Research progress on modification of cathodes for aqueous zinc ion batteries

Abstract

Aqueous zinc-ion batteries (ZIBs) have garnered much attention as promising candidates for future large-scale electrochemical energy storage solutions. Their appeal lies in their cost-effectiveness, low emissions, inherent safety, and competitive energy density. Therefore, the design and improvement of high-performance AZIBs have been extensively studied. In this review, we categorize and compare the design strategies, electrochemical performance, challenges, and modifications of various cathodes including manganese (Mn)-based materials, vanadium(V)-based materials, Prussian blue analogs (PBAs), layered transition metal dichalcogenides, and organic materials. Meanwhile, strategies for enhancing performance are discussed. Finally, we summarize the challenges faced by cathodes in AZIBs and propose future research directions. Overall, exploring different cathodes provides researchers with guidance in selecting appropriate materials to further enhance the AZIBs’ performance.

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Qing Li

Qing Li received her PhD degree from Yangzhou University in 2021. She is now an associate professor at Guangling College, Yangzhou University. Her current research mainly focuses on the design and synthesis of metal–organic framework composites, and their potential applications.

Lizhen Chen

Lizhen Chen is now a student under Prof. Li's supervision, Yangzhou University of Guangling College, in Jiangsu province in China. Her current research mainly focuses on the design and synthesis of metal–organic framework composites, and their potential applications.

Yingying Wang

Yingying Wang is now a student under Prof. Pang's supervision, Yangzhou University of chemistry and chemical engineering, in Jiangsu province in China. Her current research mainly focuses on the design and synthesis of metal–organic framework composites, and their potential applications.

Huan Pang

Huan Pang received his PhD degree from Nanjing University in 2011. He is now a distinguished professor at Yangzhou University and a Young Changjiang Scholar of the Ministry of Education in China. He is the managing editor of EnergyChem, the editorial board member of National Science Review, FlatChem and Rare Metals, and the youth editorial board member of Nano Research, Nano Research Energy, eScience among other distinguished academic journals. He was recognized as a highly cited researcher in Cross-Field by Clarivate Analytics in 2020, 2021 and 2022. His research area mainly focuses on metal–organic frameworks (MOFs) related materials.

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1. Introduction

Electrochemical energy storage, with a particular emphasis on lithium-ion battery storage, boasts advantages such as being pollution-free, highly efficient, power-flexible, and long-lasting, experiencing rapid development in recent years.^1–4 Since the 1990s, lithium-ion batteries have held a dominant position in fields such as portable electronics and electric vehicles.^5–7 The limited availability of lithium on our planet poses challenges for this energy storage system to fulfill the requirements of large-scale implementations.^8,9 At the same time, serious safety issues like explosions and fires also constrain the long-term development and application of lithium-ion battery systems.^10–12 Therefore, there is an urgent need to develop high-energy non-lithium batteries.

During the development of non-lithium batteries, researchers discovered that metallic zinc possesses significant advantages in resource abundance, theoretical capacity, chemical stability, electrochemical stability, and safety:^13–15 (1) The Earth's crust contains a substantial amount of zinc, with a concentration of 79 parts per million, which serves as a strong foundation for the development of energy storage technology on a large scale. (2) The zinc anode exhibits a relatively low redox potential (−0.76 V vs. SHE), enabling the battery to achieve a higher operating voltage. (3) The ionic radius of Zn²⁺ is 0.74 Å, close to that of Li⁺ (0.76 Å), facilitating insertion/extraction in the crystal structure of cathode materials. (4) The theoretical capacity of metallic zinc is significantly high, with a specific capacity of 820 mA h g⁻¹ and a volumetric capacity of 5855 mA h cm⁻³. The comparison with other charge carriers is shown in Table 1. Consequently, AZIBs have emerged as one of the most promising emerging technologies, primarily attributed to their exceptional electrochemical performance and remarkable safety features.

Table 1 Comparison of several charge carrier ions¹⁶

Charge carrier	Standard electrode potential (V vs. SHE)	Ionic radius (Å)	Specific capacity (mA h g^−1)	Volumetric capacity (mA h cm^−3)
Li	−3.04	0.76	3862	2066
Na	−2.71	1.02	1166	1129
K	−2.93	1.38	685	586
Mg	−2.37	0.72	2205	3832
Ca	−2.87	1.00	1337	2072
Zn	−0.76	0.74	820	5855
Al	−1.66	0.54	2980	8046

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The AZIBs is a device that converts chemical energy into electrical energy. It typically consists of a positive electrode, negative electrode, electrolyte, separator, battery casing, and other components.^17,18 Specifically, the zinc storage material, conductive additives, and binder are thoroughly mixed and evenly coated on the fluid collector to form the positive electrode of the battery. It is essential for the positive electrode materials to exhibit high performance in terms of electrolyte ion compatibility, electron transfer characteristics at the interface, as well as structural stability and integrity. Commonly utilized fluid collectors primarily consist of titanium foil, graphite foil, and stainless-steel foil. The anode material of the battery is directly fabricated from high purity metallic zinc. Common electrolytes encompass ZnSO₄, Zn(CF₃SO₃)₂ or Zn(OTf)₂), ZnCl₂, etc., which serve as charge carriers. The diaphragm typically employs a porous glass fiber structure, serving to facilitate ion passage while preventing internal battery short circuits. The battery case serves the purpose of safeguarding the battery components.

The energy storage mechanism of AZIBs encompasses several recognized reaction mechanisms, including the Zn²⁺ insertion/extraction mechanism, the H⁺/Zn²⁺ insertion/extraction mechanism, the H₂O/Zn²⁺ insertion/extraction mechanism, chemical conversion reaction mechanism, and solution–deposition reaction mechanism.^19,20 Among them, the classical Zn²⁺ insertion/extraction mechanism resembles that of lithium-ion batteries, wherein Zn²⁺ is reversibly inserted/extracted into/from the main material. During discharge, Zn²⁺ acts as a carrier and becomes embedded in the positive electrode while accepting electrons as the oxidation state of the active material decreases. In the process of back-charging, under the influence of an electric field, the positive electrode releases Zn²⁺ and undergoes oxidation. Materials possessing tunnel structures, layered structures or open frame structures such as V-oxides and Mn-oxides, vanadates, PBAs and NASICON compounds can provide pathways for Zn²⁺ insertion/extraction.

Similar to lithium-ion batteries, the energy density, power density, and cost of AZIBs largely depend on the cathodes.^21–24 Their attractiveness is based on their cost efficiency, sustainability, built-in safety features, and favorable gravimetric energy density.^25–28 The rate performance and power density of a battery are affected by the transport properties of ions and electrons at the cathode–electrolyte interface, as well as the redox reaction kinetics occurring at the cathode.^28–31 The maintenance of the battery system's cycling stability relies heavily on the cathode's structural durability and soundness^28–30 Therefore, the development of various cathodes closely influences the advancement of AZIBs. Fig. 1 illustrates the primary categories of cathodes that have been studied in recent years, along with their historical development. This paper focuses on the latest advances in Mn-based materials, V-based materials, PBAs, transition metal dichalcogenides, and organic redox compounds used as cathodes for AZIBs. We comprehensively discuss the key factors in controllable synthesis of these types of materials, mechanisms for enhancing AZIBs’ performance, and propose main challenges and prospective solutions for designing tactics. We anticipate that this review will serve as a valuable point of reference, offering direction and motivation for the advancement of novel cathode materials in forthcoming AZIBs.

Fig. 1 The brief development history of cathodes for AZIBs.

