The progress of cathode materials in aqueous zinc-ion batteries

2.1 Composition

The main structure of AZIBs includes cathode [40], anode [41,42,43,44,45,46,47,48], electrolyte [49,50,51,52,53], and separator [54,55,56]. Cathode materials can store Zn²⁺ including manganese-based compounds, vanadium-based compounds, MXene and its composites, Prussian blue analogs (PBAs), and organic compounds. Improving the properties of cathodes is extremely important for AZIBs [57]. As shown in Table 1, compared with other metals, metal zinc has good chemical stability in the aqueous solution. Compared with the standard hydrogen electrode, the zinc anode has a higher theoretical capacity (820 mA h g⁻¹) and a lower redox potential (0.76 V), so metal zinc can be used as anode.

The electrolyte serves as a medium for ion transfer between the cathode and anode and is typically an aqueous solution of zinc salt, including ZnSO₄ [58], Zn(OTf)₂ [59], and Zn(TFSI)₂ [60]. The choice of water as a solvent electrolyte can obtain higher ionic conductivity and safety performance. Research has indicated that the addition of appropriate additives to the electrolyte can significantly enhance the chemical performance of the electrolyte and improve the stability of the interface [61,62]. The separator also plays an important role in the battery system. The separators in AZIBs are typically made of glass fiber, filter paper, or polypropylene. These traditional separators usually do not affect the chemical performance of the battery, but in recent years, some newly developed functional separators can inhibit the growth of anode dendrites and improve the stability of the battery.

2.2 Working principle

The traditional AZIBs also belong to rocking-chair batteries [63], and their charging and discharging mechanism is similar to conventional LIBs. Zn²⁺ can move between the cathode and the anode (Figure 3). During the discharge process, the anode (Zn) loses electrons and transforms into Zn²⁺, which then enters the electrolyte. Simultaneously, electrons flow through the external circuit and enter the cathode, where the cathode is reduced by gaining electrons, and at the same time, Zn²⁺ from the electrolyte becomes embedded within the cathode. During the charging process, the cathode undergoes oxidation as it loses electrons, causing Zn²⁺ to detach from the cathode and enter the electrolyte. Meanwhile, electrons from the external circuit enter the anode, facilitating the deposition of Zn²⁺ from the electrolyte onto the anode surface.

Figure 3

Schematic diagram of the working principle of AZIBs [64]. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3.1 Zn²⁺ (de)intercalation mechanism

During the early stages of AZIBs research, the most widely proposed and accepted mechanism involved only the Zn²⁺ (de)intercalation mechanism. This mechanism was predominantly observed in manganese-based materials, vanadium-based materials, PBAs, and organic cathode materials. Due to the small radius (0.74 Å) of Zn²⁺, many materials with layered, channel, and frame structures can withstand its (de)intercalation. α-MnO₂ [70], β-MnO₂, γ-MnO₂, δ-MnO₂ [71,72], and ZnMn₂O₄ in the spinel structure, V₂O₅·nH₂O in the layered structure, Ca_0.25V₂O₅·nH₂O in the layered structure, and PBAs in the frame structure have been reported to follow the energy storage mechanism of Zn²⁺ (de)intercalation.

Kim’s group investigated the reversible storage of Zn²⁺ in α-MnO₂ nanorod electrodes [66]. The tunnel of the α-MnO₂ structure expanded alternately during the process of Zn²⁺ intercalation and Zn²⁺ ejection like “breathing.” The volume of the adjacent (110) plane expanded by 3.12% after Zn²⁺ intercalation (Figure 4a). The electrode was stable during the (de)intercalation of Zn²⁺. In 2020, Long et al. used layered porous nanorods composed of twin α-(Mn₂O₃–MnO₂) heterostructures as the cathode of AZIBs [67]. According to X-ray diffraction (XRD), during the first cycle, the Zn²⁺ intercalation and removal processes were experienced, and the peak characteristics recovered from the full discharge state to the full charge state, indicating that α-(Mn₂O₃@MnO₂)-500 electrode has high reversibility in the AZIBs (Figure 4b and c).

Figure 4

(a) Schematic illustration for the Zn²⁺ intercalation into tunnel structure α-MnO₂, which causes the expansion of tunnel and hence increases the interplanar spacing of adjacent (110) planes [66]. Copyright 2015 Elsevier B.V. All rights reserved. (b) XRD of the α-(Mn₂O₃–MnO₂)-500 electrodes at different charge/discharge states during the first cycle of the AZIBs. (c) Typical charge and discharge curves of the α-(Mn₂O₃–MnO₂)-500 electrode [67]. Copyright 2020 American Chemical Society. (d) Schematic illustration of reversible Zn²⁺ intercalation/deintercalation in CoFe(CN)₆ frameworks during electrochemical process. (e) Quantitative Zn²⁺ and H⁺ contribution to capacity delivery according to inductively coupled plasma (ICP) results [68]. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (f) Quantitative Zn²⁺ and H⁺ contribution to capacity delivery according to ICP results [69]. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ma et al. formed a CoFe(CN)₆ skeleton by in situ extraction of K⁺ from KCoFe(CN)₆, and during the subsequent charging and discharging process, Zn²⁺ was deintercalated in the skeleton as the Zn²⁺ concentration was much higher than K⁺ [68]. Due to the presence of two active couples of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ in this system, two steps were required for the intercalation/delamination of Zn²⁺, and this process is shown in Figure 4d. The presence of these two active couples resulted in a high-capacity, high-voltage system. After 2,200 cycles at a current density of 3 A g⁻¹, the coulombic efficiency is approximately 100%, showing excellent cycling stability (Figure 4e). This Zn/PBA battery with almost no capacity degradation after long cycles highlights the great potential for the future development of high-voltage, high-capacity AZIBs.

