Manganese dioxide as cathode for aqueous zinc-ion batteries: reaction mechanisms, optimization strategies and further prospects

Zn²⁺ insertion/extraction mechanism

During the discharge process of AZIBs, zinc foil at the anode loses electrons, releasing zinc ions (Zn²⁺), which then migrate toward the cathode material. The cathode material accepts electrons and undergoes one or more reversible reduction reactions. Manganese (Mn) undergoes a valence reduction from +4 to +3, while Zn²⁺ intercalates into the cathode material. During the charge process, the cathode undergoes an oxidation reaction, releasing the previously intercalated Zn²⁺, which migrates back to the anode and undergoes a reduction reaction to form metallic zinc or intercalate into the zinc foil lattice. This is referred to as the Zn²⁺ insertion/extraction mechanism, one of the earliest discovered and most prevalent energy storage mechanisms in AZIBs. Early studies revealed that during discharge, zinc at the cathode is oxidized to Zn²⁺, which migrates through the electrolyte and intercalates into the tunnel structure of α-MnO₂, reacting with MnO₂ to form ZnMn₂O₄. Concurrently, Mn is reduced from the +4 to the +3 oxidation state. The electrochemical reaction can be expressed as follows:

During charging, the process is reversed: Zn²⁺ is deintercalated from the cathode, returns to the electrolyte, and is ultimately reduced to metallic zinc, which deposits at the anode. Subsequent studies have revealed that distinct mechanisms govern Zn²⁺ intercalation into α-MnO₂. As shown in Figure 2A, in investigations of zinc-ion batteries using α-MnO₂/graphene scrolls (MGS) as cathode materials, Zn²⁺ was found to intercalate into the α-MnO₂ tunnels during the first cycle, converting it into layered Zn-buserite until Zn²⁺ accommodation is no longer feasible. This process is irreversible. In subsequent cycles, Zn²⁺ undergoes reversible intercalation/extraction within the generated layered structure^[83]. MnO₂ encompasses α-MnO₂(2 × 2 tunnel), β-MnO₂ (1 × 1 tunnel), γ-MnO₂ (1 × 2 and 1 × 1 tunnel), λ-MnO₂ (1 × 3 tunnel), etc. As depicted in Figure 2B, these tunnel structures provide sites for Zn²⁺ accumulation and exhibit a Zn²⁺ intercalation/extraction process analogous to that of α-MnO₂^[84]. Among these, γ-MnO₂ is particularly distinctive: during Zn²⁺ intercalation, continuous structural phase transitions occur. Specifically, Mn is reduced from the +4 to the +3 and +2 oxidation states, first transforming into spinel-type ZnMn₂O₄, then into tunnel-type γ-Zn_xMnO₂, and ultimately forming layered L-Zn_xMnO₂. As shown in Figure 2C, following complete Zn²⁺ intercalation, ZnMn₂O₂, γ-Zn_xMnO₂, and L-Zn_xMnO₂ coexist^[85]. A unique structural characteristic is that the cathode surface comprises a high-valence Mn phase, whereas low-valence Zn²⁺ ions are present in the interior. It is hypothesized that surface Mn²⁺ ions are unstable and prone to oxidation in an oxygen-rich environment. The Zn²⁺ intercalation reaction can be described by the following electrochemical equation^[86]:

Figure 2. (A) The two-step intercalation mechanism of MGS cathode. Reprinted with permission^[83]. Copyright 2018, John Wiley and Sons; (B) Different crystal structures of MnO₂ and the zinc storage mechanisms. Reprinted with permission^[84]. Copyright 2020, John Wiley and Sons; (C) The reaction pathway of Zn-insertion in γ-MnO₂ cathode. Reprinted with permission^[85]. Copyright 2015, American Chemical Society; (D) Cell structure and working mechanism of AZIBs with δ-MnO₂ cathode. Reprinted with permission^[87]. Copyright 2024, John Wiley and Sons. MGS: α-MnO₂/graphene scrolls; AZIBs: aqueous zinc-ion batteries.

Among MnO₂ with layered crystal structures, δ-MnO₂ features a two-dimensional planar configuration with large interlayer spacing, facilitating faster diffusion and intercalation of Zn²⁺. As shown in Figure 2D, in zinc-ion batteries (ZIBs), the layered δ-MnO₂ nanosheet cathode undergoes structural transformation during Zn²⁺ intercalation, converting into spinel-type ZnMn₂O₄ and the intermediate Mn(II) phase-specifically, layered δ-Zn_xMnO₂. The overall electrochemical reaction can be described by the following equation^[87]:

Manganese is reduced from the +4 to the +3 and +2 oxidation states, respectively. Subsequent studies have revealed that zinc cations are progressively intercalated into the δ-MnO₂ layers in the aqueous electrolyte through a two-stage process, while Zn²⁺ deintercalation from the MnO₂ matrix occurs in two steps^[88]. It is noteworthy that research on the Zn²⁺ intercalation/deintercalation mechanism of tunnel-structured MnO₂ is relatively mature. In recent years, emerging studies have primarily focused on co-intercalation mechanisms and other related mechanisms. However, the Zn²⁺ intercalation/deintercalation mechanism of layered MnO₂ still warrants further investigation.

