Advances in manganese-based cathode electrodes for aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) are emerging as a promising option for next-generation energy storage due to their abundant resources, affordability, eco-friendliness, and high safety levels. Manganese-based cathode materials, in particular, have garnered significant attention because of their high theoretical capacity and cost-effectiveness. However, they still face substantial challenges related to rate performance and cycling stability. To address these issues, researchers have developed various strategies. This review focuses on the key advancements in manganese-based cathode materials for AZIBs in recent years. It begins with a detailed analysis of the energy storage mechanisms in manganese-based cathodes. Next, it introduces a variety of manganese-based oxides, highlighting their distinct crystal structures and morphologies. It also outlines optimization strategies, such as ion doping (both monovalent ions and multivalent ions), the preparation of Mn-based metal-organic frameworks (MOFs), carbon materials coatings, and electrolyte optimization. These strategies have significantly improved the electrochemical performance of manganese-based oxide cathodes. By systematically analyzing these advancements, it aims to provide guidance for the development of high-performance manganese-based cathodes. Finally, it discusses prospective research directions for manganese-based cathodes in AZIBs.

Graphical abstract

Keywords

AZIBs / manganese-based cathode materials / manganese oxide / ion doping / carbon coating / electrolyte optimization

1 Introduction

As the global demand for renewable energy and electric vehicles rises, traditional lithium-ion batteries, despite their dominance in electric vehicles and large-scale energy storage applications [1‒5], are facing challenges related to limited resources, high costs, and safety risks associated with organic electrolytes [6,7]. These challenges have spurred extensive research into alternative battery technologies. Various types of batteries, including those based on monovalent ions like Na⁺ and K⁺ as well as multivalent ions such as Mg²⁺, Zn²⁺, and Co²⁺, have rapidly emerged as viable alternatives to lithium-ion batteries [8]. Among these, sodium-ion and potassium-ion batteries have been widely studied, but their reliance on flammable and toxic organic electrolytes raises safety concerns. In contrast, aqueous zinc-ion batteries (AZIBs), which use non-flammable aqueous electrolytes and leverage zinc’s abundance, affordability, eco-friendliness, and safety, have become a promising option for next generation energy storage systems [9,10].

A variety of positive electrode materials have been investigated for AZIBs, including manganese-based materials [11,12], vanadium-based materials [13–15], Prussian blue analogs [16,17], nickel-based materials [18], and organic compounds [19,20]. Vanadium-based materials offer high theoretical capacities and good electrochemical stability; however, their relatively high cost and environmental concerns limit their widespread use [20]. Prussian blue analogs are attractive due to their unique open framework structure and excellent ion exchange capabilities, but they can suffer from structural instability under high-rate conditions [21]. Nickel-based materials offer high capacitance and good electrochemical stability, yet their complex synthesis methods hinder large-scale applications [22]. Organic cathode materials are customizable and have high theoretical capacities, but their low conductivity and poor cycling stability present significant challenges [23].

In comparison, manganese oxides offer several unique advantages for AZIBs. Manganese is abundant, inexpensive, and environmentally benign, making manganese-based oxides both cost-effective and eco-friendly [24,25]. Moreover, the versatile crystal structures and multiple valence states of manganese enable high theoretical capacities and strong structural stability during electrochemical cycling. These characteristics make manganese oxides highly promising as cathode materials for AZIBs. However, some inherent drawbacks remain. The relatively low electrical conductivity of manganese oxides can hinder high-rate performance, and the release of manganese in the aqueous electrolyte may result in capacity degradation and reduced cycle life. Although various strategies such as electrolyte optimization, ion doping and the incorporation of carbon-based materials have shown significant progress in mitigating these issues, further research is required to fully overcome these limitations [26,27]. Striking a balance between these advantages and challenges will be crucial for the future development and commercialization of manganese-oxide-based AZIBs.

Significant advancements have been made in manganese-based cathode materials for AZIBs [28–31] in recent years. By rationally designing material structures, optimizing electrode fabrication processes, and modifying electrode interfaces, the electrochemical performance of manganese cathodes has been significantly improved, resulting in higher energy densities, longer cycling lives, and improved safety. Various manganese compounds, such as manganese oxides and phosphates, have been extensively investigated [32,33]. Among these, manganese oxides, with their multiple oxidation states, high capacity and excellent structural stability, have attracted the most attention as cathode materials for AZIBs [34].

Several strategies have been employed to further enhance the performance of manganese-based cathodes, including electrolyte optimization [35,36], morphology refinement [37], ion doping [38,39], and carbon material compositing [40]. Comparable approaches have been effectively utilized in different energy storage systems, such as the rational design of FeF₂-based cathodes for potassium storage, which demonstrated enhanced stability and high performance [41]. Researchers are also exploring novel manganese-based materials, such as composites and nanostructured materials, to further improve battery performance [42]. Despite these advancements, challenges remain in improving the cycling stability, conductivity, and electrochemical activity of manganese-based cathodes. Therefore, in-depth studies of the electrochemical properties and reaction mechanisms of these materials, as well as the exploration of new material designs and preparation strategies, are essential for advancing the commercialization of AZIBs.

This review, as illustrated in , systematically summarizes the latest advancements in manganese-based cathode materials for AZIBs. First, it explores the intricate energy storage mechanisms in AZIBs, which include the Zn²⁺ deintercalation mechanism, the co-deintercalation mechanism of H⁺ and Zn²⁺, the chemical conversion reaction mechanism, and the dissolution-deposition reaction mechanism. Next, it discusses the electrochemical properties of various manganese oxides, such as α-MnO₂, β-MnO₂ and γ-MnO₂ emphasizing their roles in enhancing battery performance. It then examines the effects of ion doping, including both monovalent ions (such as K⁺, Na⁺, and NH₄⁺) and multivalent ions (such as Co²⁺, Zn²⁺, and Mg²⁺). These doping strategies effectively improve the electrical conductivity of manganese oxides and accelerate Zn²⁺ ion diffusion within the electrode. Moreover, it explores the application of metal-organic frameworks (MOFs) in manganese compounds. MOFs have a high specific surface area and porous nature, providing ample active sites that significantly enhance the electrochemical performance of manganese-based materials. By altering the metal centers and organic ligands within MOFs, the characteristics of electrode materials can be precisely controlled.

