limit the dissolution of NQ molecules in the electrolyte, thus improving the cycling performance of AZIBs. Modification of dichlone and 2-((4-hydroxyphenyl) amino) naphthalene-1,4-dione (APh-NQ) molecules with Cl- or 2-(4-hydroxyphenyl) amino units exhibits better performance than the NQ cathode, originating from the electron-withdrawing or -donating units existing in their structure. Meanwhile, the effective combination further endows the NQ-related composite cathode with enhanced electrochemical properties, in terms of the high capacity of 148.8 and 136.9 mAh g⁻¹ retained after 1000 cycles at 0.5C for the dichlone/CNT and APh-NQ/CNT cathodes, with a capacity retention of 70.9% and 68.3%, respectively. More importantly, the efficient storage capability of the NQ-related cathode has also been demonstrated to be a result of the effective reaction between the Zn ions and the active C=O units in the structure. Besides, Liu and coworkers explored the properties and related mechanisms of the series of dinitrobenzene (o-DB, m-DB, and p-DB) composites as cathode materials.¹⁰⁸ The nitro positional isomers in the DB structure have significant effects on the zinc affinity activity and redox kinetics, and their combination with carbon materials can further improve its electrochemical properties. High capacity (402 mAh g⁻¹) and excellent cycling properties (∼375.9 mAh g⁻¹ after 25 000 cycles) can be achieved for the composite of p-DB with carbon nano-flower materials, which also delivers fast redox kinetics and a large energy density of 230 Wh kg⁻¹. The anion co-insertion charge storage mechanism has been obtained for this composite cathode, with a stepwise coordination/decoordination process of Zn ions with nitroxide units in the structure of p-DB. The unique structure of carbon nanoflowers can inhibit the dissolution of active substances in the electrolyte, thus enhancing the stability. Moreover, the carbon nano-flower materials, as a conductive substrate, can also build a continuous electron transport path. Meanwhile, the porous structure of the carbon nano-flower materials can promote the penetration of the electrolyte and ionic diffusion, which can further optimize the reaction kinetics, and ultimately achieve improvement on comprehensive performance of the battery.

3.2.2. OSMs with graphene. Reduced graphene oxide (rGO) delivers single or few-layered two-dimensional structures, which can act as effective based materials for loading or decorating OSM materials. Especially, the π–π stacking interaction achieved by the interaction between the rGO structure and carbonyl units from the OSM molecules can be easily formed, promoting the good electrical conductivity for the composites. In 2022, Zhu and coworkers investigated a composite consisting of 6,15-dihydrodinaphtho[2,3-a:2′,3′-h]phenazine-5,9,14,18-tetraone (DNPT) and rGO as a cathode for AZIBs.¹⁰⁹ This composite cathode delivers a capacity of 120 mAh g⁻¹ after 1000 cycles at 500 mA g⁻¹, with a capacity retention of close to 100%. External field analysis and DFT calculations were used to elucidate the synergistic storage mechanism of H⁺ and Zn²⁺ ions in the structure of DNPT/rGO. In this mechanism, the DNPT₂(H⁺)₆(Zn²⁺) structure is formed by two neighbouring DNPT molecules combined with one Zn²⁺ ion and six H⁺ ions and is optimal in the charge–discharge process. Besides, the quinone-based OSMs can be combined effectively with rGO, with the π–π stacking interaction between the C=O units and rGO. In 2024, a composite cathode of SAS@rGO was constructed through the host–guest interactions between sodium anthraquinone-2-sulfonaterationate (SAS) and rGO.⁶³ As shown in Fig. 11a, a self-supported electrode is obtained by combining the conductive main body of rGO and the small-molecule quinone SAS (sodium anthraquinone-2-sulfonaterationate), in which the interaction between the guest molecules and main body substance can inhibit the dissolution and promote the electron transport. In addition, 1,4-butyrolactone was used as a co-solvent to break the hydrogen-bonding network and achieve rapid ion transport at low temperature. Thus, this SAS@rGO composite cathode for AZIBs can achieve excellent cycling performance (0.032% capacity decay for each cycle up to 300 cycles at 0.5C) (Fig. 11c) and maintains good performance at low temperature (3000 cycles at −40 °C even under 1 A g⁻¹) (Fig. 11d). With the presence of the rGO materials, enhanced conductivity and stability can be achieved, leading to the H⁺ and Zn²⁺ ion co-storage mechanism (Fig. 11b).