H⁺/Zn²⁺ Co-insertion mechanism

The co-insertion mechanism of hydrogen ions (H⁺) and Zn²⁺ is crucial for enhancing the electrochemical performance of batteries. As shown in Figure 3A, the co-insertion mechanism in AZIBs involves the simultaneous intercalation of H⁺ and Zn²⁺ into cathode materials such as MnO₂^[89]. Distinct from traditional Zn²⁺-based energy storage mechanisms, this co-insertion strategy enables higher capacity and faster reaction kinetics. Owing to their smallest ionic radius and lowest mass, H⁺ typically contributes to high capacity and rapid dynamics in batteries. In Figure 3B, a reversible H⁺/NH⁴⁺ co-insertion/extraction mechanism was established in the zinc-manganese dioxide aqueous battery system, and it was found that such a mechanism significantly improves the rate capability and cycling stability of AZIBs^[32]. For Figure 3C, the co-insertion mechanism enhances ion diffusion within MnO₂ through a synergistic effect. Moreover, benefiting from reversible structural evolution, the battery maintains long-term stable operation^[90]. Several approaches have been reported to reduce the detrimental incorporation of H⁺ and promote Zn²⁺ uptake in aqueous systems. Notable progress has been achieved in this field, particularly through the modulation of water activity, regulation of Zn²⁺ hydration behavior, and development of subzero-temperature batteries^[91]. In Figure 3D, a strategy to tailor the electrochemical behavior of layered δ-MnO₂ by introducing Cu²⁺ ions was developed^[92]. This approach enables a reversible process involving Zn²⁺ insertion and H⁺ conversion within the modified δ-MnO₂ electrode, thus forming an efficient H⁺/Zn²⁺ co-insertion/extraction mechanism. The findings indicate that Cu²⁺ incorporation into δ-MnO₂ facilitates favorable ion intercalation/deintercalation, which is attributed to strong ionic interactions with oxygen atoms.

Figure 3. (A) Zn²⁺ intercalation and H⁺ conversion reaction mechanism in δ-MnO₂. Reprinted with permission^[89]. Copyright 2019, John Wiley and Sons; (B) The insertion energies of H⁺ or NH⁴⁺ ions in MnO₂. Reprinted with permission^[32]. Copyright 2021, John Wiley and Sons; (C) The discharge of tunnel-like ε-MnO₂ stockpiled H⁺ and Zn²⁺. Reprinted with permission^[90]. Copyright 2022, Elsevier; (D) The synthetic process of CuMO electrode. Reprinted with permission^[92]. Copyright 2022, Elsevier; (E) Schematic illustration of the preparation of TMA-MnO₂. Reprinted with permission^[1]. Copyright 2024, Elsevier.

Given the significant difference in ionic radii between Zn²⁺ and H⁺, the interlayer distance of MnO₂ is expected to play a critical role in regulating the intercalation competition between Zn²⁺ and H⁺ at the MnO₂ surface. Based on this premise, it has been demonstrated that a MnO₂/tetramethylammonium (TMA) composite with expanded interlayer spacing (denoted as TMA-MnO₂) serves as a robust cathode material for AZIBs [Figure 3E]^[1]. These studies suggest that expanding the interlayer spacing is a viable strategy to promote Zn²⁺ uptake while suppressing the competing intercalation of H⁺. This approach effectively inhibits the concurrent formation of zinc hydroxide sulfate (ZHS) and enhances the structural stability of MnO₂. Sluggish Zn²⁺ insertion kinetics in existing MnO₂-based materials have been identified as a key issue, leading to the formation of irreversible inert phases on the electrode surface and thus substantial capacity loss. To address this, researchers have emphasized that preventing the formation of such irreversible inert phases is essential for improving the cycling performance of zinc-manganese batteries^[93]. They proposed that engineering MnO₂ with a layered structure can accelerate Zn²⁺ intercalation/deintercalation and preserve structural integrity, thereby inhibiting the formation of the inert ZnMn₂O₄ phase.

Zn²⁺ insertion mechanism

The pH value of Zn/MnO₂ batteries varies during charge-discharge cycles, which confirms the occurrence of H⁺ transformation. The formation of MnOOH also provides strong evidence for the H⁺ intercalation-induced transformation reaction mechanism. As shown in Figure 4A, compared with the traditional Zn²⁺ intercalation/deintercalation mechanism, the transformation reaction mechanism enables direct charge transfer involving two or more charge carriers. Under acidic conditions, MnO₂ reacts with H⁺ to form MnOOH^[94]. However, excess OH^- will combine with Zn²⁺ to form precipitates on the cathode surface {such as Zn₄SO₄(OH)₆·nH₂O shown in Figure 4B and ZnSO₄[Zn(OH)₂]₃·xH₂O shown in Figure 4C}^[93,95]. Additionally, under specific conditions, MnOOH can further react with H⁺ to generate MnO₂ and Mn²⁺. The equation for this conversion reaction is as follows:

Figure 4. (A) Conversion reaction mechanism. Reprinted with permission^[94]. Copyright 2022, Elsevier; (B) A side reaction of the AZIBs conversion reaction to form Zn₄SO₄(OH)₆·nH₂O. Reprinted with permission^[93]. Copyright 2022, Elsevier; (C) A side reaction of the AZIBs conversion reaction to form ZnSO₄[Zn(OH)₂]₃·xH₂O. Reprinted with permission^[95]. Copyright 2022, Elsevier; (D) Dynamic compensation mechanism for the AZIBs with the Zn-Ce electrolyte. Reprinted with permission^[97]. Copyright 2023, Royal Society of Chemistry; (E) Diagram illustrating phase transformations with MnO₂ cathode in zinc sulfate. Reprinted with permission^[98]. Copyright 2022, Elsevier. AZIBs: Aqueous zinc-ion batteries.