Additionally, it discusses the integration of carbon materials with manganese-based oxides, a strategy that minimizes Mn²⁺ dissolution, enhances cycling stability, and improves material conductivity. The addition of carbon not only increases the electrode’s overall conductivity but also reduces undesirable side reactions at the electrode-electrolyte interface. Finally, it explores the optimization of electrolytes, which effectively stabilizes the Zn anode, reduces Mn²⁺ dissolution, and minimizes side reactions during battery cycling. Adjusting the electrolyte composition promotes the formation of a stable solid electrolyte interphase (SEI) layer on the zinc surface, enhancing the stability of the electrode-electrolyte interface. This adjustment also accelerates Zn²⁺ ion transport, leading to improved cycling stability and overall battery performance. Through these optimization strategies, significant progress has been made in enhancing the properties of manganese-based cathode materials in AZIBs. It aims to provide guidance for the development and optimization of advanced manganese-based electrodes for future AZIBs applications.

2 Storage mechanisms

The energy storage processes in AZIBs are complex and differ significantly from those in other battery systems, often sparking ongoing discussions within the research community. Four primary reaction mechanisms have been proposed to explain the underlying mechanisms: the deintercalation of Zn²⁺, the co-deintercalation of H⁺ and Zn²⁺, the chemical conversion reaction, and the dissolution-deposition reaction (Fig.1(a)‒Fig.1(d)) [43,44].

2.1 Zn²⁺ deintercalation mechanism

The Zn²⁺ deintercalation mechanism involves the reversible embedding (during discharge) and extraction (during charge) of Zn²⁺ from the crystalline structure of manganese-based cathode materials during electrochemical processes. This mechanism primarily occurs in manganese-based materials with layered or tunnel structures, which offer ample space and pathways for Zn²⁺ to intercalate into or deintercalated from the lattice. During the intercalation process, Zn²⁺ ions migrate through the electrolyte and reach active sites within the cathode material to embed within the crystal structure. Conversely, during charging, the embedded Zn²⁺ ions are extracted from the crystal structure and return to the electrolyte, driven by the electrical potential. This highly reversible mechanism plays a key role in ensuring excellent capacity and reliable cyclic performance in AZIBs. Khamsanga et al. [45] outlined the Zn²⁺ deintercalation mechanism within δ-MnO₂. During operation, zinc at the anode dissolves into the water-based electrolyte with zinc ions, rapidly forming a hydrated state. These ions then migrate across the separator to the δ-MnO₂ cathode material, with the reaction process being fully reversible (Fig.1(e)).

2.2 Co-deintercalation mechanism of H⁺ and Zn²⁺

Due to the weakly acidic nature of the electrolyte in AZIBs, dissociated H⁺ ions are present and actively participate in the reaction process. With a small ionic radius of approximately 0.23 Å, H⁺ ions play a significant role during the reaction phase. Pan et al. [46] validated the co-deintercalation mechanism of Zn²⁺ and H⁺ in a new phase of manganese oxide, MnO₂H_0.16(H₂O)_0.27. Additionally, Wang et al. [47] utilized the galvanostatic intermittent titration technique (GITT) to propose the co-deintercalation mechanism of H⁺ and Zn²⁺ ions, observing enhanced kinetics during the initial discharge plateau (Fig.1(f)). The researchers concluded that the first discharge plateau was attributed to H⁺ intercalation, due to its smaller ionic radius, while the subsequent plateau was associated with Zn²⁺ intercalation. Further confirmation of this co-deintercalation mechanism was provided by X-ray diffraction (XRD) analysis (Fig.1(g)), which revealed the presence of MOOH peaks following H⁺ intercalation and ZnMn₂O₄ peaks after Zn²⁺ intercalation. These findings collectively support the involvement of both H⁺ and Zn²⁺ ions in the energy storage process of AZIBs.

2.3 Chemical conversion reaction mechanism

The chemical conversion reaction mechanism primarily involves the reversible electrochemical behavior between MnO₂, MnOOH, and basic zinc sulfate (ZHS). Liu et al. [48] confirmed this mechanism. At the beginning of the discharge phase, β-MnO₂ reacts with H⁺ to form MnOOH, which subsequently converts to Mn²⁺. During the first charging cycle, MnOOH and Mn²⁺ react and are deposited as ε-MnO₂. Simultaneously, β-MnO₂ reacts with Zn²⁺, SO₄²⁻, and water molecules in the electrolyte during discharge to form ZHS, which disappears during the charging process (Fig.1(h)).

2.4 Dissolution and deposition reaction mechanism

The dissolution–deposition reaction mechanism offers a different perspective by focusing on the reversible dissolution and redeposition of cathode active materials during the charging and discharging cycles. Guo et al. [49] investigated this mechanism for both α-MnO₂ and δ-MnO₂. During the initial discharge, α-MnO₂ and δ-MnO₂ react with H₂O from the electrolyte, producing Mn²⁺ and OH⁻ ions. These ions then interact with ZnSO₄ and H₂O to generate ZHS. During the first charging cycle, ZHS reacts with Mn²⁺ ions to form birnessite-MnO₂. In subsequent charge and discharge cycles, birnessite-MnO₂ replaces the original MnO₂ (Fig.1(i)).

3 Manganese-based oxides

Manganese (Mn) is a versatile transition metal with multiple valence states (+2, +3 and +4), leading to a variety of stable compounds (Fig.2) [33]. In AZIBs, manganese-based oxides have garnered significant interest thanks to their remarkable capacity, eco-friendliness, and affordability. Manganese dioxide falls under three main categories: tunnel structure (e.g., β-MnO₂, γ-MnO₂, and α-MnO₂), layered structure (e.g., δ-MnO₂), and spinel structures (e.g., λ-MnO₂, Mn₃O₄, and ZnMn₂O₄). Tab.1 summarizes their electrochemical properties for AZIBs.

3.1 Tunnel structure

3.1.1 β-MnO₂

β-MnO₂ features a [1 × 1] tunnel structure [51] and is regarded as the most stable form of MnO₂. However, its narrow tunnels (~2.3 Å) restrict the diffusion of hydrated Zn²⁺ (~4.3 Å with its hydration shell), limiting its electrochemical performance in AZIBs. To address this limitation, researchers have focused on strategies like morphological control and defect engineering to enhance Zn²⁺ diffusion and improve electron transport. Liu et al. [48] fabricated β-MnO₂ with a nanorod morphology (Fig.3(a)), which significantly increased the specific surface area, enhanced Zn²⁺ transport efficiency, and provided more active sites for Zn²⁺ intercalation. The Zn//β-MnO₂ battery maintained a capacity of 110 mAh·g⁻¹ after 1000 cycles (Fig.3(b)), highlighting the key role of morphology regulation in enhancing cycling stability.