3.2 Zn²⁺/H⁺ co-intercalation

Materials with channel, layered, or framework structures can achieve co-intercalation of H⁺ and Zn²⁺. During the charging state, H⁺ is stored through the formation of hydrogen bonds with lattice oxygen. However, during the discharging state, H⁺ is released and enters the electrolyte. Due to the relatively small ion radius of H⁺, rapid (de)intercalation is achievable, leading to enhanced ion diffusion kinetics and contributing to a higher capacity. The kinetic properties of Zn²⁺ and H⁺ are different. Compared with H⁺, Zn²⁺ has a larger radius and the characteristics of multivalent ions, so that it moves relatively slowly and has a greater electrostatic force with the intercalation host.

Through a series of characterization and analysis of charge–discharge curve, XRD, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (TEM), Zhao et al. proved that the intercalation mechanism of Zn/MnO₂H_0.16(H₂O)_0.27 (MON) aqueous solution battery system is Zn²⁺/H⁺ co-intercalation. Quantitative chemical analysis of the Zn²⁺/H⁺ co-intercalation was also carried out using ICP mass spectrometry [69]. Approximately 0.255 mol H⁺ and 0.086 mol Zn²⁺ were inserted into 1 mol MON in the first discharge plateau at 1.8–1.3 V as shown in Figure 4f. In the second discharge plateau at 1.3–1.0 V, another 0.204 mol H⁺ and 0.142 mol Zn²⁺ were inserted into 1 mol MON. Therefore, the cathodic electrochemical reaction equation of the Zn/MON cell is given as follows:

This H⁺ and Zn²⁺ synergistic intercalation mechanism, especially the fast diffusion kinetics of H⁺, has enabled this battery to exhibit excellent electrochemical performance.

3.3 Chemical transformation reaction mechanism

The chemical conversion reaction mechanism generally exists in manganese-based materials, which means that H⁺ participates in the charging and discharging process of MnO₂ to generate MnOOH. The reaction formula is:

In contrast, Zn²⁺ does not intercalate into MnO₂. Instead, it reacts with OH⁻ ions in an equilibrium system to form ZnSO₄[Zn(OH)₂]³⁻·xH₂O (ZHS), a lamellar layered hydroxide of Zn. ZHS is a complex of electrolyte salts and hydroxides with crystalline water, the molecular formula of which varies according to the type of electrolyte. Up to now, the formation of ZHS has been observed in many electrolyte systems. Typically, it is generated on the electrode surface during the discharge process and dissolves during the charging process. It is generally accepted that ZHS is generated to maintain the acid–base balance of the electrolyte.

Pan et al. characterized the structure and morphology of α-MnO₂ electrodes using TEM and scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDS) mapping to elucidate the electrode’s electrochemical behavior and reveal the mechanism of chemical transformation [73]. The initial nanofiber-like α-MnO₂ transformed into shorter nanorods and nanoparticle aggregates (Figure 5a and b). HR-TEM analysis revealed lattice fringe spacings of 0.33 and 0.26 nm in the discharge products, corresponding to the (210) and (020) planes of monoclinic MnOOH, respectively (Figure 5c and d). In addition, the STEM-EDS mapping in Figure 5e demonstrated the absence of Zn elements in the nanorod structure, further confirming the formation of MnOOH through the chemical transformation reaction. This mechanism is identified as the primary reason for the excellent performance of the battery, with a capacity retention rate of 92% after 5,000 cycles, suggesting that the combination of Zn anode and MnO₂ cathode could potentially form a high-performance and long-life AZIBs.

Figure 5

(a) TEM image for α-MnO₂ nanofiber. (b–d) TEM/HR-TEM images of MnO₂ electrodes during electrochemical process. (e) STEM-high-angle annular dark-field image of short nanorods and STEM-EDS mappings of the elemental distributions of Mn, O, and Zn in the MnO₂ electrode in the discharged state during the first cycle [73]. Copyright 2016 Macmillan Publishers Limited. All rights reserved. (f) Rate performance of graphene oxide wrapped CuV₂O₆ nanobelts and CVO NBs and Scheme showing reversible Zn²⁺ deintercalation between CuV₂O₆ and ZnV₂O₆ [74]. Copyright 2022 American Chemical Society. (g) Schematic illustration of the displacement/intercalation reaction mechanism in the first cycle, and the (de)intercalation mechanism of Zn²⁺ in subsequent electrochemical discharge/charge processes. (h) Galvanostatic charge/discharge profiles of the Zn/Na_0.55Mn₂O₄·0.57H₂O (NMOH) cell tested between 0.8 and 1.9 V with different current densities [75]. Copyright 2020 Springer.