Fig. 11 SAS@rGO cathode for AZIBs: (a) synthesis process with SEM image and structural details; (b) three ways of ions storage; cycling performances at (c) 0.5C and (d) 1 A g⁻¹. Reproduced with permission,⁶³ Copyright 2024, American Chemical Society.

3.2.3. OSMs with MXene. MXene (Ti₃C₂T_x, T stands for surface terminals), as an emerging two-dimensional material, has better electrical conductivity (∼104 S cm⁻¹) and adsorption capacity than traditional carbon-based conductive agents of rGO and CNTs.¹²⁴ Especially, the modification with polar terminal units (–F, –OH, etc.) endows the MXene materials with better hydrophilicity, which can achieve an effective contact between the active material and aqueous electrolyte.¹²⁰ Originating from the large surface and abundant flexibility, MXene materials have been adopted to modify OSMs with enhanced conductivity and mechanical stability.¹²⁵ Huang and coworkers reported on a new imine-based electrode material, imine-based tris(aza)pentacene (TAP) and its composite electrode with MXene for AZIBs.¹¹¹ TAP, with its extended conjugate structure, delivers a low LUMO energy level, narrow energy gap, a high theoretical capacity, and effective selectivity for H⁺/Zn²⁺ storage. The obtained TAP/Ti₃C₂T_x composite was prepared by in situ injection of TAP into the layered MXene, forming a two-dimensional structure with hierarchical porous characteristics. Thus, the significantly enhanced electrochemical performance can be achieved for the TAP/Ti₃C₂T_x composite cathode, which delivers a capacity of 303 mAh g⁻¹ with a retention of 81.6% after 10 000 cycles. The enhanced cycling stability for the TAP/Ti₃C₂T_x composite cathode should be attributed to the tight electronic interactions between Ti₃C₂T_x and TAP. Meanwhile, the mechanism of the selective H⁺/Zn²⁺ co-embedding/de-embedding can be ascribed to the existence of MXene, which can also provide enhanced conductivity and promoted ion diffusion in the composite cathode materials. In 2024, the 6CN-HAT molecule containing π–π conjugated aromatic structures was selected for combination with MXene nanosheets to form a self-supporting composite thin-film electrode 6CN-HAT@MXene (Fig. 12a).³⁹ The composite electrode delivers multiple mechanisms for active site expansion, and electron/ion transport optimization with structure stabilization. The 6CN-HAT@MXene cathode exhibits a high specific discharge capacity (413 mAh g⁻¹ at 0.05 A g⁻¹) with excellent cycling stability (88.9% capacity retained after 100 cycles at 0.1 A g⁻¹) for AZIBs (Fig. 12b), which is significantly higher than that for the 6CN-HAT cathode without MXene. The mechanism with the coordination and non-coordination reactions between the active sites and H⁺/Zn²⁺ ions illuminates new ideas and material options for further improvement on AZIBs (Fig. 12c).

Fig. 12 6CN-HAT@MXene cathode for AZIBs: (a) synthetic route; (b) cycling performance at 0.1 A g⁻¹; (c) H⁺/Zn²⁺ storage mechanism during cycling. Reproduced with permission,³⁹ Copyright 2024, Wiley.

OSM composites with carbonaceous materials can enhance the structural integrity and inhibit the dissolution of OSMs in electrolytes, originating from the effective combination and fixation between the carbonaceous and OSM materials. Moreover, the carbonaceous materials can provide continuous electron conduction paths, facilitating electrolyte penetration, lower ionic diffusion resistance, and buffer volume changes. Thus, these features endow the OSM composite-based cathodes for AZIBs with enhanced cycle life, capacity and multiplicity performance. However, carbonaceous materials have no capacity contribution, and their excessive addition may reduce the overall energy density of the composite cathodes. Thus, more kinds of carbonaceous materials should be explored with the aim of achieving higher electrochemical capacities and performance. The effect of the combination between the carbonaceous materials and OSM materials also needs to be further analyzed with a broader range of theoretical calculations and corresponding experiments. Subsequent studies can start from the functionalization of carbonaceous materials, enhancement of the interaction between carbon and OSMs through heteroatom doping or surface modification to optimize charge transfer, the construction of hierarchical porous carbon or three-dimensional conductive networks to balance the ion/electron transport and active substance loading, the combination of in situ characterization and theoretical calculations to reveal the role of carbon–organic interfaces in zinc-ion storage, and the development of biomass-derived carbon or recycled carbon materials to reduce the production cost and promote practical applications.