Deng et al. [52] enhanced the crystallinity of β-MnO₂ by depositing nanolayers on carbon cloth and calcining them at 320 °C (Fig.3(c)). The sample calcined at 320 °C (MnO₂@CC320) exhibited higher specific capacity and lower charge transfer resistance (R_ct) compared to the one calcined at 250 °C (MnO₂@CC250) (Fig.3(d)‒Fig.3(f)). The improved crystallinity reduced structural defects, facilitating electron transport and enhancing electrochemical performance.

Han et al. [53] prepared oxygen-deficient β-MnO₂ (D-β-MnO₂) through efficient oxygen defect engineering. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirmed the one-dimensional homogeneous nanorod morphology of D-β-MnO₂ (Fig.3(g) and 3(h)). Oxygen vacancies enhanced the specific surface area and created additional pathways for Zn²⁺ diffusion. At a scan rate of 0.2 mV·s⁻¹, D-β-MnO₂ exhibited lower polarization potentials (0.21 and 0.31 V) compared to commercial β-MnO₂ (0.23 and 0.40 V, Fig.3(i)). The specific capacity reached 302 mAh·g⁻¹, significantly higher than that of commercial β-MnO₂ (Fig.3(j)), and after 50 cycles, D-β-MnO₂ maintained superior capacity retention (Fig.3(k)). The oxygen vacancies effectively improved Zn²⁺ intercalation kinetics by facilitating charge transfer and providing more active sites.

Pan et al. [54] synthesized β-MnO₂ and examined the effect of microwave hydrothermal time (MHT) electrochemical performance. They found that the MHT of more than 120 min was optimal (Fig.3(l)). However, capacity degradation was observed during cycling tests at 0.5 C and 4 C (Fig.3(m) and 3(n)), primarily due to the generation of the inactive ZnMn₂O₄ spinel phase (Fig.3(o)), which impeded Zn²⁺ diffusion. Therefore, avoiding the formation of ZnMn₂O₄ is crucial for maintaining the capacity of β-MnO₂.

The performance of β-MnO₂ in AZIBs is significantly influenced by its structural properties. Strategies such as morphological control, crystallinity enhancement, and defect engineering have proven effective in improving its electrochemical performance. However, challenges persist in suppressing unwanted phase transformations and ensuring long-term cycling stability. Combining multiple modification strategies may offer synergistic effects, leading to the development of high-performance β-MnO₂ cathodes. For example, Ding et al. [55] prepared β-MnO₂ with an abundance of oxygen vacancies, encapsulated in graphene oxide (GO). The introduction of oxygen vacancies improved electronic conductivity and facilitated ion diffusion, while the GO coating effectively suppressed Mn dissolution.

3.1.2 γ-MnO₂ and α-MnO₂

γ-MnO₂ features a structure composed of mixed of [1 × 1] and [1 × 2] tunnels, with the larger tunnels facilitating Zn²⁺ diffusion [56]. Shi et al. [57] developed a novel 3D high-density MXene-MnO₂ composite material (3D Ti₃C₂T_x@MnO₂, Fig.4(a)), where a robust 3D micro-flower-like structure was formed by MnO₂ nanoparticles enclosed within crumpled and rippled Ti₃C₂T_x nanosheets (Fig.4(b)). Despite the significantly increased mass loading, the 3D Ti₃C₂T_x@MnO₂ micro-flower maintained a satisfactory rate and cycling performance in AZIBs (Fig.4(c))

This 3D Ti₃C₂T_x@MnO₂ composite demonstrated exceptional cycling stability, retaining 92.7% and 90.6% of its capacity after 800 and 2000 charge–discharge cycles, respectively (Fig.4(d) and Fig.4(e)), significantly outperforming pure MnO₂, which retained only 38.5% and 21.4%. The R_ct value of 3D Ti₃C₂T_x@ MnO₂ was measured at 42.1 Ω, substantially lower than that of pure MnO₂ (98.7 Ω) (Fig.4(f)). Additionally, the composite exhibited a steeper slope in the low-frequency region, indicating enhanced ion and electron transfer rates. These findings underscore the importance of composite material design in improving both ion/electron transport and structural stability, which are vital for boosting the electrochemical performance of manganese-based materials.

α-MnO₂ features a [2 × 2] large tunnel framework formed by four edge-sharing MnO₆. Owing to its large pore structure (4.6 Å × 4.6 Å), Zn²⁺ can freely insert and extract from the material [58]. Liu et al. [59] synthesized α-MnO₂ nanofibers and introduced MnSO₄ into the ZnSO₄ electrolyte to inhibit Mn²⁺ dissolution. This modification resulted in excellent long-term cycling stability, with a capacity retention of 92% after 5 000 cycles at a rate of 5 C (Fig.4(g)). The presence of Mn²⁺ in the electrolyte helps maintain the electrode structure by reducing Mn²⁺ dissolution. Kim et al. [60] employed a solventless synthesis method to prepared tunnel-type α-MnO₂, resulting in a uniform rod-like morphology (Fig.4(h)). The α-MnO₂ nanorods delivered discharge and charge capacities of 104 and 101 mAh·g⁻¹, respectively, and achieved nearly 100% coulombic efficiency (CE) during long-term cycling (Fig.4(i)). Gao et al. [61] synthesized ultra-long nanowires of α-MnO₂ (Fig.4(j)), which provided continuous channels, facilitating the transport of electrons and ions. The battery exhibited a high specific capacity of 384 mAh·g⁻¹ at 0.1 A·g⁻¹ and maintained 165 mAh·g⁻¹ at a high current density of 2 A·g⁻¹ (Fig.4(k)), exhibiting outstanding rate capability. These findings validate the effectiveness of morphology optimization in improving battery performance.

The larger tunnel sizes in α-MnO₂ and γ-MnO₂ facilitate Zn²⁺ diffusion, enhancing their suitability as cathode materials for AZIBs. By combining these structural advantages with morphological control and composite formation, further improvements in their electrochemical performance can be achieved. Future studies should focus on optimizing these parameters and gaining a deeper understanding of the ion transport mechanisms to develop high-performance cathodes for AZIBs.