3.4 Replacement reaction mechanism

The replacement reaction mechanism is commonly found in some pre-inserted vanadium oxides containing metal ions (Ag⁺ and Cu²⁺). During the reaction process, Zn²⁺ can partially replace these metal ions, forming metal or metal ion compounds. Liu et al. investigated the mechanism of Zn²⁺ storage in CuV₂O₆ cathode materials [74]. A reversible phase transition occurs between CuV₂O₆ and ZnV₂O₆, accompanied by the (de)intercalation of Zn²⁺ and the reduction/oxidation of metallic copper nanoparticles (Figure 5f), with the main electrochemical reactions being

This reaction process helps to improve the electrochemical performance of the battery. At a current density of 0.1 A g⁻¹, the battery has a specific capacity of up to 427 mA h g⁻¹, and after 3,000 cycles at a high current density of 5 A g⁻¹, the battery still has a capacity retention rate of 99.3%. In addition, CuV₂O₆/GO composite material was also prepared. Due to the addition of GO, which can increase reactive sites and enhance structural stability, the electrochemical performance of the material was significantly improved, leading to a minimum increase in composite material capacity by 30%.

In addition to the four common Zn²⁺ storage mechanisms mentioned in this article, there are also solution-deposition mechanisms and some composite intercalation mechanisms. In 2020, Zhai et al. demonstrated the replacement/intercalation mechanism in the cathode of a manganese-based compound in AZIBs [75]. The layered NMOH was synthesized by selective etching of the siliceous tetrahedron of manganese silicate with the NaOH solution. After a series of ectopic XPS, XRD, EDS, and ICP-AES characterization, it was found that Zn²⁺ could be inserted into NMOH during the first discharge, and then, a small amount of Na⁺ was replaced by Zn²⁺, and Zn²⁺ became the pillar of the layered structure, thus improving the structural stability. In the subsequent charge–discharge cycle, reversible desorption of Zn²⁺/H⁺ is accompanied by adsorption/desorption of Na⁺ and reversible formation of Zn₄SO₄(OH)₆·0.5H₂O (Figure 5g). Na_0.55Mn₂O₄·0.5H₂O nanosheet has a high reversible capacity of 389.8 mA h g⁻¹ at a current density of 200 mA g⁻¹ (Figure 5h). The displacement/intercalation mechanism results in better electrochemical performance of the NMOH electrode. The intercalation/insertion mechanism has also been observed in various materials [76]. Zn₃V₃O₈@ZnO@NC heterostructure was successfully synthesized using a well-designed approach and employed as the positive electrode material for AZIBs [77]. After two cycles of reaction, the cathode material underwent an irreversible transformation from Zn₃V₃O₈@ZnO@NC to Zn₃(OH)₂V₂O₇⋅2H₂O (ZOVO), followed by a subsequent (de)intercalation mechanism of Zn²⁺ within ZOVO. Remarkable rate performance and exceptional cycling stability were exhibited by this material.

In summary, the storage mechanisms of the majority of layered, tunnel, and framework-structured materials, when employed as cathode materials, involve Zn²⁺ (de)intercalation, with a portion also involving the co-intercalation of H⁺ and Zn²⁺. Compared to the singular (de)intercalation of Zn²⁺, the incorporation of smaller H⁺ enables faster (de)intercalation, exhibiting enhanced ion diffusion kinetics and contributing to a certain capacity. On the other hand, the chemical transformation reaction mechanism is commonly observed in manganese-based materials, wherein Zn²⁺ does not intercalate into MnO₂; instead, H⁺ participates in the charging and discharging process of MnO₂, leading to the formation of MnOOH. Zn²⁺ then reacts with OH⁻ in a balanced electrochemical system, generating ZHS. For some vanadium oxides containing pre-intercalated metal ions, the replacement reaction mechanism is usually present. Zn²⁺ undergoes replacement reaction mechanism within the crystal lattice, producing metal or metal-ion compounds. Besides, there are some less common storage mechanisms, which require comprehensive analysis based on the material’s structural characteristics and characterization results. Research in this direction is still in its infancy, and new mechanisms are being proposed. With the continuous improvement of characterization means and research methods, this research can be continuously strengthened, and more contents of the storage mechanism of AZIBs can be mastered.

4.1 Manganese-based compounds

The element manganese (Mn) is abundant in the earth’s crust, and manganese-based materials have been of interest in the development of AZIBs [96]. This material is abundant and readily available, relatively low cost, and environmentally friendly, and therefore has a wide range of applications for large-scale energy storage applications. In addition, manganese has a high voltage plateau (1.2–1.4 V) exhibiting a high energy density and exists variety of valence states including Mn²⁺, Mn³⁺, Mn⁴⁺, and Mn⁷⁺, so manganese-based materials can accommodate the embedding of ions. A variety of manganese-based materials, including MnO₂ of various crystalline forms (α-, β-, γ-, ε-, δ-, γ-, todorokite), other manganese oxides (MnO, MnO₂, Mn₂O₃, Mn₃O₄, etc.), and spinel-phase ZnMn₂O₄, MgMn₂O₄, have been reported to be used in the cathode of AZIBs, but the most widely used is MnO₂ of various crystalline forms [97]. They have a different crystal space structure. The basic building block consists of one Mn atom and six O atoms forming an octahedral MnO₆ unit. Different MnO₂ crystal structures can be formed by connecting these units through the sharing of octahedral edges and corners.

Despite the widespread use of manganese-based materials, there are still some problems in the charging and discharging processes. First, MnO₂ with different crystal types is prone to structural transformation, damaging the crystal structure and then leading to capacity attenuation during the battery cycle [102,103]. Furthermore, issues such as the dissolution of Mn²⁺ during the reaction process and the generation of irreversible by-products are significant factors limiting the development of manganese-based compounds as cathode materials for AZIBs [104,105].