Additionally, the detection of MnOOH diffraction peaks further confirms the existence of a conversion reaction mechanism.

By investigating the ion transport and reaction behaviors during cycling, researchers identified diffraction peaks corresponding to Zn₄SO₄(OH)₆·nH₂O and MnOOH in the δ-MnO₂-4 h/CC system during charge-discharge processes^[96]. These results confirm that H⁺ is inserted into MnO₂, and the increase in electrolyte pH induced by H⁺ insertion promotes the formation of Zn₄SO₄(OH)₆·nH₂O on the electrode surface. Similarly, to elucidate the electrochemical energy storage mechanism of the A-MnO₂ cathode, researchers observed that the phase composition of the A-MnO₂ cathode during the reaction process includes thin sheets with reversible formation/dissolution behaviors, specifically basic zinc sulfate {Zn₄SO₄(OH)₆·4H₂O, BZS}. The formation of BZS is associated with increased pH and Zn²⁺ concentration in the vicinity of the cathode^[92]. Additionally, the detection of MnOOH diffraction peaks further confirms the existence of a conversion reaction mechanism.

Studies have shown that the capacity of MnO₂ cathodes often degrades due to manganese dissolution. To address this, researchers introduced Ce(SO₄)₂ as an additive into the electrolyte of Zn/MnO₂ batteries. They found that MnOOH, formed via the reaction between Ce⁴⁺ and Mn²⁺, is attracted by H⁺ and deposited on the cathode [Figure 4D]. This MnOOH is subsequently converted into MnO₂, achieving dynamic compensation and effectively ensuring capacity stability^[97]. Furthermore, researchers investigated the irreversibility of different MnO₂ polymorph cathodes during charge-discharge cycles in weakly acidic ZnSO₄ aqueous electrolytes, and analyzed its close correlation with conversion reactions. A schematic diagram illustrating the phase transition of the MnO₂ cathode in aqueous zinc sulfate solution is presented in Figure 4E. It is worth noting that current studies on the conversion reaction mechanism remain limited, with few reports available, and research in this field is still dominated by the traditional intercalation-deintercalation mechanism^[98].

Deposition-dissolution mechanism

The deposition-dissolution mechanism of MnO₂ as a cathode in AZIBs essentially involves a redox reaction coupled with Mn valence state changes and Zn²⁺ migration during cycling, accompanied by the dissolution and re-deposition of MnO₂^[51]. The reversibility of MnO₂ in the deposition-dissolution mechanism is jointly regulated by the intrinsic properties of the material (e.g., structure and morphology), electrolyte characteristics (e.g., pH and ionic composition), and operating conditions (e.g., temperature and voltage). Among these factors, Mn dissolution, Mn loss, and structural phase transitions are core challenges^[49]. Strategies such as crystal structure optimization, electrolyte regulation, and interface engineering can effectively suppress side reactions, enhance the reversibility of the mechanism, and facilitate the practical application of aqueous zinc-ion batteries^[35]. As in-depth research into the role of MnO₂ in AZIBs advances, growing evidence indicates that the dissolution-deposition phenomenon of MnO₂ is widespread. However, there remain differences in understanding regarding the specific details of reactions during charge-discharge processes^[38].

Researchers have noted that ZHS, often regarded as a by-product of proton consumption at the electrolyte interface, can potentially be utilized as a cathode material, exhibiting cycling characteristics similar to those of MnO₂ cathodes^[61]. As shown in Figure 5A, researchers analyzed the reversible formation and dissolution of ZHS during charge-discharge cycles. The ZHS cathode exhibits markedly reduced capacity in electrolytes lacking Mn^2+[99]. During the interaction between ZHS and Mn²⁺, the presence of abundant OH^- lowers the electrooxidation potential of Mn²⁺. Thus, with the assistance of ZHS, Mn²⁺ may be re-deposited as manganese oxides [Figure 5B]^[100]. This mechanism emphasizes that the electrochemical activity of ZHS stems from the liberation and reintegration of Mn²⁺, providing a new perspective for understanding the energy storage mechanism of MnO₂.

Figure 5. (A) The proposed charge storage mechanism of the β-MnO₂ cathode. Reprinted with permission^[99]. Copyright 2018, Royal Society of Chemistry; (B) The reaction mechanism for the α-MnO₂. Reprinted with permission^[100]. Copyright 2022, Elsevier; (C) The dissolution of MnO₂ in acidic electrolytes. Reprinted with permission^[101]. Copyright 2022, Royal Society of Chemistry; (D) Cycling profiles of the ZHS and the corresponding SEM images. Reprinted with permission^[102]. Copyright 2023, Royal Society of Chemistry; (E) The reaction mechanism of the MnO₂/CC electrode. Reprinted with permission^[103]. Copyright 2022, Royal Society of Chemistry; (F) Mn-based competitive capacity evolution protocol. Reprinted with permission^[104]. Copyright 2020, Elsevier. SEM: Scanning electron microscopy; ZHS: zinc hydroxide sulfate.