3.2 Layered structure

δ-MnO₂ has garnered significant attention due to its favorable structural characteristics for Zn²⁺ storage and diffusion. Its two-dimensional layered structure provides ample interlayer spacing, which can accommodate Zn²⁺ and facilitate their transport [62,63]. Compared to other manganese-based compounds, δ-MnO₂ is theoretically more suitable as a host for Zn²⁺. Wang et al. [64] successfully synthesized flower-like δ-MnO₂ on carbon cloth (δ-MnO₂-CC) using a simple hydrothermal method (Fig.5(a)). The δ-MnO₂ displayed a nanoflake morphology, with some nanoflakes forming micro-flower-like structures (Fig.5(b)). This hierarchical architecture enhanced the active surface area and facilitated electrolyte penetration, thereby improving ion diffusion and electron transport. Consequently, the δ-MnO₂-CC electrode demonstrated an impressive discharge capacity of 238.4 mAh·g⁻¹ at 5 A·g⁻¹, with no capacity fade after 5000 cycles (Fig.5(c)). Moreover, the specific capacity nearly returned to its original value when the current density was restored to its initial level, indicating excellent rate capability (Fig.5(d)).

However, δ-MnO₂ often suffers from irreversible structural transformations during charge–discharge cycles, leading to lattice collapse and performance degradation. To address this issue, structural modifications are necessary to enhance its electrochemical properties. Zhang et al. [65] intercalated benzyl trimethylammonium (BMA) into the layers of δ-MnO₂ (BMO) to improve ion diffusion, reaction dynamics, structural robustness, and electronic conductivity (Fig.5(e)). The presence of Mn³⁺ imparted negative surface charges to the nanosheets (Fig.5(f)), facilitating the electrostatic assembly of BMA cations between the layers. This intercalation resulted in a nanoflower morphology (Fig.5(g)), enhancing the surface area and conductivity. Cyclic voltammetries (CVs) indicated that BMO possessed good electrochemical reversibility, with larger current responses than pristine δ-MnO₂ (Fig.5(h)). Density functional theory (DFT) calculations showed that electrons from the C, H, and N atoms in BMA were transferred to δ-MnO₂ and accumulated around the O atoms, thus enhancing the material’s conductivity (Fig.5(i)). Furthermore, BMO exhibited higher rate capability than δ-MnO₂ at various current densities (Fig.5(j)). The discharge curves showed two distinct plateaus for BMO, with the inflection points occurring at higher voltages than those for δ-MnO₂ (Fig.5(k)), possibly due to reduced H⁺ insertion. BMO also had a significantly lower R_ct compared to δ-MnO₂ (Fig.5(l)), indicating that BMA intercalation effectively accelerated diffusion kinetics and improved electrochemical reaction rates. The intercalation of large organic cations like BMA into δ-MnO₂ layers represents an efficient approach to enhance structure stability and electrical conductivity. Future research should focus on exploring other suitable intercalants and optimizing interlayer spacing to further improve performance.

3.3 Spinel structures

Spinel-type manganese oxides, such as λ-MnO₂, ZnMn₂O₄, and Mn₃O₄, offer key advantages over layered and tunnel structures due to their three-dimensional ion transport channels and higher structural stability [66]. The spinel structure can stabilize multivalent metal states, resulting in higher specific capacities, making these materials ideal candidates for AZIBs [67]. While λ-MnO₂ possesses a spinel structure, its lack of tunnels or layers limits its zinc storage capacity, resulting in fewer studies exploring its application in AZIBs. Consequently, research has shifted towards other spinel oxides, such as ZnMn₂O₄ and Mn₃O₄.

ZnMn₂O₄ has attracted attention due to its higher specific capacity compared to monometallic manganese oxides [68]. The synergistic interaction between Zn and Mn enhances energy storage capabilities. However, ZnMn₂O₄ is prone to volume expansion and capacity loss over cycling, which affects its long-term stability. Defect engineering and morphology control have proven to be effective strategies to mitigate these challenges. Baby et al. [69] synthesized nanoscale ZnMn₂O₄ using a template-free solution combustion method (Fig.6(a)). The battery assembled with this ZnMn₂O₄ material achieved a discharge capacity of approximately 100 mAh·g⁻¹ (Fig.6(b)). Wu et al. [70] successfully prepared hollow porous ZnMn₂O₄ (Fig.6(c)), where the hollow porous structure increased the active surface area and accommodated volume changes during cycling. This configuration achieved a specific capacity of 106.5 mAh·g⁻¹ at 0.1 A·g⁻¹ (Fig.6(d)), demonstrating improved electrochemical performance. Gao et al. [71] anchored polypyrrole-coated ZnMn₂O₄ on reduced graphene oxide (ZMO/rGO-PPy). This composite displayed larger, uniformly distributed particles due to the modification (Fig.6(e)). The introduction of rGO and PPy did not alter the spinel structure of ZMO, as evidenced by the absence of clear peaks from rGO and PPy (Fig.6(f)). CVs analysis revealed that ZMO/rGO-PPy had a larger integrated area, indicating higher discharge capacity (Fig.6(g)). The composite exhibited the highest discharge capacity of 269.6 mAh·g⁻¹ (Fig.6(h)), demonstrating that the integration of electrically conductive polymers and carbon-based materials effectively enhances electrochemical performance.

Mn₃O₄ is considered a promising candidate for rechargeable AZIBs due to its low cost, environmental friendliness, and high capacity. However, its inherently low electrical conductivity and structural instability, stemming from substantial volume changes result in rapid capacity loss and poor rate capability during cycling. To enhance the electrochemical performance and cycling stability of Mn₃O₄, Wu et al. [72] employed a solvothermal reaction with glycerol as a solvent (Fig.6(i)). The D-Mn₃O₄ exhibited a contact angle of 94°, smaller than that of pristine Mn₃O₄ (127°), indicating improved electrolyte wettability (Fig.6(j)). The modified Mn₃O₄ delivered 116 mAh·g⁻¹ at 2 A·g⁻¹ over 3000 cycles, showing improved cycling stability (Fig.6(k)). In another study, Zhang et al. [73] synthesized Cu²⁺-doped Mn₃O₄ (CMO) via a straightforward hydrothermal technique (Fig.6(l)). Compared to Mn₃O₄, CMO displayed a larger area beneath the CV curves, indicating higher capacity and faster ion transport, as evidenced by a larger peak current (Fig.6(m)). The rate performance of CMO surpassed that of Mn₃O₄ across various current densities (Fig.6(n)), demonstrating the effectiveness of ion doping in improving performance.

The spinel structure offers a promising pathway for developing high-performance cathodes for AZIBs. By employing strategies such as morphology control, composite formation, and ion doping, the challenges associated with volume changes and conductivity can be mitigated. Further research should focus on understanding the underlying mechanisms and optimizing these strategies to achieve better performance.