As an effective strategy to improve the electrochemical performance of AZIBs, defect engineering has been widely used in recent years [106]. Defect engineering usually includes two categories: anionic defect and cationic defect. In general, defect engineering can speed up ion movement and enhance electron transfer kinetics by adjusting the structure.

In 2020, Wang et al. prepared highly defective MnO₂ (D-MnO₂) ultrafine nanowires with an ultra-thin structure (approximately 10 nm in diameter) using a solvation method. These nanowires exhibited abundant oxygen vacancies and lattice holes (Figure 6a and b) and demonstrated a high energy density of 406 W h kg⁻¹ [98]. Through the theoretical calculation of adsorption energy and charge density difference distribution (Figure 6c–e), it is proved that the ion/electron transfer rate can be increased by decreasing adsorption energy, which makes D-MnO₂ cathode show better performance than MnO₂ cathode without defect. Among the many crystalline forms of manganese dioxide, the crystal structure of ε-MnO₂ is a hexagonal soft manganese ore with a dense stacking structure. However, its structure is unstable and can change when the temperature exceeds 300°C. Zhang et al. used a defect engineering approach to prepare nitrogen-doped ε-MnO₂ (MnO₂@N) to improve the electrochemical performance of manganese-based cathodes [99]. The electrochemical performance of the N-doped MnO₂ cathode is improved by the synergistic effect of increased electrical conductivity and structural stability. Figure 6f shows the Raman spectra of the MnO₂ and MnO₂@N electrodes. It can be seen that the peak position of the Mn–O bond at 665 cm⁻¹ shifts toward the lower wave number after nitrogen doping, demonstrating the creation of oxygen vacancies and therefore increasing the diffusion coefficient of the ions, resulting in accelerated charge transfer and increased conductivity. Moreover, the nitrogen doping creates an Mn–N bond, which also enhances the electrochemical stability of the cathode. After 500 cycles at 0.5 A g⁻¹, the capacity retention was close to 100%, compared to 62% for the MnO₂ cathode (Figure 6g). This work provides a new idea for progressing the stability of manganese-based AZIBs.

Figure 6

(a and b) Adsorption structures of Zn in perfect MnO₂ and D-MnO₂; Dashed green circle in (b) represents the O vacancy in D-MnO₂. (c) Adsorption energies of Zn and H in perfect MnO₂ and D-MnO₂. (d–e) Charge density difference distribution diagrams for Zn in MnO₂ and D-MnO₂. Yellow area: charge accumulation; cyan area: charge depletion [98]. Copyright 2020 American Chemical Society. (f) Comparison of Raman corresponding to MnO₂ and MnO₂@N. (g) Comparison of specific capacity of MnO₂//Zn and MnO₂@N//Zn cells at 0.5 A g⁻¹ [99]. Copyright 2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. (h) E-pH diagram of the MnO₂ in different Mn²⁺ concentration. (i) Schematic showing the MnO₂/CNT foam cathode that features a reversible chemical conversion and a 3D hierarchical structure for mass and charge transport [100]. Copyright 2021 Wiley-VCH GmbH. (j) Comparison of manganese dissolution of MnO@C with and without (Zn(OH)₂)₃(ZnSO₄)(H₂O)₅ in the ZnSO₄ electrolyte [101]. Copyright 2021 Elsevier B.V.

Researchers are continually making improvements to retard the dissolution of manganese-based materials, thus allowing them to demonstrate excellent cycling performance. Jaekook Kim’s team demonstrated that the Mn²⁺ additive can be used to prevent the dissolution of manganese, improve the stability of the ZnMn₂O₄ cathode, and enhance its electrochemical performance [107]. Figure 6i shows the current strategy used to construct a new Zn–MnO₂ battery chemistry, namely, a combination of two redox reactions, MnO₂/Mn²⁺ (two-electron process) and MnO₂/Mn³⁺ (single-electron process).

According to thermodynamic calculation, the PH value at three points (MnO₂–MnOH–Mn²⁺) increases with the decrease of Mn²⁺ concentration (Figure 6h). Therefore, reversible MnO₂/Mn²⁺ redox is likely to occur when the concentration of Mn²⁺ in mild electrolytes is low. By adjusting Mn²⁺ concentration within the appropriate range, Shen et al. can realize the double electron redox reaction without oxygen evolution reaction and recycle the by-products produced by MnOOH disproportionation [100]. Thus, high rate capacity and long cycle life are obtained, namely, 430 mA h g⁻¹ at 19.5 A g⁻¹ and at 16,000 cycles without significant capacity attenuation. In addition, the three-dimensional (3D) carbon nanotube foam skeleton was used as a conductive scaffold, so that the material could adapt to the volume change during the deposition/dissolution of MnO₂ and provide a charge transport pathway. This provides the direction for further development of AZIBs.

In 2022, in situ electrochemically induced artificial solid electrolyte mesophase (Zn(OH)₂)₃(ZnSO₄)·(H₂O)₅ on MnO@C cathode material was reported in AZIBs [101]. No additional Mn²⁺ was added to the electrolyte, but the solution of Mn²⁺ was inhibited by the homogeneous coating of (Zn(OH)₂)₃(ZnSO₄)·(H₂O)₅ on MnO@C. The results of ICP emission spectrometry (Figure 6j) show that coating (Zn(OH)₂)₃(ZnSO₄)·(H₂O)₅ on MnO@C can effectively inhibit the dissolution of Mn²⁺ in aqueous electrolyte, thus improving the cycling stability of MnO@C in AZIBS. Capacity attenuation of each cycle is 0.003% in 10,000 cycles. This in situ electrochemically induced artificial solid electrolyte interface is a new way to inhibit the dissolution of electrode materials.