4. Design strategy of OSMs

Organic small molecules (OSMs) and its derivatives have been considered as a promising choice as electrode materials for AZIBs, originating from their structural diversity, environmental friendliness, and facile synthesis process. Moreover, based on the flexible molecular design, the voltage, capacity, electrical conductivity, and redox kinetics of OSM electrodes can be systematically regulated when applied as the electrode for batteries. Thus, a high density of the redox sites and resultant high theoretical capacity can be achieved for AZIBs with OSM electrodes. However, the OSMs and related materials are also limited by the low conductivity and solubility in electrolytes, significantly limiting the reversible capacities of batteries and their cycling performance. It is noted that the performance of organic electrodes also depends on their diffusion/coordination kinetics as charge carriers. It is a major challenge to design organic small-molecule electrodes that combine high-density redox sites, high electronic conductivity and stable charge storage. So, considering the large specific structures, functional groups decoration, or combination with other materials, an optimized design of the organic small-molecule materials has been explored to endow the OSM electrodes with fast electronic transfer and alleviated solubility in electrolytes, leading to enhanced battery performance.

4.1. Structure design of OSMs

When designing OSM electrodes for AZIBs, there should be a comprehensive consideration of the conjugation system, functional group modification, molecular conformation control and molecular aggregation state modulation (Fig. 13). An enhanced conjugation system and molecular aggregation state can reduce the solubility of the OSM materials in the electrolyte, while the functional group modification can improve the electron transportation in the molecular structure and provide more redox interaction potential with the zinc ions, so as to achieve superior performance for AZIBs.

Fig. 13 Rational design strategies of OSM-based cathode materials for AZIBs with enhanced performances.

A rational design with an enhanced conjugation system and molecular aggregation state should be first considered when adopting the OSM materials as cathodes for AZIBs, to overcome the fatal problem of solubility in electrolytes. By introducing structures such as conjugated double bonds, conjugated triple bonds or aromatic rings into the OSM molecules, the degree of conjugation of the OSM materials can be significantly increased. Moreover, the introduction of conjugated structures containing N- or O-involving functional groups with lone pairs of electrons can also endow the OSM materials with an increased conjugation degree. The extended conjugated system can help improve the stability of the OSM electrode with the alleviated solubility in electrolyte, and promote the fast electron transport properties in the structure of the OSM materials. With the enhanced conjugated interaction and enhanced interaction between the molecules, the molecular aggregation state of the OSM materials can also be adjusted. In some cases, the formation of ordered aggregation structures through intermolecular interactions of hydrogen bonding or π–π stacking can increase the surface area and reactivity of molecules. For example, it has been found that the optimization of aggregation-dependent properties can be achieved by modulating the molecular aggregation state through a dynamic self-assembly process.⁷⁵ Furthermore, the structural and electrochemical stability of OSM materials can be significantly enhanced by weakening the interaction between the solvent and OSMs through intramolecular hydrogen bonding. However, an over-aggregated molecular mode will also decrease the surface area and active sites for zinc-ion storage,⁷⁵ so appropriate strategies should be applied for the modification upon aggregation.