2. Mn-based cathodes for AZIBs

Among the many cathodes, Mn-based compounds have received significant attention.^26,32–34 (1) Manganese is found in significant quantities in the Earth's crust and can be easily obtained due to its prevalence near the surface as Mn-oxide minerals. (2) Due to Mn having three common oxidation states (+2, +3, and +4), Mn-oxide minerals exhibit atomic structural diversity and multivalent phases. (3) Mn-based oxides have relatively high operating potential and theoretical capacity. Different types of Mn-based materials, such as various crystalline structures of MnO₂, Mn₃O₄, and spinel phases like ZnMn₂O₄ and MgMn₂O₄, have been examined for their potential use as cathodes in AZIBs.^35–37 However, the utilization of Mn-based cathodes in AZIBs is constrained by certain factors that limit their practical implementation. (1) The ionic radius of Zn²⁺ (0.75 Å) is similar to that of Li⁺ (0.76 Å), but Zn²⁺ carries two charges, leading to a high charge density. This results in strong electrostatic interactions with the host material during insertion, causing slow Zn²⁺ diffusion kinetics. Consequently, cathodes in aqueous ion batteries should possess well-developed layered or porous structures to facilitate the reversible incorporation and removal of Zn²⁺. (2) Mn-based cathodes typically have insufficient conductivity of electricity and ions, leading to slow electrode reaction kinetics and suboptimal performance. (3) Mn-based materials exhibit phase changes and manganese dissolution issues during cycling.

2.1 MnO₂

MnO₂ has a tunnel or layered structure, which can stably facilitate the reversible insertion/removal of Zn²⁺, making it the most popular Mn-oxide material for AZIBs.^38–40 However, its performance enhancement is still constrained by the sluggish kinetics of ion diffusion, limited electronic conductivity, and the dissolution of manganese.^41–43 In recent years, researchers have made significant efforts to improve the use of MnO₂-based cathodes in AZIBs by optimizing their structure and composition, thereby improving the cycling stability of the cathode.^44–46

Research has indicated that the incorporation of metal atoms and oxygen vacancies in MnO₂-based materials through doping techniques often leads to a synergistic impact, resulting in enhanced kinetics and stability for aqueous zinc–MnO₂ batteries. Using a straightforward hydrothermal technique, Li et al. developed an α-MnO₂ structure doped with Mg and featuring an insufficient amount of oxygen vacancies.⁴⁷ After 700 cycles, the electrode demonstrated a decrease in reaction resistance, polarization, and ion diffusion hindrance at 0.6 A g⁻¹. Moreover, there was an improvement in ion diffusion coefficient and stability, leading to a capacity retention of 77.2%, which significantly surpasses that of the bare α-MnO₂ electrode. Ye's group proposed a new method to enhance the cycling stability of δ-MnO₂ through selenium doping (Se–MnO₂).⁴⁴ The study shows that adjusting the se-doping content can regulate the proportion of H⁺ intercalation in MnO₂, thereby inhibiting the formation of the ZnMn₂O₄ by-product. Doping with selenium modifies the Mn–O octahedral geometry, optimizes the Mn–O bond lengths during Zn²⁺ intercalation, and alleviates Mn dissolution associated with the Jahn–Teller effect, thus alleviating lattice strain during Zn²⁺ intercalation/deintercalation (Fig. 2(a)). The carbon nanotubes were coated with an optimized Se–MnO₂ composite containing 0.8 atomic percent selenium. This composite reached 386 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 2(h)). Moreover, it sustained 102 mA h g⁻¹ at the 5000th cycle at 3.0 A g⁻¹, showcasing remarkable durability and stability throughout extended cycling.

Fig. 2 (a) MnO₂ and Se–MnO₂ after Zn²⁺/H⁺ co-intercalation.⁴⁴ Copyright 2024 Wiley-VCH GmbH. (b) Schematic depiction of the manufacturing procedure of Al-doped MnO₂.⁴⁸ Copyright 2024 The Royal Society of Chemistry. (c) Simulations investigating the adsorption energy and charge density variations for Mn²⁺ on different surfaces.⁴⁹ Copyright 2023 Wiley-VCH GmbH. (d) The formation scheme of different α-MnO₂ materials.⁵⁰ Copyright 2024 Wiley-VCH GmbH. (e) Schematic depiction of the crystal growth mechanism of BiO/MnO₂ and d-BMO.⁵¹ Copyright 2024 The Royal Society of Chemistry. (f) Schematic diagram for the preparation of CP.³² Copyright 2024 Springer Singapore. (g) Energy barrier of Mn atom removed from the structure of C@PODA/MnO.⁵² Copyright 2022 Wiley-VCH GmbH. (h) The cyclic of Se–MnO₂ and MnO₂ measured at 0.1 A g⁻¹.⁴⁴ Copyright 2024 Wiley-VCH GmbH. (i) GCD curves of MnO₂@CNTs and MnO₂ electrodes at 0.1 A g⁻¹.⁵³ Copyright 2024 Wiley-VCH GmbH. (j) Discharge mass capacity of 3DP MnO₂.⁴⁹ Copyright 2023 Wiley-VCH GmbH.

Zhao et al. overcame the compromise between charge–discharge kinetics and stability by designing defect-rich and Al-doped layered MnO₂ nanosheet (Al_x–MnO₂) electrodes (Fig. 2(b)).⁴⁸ The Al-rich ion vacancies generated by the electrochemical oxidation of Mn-based layered double hydroxides (MnAl-LDHs) provide three-dimensional diffusion channels, and the remaining Al atoms inhibit Jahn–Teller distortion, thereby enhancing structural stability. The optimized electrode (Al_0.1–MnO₂) exhibited excellent specific capacity, rate performance, and cycling stability, significantly outperforming most reported manganese and V-based cathodes. Zheng's group selected Mo-doped α-MnO₂ as the cathode, where NH₄⁺ intercalation forms stable N–H⋯O interactions, stabilizing the 2 × 2 tunnel structure of Mo–MnO₂, effectively suppressing Mn³⁺ dissolution and reducing lattice distortion, thus enhancing the cycling stability of the cathode.⁵⁴ Experiments showed that Mo–MnO₂ exhibited 265.2 mA h g⁻¹ with 364.3 W h kg⁻¹ at 100 mA g⁻¹, and retained 95.2% at 1000th cycle at 2.0 A g⁻¹. This work provides fresh perspectives on the advancement of AZIBs with enhanced energy density and extended lifespan capabilities.

Doped carbon-based materials have also been highly effective in improving the structure, conductivity, and electronic properties of manganese-based oxides. Yang and co-workers reported a ultrathick MnO₂ electrodes using 3D-printed carbon micro-lattices composed of graphene and carbon nanotubes, achieving a high specific capacity of 282.8 mA h g⁻¹ even at high mass loading (Fig. 2(j)).⁴⁹ The approach enhances conductivity and ion transfer dynamics (Fig. 2(c)), providing a promising strategy for high-performance ZIBs. Fan et al. developed symbiotic composite microspheres composed of α-MnO₂ nanowires and carbon nanotubes (Fig. 2(d)), effectively inhibiting the excessive crystallization of ZnSO₄·3Zn(OH)₂·nH₂O.⁵⁰ The assembled electrode exhibited a high initial capacity at a current density of 0.05 A g⁻¹ and maintained a significant capacity retention rate of 88% after 2500 cycles. Furthermore, through quantitative and qualitative analysis, the novel energy storage mechanism involving the synergistic effect of multivalent manganese oxides was comprehensively studied. Zhou et al. took an alternative approach to the use of carbon-based materials.⁵³ They developed a 3D framework structure of MnO₂@CNTs. In this design, carbon nanotubes (CNTs) were grown in situ on carbon cloth, serving to anchor and interconnect the electrodeposited spherical MnO₂. This innovative approach ensures enhanced structural integrity and efficient electron transport within the electrode. Thanks to this unique 3D structure, the issue of MnO₂ structural fragmentation and the electron and ion transport pathways were considerably improved. The optimized MnO₂@CNTs cathode exhibited remarkable cycling stability and an impressive specific capacity of up to 256.35 mA h g⁻¹ (Fig. 2(i)), and the mechanism was revealed through in situ Raman spectroscopy and COMSOL finite element analysis.