Manganese-based materials have shown great promise in AZIBs due to their advantages of high voltage platform and high energy density. However, the development of these materials is still constrained by issues such as electrode material dissolution and low ionic conductivity. Effective approaches to address the current challenges in manganese-based material development include defect engineering, pre-doping of ions, and synthesis of composite materials. Despite these efforts, further endeavors are required to fully realize the application of manganese-based materials in AZIBs.

4.2 Vanadium-based compounds

Vanadium-based material is an excellent cathode material for AZIBs. It is characterized by abundant resource reserves, diverse structures, and various valence states (V²⁺, V³⁺, V⁴⁺, and V⁵⁺). Since vanadium-based material in polyvalent states can achieve multistep redox, it is endowed with a high theoretical specific capacity (>300 mA h g⁻¹) [108]. Vanadium-based materials are usually composed of V–O polyhedrons interlinked with each other. Due to its easy deformation, a variety of vanadium oxides with different compositions and structural frames can be formed, including VO₂ [109], V₂O₃ [110,111], V₂O₅ [112,113], and V₆O₁₃ [114,115]. Most of these vanadium oxides are layered or tunnel structures, which are conducive to the embedding and exfoliation of Zn²⁺, but a large amount of Zn²⁺ embedding will cause great structural changes and poor stability [116]. To improve the stability of the crystal structure, some metal ions (such as Na⁺, K⁺, and Ca²⁺) or H₂O molecules are usually introduced as pillars to enhance the structural stability [117,118,119]. Qi et al. achieved the insertion of Cs⁺ into V₂O₅·nH₂O using the hydrothermal method, resulting in the formation of strong Cs–O bonds between the layers. This significantly enhanced the stability of the cathode material and achieved a long cycle (10,000 cycles at 20 A g⁻¹ current density, with a capacity retention rate of 89%) [120]. Guan et al. prepared Mg_0.2V₂O₅·nH₂O nanoribbons derived from the conducting compound V₄C₃ MXene [121]. The intercalated Mg²⁺ and structural water can not only enhance the conductivity of the material and accelerate the ion diffusion but also improve the structural stability, so that the battery can obtain good rate performance and long cycle stability. In 2021, Wan’s team proposed a general compensation strategy that uses polar organic molecules to displace some of the crystalline water in Al _x V₂O₅·nH₂O [122]. The polar groups in the organic molecule can generate electrostatic attraction with the pre-intercalated Al³⁺, ensuring its anchoring in the sandwich of the hydrated vanadate (Figure 7b). However, the low polar groups in organic molecules can produce weak interaction with Zn²⁺ during the cyclic process to ensure that Zn²⁺ can be (de)intercalated (Figure 7a). This work makes the cathode electrode material obtain stronger structural stability.

Figure 7

(a) Schematic illustration of N,N-dimethylformamide (DMF) intercalation enhancing Zn²⁺ transfer kinetics and structural stability. (b) Calculated molecular electrostatic potential distribution of H₂O and DMF molecules [122]. Copyright 2021 Wiley-VCH GmbH. (c) Illustration of V dissolution in the aqueous electrolyte of V₂O₅ nanoplates and V₂O₅ nanoplates/MXene hybrids cathodes. (d) Rate capacities of VPMX73/VPMX82/V₂O₅ nanoplates (0.1–5 A g⁻¹) [123]. Copyright 2022 American Chemical Society. (e) Rate properties of NVO₂ Electrode (0.1–10 A g⁻¹) [124]. Copyright 2022 Elsevier B.V. (f) Cyclic voltammetry (CV) curves at different scanning rates. (g) Log(i) and log(v) plots at specific peak currents. (h) The contribution ratio of the capacitive and diffusion-limited capacities at different scan rates. (i) Galvanostatic intermittent titration technique (GITT) curves at 0.2 C and the calculated diffusion coefficient of N3VPF and N3VPF@5% rGO [125]. Copyright 2023 Wiley-VCH GmbH.

Despite the promising applications of vanadium-based oxides, the dissolution of vanadium is still one of the main challenges in achieving its stable performance in AZIBs [126,127]. Liu et al. used a van der Waals self-assembly method to coat a layer of Ti₃C₂T _x MXene layer on the surface of V₂O₅ nanoplates (VPMX) to inhibit vanadium dissolution, which helped maintain cathode integrity and therefore improve the storage performance of Zn²⁺ (Figure 7c) [123]. The MXene layer was shown to promote rapid electron transfer at the solid–solid heterogeneous interface between V₂O₅ and MXene. As shown by in situ XRD results, the MXene layer allows reversible co-intercalation of water molecules with Zn²⁺, thus reducing electrostatic repulsion, promoting electrochemical kinetics, and enhancing the rate capability of the cell to 243.6 mA h g⁻¹ at 5.0 A g⁻¹ (Figure 7d). This composite cathode exhibits long-term cycling stability in excess of 5,000 cycles. The coating method proposed in this work to inhibit vanadium dissolution presents an effective strategy for enhancing the electrochemical performance of vanadium-based aqueous batteries.