On the other hand, the decoration of suitable functional groups/units in the electrode structure is also crucial for the enhanced performance of OSM-based aqueous zinc-ion batteries. The selection and design of functional units will significantly affect the electrochemical properties and stability of the battery.¹²⁶ Firstly, the carbonyl units (C=O) have strong polarity and are able to achieve charge storage through coordination with Zn²⁺.³² Thus, organic small-molecule materials with carbonyl units have been considered as the most widely-used cathode materials for AZIBs, including the series of quinone or ketones-based OSMs. For the energy-storage mechanism, C=O can act as the active site for Zn-ion storage based on the reversible evolution between C=O and C–O, along with the insertion and extraction of Zn²⁺ ions.⁶⁷ The introduction of more carbonyl functional groups in the structure of OSM cathodes can increase the binding sites with Zn ions and further improve the charge storage capacity. Moreover, the location of the carbonyl groups can be reasonably adjusted to optimize the electronic structure of the OSM molecule, and promote its charge transfer and storage. Secondly, several nitrogen-containing functional groups, such as the imino group (C=N), cyano group (–CN) and nitro group (–NO₂), can also be considered as the effective reaction sites for Zn-ion storage.^103,108 Nitrogen atoms in imino or cyano groups can provide a lone pair of electrons, which can enhance the electronic conductivity and redox activity of the OSM molecule, along with the effective and fast storage and release of Zn²⁺ ions.⁸² The low electronegativity of these N-containing units can also reduce the electrostatic interaction with zinc ions and promote rapid ion transfer. It is worth noting that the strong electron-withdrawing ability of the cyano functional groups (–CN) can be further regulated by modifying the chemical environment around them, which can further change the electron cloud distribution and affect the energy level structure and redox potential of the OSM molecules.⁸³ Moreover, the properties of the cyano functional groups-modified OSM molecules can be further optimized through synergistic interactions with other functional groups. Thirdly, other functional groups, such as the tetrasulfanyl and benzene rings, can enhance the Zn-ion storage performance of AZIBs with OSM cathodes. The tetrasulfanyl functional group has attracted much attention due to its enhanced electron-donating ability and abundant sulfur atom sites.¹⁰⁴ Sulfur atoms can form relatively stable chemical bonds with zinc ions and is important for charge storage. Moreover, the presence of the tetrasulfur group can regulate the energy level structure of the molecule and optimize the redox potential, resulting in a better voltage plateau and energy efficiency of the battery during charging and discharging. As a classical aromatic structure, the benzene ring is also indispensable in organic cathode materials. The π-electron cloud of the benzene ring can participate in the interaction with zinc ions and enhance the intermolecular π–π stacking.¹⁰⁵ This stacking effect not only improves the crystallinity and stability of the material, but also can endow ordered channels for Zn-ion transportation, which is conducive to the increased ion diffusion rate and improved multiplication performance of the battery.

For enhancing the performance of AZIBs with OSM cathodes, the synergistic effect of functional groups has been considered as an effective strategy during the synthesis of OSM materials. This multifunctional synergy combines different types of functional groups in the same organic molecule and utilizes their synergistic effect to enhance the electrochemical performance. In the combination of the nitro (–NO₂) and carbonyl (C=O) units, the nitro group has a two-electron redox activity that can significantly increase the specific capacity, while the carbonyl group provides an additional active site that enhances the ion adsorption capacity.¹²⁷ The combination of the cyano (–CN) unit and an aromatic ring can also achieve an increased discharge voltage along with a reduced LUMO energy level. The combination of a hydroxyl group (–OH) and carbonyl group (C=O) can lead to enhanced hydrophilicity and the promoted penetration of the electrolyte and ionic transfer.¹²⁸ Moreover, the reasonable regulation of the ratio of different functional groups in the molecule can balance key performance indicators, such as the charge storage capacity, electrical conductivity and stability. In nitro compounds, the electrochemical properties can be optimized by changing the number and position of the nitro groups.¹²⁷ Through theoretical calculations and experimental verification, the ratio of functional groups can be precisely optimized to achieve the best comprehensive performance. To optimize the transport and storage efficiency of zinc ions, it is necessary to avoid hindering the accessibility and storage of zinc ions due to large site resistance, and ensure that the functional groups have reasonable spatial orientation and accessibility.¹²⁸