The use of biomass waste to create biocompatible cathode materials has expanded the approaches to optimizing MnO₂-based materials. Lv et al. prepared a γ-MnO₂ composite cathode uniformly loaded on grapefruit peel-derived N-doped carbon (Fig. 2(f)), which exhibited an enhanced specific capacity of up to 391.2 mA h g⁻¹ and excellent durability, maintaining 92.17% with 3000 cycles.³² Additionally, the composite exhibited a significant energy density and nearly 100% coulombic efficiency, and its immense potential in clinical medicine was verified through in vitro cytotoxicity tests.

Zhao's group introduced a new organic/inorganic hybrid cathode, C@PODA/MnO₂, for AZIBs.⁵⁵ The C=N within the PODA chain significantly enhances ion and electron mobility, as well as Zn²⁺ storage capacity. Moreover, the creation of Mn–N significantly boosts Zn²⁺ diffusion and minimizes the dissolution of Mn, thus improving redox kinetics and preserving the structural integrity of MnO₂ (Fig. 2(g)). The C@PODA/MnO₂ cathode demonstrated 321 mA h g⁻¹, coupled with outstanding rate capability and exceptional stability, enduring over 2000 cycles without significant degradation, showcasing the potential of organic/inorganic cathode. Constructing heterostructures to optimize MnO₂-based materials is a promising approach. Zhao's group synthesized a novel Mn-based heterostructure BiO/MnO₂ as a cathode for ZIBs (Fig. 2(e)), using BiO as a metal ion reservoir. Through the formation of a dynamic BMO/MnO₂ heterostructure during cycling, they effectively suppressed Mn dissolution, increased conductivity, and reduced the formation of irreversible byproducts ZnMn₂O₄.⁵¹ This dynamic transformation also improved the capacity and reaction kinetics, ultimately resulting in the BiO/MnO₂ cathode achieving a maximum specific capacity of 720.6 mA h g⁻¹ even at 0.1 A g⁻¹ and exhibiting 474.4 mA h g⁻¹ at 0.3 A g⁻¹ with excellent lifespan over 160 cycles.

In summary, the design of MnO₂-based cathodes should primarily address challenges related to active material dissolution, low intrinsic conductivity, structural collapse, and deterioration of ion diffusion kinetics.^56,57 (1) The nanoscale modification of MnO₂-based materials can be considered as a potential solution to mitigate volume expansion issues and maintain structural stability. (2) The incorporation of carbon-based materials and organic compounds can enhance the conductivity of electrode materials, while simultaneously providing a larger specific surface area and more active sites. (3) In the case of δ-MnO₂ with a layered structure, pre-embedding guest species is deemed advantageous for optimizing material architecture. Introducing design defects in the crystal structure can influence the distribution of electronic structures. (4) Doping strategies offer an effective means to regulate both electronic configuration and surface morphology of MnO₂-based structures, thereby preventing collapse during phase transitions.

2.2 Other Mn-based materials

Besides MnO₂, other Mn-based materials have also achieved good performance in aqueous zinc-ion systems.^58–60 For example, ZnMn₂O₄ (less than 250 mA h g⁻¹), MgMn₂O₄ (which can achieve 269 mA h g⁻¹ after activation), Mn₃O₄ (∼300 mA h g⁻¹), Mn₂O₃ (∼250 mA h g⁻¹), MnO (∼300 mA h g⁻¹), and MnO_x (∼300 mA h g⁻¹). Among these, the two spinel-phase materials, ZnMn₂O₄ and MgMn₂O₄, follow the same energy storage mechanism: first, cations (Zn²⁺/Mg²⁺) are released during charging, and then they are reversibly intercalated into the spinel structure during discharging. Therefore, MgMn₂O₄ involves both Mg²⁺ and Zn²⁺ in the electrochemical reactions. MnO requires an increase in the Mn oxidation state during the first charge, converting it into a higher oxidation state Mn-oxide. In subsequent cycles, the reaction mechanism is quite similar to that of Mn₃O₄, Mn₂O₃, MnO_x, and even MnO₂, with Zn²⁺ and/or H⁺ participating in the electrochemical reactions.

To alleviate the problems of poor structural reversibility and sluggish reaction kinetics in ZnMn₂O₄, Wang and colleagues introduced a microstructural strain approach by incorporating nickel into the tetrahedral sites of ZnMn₂O₄, which effectively controlled the MnO₆ microstructure to improve both structural stability and kinetics in the material (Fig. 3(a) and (b)).⁶¹ The nickel-substituted ZN_0.5MO/NCNTs cathode displayed a high specific capacity of 239.2 mA h g⁻¹ at 0.1 A g⁻¹, along with excellent rate performance and a long lifespan exceeding 3000 cycles (Fig. 3(i)). This work showcases the considerable utility of employing the microstructural strain approach to develop durable and high-capacity Mn-based cathodes for zinc storage. Deng et al. employed an electrochemical induction technique for the integration of ZnMn₂O₄ quantum dots (ZMO QDs) into a porous carbon structure, resulting in the formation of a composite comprising ZMO QDs and carbon (Fig. 3(c) and (e)).⁶² This arrangement reduces the distance for Zn²⁺ to diffuse and enhances the number of available sites, and the carbon framework facilitates the rapid transport of electrons. The formation of Mn–O–C bonds effectively suppresses the Jahn–Teller effect and the dissolution of manganese, significantly improving the electrochemical performance (Fig. 3(k)).

Fig. 3 (a), (b) and (i) ZN_0.5MO/NCNTs: illustration of the synthesis process, TEM and rate performance.⁶¹ Copyright 2024 Wiley-VCH GmbH. (c) and (e) ZMO QD@C: schematic illustration of preparing and TEM image.⁶² Copyright 2022 Wiley-VCH GmbH. (d), (h) and (k) Synthesis route for the flexible 3DP MnO_x/rGO, TEM of MnO_x nanowires and rate performance⁶³ Copyright 2023 Elsevier B.V. (f) and (g) PrO–MnO: the synthesis and TEM image for PrO–MnO.⁶⁴ Copyright 2024 Wiley-VCH GmbH. (j) Rate performance.⁶⁵ Copyright 2024 Wiley-VCH GmbH.

To address the challenges associated with active material dissolution and poor structural stability, Liu’ group developed a hollow octahedral Pr₆O₁₁–Mn₂O₃ heterostructure (PrO–MnO) by a metal–organic framework template method (Fig. 3(f) and (g), which effectively suppressed manganese dissolution, facilitated interfacial charge redistribution, and enhanced electron/ion transport, thus improving electrochemical performance.⁶⁴ As a consequence, the cathode composed of PrO–MnO demonstrated remarkable cycling stability and rate performance, exhibiting a superior reversible capacity of 140.8 mA h g⁻¹ even after undergoing 2000 cycles at an intensity of 1 A g⁻¹, surpassing the performance of the Mn₂O₃ cathode.