Additionally, traditional views suggest that the incorporation of Mn²⁺ can suppress manganese dissolution, thereby enhancing the battery’s ability to maintain cycling performance over multiple charge-discharge cycles. However, this explanation does not fully account for the observed fluctuations in battery capacity following the addition of Mn²⁺ to ZnSO₄ electrolytes. To address this phenomenon, an in-depth study monitored changes in Mn²⁺ concentration and electrolyte pH at different stages of cycling processes. As shown in Figure 5C, the cycle curves are divided into four regions: the activation region, the H⁺/Zn²⁺ dominated region, the Mn²⁺ dominated region, and the final attenuation region^[101]. Proton-coupled reactions (PCRs) are closely associated with energy conversion and storage in AZIBs. Specifically, pH changes regulate the formation and dissolution of Mn²⁺ and MnO₂, which in turn affect the cycling capacity of AZIBs. Consequently, the behavior of Mn²⁺ is tightly linked to the pH level in the vicinity of the electrode/electrolyte interface. Notably, during battery charging, proton concentration significantly impacts the deposition potential of Mn²⁺. Due to the high electrolyte pH, manganese oxides are more readily formed [Figure 5D]^[102]. During discharge, hydrogen evolution reactions and irreversible H⁺ deintercalation reactions consume H⁺, which cannot be regenerated during subsequent charging. This leads to an increase in electrolyte pH, thereby promoting the deposition of manganese oxides onto the cathode [Figure 5E]^[103].

Therefore, elucidating the intrinsic mechanism of the MnO₂/Mn²⁺ dissolution-deposition phenomenon, clarifying the reaction mechanism of aqueous zinc-manganese batteries [Figure 5F], explaining capacity fluctuations in Zn-MnO₂ batteries, and developing high-performance aqueous zinc-manganese secondary batteries are critical issues requiring in-depth investigation at present^[104].

Structural control

When MnO₂ serves as the cathode material for AZIBs, variations in its crystal structure (e.g., α, β, γ, δ types) exert a substantial influence on the electrochemical performance, and crystal transformation may take place during the cycling process^[60]. α-MnO₂ features a layered structure characterized by a substantial interlayer spacing, which facilitates the intercalation of Zn²⁺ and endows it with a relatively high initial capacity. Nevertheless, during the cycling process, the structure is liable to transform into β-MnO₂ as a result of the dissolution of Mn²⁺, thereby leading to capacity attenuation^[53]. The compact tunnel structure of β-MnO₂ restrains the dissolution of Mn and exhibits excellent cycling stability. However, the narrow tunnels restrict the diffusion of Zn²⁺ and result in a relatively low capacity. γ-MnO₂ is a mixed structure of α and β, characterized by both high capacity and a certain degree of cycling stability^[58]. However, δ-MnO₂ (layered hydrated structure) exhibits poor capacity and rate performance because the interlayer water molecules impede ion transport. Crystal form transformation is typically driven by the dissolution-deposition mechanism (e.g., the re-deposition of Mn²⁺ into different crystal forms subsequent to dissolution), or induced by lattice stress resulting from the intercalation of Zn^2+[55]. Among these, β-MnO₂ exhibits superior reversibility owing to its robust structural rigidity and the arduousness of crystal form transformation. In contrast, the layered structure of α-MnO₂ endows it with a high capacity. Nevertheless, the cycle life is restricted because of the irreversibility of crystal form transformation. The equilibrium between capacity and cycling stability can be optimized by regulating the synthesis conditions (e.g., temperature and pH value) or introducing heterogeneous ions (such as Zn²⁺ and Co²⁺) to stabilize the target crystal form^[59].

The structural collapse of MnO₂ can be effectively alleviated, and its structural stability enhanced, through structural engineering strategies including interlayer intercalation, in-situ coordination, and introduction of oxygen vacancies^[105]. Researchers designed ammonium ion pre-embedded layered δ-MnO₂ nanosheets (denoted as AMO) as the cathode for AZIBs^[106]. During ion-exchange reactions, ammonium cations can replace the initially intercalated potassium ions in pristine δ-MnO₂ nanosheets. As shown in Figure 6A, the embedded ammonium ions act as interlayer pillars, expanding the lattice spacing of AMO and forming a hydrogen-bonding network. Consequently, AMO mitigates the Jahn-Teller effect and enables fast Zn²⁺ diffusion kinetics. The robust AMO cathode exhibits excellent electrochemical performance, including high specific capacity, superior rate capability, and extended cycle life. As illustrated in Figure 6B, typical flower-like δ-MnO₂ nanostructures with optimized oxygen vacancies were constructed via hydrothermal and reduction treatments^[107]. Moderate oxygen vacancies in δ-MnO₂ effectively maintain the equilibrium of H⁺/Zn²⁺ chemical adsorption-desorption, facilitating redox reactions and accelerating electron transfer kinetics. Furthermore, they fundamentally enhance electrochemical activity and conductivity, leading to faster reaction kinetics and more reversible redox processes. This cathode material retains high specific capacity, remarkable rate capability, and excellent cycle stability. Through interlayer engineering, PVP-intercalated MnO₂ hybrid superlattices with significantly expanded (001) crystal planes were successfully synthesized. In these structures, electronic entropy and discriminative plasmonic Grotthuss intercalation behavior can be modulated during Zn²⁺/H⁺ co-insertion. As shown in Figure 6C, the increased electronic entropy triggers more t2g-eg transitions, thereby enhancing reactivity. Meanwhile, plasmonic Grotthuss-selective intercalation arises from the hybrid superlattice architecture and optimized charge distribution^[108]. The significant improvements in specific capacity and rate performance of δ-MnO₂ are attributed to both enhanced electronic entropy of Mn and selectively accelerated proton transport dynamics. An energy barrier favorable for H⁺ transport and suppression of strong Coulombic interactions with Zn²⁺ collectively promote fast proton dynamics and cyclic stability.