Lastly, the optimization of the pore structure and specific surface area should be considered for the design of OSM cathodes with enhanced electrochemical performance. Having a suitable pore size is important for Zn-ion storage for the AZIBs. Pores that are too large will lead to disorder of the zinc-ion storage or the aggregation or leakage of zinc ions,⁶² while pores that are too small will impede the diffusion of ions. Moreover, the size of the zinc ions should be fully considered to ensure that the channels or pores can accommodate the embedding and de-embedding of zinc ions. Considering the size of Zn ions (0.74 Å), it is suggested that channels or pores with a size range of 50–100 nm can ensure rapid ion diffusion, while effectively limiting the aggregation of Zn ions. Moreover, an enlarged specific surface area can provide more exposed active sites for the Zn-ion reaction or the embedding of AZIBs, further improving the Zn-ion storage properties. By designing multilevel pore structures (combination of micro-, meso-, and macro- pores), the specific surface area and ion transport efficiency of the materials will be significantly improved.^4,129 For achieving the enlarged surface area, dispersed micro-size or nano-size morphologies or hollow/core–shell spherical morphologies are suggested for the design or synthesis of OSM materials.

4.2. Composites with other materials

Another effective strategy to overcome the solubility problems in liquid electrolytes of OSM materials is the combination with other materials with a network or super-molecular structures. The combination with carbonaceous materials or cations can also promote electrical conductivity of OSM materials and further promote full utilization of their capacities. In the development process of aqueous zinc-ion batteries, the composite construction of OSM materials has shown significant advantages.

Carbonaceous materials, including carbon nanotubes and graphene, have been the best choice for enhancing the electrical conductivity of organic small-molecule materials via a conductive network construction.¹³⁰ The advantages of carbonaceous materials, in terms of their high electrical conductivity, good mechanical stability and large specific surface area, can also improve the electrochemical properties of OSM electrodes of AZIBs by providing more channels for electron and ion transfer or some potential active sites for Zn-ion storage.¹⁰⁷ One-dimensional carbon nanotube or two-dimensional graphene can also act as the substrate material to adjust the morphology or arrangement of OSM materials during their synthesis process, in order to provide fast diffusion channels for zinc ions while enhancing the overall stability of the composite material.⁶³ MXene materials, a class of two-dimensional transition metal carbides with high electrical conductivity, good chemical stability, and a large layer spacing, can also been selected to combine with OSM materials for AZIBs to provide an excellent physicochemical environment for the storage and transport of zinc ions.¹¹¹ The high conductivity of MXene can effectively enhance the electron transfer efficiency of the OSM electrode, thus promoting the multiplicity performance of the battery. Meanwhile, the stability structure of MXene can provide a stable support for OSM materials, preventing their dissolution and agglomeration in the electrolyte. In summary, the composite of carbonaceous materials as the cathodes of AZIBs is an efficient strategy to promote the electrochemical performance. Furthermore, the conductivity, stability and ion transport efficiency of the composite electrode can be significantly improved by optimizing the composite structure design in terms of the combination mode and morphology adjustment.

Metal ions or other cation ions (Mn²⁺, Cu²⁺, NH₄⁺) have excellent redox properties and ion diffusion ability, and can also be selected to combine with OSM materials to improve their reversible capacity and cycling performance. Firstly, the doping of metal ions or other cations can extend the lattice spacing of the OSM structure by connecting with the OSM materials into the inner structure, thus enhancing the ion diffusion and electron conductivity of OSM composite cathodes. This results in an increase in the Zn²⁺ diffusion coefficient of the cathode material, a substantial decrease in the charge transfer resistance, and a significant increase in the capacity and multiplicity performance. Especially, the hydrogen-bonding of NH₄⁺ is able to weaken the Zn²⁺ electrostatic interactions with oxygen atoms, thereby accelerating the diffusion of Zn²⁺, lowering the formation energy of the material, narrowing the band gap, and thus enhancing the cycling performance. Moreover, the presence of several metal ions with excellent electrical conductivity can promote the improved electron transfer efficiency of the OSM composite cathode materials. Meanwhile, some noble metal ions can be used as a catalyst to improve the redox reaction of OSMs to improve the charging and discharging efficiency of OSM composite cathode-based AZIBs.