Mn₃O₄, serving as a cathode in AZIBs, presents numerous benefits such as its affordability, eco-friendliness, and exceptional safety features. However, its practical electrochemical capacity falls significantly short of its theoretical value. Song et al. employed an in situ hydrothermal technique to synthesize a composite material consisting of Mn₃O₄ nanoparticles and nitrogen-doped carbon dots (NCDs). Subsequently, calcination was conducted to yield nanocomposites of Mn₃O₄/NCDs.⁶⁶ Despite possessing a mere 3.9 wt% carbon content, the NCDs facilitated the formation of a conductive network and ensured structural stability for the composites. This endowed the Mn₃O₄/NCDs composite with several advantageous properties, including a significant surface area, numerous sites with activity, exceptional water affinity, and commendable conductive properties. These characteristics played a crucial role in enhancing the electrochemical efficiency of the Mn₃O₄/NCDs electrode. It exhibited excellent specific capacities of 443.6 mA h g⁻¹ at a current density of 0.1 A g⁻¹ and 123.3 mA h g⁻¹ at 1.5 A g⁻¹, which are markedly higher than those of pure Mn₃O₄ nanoparticles synthesized under similar conditions. This enhancement highlights the efficacy of NCDs in improving the electrochemical characteristics of Mn₃O₄ for utilization in AZIBs.

Xu's team employed an acid etching technique to fabricate a cathode made of nanostructured MnO_x, resulting in the creation of ample Mn(III) sites and numerous oxygen vacancies. This facilitated the reversible adsorption/desorption of Zn²⁺ and enhanced electron transport efficiency.⁶⁷ The abundant active sites and reduced diffusion paths of Zn²⁺ and H⁺ were facilitated by the mesoporous structure of this material, leading to exceptional reaction kinetics and long-lasting durability. As a result, this MnO_x cathode exhibited an impressive reversible specific capacity of 388.7 mA h g⁻¹ at 0.1 A g⁻¹. Additionally, it demonstrated excellent cycling stability, retaining 72.0% of its capacity after 2000 cycles. Wu's group synthesized a highly flexible and puncture-resistant 3D porous MnO_x/rGO hydrogel as a cathode for AZIBs using a repeated freeze–thaw method with polyvinyl alcohol, MnO_x nanowires, and reduced graphene oxide (rGO) (Fig. 3(d) and (h)).⁶³ This 3D self-supporting hydrogel created a complex 3D interconnected system through hydrogen bonding and the reinforcement of inorganic nanowires, optimizing its electrochemical and mechanical properties. It demonstrated a remarkable capacity of 297.6 mA h g⁻¹ and good long-term stability after 500 cycles at 1 A g⁻¹ (Fig. 3(k)). Additionally, this work employed a finite element method to confirm that the hydrogel material can mitigate stress distribution induced by the zinc plating process, and soft-packaged AZIBs exhibited excellent cycling performance and stable power supply at different bending angles.

Li et al. synthesized N-Mn₃O₄/MnO via a two-step solvothermal method, proposing a synergy to boost capacity and cycling stability in AZIB cathodes.⁶⁸ The material achieved 227.8 mA h g⁻¹ even at 5 A g⁻¹, retaining 92% at the 1500th cycle. Even at 10 A g⁻¹, it demonstrated a retention of 77.2% over 2500 cycles. Additionally, the study employed a novel water-soluble binder, providing a robust basis for the practical implementation of AZIBs. In exploring energy storage mechanisms, Liu's group proposed a novel concept of spatial deposition mechanism by adding MnSO₄ additive to MnO-doped graphene blocks (G-MnO). This mechanism constructed an internal micro-electric field through the covalent interface between MnO nanoparticles and multilayer graphene nanosheets, facilitating continuous electron transport. The constructed Zn//G-MnO battery demonstrated excellent rate performance, reaching 281.5 mA h g⁻¹ at 0.1 A g⁻¹, and maintained good cycling stability, retaining 106 mA h g⁻¹ at 20 A g⁻¹ (with 93 mA h g⁻¹ remaining after 5000 cycles) (Fig. 3(j)).⁶⁵

In summary, Mn-based cathodes have an elevated voltage platform, which is highly beneficial for enhancing the energy density of AZIBs. However, numerous obstacles persist, including the comparatively limited specific capacity (200–350 mA h g⁻¹) and inadequate cycling stability.^69,70 To address these problems, researchers have proposed several improvement mechanisms: (1) adjusting diffusion pathways through morphology control and nano-engineering to facilitate ion and electron transfer and mitigate volume changes during cycling; (2) improving structural stability and enhancing material conductivity through uniform doping, carbon material composites, and defect engineering. Additionally, the lack of a consistent understanding of the mechanism has significantly hindered the development of Mn-based material systems. Although Mn-based cathodes for AZIBs have been widely reported, they are still in the early stages. Research on Mn-based AZIBs with high energy density and long cycle life, and clarifying the reaction mechanism of Mn-based materials, remains a highly significant research topic.

3. V-based cathodes

AZIBs greatly benefit from the exceptional quality of V-based cathodes.^71–73 The multivalence of vanadium allows for multi-step redox processes, granting V-based materials a high theoretical specific capacity.^74–77 Additionally, V-based cathodes demonstrate a suitable voltage plateau, approximately 0.8–1.0 V (vs. Zn/Zn²⁺).^78,79 Meanwhile, the structure formed by the interconnection of easily deformable V–O polyhedra gives V-based materials a high degree of design flexibility.^80–82 Common V-based materials primarily include vanadium oxides and metal vanadates.^83–86 Similar to Mn-based compounds, the strong electrostatic interactions between Zn²⁺ and V-based materials lead to sluggish Zn²⁺ diffusion kinetics in the cathode.^87,88 Unmodified vanadium oxides are typically limited by their low electronic conductivity.⁸⁸ As cathodes for AZIBs, V-based materials operate at a significantly lower voltage compared to manganese-based and PBA materials.^89,90

3.1 V₂O₅

The utilization of V₂O₅ as a cathode material in AZIBs is hindered by its inadequate structural stability and low conductivity (4 × 10⁻⁴ S cm⁻¹), leading to unstable cycling performance.^75,91–93

Zhang et al. prepared a “two-in-one” strategy, inserting conductive PANI pillars into porous V₂O₅ derived from V-MOF to boost AZIBs performance.⁹⁴ The incorporation of PANI not merely widened the pathways for zinc ion diffusion, reinforcing the structural integrity, but also significantly augmented the overall conductivity, thereby leading to a remarkable enhancement in both the rate capability and cycling durability of the PVO (Fig. 4(l)). The research results offer pivotal understandings that can guide the development of MOF-derived cathode materials aimed at achieving superior performance in advanced AZIBs.

Fig. 4 (a) The synthesis process for layered V₂O₅.⁹⁵ Copyright 2024 Elsevier B.V. (b) The formation of Od-Ce@V₂O₅.⁹⁶ Copyright 2022 Springer Singapore. (c) The process of synthesizing V₂O₅@NC.⁹⁷ Copyright 2023 Wiley-VCH GmbH. (d) The supramolecular self-assembly strategy.⁹⁸ Copyright 2024 Wiley-VCH GmbH. (e) Diffusion barrier of Zn²⁺ migration in ZVO and Ov-ZVO.⁹⁹ Copyright 2023 Wiley-VCH GmbH. (f) The calculated Zn²⁺ adsorption energy.¹⁰⁰ Copyright 2023 Wiley-VCH GmbH. (g) Preparation and morphology of V₂O₅ nanoplates/MXene hybrids.¹⁰¹ Copyright 2022 American Chemical Society. (h) Long-term cycling and coulombic efficiency of V₂O₅@NC-40.⁹⁷ Copyright 2023 Wiley-VCH GmbH. (i) EPR: Od-Ce@V₂O₅.⁹⁶ Copyright 2022 Springer Singapore. (j) GITT analysis.⁹⁸ Copyright 2024 Wiley-VCH GmbH. (k) Rate capacities.¹⁰¹ Copyright 2022 American Chemical Society. (l) Long-term cyclic performance and coulombic efficiency of V₂O₅ and PV.⁹⁴ Copyright 2023 Elsevier B.V.