In addition to vanadium oxides, some layered vanadates, polyanion framework (NASICON) structure [88,128,129], and vanadium-based compounds have been shown to have excellent Zn²⁺ storage capacity. Amongst these, ammonium vanadate (NH₄V₄O₁₀) is a potential cathode material for AZIBs due to its stable bilayer structure and high theoretical capacity. As in the case of vanadium oxides, the introduction of H₂O molecules or some metal ions (K⁺, Na⁺, Al³⁺, etc.) can expand the layer spacing and improve the stability of the crystal structure. The NH₄ ⁺ between the layers can also increase the layer spacing and mitigate the lattice changes during charging and discharging. However, when the NH₄ ⁺ content is high, it limits the diffusion efficiency of Zn²⁺. Therefore, Chen et al. used an acid treatment to remove some of the ammonium ions to obtain NH₄ ⁺-deficient ammonium vanadate (NVO₂) [124]. This measure could increase the interlayer distance without affecting the diffusion efficiency of Zn²⁺. The experimental results show that the NVO₂ electrode has a specific capacity of up to 472.5 mA h g⁻¹ at 0.1 A g⁻¹ and up to 195.5 mA h g⁻¹ when the current density is increased by a factor of 100, with a capacity retention rate of 41.4% (Figure 7e). Polyanionic materials are also important Zn²⁺ storage materials because of their high ion transfer kinetics due to their stable ion framework and large ion channels. In 2023, Guan et al. prepared Na₃V₂(PO₄)₂F₃ (N3VPF) nanospheres, which were then coated with reduced graphene oxide (rGO) to obtain a 3D composite N3VPF@rGO [125]. The addition of rGO not only stabilizes the structure of the composite material but also enhances electron conductivity and accelerates the ion transfer rate, thereby significantly optimizing the capacity. Figure 7f shows CV curves at different sweep speeds. The migration dynamics of Zn²⁺ can be quantitatively analyzed by the following formula:

When b is close to 0.5, it is a diffusion control process, while when b is close to 1, it is a surface-controlled pseudocapacitive behavior. As shown in Figure 7g, the values of peaks 1–4 are all around 1, so the migration behavior of Zn²⁺ is mainly controlled by the pseudocapacitive behavior. The contribution of diffusion and surface capacitance can be calculated quantitatively by the following equation:

When the scan rate increases from 0.1 to 0.4 mV s⁻¹, the capacitance contribution increases from 56.9 to 88.3% (Figure 7h). The increasing contribution of pseudocapacitive behavior indicates the faster reaction kinetics of N3VPF@rGO in AZIBs. In addition, the GITT method can be used to calculate the ionic diffusion coefficient in the reaction (Figure 7i), which can be computed by the following formula:

The results show that the Zn²⁺ of N3VPF@rGO is about 10⁻¹¹ to 10⁻¹⁰ cm² s⁻¹, indicating its rapid reaction kinetics. It can be seen from the aforementioned characterization and calculation that the excellent pseudocapacitive behavior and the fast chemical reaction kinetics indicate that N3VPF@rGO has excellent electrochemical performance.

Vanadium-based cathode materials exhibit high reversible capacity and excellent rate performance due to their multiple valence states and diverse structures. To fully exploit the advantages of vanadium-based materials, various approaches have been employed to address issues such as structural instability, poor conductivity, and structural dissolution. These approaches include ion intercalation, molecular intercalation, and material encapsulation. With the continuous improvement of characterization methods and research techniques, the development of vanadium-based cathode materials can be undoubtedly accelerated further.

4.3 PBA

In addition to the common manganese and vanadium-based compounds, PBAs have been developed for use in the cathode of AZIBs [130,131]. Prussian blue and PBAs can be collectively referred to as metal-iron cyanides metal hexacyanoferrates. By replacing Fe²⁺ and Fe³⁺ in the Prussian blue material with other transition metal ions, PBAs can be obtained. Most PBAs have the Fm3m space group and face-centered cubic structure, usually denoted as A _x M₁[M₂(CN)₆]_1−y ·nH₂O, where A is an alkali metal element such as Na and K, and M₁ and M₂ are transition metal elements including Mn, Cu, Ni, Co, and Zn. Because of its stable 3D open skeleton structure and large lattice gap, it can provide more diffusion paths, enabling rapid deintercalation of Zn²⁺ [132]. In 2019, Ma et al. used cobalt hexacyanoferrate (CoFe(CN)₆) as the cathode of AZIBs, in which there are two pairs of Co^2+/3+ and Fe^2+/3+ redox reactions, so the removal of Zn²⁺ requires two steps [68]. The presence of these two active couples provides AZIBs with A high voltage platform (1.75 V) and a high discharge capacity (109.5 mA h g⁻¹ at 6 A g⁻¹). This variety of redox reactions can increase battery capacity, so it can be extended to other battery systems.

Due to the high working voltage platform (∼1.5–1.8 V) of PBAs, this kind of material has high ionic conductivity and rapid charge and discharge ability [133]. Despite the relatively low specific capacity of this material (<200 mA h g⁻¹), its low cost, environmental friendliness, and simple synthesis still offer a relatively good prospect for AZIB cathode materials.