5. Conclusions and outlooks

The effective utilization of OSMs as cathode materials for AZIBs has garnered significant attention, originating from their inherent advantages, including resource renewability, environmental friendliness and structural diversity. These materials offer promising alternatives to traditional inorganic/organic cathodes, which are often hindered by their limited resources, high costs, and environmental concerns. OSM materials, with their diverse molecular structures and tunable properties, provide a unique platform for optimizing the electrochemical performance of AZIBs. However, several challenges persist in the practical application of OSMs as cathodes, including the dissolution in aqueous electrolytes, relatively low electrical conductivity and the unclear electrochemical mechanisms. Developing novel molecular designs of OSM materials with enhanced stability and higher conductivity is an effective strategy that should be further explored. Researchers have explored various strategies to enhance the stability and conductivity of OSM cathodes with rational molecular design to increase the conjugation and planarity, and hybridization with inorganic materials to improve the mechanical stability and electrical conductivity. Especially, the addition of electron-withdrawing groups and the extension of π-conjugation systems are suggested as approaches to modify the structure of the organic small molecules. Meanwhile, optimizing the electrolyte composition for enhancing the stability of the organic electrodes and improving ionic conductivity is essential. Exploring non-aqueous or hybrid electrolytes that can reduce the solubility of OSM materials may offer a viable solution. Moreover, constructing porous or hierarchical porous structures with more exposed active sites and introducing a flexible charge carrier with a lower potential barrier for ion transport have also been proven to be effective strategies for achieving OSM-based cathodes for AZIBs with enhanced battery performances. The creation of composite materials by combining OSMs with conductive polymers or inorganic carbonaceous materials can also significantly improve their mechanical stability and electrical conductivity, which offers an effective strategy for improving the performance of AZIBs with the enhanced dissolution properties of OSM cathodes. In addition, more explorations have been directed toward these issues for OSM-involving AZIBs to achieve enhanced electrochemical capacities with high Coulombic efficiency, cycling performances and rate properties, or clearly illuminated storage mechanism involving Zn²⁺, H⁺, or other anion intercalation processes. The use of advanced spectroscopic and microscopic techniques will aid in the rational design of better-performing materials by allowing researchers to gain deeper insights into the electrochemical mechanisms and degradation pathways of OSM-based cathodes.

By focusing on the molecular design, composite development, and electrolyte optimization, it is possible to realize the full potential of organic small-molecule cathodes and pave the way for commercialization of high-performance and sustainable aqueous zinc-ion batteries. Cost-effective synthesis routes and scalable production methods are crucial toward achieving the wide application of OSM-based cathode materials, which can further reduce costs and improve the overall economic viability of AZIBs.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