Additionally, Zhu's group promoted the utilization of polyaniline by simultaneously pre-intercalating polyaniline and water into V₂O₅ nanowires (Fig. 4(a)), significantly enhancing the capacity and charging performance of the zinc-ion battery, increasing the capacity up to 384 mA h g⁻¹.⁹⁵ The fabricated zinc-ion micro-batteries exhibited high areal capacity, energy density, and power density, along with excellent self-discharge performance. This research offers a viable approach for improving the electrochemical performance of planar micro-batteries, promoting the development of advanced portable electronic devices. Liu et al. developed carbon-coated V₂O₅ microspheres through a chitosan-assisted approach.¹⁰² The carbon layer significantly improved the conductivity of the active material, enabling the battery to reach a specific capacity of 532.4 mA h g⁻¹ and 354.9 W h kg⁻¹ at 0.2 A g⁻¹. The assembled battery retained 86% of its capacity after 3000 cycles at a current of 5 A g⁻¹. This design significantly enhanced the long-term cycling stability of the battery.

The specific surface area is a crucial factor in optimizing the structure and composition of V₂O₅-based materials. Zhang et al. achieved notable breakthroughs in their research. They designed a novel graded porous spindle-shaped Ag–V₂O₅ with a unique heterogeneous structure.¹⁰³ This particular design offers an increased surface area that is specific to its purpose, a heightened ability to transport Zn²⁺, and improved structural strength, resulting in outstanding electrochemical performance. This includes a notable capacity that is specific to the application, exceptional performance at various rates, and remarkable stability throughout multiple cycles.

The design of hierarchical structures and amorphous components can enrich the diffusion and adsorption pathways for Zn²⁺. Jia et al. developed MIL-88B(V)@rGO composites as cathodes for AZIBs, where reduced graphene oxide (rGO) sheets serve as a platform for anchoring MIL-88B(V) nanorods. These nanorods undergo irreversible transformation into amorphous V₂O₅ during the initial charge–discharge cycle, thereby serving as active sites for subsequent insertion/extraction of Zn²⁺.¹⁰⁰ Density functional theory calculations revealed that the presence of rGO sheets resulted in enhanced conductivity and lowered the energy barrier for Zn²⁺ migration (Fig. 4(f)). Ultimately, this cathode demonstrated exceptionally high reversible capacity and outstanding rate performance. Guo's group prepared an amorphous calcium-doped V₂O₅ (a-Ca–V₂O₅) cathode through calcium doping and self-supporting structural design.²⁴ The introduction of calcium reduced the formation energy of VO₂, promoting the reversible transition from amorphous to crystalline state, thereby enhancing the reversible capacity of the battery. The amorphous structure provided abundant active sites, supporting the realization of high volumetric capacity while ensuring excellent rate performance. This design strategy effectively addressed the issue of insufficient conductivity in amorphous metal oxides. Lashari et al. prepared a simple in situ construction strategy to synthesize a layered oxide nanosheet/nitrogen-doped carbon nanosheet (NC) heterostructure (Fig. 4(c)).⁹⁷ Compared to traditional layered oxides, this structure offers larger interlayer spacing and improved electrical conductivity, which enhances Zn²⁺ diffusion between layers and increases the utilization efficiency of active storage sites. The NC layer is bonded to the oxide nanosheets through C–O bonds, creating a stable structure that enhances cycling stability. The electrode composed of V₂O₅@NC-400 exhibits remarkable performance in terms of capacity and cycling stability (Fig. 4(h)).

The pre-insertion of Mⁿ⁺ not only increased the distance between layers and enhanced the stability of the material's structure, but also promoted the movement of Zn²⁺. As a result, the materials exhibit notable enhancements in terms of specific capacity and long-cycle performance when compared to V₂O₅. M_xV₂O₅·nH₂O generally follows the Zn²⁺ deintercalation reaction mechanism. In some materials, a small portion of the pre-intercalated cations may exit during the first cycle, but this does not affect the Zn²⁺-dominated energy storage mechanism.¹⁰⁴ In the investigation of methods to improve the Zn²⁺ diffusion coefficient of V₂O₅ as a cathode in AZIBs, Liu's group proposed a new strategy involving nanoscale confinement, where ultrathin V₂O₅ nanosheets containing either a single cation or multiple cations were prepared via supramolecular self-assembly (Fig. 4(d)).⁹⁸ This approach overcomes the challenges of uncontrollable phase structures and the difficulty of co-confining multiple cations associated with traditional hydrothermal/solvothermal methods. The study found that single-cation confinement significantly improved specific capacity and Zn²⁺ diffusion kinetics, while multiple-cation confinement enhanced structural and cycling stability through synergistic support effects. The optimized Zn²⁺ diffusion coefficient can reach 7.5 × 10⁻⁸ cm² S⁻¹ (Fig. 4(j)).

Ye et al. developed an oxygen vacancy-rich V₂O₅ structure (Ov-ZVO), which demonstrated exceptional electrochemical performance through increased interlayer spacing and enhanced structural stability.⁹⁹ The study indicates that the presence of oxygen vacancies reduces the energy barrier for Zn²⁺ diffusion and reduces the interaction between zinc and oxygen atoms (Fig. 4(e)), leading to improve the performance. The assembled Zn||Ov-ZVO battery achieved a capacity of 402 mA h g⁻¹ at 0.1 A g⁻¹ and delivered 193 W h kg⁻¹ at 2673 W kg⁻¹. These results present a promising strategy for creating oxygen vacancy structures and highlight their role in boosting electrochemical performance, paving the way for the development of defect-functionalized advanced materials. Bao et al. synthesized cerium-doped and oxygen-deficient V₂O₅ nanobelts via a one-step hydrothermal method, achieving dual-enhanced electrochemical performance (Fig. 4(b)).⁹⁶ The incorporation of cerium ions and the creation of oxygen vacancies within the material resulted in a substantial improvement in both its electrical conductivity and structural robustness (Fig. 4(i)), while also reducing the adsorption energy of Zn²⁺ and accelerating ion diffusion. The findings indicated that the electrode exhibited 444 mA h g⁻¹ at 0.5 A g⁻¹, accompanied by exceptional coulombic efficiency. Furthermore, this material surpassed expectations by achieving 304.9 W h kg⁻¹.

Achieving high practical energy density by increasing the ultrahigh capacity of cathode materials is a key challenge. Zhao et al. improved the capacity of the classic vanadium oxide cathode by designing a two-dimensional structure with atomic thickness.¹⁰⁵ Theoretical calculations and experimental results confirmed that the prepared graphene-like V₂O₅·nH₂O (GAVOH) achieved a high capacity of 714 mA h g⁻¹. Studies have shown that pseudocapacitance effects mainly contribute to this ultrahigh capacity. Subsequently, CNTs were introduced to design GAVOH-CNT gel inks, enabling large-scale cathode preparation, and demonstrating ultra-stable cycling performance and excellent rate capability.