However, due to the lack of redox sites and structural damage during ion dissociation, their capacity is limited and their cyclic stability is poor [134]. Of these, the rapid capacity decay of Mn-PBAs is mainly due to the dissolution of metal ions and the Jahn-Teller distortion of the Mn-N₆ octahedron during repeated cycling. Zeng et al. after etching with tannic acid, prepared Cu-substituted Mn-PBA double-shelled nanoboxes (CuMn-PBA DSNBs) by the cation exchange method (Figure 8a) [89]. The high specific surface area of 227.6 m² g⁻¹ of CuMn-PBA DSNBs was much higher than the 7.7 m² g⁻¹ of the original Mn-PBA NCS (Figure 8c). The abundance of mesopores may have been created during the tannic acid etching process. The Jahn-Teller distortion of the Mn-N₆ octahedron is mitigated by the substitution of Cu and the creation of vacancies in Mn during the preparation process, so the stability of this cathode material is improved and the cycle life of the battery is enhanced. Tannic acid etching allows this unique hollow structure to be obtained, exposing its active site. The large volume changes generated during ion (de)intercalation can be therefore mitigated, thereby increasing the volume capacity and cycling stability. In addition, partial Cu substitution and the induction of Mn vacancies can suppress the Jahn-Teller distortion of the Mn-N₆ octahedron, thereby extending its lifetime. The results show that the CuMn-PBA DSNB electrode has good electrochemical properties and exhibits a high specific capacity, with a discharge capacity of 116.8 mA h g⁻¹ at 0.1 A g⁻¹ (Figure 8b). In addition, its excellent multiplier performance and good cycling properties surpass those of the Mn-PBA NC and Mn-PBA DSNB electrodes.

Figure 8

(a) Field emission scanning electron microscopy (FESEM) images of Mn-PBA NCs. (b) Charge–discharge voltage profiles of the CuMn-PBA DSNB electrode at various current densities. (c) N₂ adsorption–desorption isotherms and corresponding pore size distribution of CuMn-PBA DSNBs [89]. Copyright 2022 WILEY-VCH. (d) FESEM image of as-prepared ZMO. (e) XRD pattern of ZMO, Ti₃C₂T _x , and ZMO@Ti₃C₂T _x composite [135]. Copyright 2020 Elsevier B.V. (f) SEM images of VSe₂@V₂CT _x . (g) Schematic illustration of the surface selenization of V₂CT _x for the preparation of VSe₂@V₂CT _x nanohybrids. (h) XRD patterns of V₂AlC, V₂CT _x , neat VSe₂, and VSe₂@V₂CT _x . (i) Raman spectrum of VSe₂@V₂CT _x [136]. Copyright 2022 American Chemical Society.

PBA materials possess a high voltage platform and their internal 3D diffusion channels can accelerate ion transport, endowing them with rapid charging and discharging capabilities. However, as cathode materials for AZIBs, PBAs undergo significant volume changes during the charge–discharge process, thereby impacting their cycling stability. Moreover, their relatively poor conductivity also constitutes a major hindrance to their development. Therefore, it is imperative to enhance both their conductivity and effectively mitigate volume fluctuations. Both implementing rational strategies to adjust the structure of PBAs and fabricating high-conductivity composite materials are worthwhile measures that merit attention.

4.4 MXene and its composites

MXene is a two-dimensional (2D) transition metal carbide/nitride [137,138]. The shape is similar to potato chips stacked on top of each other, where “MX” refers to the MAX phase from which the A-layer elements have been etched away and “exe” refers to the graphene-like nanolayer structure [139]. The MAX phase is a precursor of MXene in a layered hexagonal structure, denoted as M _n+1AX _n (MAX), where N is 1–3, M is a transition metal (e.g., Mo, Ti, Zr), A is a group IIIA or IVA element such as Al, Ge, and Si in the periodic table, and X is C, N, or a mixture of them [140]. The laminated hexagonal MAX phase consists of an almost dense stacking of M layers, with the octahedral positions occupied by X atoms, and the M and X layers bonded together by A atoms. MXene is obtained by selective etching of “A” metal elements in the MAX phase. As a 2D material with three or more atomic layers, MXene has different properties compared to its 3D precursors. The general formula of MXene is M _n+1X _n or M _n+1X _n T _n . M and X have the same meaning as represented in the MAX phase, while Tx are surface terminations such as –O, –OH, –F, and –Cl. The wide variety of surface terminations increases the number of possible compositions of MXene and allows adjustments to be made to the physical and chemical properties of MXene. The n + 1 layer M is interleaved with n layers of X when etching A elements. The creation of MXene further extends the family of 2D inorganic materials to include Ti₃C₂T _x , Ti₂CT _x , and Ta₄C₃T _x .

The diverse composition of chemical elements and unique layered structure allows MXene to enjoy extraordinary compositional diversity and tunable properties, thus giving it excellent properties such as high electronic conductivity, unique morphology, rich chemical properties, and excellent mechanical properties, which makes it extremely important for various applications such as energy storage [141,142], chemical sensors [143,144], and supercapacitor.

However, because of its low voltage and capacity, MXene is not used alone. Instead, it is applied to AZIBs with other active materials [92,145,146]. MXene is easy to form composites with other materials due to its good flexibility, as well as its 2D form and layered structure. In addition to its high conductivity and excellent electrochemical activity, MXene and MXene-based composites have been greatly applied in the field of energy and have become high-performance electrode materials for lithium–sulfur batteries, ZIBs, and supercapacitors [147].

Shi et al. designed and prepared a new composite material of zinc manganate (ZnMn₂O₄ and ZMO) and Ti₃C₂T _x MXene. Figure 8d shows the FESEM image of the synthesized ZMO. The XRD pattern verifies the effective combination of ZMO and Ti₃C₂T _x in the composite (Figure 8e). Ti₃C₂T _x is highly conductive, and its use as a scaffold can alleviate the irreversible structural changes of zinc manganate during the chemical reaction as well as reduce the generation of side reactions [135]. The prepared product ZMO@Ti₃C₂T _x was applied to AZIBs and demonstrated electrochemical properties superior to those of MXene itself. The product exhibits a large reversible specific capacity of 172.6 mA h g⁻¹, a Coulombic efficiency of approximately 100%, and a capacity retention rate of up to 92.4% after 5,000 cycles. It has great potential for wearable electronics due to its ability to show high electrochemical stability in different states of deformation.