3.4 Synthesis methods of manganese oxides

Manganese oxides, with their diverse crystal structures and excellent electrochemical properties, are widely used in AZIBs. The synthesis methods of manganese oxides play a crucial role in determining their structure and performance. In recent years, various synthesis methods have been developed, including the hydrothermal method [48,53,59,61,65,71], chemical precipitation method [52,64], microwave-assisted method [54], aerosol spray method [57], solvothermal method [60,69], and solvent-free method [70,72]. These methods not only control the crystal structure and morphology of the materials, but also have significant implications for enhancing their electrochemical performance.

The hydrothermal method allows for precise control of nanostructures, making it ideal for synthesizing manganese oxides with high surface areas. The chemical precipitation method is simple and cost-effective, making it well-suited for large-scale production. The microwave-assisted method accelerates the synthesis process through rapid and uniform heating, providing a new option for preparation of manganese oxides. The aerosol spray method is suitable for large-scale production of films or particles. The solvothermal method facilitates the synthesis of high-purity, uniform manganese oxides under mild conditions. The solvent-free synthesis method, due to its environmentally friendly and straightforward nature, has gained significant attention in sustainable development research. These methods, by precisely controlling synthesis conditions and precursor concentrations, enable manganese oxide materials to exhibit excellent performance in battery applications. Future research can explore synergistic effects by combining multiple methods, further enhancing the overall performance of manganese oxides and promoting their broader application in AZIBs.

4 Ion-doped manganese-based materials

Doping ions is an effective strategy to enhance the electrochemical properties of manganese-based cathode materials in AZIBs. By introducing foreign ions into the crystal lattice, ion doping can increase interlayer spacing, reduce electrostatic repulsion against Zn²⁺, and stabilize the crystal structure. The doped ions often act as “pillars,” preventing structural collapse during charge–discharge cycles, thereby improving cycling stability. This section explores the effects of monovalent and polyvalent cation doping on manganese-based oxides, as summarized in Tab.3.

4.1 Monovalent cation

Monovalent cations, such as K⁺, Na⁺, and NH₄⁺, are commonly used to modify manganese-based cathodes due to their ability to expand interlayer spacing and enhance Zn²⁺ diffusion. By doping monovalent ions into the crystal lattice, interlayer distance increases, reducing electrostatic repulsion between the host material and Zn²⁺ ions. This facilitates improved Zn²⁺ intercalation and de-intercalation kinetics. Additionally, the incorporation of these ions can introduce defects that serve as active sites for Zn²⁺ storage, thereby improving electrochemical performance. Yang et al. [74] synthesized potassium-ion-doped manganese dioxide nano-scrolls (K-MnO₂). The incorporation of K⁺ did not alter the α-MnO₂ crystal structure but resulted in a wire-like morphology (Fig.7(a) and Fig.7(b)). The K-MnO₂ exhibited higher redox peak currents in CVs compared to pristine α-MnO₂, indicating enhanced electrochemical activity (Fig.7(c)). The specific capacity of K-MnO₂ exceeded that of α-MnO₂ (Fig.7(d)), with the improvement attributed to the introduction of defects by K⁺ doping, which provided additional active sites for Zn²⁺ storage.

Wang et al. [75] demonstrated that pre-intercalation of Na⁺ and water molecules into δ-MnO₂ (δ-NMOH) effectively activated stable Zn²⁺ storage. The Na⁺ and H₂O served as pillars, expanding the interlayer spacing to 0.72 nm, which enhanced the electrochemical performance (Fig.7(e)). The δ-NMOH exhibited exceptional cycling stability with a capacity retention of 98% after 10 000 cycles (Fig.7(f)), which was attributed to the smooth Zn²⁺ diffusion facilitated by the pre-intercalated structure. The combined doping effect of Na⁺ and H₂O greatly enhanced the overall electrochemical stability.

The synergistic combination of different monovalent ions can further optimize the material’s properties. Zeng et al. [76] developed a sodium-potassium co-doped layered manganese oxide, K_0.37Na_0.18MnO₂·xH₂O (KNMOH), via a liquid-phase synthesis. The CVs of KNMOH showed small overpotentials even at the increased scanning rates, indicating favorable electrochemical kinetics (Fig.7(g)). The nanosheet morphology provided a large surface area for electrolyte contact, enhancing Zn²⁺ transport. KNMOH had a smaller R_ct compared to Na-only doped NMOH, confirming that co-doping substantially improved diffusion dynamics and stability (Fig.7(h)).

Yao et al. [77] incorporated ammonium ions (NH₄⁺) into MnO₂ nanosheets (AMO) using a two-step strategy (Fig.7(i)). The AMO nanosheets interconnected to form an ordered nanoarray on carbon cloth (Fig.7(j)), enhancing electronic conductivity and structural integrity. At 0.5 A·g⁻¹, the AMO exhibited superior cycling stability compared to potassium-doped MnO₂ (KMO), maintaining a higher capacity over prolonged cycles (Fig.7(k)). AMO also had a significantly lower R_ct (268 Ω) compared to KMO (536 Ω), demonstrating more efficient ion transport (Fig.6(l)).

Monovalent cation doping effectively expands the interlayer spacing of manganese-based oxides, facilitating Zn²⁺ intercalation and de-intercalation, while improving electrochemical kinetics. The choice of dopant and its concentration are crucial in optimizing the material’s performance. Na⁺ doping primarily improves Zn²⁺ diffusion kinetics by significantly expanding the interlayer spacing of δ-MnO₂, promoting smoother ion transport. On the other hand, K⁺ doping enhances Zn²⁺ storage by introducing lattice defects that create additional active sites for Zn²⁺ ions. These differences in doping mechanisms highlight the complementary roles of Na⁺ and K⁺, suggesting potential benefits from their synergistic doping. Future research should focus on systematically studying the effects of various monovalent ions and their synergistic combinations to further enhance the electrochemical of manganese-based cathodes.

4.2 Polyvalent cations

Multivalent ions, including divalent and trivalent cations, are frequently used to dope manganese-based oxides due to their diverse valence states, which offer various possibilities for doping modifying the material’s properties. These doped manganese-based compounds can improve electrochemical performance, enhance cycling stability, increase safety, reduce costs, and broaden their application range, making them highly promising materials with great potential and broad prospects [78,79].

Lin et al. [80] synthesized ultra-long Co-doped MnO₂ nanowires on carbon cloth through a combination of hydrothermal methods and plasma treatment (Fig.8(a)). The plasma treatment induced oxygen vacancies that enhanced electrical conductivity and created additional active sites. The nanowire network, uniformly distributed, facilitated efficient electron flow during electrochemical processes (Fig.8(b)). Co-MnO₂-2 showed a larger integrated area in CVs, indicating higher capacitance without altering the charge storage mechanism (Fig.8(c)). The material maintained a specific capacity of 249 mAh·g⁻¹ when the current density was reduced from 5 to 1 A·g⁻¹, showcasing outstanding rate capability and reversibility (Fig.8(d)).