C4Q	Calix[4]quinone
PTO	Pyrene-4,5,9,10-tetraone
PQ-Δ	Triangular phenanthrenequinone-based macrocycle
DTT	Sulfur heterocyclic quinone dibenzo[b,i]thianthrene-5,7,12,14-tetraone
4S6Q	Sulfur heterocyclic quinones (6a,16adihydrobenzo[b]naphtho[2′,3′:5,6] [1,4]dithiino[2,3-i]thianthrene5,7,9,14,16,18-hexaone)
TDT	2,3,7,8-Tetraamino-5,10-dihydrophenazine-1,4,6,9-tetraone
4S4Q	Benzo[b]naphtho[2′,3′:5,6][1,4] dithiino[2,3-i]thianthrene-5,9,14,18-tetraone
BDTD	Benzo[1,2-b:4,5-b′] dithiophene-4,8-dione
C8Q	Calix[8]quinone
DBPTO	Dibenzo[b,i]phenazine5,7,12,14-tetraone
DEBQ	2,3-Diethylbenzo[g]quinoxaline5,10-dion
m-DMBQ	Polar 2,6-dimethoxy-1,4-benzoquinone
TAHQ	6,8,15,17-Tetraaza-heptacene-5,7,9,14,16,18-hexaone
p-PNQ	2,2′-(1,4-phenylenedithio) bis(1,4-naphthoquinone)
IDT	Indanthrone
NI-DAQ	1,4,5,8-Naphthalenetetracarboxylic dianhydride-2-aminoanthraquinon
Me-NQ	Menadione
DC-PDESA	N,N′-(2,5-dichloro-1,4-phenylene) diethanesulfonamide
HBCD	(2-hydroxypropyl)-β-cyclodextrin
NDI	1,4,5,8-Naphthalene diimide
DANTCBI	N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic bisimide
NTCDA	Pyrene-based 1,4,5,8-naphthalenetetracarboxylic dianhydride
BDB	1,4 bis(diphenylamino)benzene
HATN	Diquinoxalino [2,3-a:2′,3′-c] phenazine
PNZ	Phenazine
RF	Riboflavin
HATN-3CN	Phenazine-2,8,14-tricarbonitrile
DQP	Diquinoxalino[2,3-a:2′,3′-c] phenazine
MB	Methylene blue
TAPQ	5,7,12,14-Tetraaza-6,13-pentacenequinone
TCNAQ	Tetracyanoanthraquinodimethane
DMPZ	5,10-Dihydro-5,10-dimethylphenazine
HATNQ	Hexaazatrinaphthalenequione
HATN-PNZ	Hexaazatrinaphthalene-phenazine
HATAQ	Hexaazatrianthranylene (HATA) embedded quinone
HATTA	5,6,11,12,17,18-Hexaazatrinaphthylene-2,8,14-tricarboxylic acid
HATAN	Anthraquinone-quinoxaline derivative
HFHATN	Hexafluorohexaazatrinaphthylene
BBQPH	Benzo[a]benzo[7,8]quinoxalino[2,3-I] phenazine8,17-dione
DQDPD	Dipyrido[3′,2′:5,6;2′′,3′′:7,8] quinoxalino [2,3-i] dipyrido[3,2-a:2′,3′-c] phenazine-10,21-dione
TPHATP	Triphenazino[2,3-b] (1,4,5,8,9,-12-hexaazatriphenylene)
TABQ-PQ	Dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino [2,3-i] phenazine-10,21-dione
TQD	Triquinoxalinedio
TRT/TCT	2,4,6-Trichloro-1,3,5-triazine/2,4,6-trichloro-1,3,5-triazine
BBPD	Bisbenzimidazo[2,1-b:2′,1′-i]-benzo[lmn] [3,8] phenanthroline-8,17-dione
TFHATN\TCLHATN	Triffuorohexaazatrinaphthylene/trichlorohexaazatrinaphthylene
HCLHATN	Hexachlorohexaazatrinaphthylen
DHTAT	5,12-Dihydro-5,6,11,12-tetraazatetracene
TAT-HATTI	Tris(3-amino-1,2,4-triazolyl)hexaazatriphenylenehexacarboxy triimide
π-PMC	3,4,9,10-Perylenetetracarboxylic dianhydride
NPC	N-phenyl carbazole
TTF/TTN	Tetrathiafulvalene/tetrathianaphthalene
CP	Trisaminocyclopropenium
PDI	Perylene-based imide derivative
APh-NQ@CNT	2-((4-hydroxyphenyl)amino)naphthalene-1,4-dione@carbon nanotube
DNPT/rGO	6,15-dihydrodinaphtho[2,3-a:2′,3′-h]phenazine-5,9,14,18-tetraone/graphene oxide
p-DB/CN	para-Dinitrobenzene/carbon nanoflower
TCNQ/CCP	7,7,8,8-Tetracyanoquino dimethane/cyanuric chloride pyrene
TAP/Ti3C2Tx	Imine-based tris(aza)pentacene/Ti3C2Tx
PNZ/KB	Phenazine/ketjen black
IV@rGO	VO(ac)2@rGO
TMBQ@KMCN	Tetramethyl-benzoquinone (TMBQ) in a KOH activated MOF-derived carbon nanocage
SAS@RGO	Sodium anthraquinone-2-sulfonaterationate@reduced graphene oxide
PNO@KB	Phenoxazine@ketjen black
6CN-HAT@MXene	Hexaazatriphenylene hexacarbonitrile@MXene
PDSe/CMK-3	Phenyl diselenide/ordered mesoporous carbon

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

We are grateful to the Innovative research team of high-level local university in Shanghai and Natural Science Foundation of Shanghai (23ZR1422600 and 23ZR1423800) for financial support.

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Footnote

† Lingyan Long and Kailing Mei contributed equally to this work.

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