Sha et al. adopted a simple one-step surface selenization strategy to construct transition metal selenides (TMSes) on MXene, obtaining VSe₂@ V₂CT _x (Figure 8g) [136]. Figure 8f shows the scanning electron microscope (SEM) image of VSe₂@V₂CT _x . It can be seen that the surface of the material has an obvious layered structure. XRD and Raman of Figure 8h and i show that the main chain of V₂CT _x is retained, while VSe₂ is selenized on the surface of V₂CT _x . This approach integrates the advantages of MXenes and TMSes, so that the hybrid material obtains high capacity and good structural stability.

Traditional MXene preparation is generally obtained by etching HF. However, HF is highly toxic and corrosive, which means this preparation method is relatively dangerous and not environmentally friendly. So the researchers tried to avoid using fluoride in the etching process and went for a safer approach. In 2020, Li et al. obtained MXene by in situ etching MAX with a special fluorine-rich electrolyte inside the cell and used it directly in situ [148]. According to the characterization by SEM, TEM, XRD, and XPS (Figure 9a–e, g), the whole process includes two stages: first, V₂CT _x MXene is etched by MAX V₂AlC, and then V₂CT _x is oxidized to V₂O₅ V₂CT _x MXene has better performance than V₂AlC, while V₂O₅ has a higher capacity than V₂CT _x MXene. So as the reaction goes on, the battery’s performance improves (Figure 9f). All processes are implemented inside the battery, avoiding external environmental pollution, and the battery always works as a rechargeable battery with an increasing capacity. Combining the 2D layered structure and high electrical conductivity characteristics of MXene with other materials enables the composite to fully exploit the advantages of both components, continuously enhancing the electrochemical performance of AZIB cathode materials. As research progresses, we firmly believe that MXene will undoubtedly continue to leverage its strengths and demonstrate significant application prospects in the field of rechargeable batteries.

Figure 9

(a) SEM image of V₂AlC powders. (b) SEM images of the cathode after 400 cycles in the coin-type battery at 10 A g⁻¹. (c) TEM image of V₂O₅/V₂CT _x . (d) XRD patterns of the cathode after 5, 150, and 400 cycle. (e) XRD patterns of the cathode after 600 and 800 cycles at 5 A g⁻¹. (f) CV measurement of the battery from 0.1 to 2.1 V at 5 mV s⁻¹. (g) Original and fitted V 2p XPS spectra of the cathode after 5, 200, and 600 cycles at 5 A g⁻¹ [148]. Copyright 2020 Wiley-VCH GmbH.

4.5 Organic materials

4.5.2 Organic material containing a carbonyl group

The quinone compound containing the carbonyl group can realize the rapid movement of ions by the van der Waals force connection between molecules and has good stability in water. The storage mechanism for Zn²⁺ in quinone-based compounds is primarily an ionic coordination mechanism, whereby a cathode charged cation is coordinated to a negatively charged oxygen atom with concomitant reduction of the carbonyl group.

Sun et al. used a molecular structure design approach to synthesize novel small thioquinones (4S6Q) and (4S4Q) (Figure 10a) [151]. The introduction of a conjugated thioether (–S–) bond, which both enhances the electrical conductivity of the compounds and inhibits the solubility of the compounds by extending the π-conjugation plane and building a flexible molecular backbone, allows this small thioquinone to be less soluble and exhibit good electrochemical stability. Used as the cathode for AZIBs, the Zn//4S6Q and Zn//4S4Q cells exhibited good electrochemical performance in a 3.5 M Zn(ClO₄)₂ electrolyte. For example, Zn//4S6Q batteries have a discharge capacity of 240 mA h g⁻¹ at 150 mA g⁻¹, without capacity decay at 3 A g⁻¹ and up to 20,000 long cycles. Furthermore, thanks to the antifreeze electrolyte, the batteries can maintain a high discharge capacity of 201.7 mA h g⁻¹ at −60°C, which is equivalent to 86.2% of the discharge capacity at 25°C. Experiments show that the excellent electrochemical properties generated by these novel compounds make them promising alternative electrode materials for Zn–organic batteries.

Figure 10

(a) The synthesis processes of 4S4Q and 4S6Q compounds [151]. Copyright 2022 Springer. (b) SEM images of α-Mn₂O₃ cathodes at voltage of ⅰ–ⅷ points in Figure (ⅸ) and element concentration of A–H [152]. Copyright 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. (c) Structure of Cu₃(HHTP)₂, which when viewed down the c-axis, exhibits slipped-parallel stacking of 2D sheets with a honeycomb lattice. The cyan, red, and gray spheres represent Cu, O, and C atoms, respectively. The H atoms are omitted for the sake of clarity. (d) Expected redox process in the coordination unit of Cu₃(HHTP)₂. (e) Discharge–charge voltage profiles of Cu₃(HHTP)₂ at 50 mA g⁻¹ [153]. Copyright 2019 Nature. (f) Quantitative capacity contribution calculated from ICP-AES discharged to different voltages in the first three cycles [95]. Copyright 2021 Wiley-VCH GmbH.