Another significant factor hindering the development of V₂O₅ and its derivatives is the vanadium dissolution problem. Liu's group devised a method to construct a Ti₃C₂T_x MXene layer on the surface of V₂O₅ nanosheets via van der Waals self-assembly (Fig. 4(g)), effectively suppressing vanadium dissolution during the electrochemical process, which significantly enhances the energy storage performance of AZIBs.¹⁰¹ Compared to traditional V₂O₅/C composites, the VPMX hybrid material offers notable advantages: the MXene layer maintains structural integrity and suppresses vanadium dissolution; the heterojunction between V₂O₅ and MXene improves electrochemical kinetics; and the existence of water molecules with lubricating properties in the VPMX cathode promotes the movement of Zn²⁺ across the interface. Consequently, the VPMX cathode demonstrated outstanding long-term stability and enhanced rate performance over 5000 cycles (Fig. 4(k)).

3.2 Other V-based cathodes

Among the various vanadium oxides, VO₂(B), which has a +4 state, has also received considerable attention.¹⁰⁶ The optimization strategies typically used are similar to those employed for V₂O₅. For instance, Wang et al. proposed a cobalt substitution engineering strategy, where Co²⁺ doping and oxygen vacancy substitution were introduced to stabilize the VO₂ structure and enhance its ionic and electronic conductivity, thereby boosting zinc ion storage performance.¹⁰⁷ The synthesized Ov-CoVO mitigated the vanadium dissolution issue and maintained stability even in an acetonitrile system (Fig. 5(g)). Theoretical calculations also confirmed that Ov-CoVO has a more stable structure and faster electronic/ionic conductivity. Additionally, defect engineering is a recognized strategy for optimizing VO₂-based materials. Zhang et al. proposed electrode materials featuring controllable oxygen vacancies.¹⁰⁸ Ov-VO₂@CNF was synthesized in situ via a straightforward one-step hydrothermal process (Fig. 5(c)). This method can balance the limitations of the adsorption energy barrier.

Fig. 5 (a) SEM and TEM images of VO₂-D and VO₂-A.¹⁰⁹ Copyright 2024 Wiley-VCH GmbH. (b) SEM image of V₂O₅·4VO₂·2.72H₂O nanobelts.¹¹⁰ Copyright 2021 Elsevier B.V. (c) SEM image of the Ov-VO₂@CNF.¹⁰⁸ Copyright 2024 American Chemical Society (d) SEM and TEM images of superstructures.¹¹¹ Copyright 2024 Elsevier B.V. (e) SEM image of the a-Ca–V₂O₅@CNTF.²⁴ Copyright 2023 Wiley-VCH GmbH. (f) SEM images of Mx-V₆O₁₃.¹¹² Copyright 2022 Wiley-VCH GmbH. (g) Diffusion barrier of Zn²⁺ in VO and Ov-CoVO.¹⁰⁷ Copyright 2024 Wiley-VCH GmbH. (h) Rate performance of VO₂-D and VO₂-A.¹⁰⁹ Copyright 2024 Wiley-VCH GmbH. (i) Rate performance of the HBC650·V_x superstructures.¹¹¹ Copyright 2024 Elsevier B.V. (j) Long-term cycling properties of Al3 electrode.¹¹² Copyright 2022 Wiley-VCH GmbH.

Controlling the exposure of crystal facets and the arrangement of electrodes related to morphology is a successful approach to enhancing the ionic transport efficiency of VO₂-based cathodes. He et al. took VO₂(B) nanoribbons as an example and found that a dispersed morphology dominated by the (001) crystal facet facilitates rapid ion diffusion along the c-axis, which can enhance rate performance and cycling stability (Fig. 5(a) and (h)).¹⁰⁵ Additionally, the Pang's group synthesized ultrathin V₂O₅·4VO₂·2.72H₂O nanoribbons using a simple hydrothermal method (Fig. 5(b)), which served as cathode materials for AZIBs.¹¹⁰ Due to the expansion of interlayer spacing caused by crystal water, it exhibited a high capacity of 567 mA h g⁻¹ at 0.1 A g⁻¹, and maintained an excellent rate performance of 215 mA h g⁻¹ at 10.0 A g⁻¹. Following this, the team used a confinement strategy to embed metal ions within the interlayer space of layered vanadium oxide nanoribbons (Fig. 5(f)), maintaining their original structure and developing a range of metal-confined nanomaterials.¹¹² The results showed that Al_2.65V₆O₁₃·2.07H₂O, as a cathode for AZIBs, achieved 571.7 mA h g⁻¹, and was able to maintain 205.7 mA h g⁻¹ even at 5.0 A g⁻¹. After the 2000 cycling, the capacity retention rate remained as high as 89.2% (Fig. 5(j)). Besides hydrothermal doping, Guo et al. prepared calcium-doped VO₂ nanowire arrays using an in situ electrochemical method (Fig. 5(e)).²⁴ The design of superstructures can effectively alleviate the dissolution problems of cathode materials. Pang's group used MIL-96(Al) nanomaterials as a precursor and prepared a series of layered (AlO)₂OH·VO₃ composite superstructure materials with varying morphologies and V-oxide contents via calcination and hydrothermal synthesis methods (Fig. 5(d)).¹¹¹ Among them, the HBC650·V₄ superstructure exhibited a high specific capacity of 180.1 mA h g⁻¹ after 300 cycles at 0.5 A g⁻¹ (Fig. 5(i)), and retained a capacity retention rate as high as 99.6% at the 5000th cycle at 5.0 A g⁻¹.

Due to the relatively low voltage platform of V-based materials (0.8–1.0 V), it is essential to focus on the research of high specific capacity cathode materials to achieve suitable energy density.¹¹³ Vanadium nitride oxides (VN_xO_y) were considered unsuitable for zinc storage due to their overcrowded lattice structure, yet they can attain ultra-high specific capacities through optimization. Additionally, materials with the M_xV₃O₈ structure are among the most reported cathode materials in vanadates, where Mⁿ⁺ can be H⁺, Li⁺, Na⁺, K⁺, or NH₄⁺, serving a stabilizing role in the interlayer. The NASICON polyanionic compound with the formula M₃V₂(PO₄)₃ (M = Li, Na) features a stable three-dimensional framework and is also an important category of V-based zinc-storage materials.

Na superionic conductor (NASICON)-type compounds have been recently considered to be some of the most attractive candidates for AZIBs due to their large ionic channels and fast kinetics.^114,115 Nevertheless, as a cathode material for AZIBs, they still suffer from limitations of poor cycle life and low electronic conductivity, as well as a slow Zn²⁺ migration rate. Cation doping is considered a powerful strategy to enhance the intrinsic properties of NVP-based cathodes. Liu et al. reported an advanced cathode of Ti⁴⁺/Zr⁴⁺ as dual-supporting sites in Na₃V₂(PO₄)₃ with an expanded crystal structure, exceptional conductivity, and superior structural stability for AZIBs, which exhibits fast Zn²⁺ diffusion and excellent performance.¹¹⁶ The results of AZIBs afford remarkably high cycling stability (91.2% retention rate over 4000 cycles) and exceptional energy density (191.3 W h kg⁻¹), outperforming most NASICON-type cathodes.

In summary, V-based materials typically exhibit high electrochemical specific capacities (300–650 mA h g⁻¹) and relatively simple reaction mechanisms (solid solution reactions involving Zn²⁺ and H⁺ either separately or together), which gives them the potential for large-scale energy storage applications. However, the low average voltage and dissolution issues of V-based materials remain the main factors limiting their development. To stabilize the charge–discharge performance of V-based materials and mitigate degradation issues caused by dissolution and low conductivity, strategies such as ion doping, morphology control, nano-engineering, defect engineering, carbon materials, and conductive polymer composites have been employed to enhance the overall performance of V-based materials. In conclusion, achieving high energy density and long cycling life in V-based zinc-storage materials remains a worthwhile research topic.