Figure 6. (A) Illustration of the synthesis of AMO. Reprinted with permission^[106]. Copyright 2023, John Wiley and Sons; (B) The preparation procedure for δ-MnO₂ with rich oxygen vacancies (δ-MnO_2-x). Reprinted with permission^[109]. Copyright 2023, Elsevier; (C) PVP intercalation in δ-MnO₂. Reprinted with permission^[108]. Copyright 2023, John Wiley and Sons; (D) Ti₃C₂ MXene nanosheets with good conductivity enable fast electron transfer kinetics. Reprinted with permission^[109]. Copyright 2023, Elsevier; (E) The advantages of δ-MnO₂-C NA structure. Reprinted with permission^[110]. Copyright 2023, John Wiley and Sons; (F) The synthesis process for a-MnBO_x. Reprinted with permission^[111]. Copyright 2023, John Wiley and Sons. PVP: Polyvinylpyrrolidone; AMO: ammonium ions doped manganese dioxide.

As depicted in Figure 6D, closely linked MnO₂@MXene hetero-nanosheets were synthesized via in-situ polymerization of dopamine on Ti₃C₂@MXene nanosheets, followed by a redox reaction between KMnO₄ and polydopamine^[109]. During this process, thin layer-like δ-MnO₂ nanosheets were vertically deposited on MXene nanosheets, substantially promoting rapid electron/ion transfer and enhancing structural stability. Notably, the MnO₂@MXene hetero-nanosheets exhibit a unique Zn²⁺ storage mechanism in Zn(OTf)₂/TEP organic electrolytes, overcoming the limitations and side reactions associated with unprotonated storage in traditional aqueous electrolytes. Consequently, the MnO₂@MXene hetero-nanosheet-based thin-film cathode delivers excellent Zn storage performance. MnO₂ nanocrystal-carbon hybrid frameworks were constructed via in-situ reactions between a rationally designed dimethylimidazole-based MOF array and KMnO₄. The self-supported arrays, consisting of a carbon skeleton and MnO₂ nanocrystals, endow the δ-MnO₂-C NA hybrid electrode with an enlarged electrolyte contact area, enhanced conductivity, and increased active sites. Furthermore, the carbon framework inhibits Mn dissolution and mitigates volume fluctuations, resulting in favorable electrochemical performance-particularly outstanding ultra-long cycling durability [Figure 6E]^[110].

As illustrated in Figure 6F, an amorphous manganese borate (denoted as a-MnBO_x) cathode material for AZIBs was designed, featuring irregular coordination between divalent Mn²⁺ ions and BO₃^3- planar triangular or BO₄^5- tetrahedral structures^[111]. The boric acid ligand effectively stabilizes Mn²⁺ in its divalent state and enables a-MnBO_x to form a 3D stable porous framework with a large specific surface area (217 m² g^-1) and high electrochemical reactivity. Additionally, the surface of a-MnBO_x serves as an active component, facilitating reactions such as ion insertion/extraction and reversible conversion. In summary, electronic structure modulation achieves optimized electrochemical performance, primarily reflected in enhanced main structural stability and other key aspects. This enhancement not only strengthens overall performance but also paves the way for the practical application of MnO₂ as cathodes in AZIBs, promoting their efficient integration into real-world scenarios^[112].

Morphological control

The morphological regulation of MnO₂, such as nanowires, nanosheets, hollow structures, and hierarchical structures, exerts a substantial influence on their electrochemical performance. Distinct morphologies optimize the storage behavior of Zn²⁺ by altering the specific surface area, ion diffusion path, structural flexibility, and electron transport capacity of MnO₂. Firstly, there exists one-dimensional morphology, encompassing nanowires, nanorods, and so forth. Their structural characteristics involve directional ion transport channels, robust structural flexibility, and outstanding electron conduction ability, which together contribute to stable cycling performance. Furthermore, two-dimensional morphology, such as nanosheets and nanoplates, exhibits large specific surface areas and uniform dispersion properties. These attributes facilitate efficient ion diffusion and enhanced electrochemical reactions, contributing to improved energy density and cycling stability. Additionally, three-dimensional morphologies, such as hierarchical nanostructures and porous frameworks, offer abundant active sites and effective pathways for ion and electron transport. These unique architectures further enhance the overall performance of electrochemical devices by promoting rapid kinetic processes and facilitating mass transport. Regarding material regulation, template methods, including the SiO₂ template and metal-organic framework template, are predominantly employed to precisely control the three-dimensional morphology. Additionally, self-assembly methods, such as the sol-gel method and self-assembly process, can be utilized to prepare hierarchical nanostructures and regulate the pore size distribution^[113].