Xie et al. [81] prepared Zn-doped MnO₂ with oxygen-rich vacancies (Fig.8(e). Plasma treatment increased the specific surface area and introduced oxygen vacancies, which provided more reactive sites (Fig.8(f)). Zn-MnO₂-4 exhibited a larger integral area in the CVs, indicating superior specific capacity and faster charge transfer kinetics (Fig.8(g)). Its improved rate performance across different current densities highlighted the role of plasma treatment in enhancing energy storage capabilities (Fig.8(h)).

Han et al. [82] synthesized Eu-doped β-MnO₂ (20EM, Fig.8(i)). The doped samples featured exhibited elongated rods with nanoparticles clustered within them (Fig.8(j)). 20EM had a smaller voltage difference between oxidation and reduction peaks compared to undoped β-MnO₂, suggesting superior Zn²⁺ reversibility during redox reactions (Fig.8(k)). Eu³⁺ doping reduced the R_ct, enhancing electrical conductivity and ion diffusion rates (Fig.8(l)).

Li et al. [83] prepared Mg-doped α-MnO₂ (MMO, Fig.8(m)). The MMO exhibited a nanorods structure (Fig.8(n)) and demonstrated improved electrochemical performance with a specific capacity of 311 mAh·g⁻¹ and a capacity retention of 77.2% over 700 cycles (Fig.8(o)). The improvements were attributed to the reduced R_ct, increased Zn²⁺ diffusion coefficient, and enhanced structural stability introduced by Mg²⁺ doping.

Polyvalent cation doping introduces substantial modifications to the crystal structure and electronic properties of manganese-based oxides. For example, Mg²⁺ doping reduces lattice distortion, resulting in a higher Zn²⁺ diffusion coefficient and improved structural stability, while Co²⁺ doping primarily enhances electronic conductivity through the creation of oxygen vacancies. However, excessive doping of larger ions, such as Eu³⁺, can lead to unwanted lattice distortions, requiring careful optimization of doping levels. Future studies should focus on optimizing doping levels and exploring new polyvalent ions to achieve superior performance manganese-based cathodes.

4.3 Intrinsic mechanisms of doping

Doping is an effective strategy for enhancing the electrochemical properties of electrode materials, especially in energy storage systems, such as AZIBs. By modifying various aspects of electrode materials, doping can significantly improve their performance, including their crystal structure, electronic and ionic conductivity, material stability, and electrochemical reaction kinetics [84, 85]. The mechanisms through which doping improves electrode performance, can be explored from the following perspectives:

1) Optimization of crystal structure. Doping can effectively modify the crystalline structure of electrode materials, especially by altering the interlayer spacing or introducing lattice defects, which in turn influences the performance of ion intercalation and de-intercalation [86, 87].

2) Enhancement of ionic conductivity. Doping can enhance the ionic conductivity of materials by introducing additional charge carriers or generating oxygen vacancies. These dopants accelerate ion diffusion and reduce resistance, thereby improving the capacity retention of the battery during long-term use [88, 89].

3) Enhanced structural stability. Doping can improve the resistance of electrode materials to degradation by reducing damage caused by volume expansion or structural changes during prolonged charge and discharge cycles. Some dopants strengthen the interaction forces within the lattice, allowing the material to better withstand repeated cycling and ultimately extending the battery’s lifespan [90].

4) Regulation of electrochemical reaction kinetics. Doping can regulate the kinetics of electrode reactions by reducing overpotentials and resistance in the electrode processes, thereby enhancing the efficiency of battery charging and discharging. The doped elements enhance the reaction activity of the electrodes, providing more active sites and improving the rate performance of the battery. Furthermore, doping can optimize the surface properties of the material, reducing charge accumulation and increasing the reaction rate, which further enhances the efficiency and cycling life of the battery [91, 92].

5 MOF manganese-based materials

MOFs are crystalline porous materials formed by the coordination of metal ions or clusters with organic ligands. Due to their high porosity, large specific surface area and tunable structures, MOFs have garnered significant interest in energy storage applications, including AZIBs [93, 94]. The integration of MOFs into AZIBs offers several advantages:

1) Enhancing electrolyte penetration: The high porosity facilitates electrolyte infiltration, improving ion accessibility to active sites and enhancing overall ion diffusion.

2) Improving structural stability: The robust framework can accommodate volume changes during cycling, which helps maintain the structural integrity of the material.

3) Providing abundant active sites: The large surface area of MOFs provides numerous redox-active sites, which contribute to higher capacities.

4) Tunability: MOFs provide flexibility in selecting electrode materials by varying the mater center and organic linkers, offering the potential for custom-tailored properties [95,96].

Despite these advantages, challenges such as low intrinsic conductivity and potential structural degradation under operational conditions need to be addressed. Recent studies have focused on leveraging the unique properties of MOFs and their derivatives to overcome these limitations.

Wang et al. [97] synthesized Mn₂O₃ multi-shelled hollow nanospheres (Mn₂O₃ MHS) by oxidizing Mn-MOF microspheres (Fig.9(a)). The resulting hollow structure retained its spherical shape but showed noticeable shrinkage and a rougher surface after oxidation, forming thin shells (Fig.9(b)). The CVs profiles from the first three cycles showed excellent stability, with nearly overlapping curves, indicating high reversibility (Fig.9(c)). After 500th cycles, the discharge capacity remained at 152.8 mAh·g⁻¹ with a CE above 99%, demonstrating excellent cycling stability (Fig.9(d)). The unique multi-shelled hollow structure facilitated electrolyte penetration and provided a buffer for volume expansion during Zn²⁺ intercalation, which contribute to the enhanced electrochemical performance.

Yin et al. [98] developed a manganese-based MOF using a coordination unsaturation strategy (Fig.9(e)). The Mn-H₃BTC-MOF-4 exhibited clear lattice fringes, which indicated abundant transport channels for Zn²⁺ (Fig.9(f)). By adjusting the molar ratio of Mn²⁺ to carboxyl (–COOH) groups, the coordination environment within the framework was optimized. The CV curves indicated that Mn-H₃BTC-MOF-4 exhibited a larger area compared to Mn-H₃BTC-MOF-2 and Mn-H₃BTC-MOF-6, reflecting superior electrochemical performance due to optimized coordination (Fig.9(g)). At 100 mA·g⁻¹, Mn-H₃BTC-MOF-4 delivered a discharge capacity of 138 mAh·g⁻¹ and sustained a high charge-discharge voltage plateau at 1.6 V (Fig.9(h)). The unsaturated coordination sites enhanced the redox activity and facilitated Zn²⁺ diffusion.