4. Other cathodes

4.1 PBA-based cathodes

PBA refers to a class of transition metal cyanides with an open framework structure. Due to their non-toxic, low-cost, simple synthesis methods, and flexible adjustable structures, PBAs have garnered researchers’ attention as cathodes in AZIBs. These materials have a high operating voltage range of approximately 1.5 to 1.8 V and possess rapid charge and discharge capabilities.^117,118 To augment the Zn²⁺ diffusion velocity and maintain stability during the intercalation and deintercalation cycles, the adoption of doping techniques presents a promising avenue. Zeng's group successfully synthesized cobalt-doped, Mn-rich PBA hollow spheres (CoMn-PBA HSs) through a template-guided ion exchange method (Fig. 6(a) and (b)).¹¹⁹ The hollow structure of this material provides abundant active sites, and the partial cobalt doping along with fast Zn²⁺ diffusion significantly enhances the storage performance (Fig. 6(c)). Ye et al. optimized the structural stability of Mn-PBA electrodes by regulating the concentration gradient.¹²⁰ The introduction of manganese in Mn-PBA created dual zinc ion active centers, greatly improving its specific capacity. The optimized Mn-PBA-3 displayed remarkable reversible specific capacity alongside outstanding cycling durability. Suma et al. modified PBA with cobalt–nickel co-doping as cathode material for AZIBs.¹²¹ The results indicated that the synergistic effect of cobalt–nickel co-doping and NC separators allowed the battery to retain 84% of its capacity after 5000 cycles.

Fig. 6 (a) The synthetic process of CoMn-PBA.¹¹⁹ Copyright 2021 Wiley-VCH GmbH. (b) TEM image of CoMn-PBA.¹¹⁹ Copyright 2021 Wiley-VCH GmbH. (c) Charge–discharge voltage profiles of the CoMn-PBA HSs.¹¹⁹ Copyright 2021 Wiley-VCH GmbH. (d) Schematic diagram from V₆O₁₃ nanobelts to V-PBA nanocubes.¹²¹ Copyright 2024 Wiley-VCH GmbH. (e) The theoretically most stable structural model of MoS₂/PEDOT.¹²² Copyright 2022 Wiley-VCH GmbH. (f) Initial charge/discharge curve and ex situ XRD patterns of MoS₂/PED.¹²² Copyright 2022 Wiley-VCH GmbH. (g) The Zn-migration behaviors of MoS₂/PED.¹²² Copyright 2022 Wiley-VCH GmbH. (h) Schematic synthesis of the MoS₂/rGQDs hybrid.¹²³ Copyright 2024 Wiley-VCH GmbH. (i) Rate capability of MoS₂/rGQDs.¹²³ Copyright 2024 Wiley-VCH GmbH. (j) The proposed redox mechanism of PANI/CFs.¹²⁴ Copyright 2024 Wiley-VCH GmbH. (k) Preparation of PDBS and proposed redox mechanism.¹²⁵ Copyright 2021 Wiley-VCH GmbH. (l) illustration of the synthesis of COF-PTO and the top and side view models.¹²⁶ Copyright 2024 Wiley-VCH GmbH.

Pang's group utilized a gentle in situ conversion strategy to synthesize V-based PBA nanocubes at room temperature (Fig. 6(d)), showcasing an outstanding discharge specific capacity of 200 mA h g⁻¹ in AZIBs, which is notably higher than that of other metal-based PBA nanocubes.¹²¹ Besides, many other PBAs have also been reported in aqueous zinc-ion battery systems. These materials mostly follow an energy storage mechanism dominated by Zn²⁺ intercalation/deintercalation and have very high operating voltages.^127,128 Although these materials have relatively low specific capacities (<200 mA h g⁻¹), they still show promising development potential.¹²⁹

In conclusion, PBAs with a large open tunnel size are favorable for the reversible insertion/extraction of Zn²⁺ due to their good specific capacity, high working potential, and structural flexibility. However, the practical application of ZIBs is still limited by their low specific capacity. Further research can explore PBAs and electrolyte composition regulation to address this issue.

4.2 Layered TMDs

Layered transition metal dichalcogenides (TMDs) have a 2D layered structure that allows for improved insertion and extraction of Zn²⁺.¹³⁰ MoS₂, as a typical representative of TMDs, is the most reported cathode for AZIBs.¹³¹ However, the electrochemical activity and low ionic conductivity of MoS₂ make reversible insertion of Zn²⁺ infeasible, necessitating improved strategies to lower the Zn²⁺ insertion energy barrier for MoS₂.

Li et al. creatively synthesized a MoS₂/poly(MoS₂/PEDOT) composite material (Fig. 6(e)).¹²² The insertion of PEDOT broadens the interlayer distance and enhances the electrical conductivity of MoS₂, and initiates proton intercalation reactions, further enhancing its functional properties. The research systematically explored the efficient and reversible co-intercalation/extraction behavior of H⁺/Zn²⁺ (Fig. 6(f)). The study disclosed that the co-intercalated protons act as effective barriers, mitigating electrostatic interactions between MoS₂/PEDOT and Zn²⁺, akin to lubricants, thereby expediting ionic diffusion kinetics and bolstering the overall rate capability (Fig. 6(g)). Subsequently, the team focused on the importance of active site utilization for reversible capacity, and employed a tailored MoS₂/reduced graphene quantum dots (rGQDs) composite material to uncover the “gap-filling” mechanism in aqueous Zn–MoS₂ batteries to tackle this issue (Fig. 6(h)).¹²³ The research revealed that NH₄⁺ and H⁺ ions serve as gap fillers, effectively utilizing active sites and shielding electrostatic interactions, thus accelerating the diffusion of Zn²⁺. This mechanism achieved a rate capability (439.5 mA h g⁻¹ at 100 mA g⁻¹ and 104.3 mA h g⁻¹ at 30 A g⁻¹) and an ultra-long cycle life (8000 cycles) (Fig. 6(i)).

In conclusion, the optimization strategy for Layered TMDs is analogous to that employed for other layered materials. In future studies, in situ characterization technology can be utilized to delve deeper into the internal relationship between lattice structure and performance of zinc-ion batteries, thereby elucidating the zinc storage mechanism of such materials. By fully considering the synergistic effects between various optimization strategies, such as hybridization and doping, it is possible to enhance both the structural stability and electronic/ionic conductivity of these materials.