Currently, the morphology regulation of aqueous zinc-manganese batteries encompasses three main categories. The first category includes template method, self-assembly method, and solvothermal method^[114]. These approaches can effectively modulate the morphology and structure of materials, thereby enhancing electrochemical performance. Wang et al. prepared novel manganese-deficient MnO₂micro-nano spheres with interlayer “Ca²⁺ pillars” (denoted as CaMnO-140) through a low-temperature(140 °C) hydrothermal process [Figure 7A]^[58]. This material not only improved electrochemical efficiency but also delivered excellent specific capacitance, as well as remarkable power and energy densities. The second category leverages nanostructured materials with relatively large specific surface area and excellent electrical conductivity, which has emerged as a research hotspot in the field of aqueous zinc-manganese batteries. Manganese oxide nanomaterials with diverse morphologies can be fabricated via methods such as the solvothermal method and chemical vapor deposition. As shown in Figure 7B, the researchers used the oxidized graphene as a reducing agent to reduce KMnO₄, yielding δ-MnO₂ nanosheets^[114]. These δ-MnO₂ nanosheets can enhance ion diffusion and structural stability. Yu et al. developed a manganese-cerium doped composite cathode (CS@Ce-MnO₂) with a hierarchical carbon sphere core and cerium-doped manganese dioxide nanosheets^[115].

Figure 7. (A) Structural and morphological analyses. Reprinted with permission^[58] Copyright 2024, John Wiley and Sons; (B) The illustration of the formation of δ-MnO₂ nanosheets. Reprinted with permission^[114]. Copyright 2019, Elsevier; (C) Schematic sketch of the synthesis and possible cycling process of β-MnO₂ and δ-MnO₂ submicrospheres. Reprinted with permission^[116]. Copyright 2019, Elsevier; (D) Schematic illustration of the MnO₂/MXene superlattice synthesis. Reprinted with permission^[117]. Copyright 2023, American Chemical Society.

The third category involves combining manganese oxides with conductive materials (e.g., carbon nanotubes and graphene) to effectively improve the electrical conductivity and mechanical strength of electrodes. Guo et al. constructed a high-performance Zn/MnO₂ battery based on salt anions in the aqueous electrolyte. As depicted in Figure 7C, MnO₂ herein exhibits a vertically aligned nanosheet structure^[116], which is crucial for the cycling stability of Zn/MnO₂ aqueous batteries. Wang et al. fabricated 2D MnO₂/MXene superlattices through liquid-phase preparation of single-layer MnO₂ and Ti₃C₂T_x MXene nanosheets [Figure 7D]. In this process, single-layer MnO₂ nanosheets and single-layer MXene nanosheets are arranged at intervals^[117]. The MXene nanosheets can separate the MnO₂ nanosheets and prevent them from aggregating. MXene nanosheets can separate MnO₂ nanosheets to prevent aggregation; moreover, they can enhance electrical conductivity, maintain the stability of overall structural performance, and further enable rapid electron transport. Additionally, the regularly arranged and planarly stacked MnO₂ nanosheets and MXene nanosheets can provide exposed active sites and enhance ion diffusion capability.

Overall, the morphology regulation of aqueous zinc-manganese batteries constitutes a multidimensional and comprehensive research domain. Diverse morphology regulation strategies interact synergistically, enhancing key metrics such as electrochemical performance, cycling stability, and safety of the batteries from multiple perspectives^[118]. Morphology regulation offers a significant approach for enhancing the performance of AZIBs cathode materials by optimizing the structural dimension, pore network, and interface characteristics of MnO₂. In the future, it is essential to integrate computational simulation to design morphological structures that possess both high capacity and long service life, and to advance the practical application of AZIBs. This lays a solid foundation for the practical application of aqueous zinc-manganese batteries in diverse fields, including large-scale energy storage and portable electronic devices, and facilitates their advancement toward higher performance levels.

Defect engineering

The introduction of various defects into MnO₂ cathode materials via atomic vacancies, element doping, or defect engineering can significantly modulate the electrochemical properties of AZIBs^[119]. These defects create additional ion transport channels, which reduce Zn²⁺ diffusion resistance, accelerate charge transfer, and thus enhance rate capability^[120]. The increased number of active sites facilitates electrochemical reactions, thereby improving the material’s adsorption and storage capacity for Zn²⁺ and further increasing the specific capacity of the battery^[121]. Furthermore, defects can mitigate volume changes in the electrode during cycling, leading to enhanced structural stability. Oxygen vacancies facilitate the intercalation of Zn²⁺ by modulating the intercalation process of MnO₂, offering a more accommodating pathway. Simultaneously, the oxygen vacancy, functioning as a positively charged defect, can attract Zn²⁺ via Coulombic interaction, thereby reducing the energy barrier associated with intercalation. Appropriate lattice defects can regulate the D-band center of MnO₂, which strengthens the binding energy with Zn²⁺ and enhances the reversibility of the reaction. Nevertheless, excessive distortion will disrupt the periodicity of the layered structure, leading to capacity attenuation. Manganese vacancies can effectively optimize the electronic structure, create ion storage sites, inhibit dissolution, and stabilize the structure. Optimizing the electronic structure and ionic transport path of manganese dioxide through defect engineering is a crucial strategy for enhancing the performance of AZIBs^[67].