Mao et al. [99] synthesized α-Mn₂O₃ through a MOF-derived method, resulting in rod-like structures composed of small nanoparticles (Fig.9(i)). The N₂ adsorption–desorption isotherms showed a type IV curve, indicating the presence of mesopores (Fig.9(j)). These mesopores facilitated the accommodation of Zn²⁺, enhancing ion transport kinetics. The α-Mn₂O₃ electrode maintained a specific discharge capacity of 92.7 mAh·g⁻¹ with a capacity retention rate of 53.3% after 1700 cycles at 2 A·g⁻¹ (Fig.9(k)), confirming its excellent high-rate capability and cycling stability.

Zhang et al. [100] developed a manganese-based MOF/CNT composite using an in situ solvothermal method (Fig.9(l)). The Mn-MOF particles were uniformly covered by CNTs, preventing aggregation and forming a conductive network (Fig.9(m)). The Mn-MOF/CNT//Zn battery maintained nearly 100% capacity retention after 900 cycles at a current density of 1000 mA·g⁻¹, demonstrating exceptional cycling performance (Fig.9(n)). The Brunauer-Emmett-Teller (BET) surface area of Mn-MOF/CNT was significantly higher (111.8 m²·g⁻¹) than that of Mn-MOF (23.2 m²·g⁻¹), which enhanced Zn²⁺ migration and diffusion (Fig.9(o)). EIS showed a lower R_ct for Mn-MOF/CNT, indicating improved conductivity and faster ion diffusion kinetics (Fig.9(p)).

The application of MOFs and their derivatives in AZIBs presents a promising strategy for enhancing the performance of manganese-based cathodes. The adjustable architectures and significant porosity of MOFs enhance ion transport and contribute to structural stability. However, challenges such as the stability of MOFs in aqueous environments and potential degradation remain. Future research should focus on designing MOFs with optimized pore structures and investigating synergistic interactions with other materials to enhance capacity and extend cycling life.

6 Carbon-coated manganese-based material

One of the major challenges with manganese-based cathodes is the dissolution of Mn²⁺ into the electrolyte during cycling, which leads to structural degradation and capacity fading [101]. Carbon coating is an effective strategy to mitigate this issue by providing a protective layer that inhibits Mn dissolution, enhancing electrical conductivity and improving overall electrochemical performance [102,103].

Huang et al. [104] synthesized a porous nitrogen-doped carbon-coated manganese oxide/zinc manganate composite (MZM@N-C, Fig.10(a)). At a current density of 50 mA·g⁻¹, MZM@N-C achieved an impressive specific capacity of 772.8 mAh·g⁻¹ (Fig.10(b)), attributed to the improved conductivity and structural stability imparted by the N-doped carbon coating. Zhao et al. [105] synthesized ε-MnO₂ by annealing MnCO₃, followed by coating it with a carbon layer derived from caramelized D-glucose to produce ε-MnO₂@C (Fig.10(c)). This carbon coating significantly reduced Mn²⁺ dissolution and minimized side reactions between the electrode and electrolyte. The CVs displayed high consistency and good reversibility (Fig.10(d)). The capacity retention remained at 91% after 1700 cycles at 1 A·g⁻¹, with nearly 100% CE throughout (Fig.10(e)). The improved cycling stability was attributed to the carbon layer acting as a barrier against Mn dissolution and enhancing electronic conductivity.

Graphene, known for its outstanding electrical conductivity and mechanical stability, has also been used to enhance manganese-based cathodes. Wang et al. [106] developed a γ-MnO₂-graphene composite. This composite showed an extended discharge plateau and reduced voltage hysteresis compared to unmodified MnO₂, indicating improved energy storage capability (Fig.10(f)). The specific capacity reached 301 mAh·g⁻¹, more than double that of pristine MnO₂. The reduced polarization between cathodic and anodic peaks suggested improved reversibility (Fig.10(g)). After 300 cycles, the composite retained 64.1% of its initial capacity, demonstrating enhanced cycling stability (Fig.10(h)).

Ding et al. [107] employed electrospinning to fabricate bead-shaped manganese oxide encapsulated within carbon nanofibers (MnO_x-CNFs, Fig.10(i)). The core-shell structure, consisting of uniformly distributed MnO_x beads within a carbon matrix, formed a 1D porous network (Fig.10(j)), facilitating efficient electron and ion transport. The architecture resulted in higher current densities and narrower potential gaps in CVs, indicating enhanced electrochemical reactivity and reduced voltage polarization (Fig.10(k)).

Zhai et al. [108] used chemical vapor deposition to prepare carbon-coated MnO (MnO@C, Fig.10(l)). The R_ct of MnO@C was substantially lower than that of unmodified MnO₂, indicating improved electrical conductivity after carbon modification (Fig.10(m)). The CVs from the initial three cycles displayed consistent redox peak positions (Fig.10(n)), demonstrating stable and reversible redox reactions.

Xie et al. [109] introduced graphitic carbon nitride (g-C₃N₄) nanosheets to fabricate an α-MnO₂@g-C₃N₄ composite. This modification reduced agglomeration and increased the specific surface area (Fig.10(o)), providing more active sites for electrochemical reactions. The capacitive contribution of α-MnO₂@g-C₃N₄ increased significantly with scan rate, indicating improved ion diffusion rates due to the hollow structure and enhanced active sites (Fig.10(p)). The composite sustained 5000 cycles with a CE consistently above 98.7% and sustained a recoverable capacity of approximately 100 mAh·g⁻¹, demonstrating excellent cycling stability (Fig.10(q)). In contrast, unmodified MnO₂ showed a sharp capacity decline, confirming that the structural integrity was effectively preserved in the composite during extended cycling.

Although the carbon-coating strategy has been effective in suppressing Mn dissolution, enhancing conductivity, and improving electrochemical performance, there remains considerable potential for further optimization. Achieving precise control over carbon layer thickness, pore size, and the incorporation of doping elements (such as N, S, P) can further enhance interface stability and improve electron and ion transport efficiency. By combining the carbon layer with highly conductive materials like graphene, carbon nanotubes, or g-C₃N₄, it is possible to maintain excellent conductivity while also providing higher energy density and longer cycle life.

Additionally, employing in situ characterization techniques to investigate the structural evolution of the carbon layer during cycling and understanding the mechanisms behind the suppression of Mn dissolution will provide valuable insights for the precise design of materials. Moreover, integrating the carbon-coating strategy with optimizations of the electrolyte, binder, and current collector systems can achieve systemic synergistic improvements.