4.3 Organic materials

In the past few years, organic materials with redox activity have emerged as promising zinc storage materials due to their advantages of being lightweight, environmentally friendly, and structurally diverse.^132–136

Compared to the traditional Zn²⁺ insertion/extraction mechanism, the dual-ion mechanism, similar to that of supercapacitors, often endows batteries with higher operating voltages, better rate performance, and longer cycle life. Wan et al. developed an aqueous zinc/polyaniline battery that combines the Zn²⁺ insertion/extraction mechanism with the dual-ion mechanism (Fig. 6(j)). This zinc/polyaniline battery exhibits excellent electrochemical performance, especially maintaining a high capacity of 92% after as many as 3000 cycles.¹²⁴ The slow ion diffusion and structural instability of the metal oxide cathode materials discussed above severely limit the performance enhancement of AZIBs. Sun et al. designed and applied a novel organic material, poly(2,5-dihydroxy-1,4-benzoquinone sulfide) (PDBS), as a zinc-storage cathode material, and the aqueous zinc-organic battery based on the PDBS cathode exhibited excellent cycling stability and rate performance (Fig. 6(k)).¹²⁵ Moreover, the flexible polymer structure of PDBS allows for the rotation and bending of polymer chains in an electrochemical environment, facilitating Zn²⁺ insertion/extraction, thereby demonstrating the advantages of organic electrode materials in multivalent cation storage. Zhong et al. reported a covalent organic framework (COF-PTO) containing pyrene-4,5,9,10-tetrone groups as the cathode material for aqueous self-charging zinc batteries (Fig. 6(l)).¹²⁶ The ordered channel structure of COF-PTO allows it to maintain a capacity retention of up to 98% after 18 000 cycles at 10 A g⁻¹, and it exhibits ultra-fast ion transport capability. A systematic investigation of the reaction mechanism was conducted, in which the simultaneous insertion of Zn²⁺ and H⁺ dual ions in COF-PTO promotes the self-charging reaction, thereby enhancing the self-charging performance. By the condensation of hexaketocyclohexane octahydrate (HKCO) and 1,2-diaminoanthraquinone (DQ), Zhai et al. constructed anthraquinone–quinoxaline derivatives (HATAN) with more robust pyrazine linkage and holding fully conjugated structure.¹³⁶ The fully conjugated structure endows HATAN with enhanced π-electron delocalization and strong intermolecular π–π interaction, which can increase the electronic conductivity and physicochemical/electrochemical stability. Additionally, the introduction of pyrazine and quinone increases the redox-active sites of HATAN (C=O and C=N), leading to a large theoretical capacity.

In conclusion, the diversity in the structures of organic materials grants them remarkable design flexibility, which often originates from their structural complexity, making the investigation of their charge–discharge mechanisms somewhat challenging, full of both opportunities and challenges.

5. Conclusions and prospects

This review summarizes the recent progress in cathodes for AZIBs, including V-based materials, Mn-based materials, and other materials. The corresponding performance characteristics are summarized in Table 2. The currently used cathode materials each have their advantages and disadvantages, as shown in Fig. 7. Among the AZIB cathodes, Mn-based materials exhibit relatively balanced performance in various aspects, with moderate discharge potential, rate performance, and cycling stability. V-based cathodes excel in specific capacity and rate performance, but their operating voltage is relatively low, and they also possess some level of toxicity. Other materials, such as PBAs, offer high operating voltage but low specific capacity. TMD-based materials have low electrochemical activity and poor electronic conductivity; organic materials, as rising stars, show excellent performance, but their structural complexity makes mechanism research particularly challenging. Modification strategies for the aforementioned cathode materials typically revolve around the following approaches: (1) interlayer engineering (interlayer water, interlayer metal ion, and interlayer polymer intercalation) and heterostructure design. (2) Morphology control. (3) Composite conductive materials. (4) Defect engineering design. (5) Heteroatom doping.

Table 2 Comparison of electrochemical performances of different cathodes for ZIBs

Types of materials	Materials	Capacity (mA h g^−1)	Current density (A g^−1)	Cycle number	Long-term stability (%)	Current density (A g^−1)	Ref.
Mn-based	δ-MnO2·NDs	335	0.1	1000	86.2	1	137
	δ-MnO2−x-2.0	551.8	0.5	15 000	83	10	138
	PVP intercalated MnO2	317.2	0.1	20 000	∼100	10	139
	2D MnO2/Mxene	315.1	0.2	5000	88.1	5	140
	α-Mn2O3 microrod array	103	5	2000	∼100	2	141
	Ni-doped Mn2O3	252	0.1	2500	85.6	1	142
	Od-Mn3O4	325.4	0.3	1100	∼100	3	143
	MnO@NGS	228	0.1	300	98	0.5	144
	t-MnO@C	743	0.05	20 000	97	5	145
	d-ZMO	419.7	0.1	1000	81.8	3	146
	N-ZMO NTAs	223	0.1	1500	92.1	4	147
	KMOH@C	412.7	0.5	6000	∼100	3	148
	Na0.55Mn2O4·1.5H2O	367.5		10 000	∼100		149
	NMO/VTCNTs	329	0.2	1000	∼100	2	150
	K0.16Mg0.06Mn2O4·1.4H2O	400	0.1	1000	94	3	151
V-based	V-EG	553	0.3	10 000	81.1	20	152
	V6O13/VO2	498.3	0.2	5000	96.8	10	153
	Tunnel-oriented VO2 (B)	420.8	0.1	5000	84.3	10	109
	V2O3−x-CC	587	0.1	5000	71	10	154
	Al–V2O5	532	0.1	5000	76	5	155
	VO2@NC	435.4	1	2500	99.7	10	156
	Zn0.3V2O5·1.5H2O	426	0.3	20 000	96	10	157
	Mn1.4V10O24·12H2O	456	0.2	5000	80	10	158
	SbO2/K0.43V6O13			20 000	89.3	20	28
	V6O13/CeVO4	280	1	4000	78.2	10	159
	KV3O8	556.4	0.8	5000	81.3	6	160
	VO-NVO	498.6	0.2	4000	95.1	5	161
	(NH4)2V10O25·8H2O	408	0.1	4000	94.1	5	162
	H3.78V6O13	406	0.1	15 000	72.9	10	163
	Ni0.24V5.76O13	302.6	1	10 000	96.5	8	164
PBA	NiHCF/RGO	50.1	0.2	1000	80.3	0.2	165
	CoMn-PBA HSs	128.6	0.05	1000	76.4	1	119
	G-EE-1	160	2	10 000	55	10	121
	FeNi-PBA@PAN	275.8	0.1	2000	81	2	127
	CoxNi1−xHCF	92	0.1	5000	84	1	128
TMD	MoS2/rGQDs	439.5	0.1	500	94.2	0.5	123
	MoS2/PEDOT	312.5	0.1	4000	90.1	5	122
	P-MST	138.3	0.5				130
	1T MoS2@MXene	270	0.1	2500	94.7	10	131
Organic	EDA-VO	382.6	0.5	10 000		5	166
	PoPD	318	0.05	3000	66.2	1	167
	HHTP	225	0.05	7000	90	10	168
	MXene/Bi	178.1	0.25	4500	∼100	2.5	169
	BPD	429	0.05	10 000	100		170
	VEG@MXene	360.3	0.5	3000	85.2	10	171
	TpDa-COF	96.6	0.1	10 000	98		172
	TAP/Ti3C2Tx	303	0.04	10 000	81.6		173

---

Fig. 7 Summary of the advantages and disadvantages of different cathodes for AZIB.

Additionally, most current cathodes only exhibit excellent electrochemical performance under low mass loading (<2 mg cm⁻²). However, in practical applications, a mass loading of approximately 10 mg cm⁻² of active material is often required to meet the demands. Yet, high mass loading often results in low conductivity and limited ion diffusion, and the electrochemical activity, which is closely related to specific capacity and rate capacity, significantly decreases with increased mass loading. Therefore, enhancing the electrochemical activity of cathode materials at commercial mass loading levels (around 10 mg cm⁻²) is one of the key development directions for AZIBs in the future, and also represents a significant challenge.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52371240), Natural Science Foundation of Jiangsu Province (BK20230566). We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions, Natural Science Research Project of Guangling College, Yangzhou University (ZKZD23005), the universities’ philosophy and social science researches in Jiangsu Province (2023SJYB2088), and the technical support we received at the Testing Center of Yangzhou University.

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