As shown in Figure 8A, the transformation of α-MnO₂ nanowires into Mo-Vo nanoparticles (as cathodes with rich surface oxygen defects) is achieved by adjusting annealing conditions in argon^[122]. The small size of Mo-Vo nanoparticles can effectively promote the spatially uniform distribution of volume stress during ion intercalation, thereby improving the structural stability of the Mo-Vo cathode. By regulating the surface oxygen defects of MnO₂, the Zn²⁺ intercalation pseudocapacitive behavior in the Mo-Vo cathode is significantly enhanced, which in turn improves cycling stability and redox kinetics. Amorphous MnO₂ nanosheets with abundant oxygen vacancies were prepared using sodium carboxymethyl cellulose (CMC) as a capping agent, as illustrated in Figure 8B^[123]. During growth, CMC preferentially attaches to the (003) faces of MnO₂, guiding its crystal growth and regulating its morphology. The small nanosheets expose abundant edge sites and surface oxygen vacancies, which facilitate the insertion/extraction of H⁺ and Zn²⁺ in ZIBs. Additionally, CMC stabilizes Mn³⁺ during preparation and inhibits the Jahn-Teller effect. A novel Al-intercalated MnO₂ cathode is engineered, as revealed in Figure 8C^[124]. Since Al is effectively embedded in the tunnel-type MnO₂ framework, strong Al-O bonds are formed, which significantly enhance the cathode’s specific capacity and cycle retention rate. This configuration not only optimizes the charged ion states and electronic bandgap but also ensures reversible phase transformation of the Al-intercalated MnO₂ cathode and suppresses Mn dissolution. Figure 8D depicts a method for depositing MnO₂ nanosheets on commercial carbon cloth (CC) (used as a skeleton) to obtain MnO₂@CC^[125]. The structural engineering of MnO₂@CC is achieved via n-butyllithium treatment, which forms oxygen vacancies, partially reduces Mn ions, and slightly enlarges the interlayer distance to form MnO_2-x@CC. This material exhibits high specific capacity, excellent rate capability, and long-term cycling stability. A layered β-MnO₂ cathode composed of staggered nanosheet spheres is developed via effective defect engineering (fluorine doping and oxygen vacancies), resulting in improved ion insertion/transport dynamics and enhanced conductivity [Figure 8E]^[126]. Thus, defect engineering effectively enhances the electrochemical performance of batteries.

Figure 8. (A) The transition of nanowire (α-MnO₂) to nanoparticle (MO-Vo). Reprinted with permission^[122]. Copyright 2022, American Chemical Society; (B) Schematic drawing for fabricating the different MnO₂ samples. Reprinted with permission^[123]. Copyright 2022, John Wiley and Sons; (C) Pristine MnO₂ and Al-intercalated MnO₂. Reprinted with permission^[124]. Copyright 2021, Elsevier; (D) Fabrication process of the MnO₂@CC and MnO_2-x@CC. Reprinted with permission^[125]. Copyright 2021, American Chemical Society; (E) Schematic mechanism for the formation of hierarchical spheres with interlaced nanosheets via the F-doping of β-MnO₂. Reprinted with permission^[126]. Royal Society of Chemistry.

Development of electrolytes

The electrolyte functions as a “connector” between the positive and negative electrodes, offering them a practical operating environment. With the advancement of AZIBs, diverse zinc salts have been employed as electrolytes, primarily encompassing ZnSO₄, Zn(ClO₄)₂, ZnCl₂, Zn(CF₃SO₃)₂, Zn(CH₃COO)₂, and Zn(TFSI)₂,etc.^[69]. Nevertheless, the utilization of aqueous electrolytes presents numerous challenges to the positive electrode, encompassing issues such as dissolution and by-products. To address this issue, researchers have put forward a diverse range of solutions, primarily encompassing the utilization of high-concentration salts, electrolyte additives, gel electrolytes, solid electrolytes, and ionic liquids. These methods enhance the overall electrochemical performance of AZIBs through the rational expansion of the electrolyte window, the adjustment of the solvation structure of the electrolyte, and the improvement of the stability at the interface between the electrode and the electrolyte^[54].

In general, conventional inorganic salt electrolytes for AZIBs have certain limitations, such as low Coulombic efficiency (below 75%) and a narrow electrochemical stability window (ESW)^[127]. Researchers reported a method of applying Zn(CF₃SO₃)₂ organic electrolyte in AZIBs, which exhibits high Coulombic efficiency (100%) and a wide electrochemical window. As shown in Figure 9A, oxygen evolution side reactions are significantly inhibited, which is much weaker than in ZnSO₄ electrolytes^[128]. Further experimental data demonstrate excellent reversibility and fast Zn deposition/stripping kinetics in the Zn(CF₃SO₃)₂ electrolyte system. In aqueous rechargeable zinc-ion batteries (ARZIBs), prior to Zn²⁺ deposition, the zinc plate surface has inherent roughness. During deposition, protruding tips inevitably form, which increase the electric field strength and lead to the fastest ion deposition and dendrite formation. Figure 9B shows that a small amount of diethyl ether (Et₂O) as an electrolyte additive can significantly improve the performance of Zn-MnO₂ cells. An appropriate amount of Et₂O molecules effectively inhibits zinc dendrite formation on the zinc anode^[129], thereby enhancing the cycling stability of rechargeable aqueous zinc-ion batteries. The four-in-one functional additive triethyl phosphate (TEP) was incorporated into the conventional ZnSO₄ electrolyte. Experimental results showed that TEP participates in forming a dense inorganic interface. The dendrite-free zinc anode exhibits higher stability and improved corrosion resistance. This enhances the battery’s stability and lifespan, as shown in Figure 9C^[130]. Hao et al. reported an example of improving Zn/LiFePO₄ battery performance by adding sodium benzenesulfonate (SDB) additives. In Figure 9D, the SDB-containing electrolyte enables smooth anodic Zn²⁺ deposition (instead of vertical growth of lamellar Zn dendrites) while enhancing Li⁺ migration in LiFePO₄^[131]. Researchers also reported a method to modify battery performance by adding Na⁺ to the electrolyte, with the main mechanism depicted in Figure 9E^[132]. Na⁺ can form a positive electrostatic protective layer on the surface of zinc protrusions, fundamentally suppressing zinc dendrite formation. This results in highly reversible zinc stripping/plating processes with high Coulombic efficiency.