By pursuing these advancements, carbon-coated manganese-based materials are expected to make significant progress in both performance and stability, laying a solid foundation for the development of high-performance energy storage devices.

7 Optimizing the electrolytes

Optimizing electrolytes is a crucial strategy to improving the cycling stability and reaction kinetics of manganese-based cathodes in AZIBs. The composition and concentration of the electrolyte play a pivotal role in the formation and stability of the solid electrolyte interface (SEI) layer, which act as a protective barrier to prevent manganese dissolution, reduce parasitic reactions, and maintain structural integrity during prolonged cycling.

Cao et al. [110] incorporated dimethyl sulfoxide (DMSO) into a diluted ZnCl₂ electrolyte, resulting in the formation of a distinctive SEI layer composed of Zn₁₂(SO₄)₃Cl₃(OH)₁₅·5H₂O, ZnSO₃ and ZnS upon DMSO degradation (Fig.11(a)). This SEI layer effectively inhibited Zn dendrite growth and minimized water decomposition, thereby improving the cycling stability of Zn||Zn symmetric cells (Fig.11(b)).

Chen et al. [111] explored the use of 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) as an electrolyte additive. When 0.5 mol/L [BMIM]OTF was added to a 3 mol/L Zn (OTF)₂ aqueous electrolyte, it formed an organic-inorganic hybrid SEI at the anode–electrolyte interface (Fig.11(c)). This SEI layer suppressed parasitic reactions and Zn corrosion, thereby improving the cycling stability for Zn||Zn cells with the [BMIM]OTF additive (Fig.11(d)).

Zeng et al. [112] developed a robust polydopamine (PDA) SEI layer on the Zn using dopamine as an electrolyte additive (Fig.11(e)). Huang et al. [113] incorporated saccharin (Sac) into the electrolyte to adjust the electric double layer (EDL) structure at the anode/electrolyte interface (AEI). The decomposition of Sac anions resulted in the formation of a distinct SEI on the Zn surface, which regulated Zn deposition behavior and prevented side reactions. After incorporating Sac into the electrolyte, Zn symmetric cells achieved an extended cycle life of 550 h at 10 mA·cm⁻² (Fig.11(f)). Full cells utilizing the Sac/ZnSO₄-based electrolyte maintained a reversible capacity of 100 mAh·g⁻¹ with a high coulombic efficiency of 99.9% over 7500 cycles. In contrast, cells without the Sac additive rapidly failed after 1200 cycles due to severe dendrite formation and the lack of a protective SEI (Fig.11(g)).

Although electrolyte optimization has proven to be an effective strategy, there is still limited discussion on the detailed mechanisms and practical design principles of such approaches. Future research should focus on a deeper exploration of the fundamental processes governing SEI formation and ion transport, and the interactions between electrolyte additives and electrode interfaces. Through continued interdisciplinary efforts and the application of advanced characterization techniques, a more comprehensive understanding can be developed. This understanding will ultimately guide the design of AZIBs that offer both higher energy density and enhanced longevity.

8 Summary and outlook

The development of AZIBs has gained significant momentum as a sustainable and cost-effective alternative to traditional lithium-ion batteries, particularly for large-scale energy storage systems and electric vehicles. This review systematically examined the advancements in manganese-based materials, which have attracted significant attention due to their natural abundance, low cost, and environmental friendliness. Various strategies have been employed to enhance the electrochemical performance of these materials, leading to significant improvements in capacity, rate performance and cycling stability.

Structural engineering of manganese oxides has been crucial, with diverse crystal structures such as tunnel-type (β-MnO₂, α-MnO₂, γ-MnO₂), layered (δ-MnO₂), and spinel (λ-MnO₂, Mn₃O₄, ZnMn₂O₄) being explored to optimize Zn²⁺ diffusion pathways and stabilize the electrode during cycling. These structural modifications have facilitated more efficient ion transport and enhanced the structural integrity of the cathodes. Ion doping, through the pre-intercalation of monovalent cations (K⁺, Na⁺ and NH₄⁺) and multivalent cations (Co²⁺, Zn²⁺, Mg²⁺ and Eu³⁺) into manganese oxides, has effectively expanded lattice spacing, reduced electrostatic repulsion and enhanced structural stability, resulting in improved electrochemical properties, including higher capacity and better rate performance.

The use of MOFs as precursors or templates has significantly enhanced charge storage capacities due to their porous nanostructures, substantial porosity, and extensive surface areas. By adjusting the organic ligands and metal centers, MOFs offer tunable pore sizes and functionalities, optimizing ion transport channels and electrochemical efficiency. Additionally, the incorporation of carbon materials, such as carbon nanotubes, graphene, and carbon coatings, has effectively enhanced the electrical conductivity and mitigated manganese dissolution. These conductive networks facilitate electron transport and provide mechanical support, substantially increasing specific capacity and rate performance. Furthermore, electrolyte optimization is an important strategy to enhance the performance of manganese-based cathodes, with stable SEI layer formed on the Zn anode surface to suppress manganese dissolution, reduce side reactions, and maintain structural integrity.

Despite these advancements, several challenges remain in realizing the full potential of manganese-based cathodes for AZIBs. Future research will focus on the systemic optimization and engineering of materials for AZIBs. Researchers should delve deeper into the interface reaction mechanisms of manganese-based cathode materials and explore how advanced material engineering techniques can regulate electrode structure and performance.

1) Material performance optimization: Enhancing the electrochemical performance of manganese-based cathode materials is a primary task. Enhancing their cycling stability and conductivity can be accomplished through innovative material design and surface modification strategies, such as ion doping and conductive polymer coatings, which enhance electron and ion transport efficiency.

2) Interface and structure design: The development of innovative synthesis techniques is necessary to achieve precise control over the microstructure and macroscopic morphology of manganese-based materials. Approaches like nanostructuring and the designing porous architectures are pivotal in creating more active sites and facilitating efficient ion transport pathways, thereby enhancing battery performance and prolong operational lifespan.

3) Advanced testing and simulation technologies: Utilizing advanced characterization techniques and computational models is essential for understanding the electrochemical behavior and failure mechanisms of materials. Real-time imaging, in situ spectroscopy, and simulations based on density functional theory will provide deeper insights into how materials behave under battery operating conditions.

By focusing on these research directions, AZIBs are expected to become the cornerstone of next-generation energy storage systems, providing robust support for more sustainable and environmentally friendly energy solutions.

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