Figure 9. (A) Cyclic voltammograms of Zn electrode in 1M Zn(CF₃SO₃)₂. Reprinted with permission^[128]. Copyright 2016, American Chemical Society; (B) Schematics of morphology evolution for Zn anodes with and without Et₂O additive. Reprinted with permission^[129]. Copyright 2019, Elsevier; (C) Zn deposition process in blank ZnSO₄ and TEP-containing electrolytes. Reprinted with permission^[130]. Copyright 2024, John Wiley and Sons; (D) Schematic illustration of the hybrid aqueous Zn/LFP battery. Reprinted with permission^[131]. Copyright 2019, John Wiley and Sons; (E) Schematic diagram about the effect of Na₂SO₄ additive. Reprinted with permission^[132]. Copyright 2018, Springer Nature; (F) Schematic illustration of the Zn-MnO₂ cell assembled in a UV-vis cuvette. Reprinted with permission^[105]. Copyright 2022, Elsevier; (G) Schematic diagram of additives on the capacity and voltage. Reprinted with permission^[133]. Copyright 2021, Elsevier; (H) Feasibility study for the designed Zn-Mn hybrid aqueous battery. Reprinted with permission^[134]. Copyright 2020, John Wiley and Sons; (I) Schematic diagram of formation of stable coordinated hydrated structures. Reprinted with permission^[135]. Copyright 2022, John Wiley and Sons; (J) Theory-Driven Design of a Cationic Accelerator for High-Performance Electrolytic MnO₂-Zn Batteries. Reprinted with permission^[135]. Copyright 2022, John Wiley and Sons.

In aqueous rechargeable Zn-MnO₂ cells, the electrolyte pH gradually shifts alkaline during reactions, hindering reversible electrochemical processes. As shown in Figure 9F, a straightforward electrolyte modification approach was utilized. By comparing the pH of the electrolyte containing a pH indicator with its initial value, the proton release associated with Mn²⁺ additive oxidation and the stable voltage at the end of the charging cycle were determined^[105]. The effect of floating voltage was also evaluated to maintain electrolyte pH stability. To date, electrolyte regulation via additives remains the simplest and most convenient method to modulate cathode voltage and capacity. Various additives (including Na⁺, Mg²⁺, Al³⁺, Br^-, etc.) can optimize capacity and voltage properties by enhancing reactant utilization, promoting mixed-ion intercalation, or regulating contact potentials, with a representative example shown in Figure 9G^[133]. Chao et al. developed an innovative high-rate, high-voltage Zn-Mn hybrid aqueous battery (HAB) by simply incorporating Ni²⁺ into the electrolyte, as shown in Figure 9H^[134]. They found that the Ni-MnO₂ electrode exhibits significantly lower solution and electrode resistance compared to pure MnO₂. Additionally, the charge transfer resistance at the Ni-MnO₂ electrode is notably reduced, and the electrode maintains excellent cycling stability even at high rates. These improvements greatly enhance the practicality and cost-effectiveness of aqueous Zn-ion batteries. Sluggish kinetic processes at the cathode (MnO₂/Mn²⁺) and anode (Zn/Zn²⁺) in aqueous MnO₂-Zn batteries limit their electrochemical performance. To address this, the concept of a cationic accelerator (CA) in the electrolyte was proposed. Experimental results demonstrate that CA significantly accelerates cation mobility and enhances charge transfer at the electrode-electrolyte interface. This facilitates Mn²⁺ and Zn²⁺ deposition/dissolution while boosting reaction kinetics at both the MnO₂/Mn²⁺ and anodic Zn/Zn²⁺ anode interfaces. As illustrated in Figure 9I, this innovation paves the way for developing high-performance energy storage systems. Figure 9J investigates poly(vinylpyrrolidone) as a model to realize the rational design of electrolytes via the cation promoter strategy. The cation promoter can effectively accelerate the rapid migration of cations in the electrolyte and enable sufficient charge transfer at the electrode-electrolyte interface, thereby facilitating the deposition/dissolution processes of manganese ions (Mn²⁺) and zinc ions (Zn²⁺), as well as enhancing the reaction kinetics of the cathode (manganese dioxide/manganese ions, MnO₂/Mn²⁺) and anode (zinc/zinc ions, Zn/Zn²⁺) reactions. By integrating theoretical predictions with experimental studies, this research offers promising opportunities for the advancement of aqueous zinc-ion batteries^[135].

Other strategies

The interface regulation strategy can notably enhance the electrochemical performance. This is achieved by optimizing the electrode - electrolyte interface through the use of surface-coated carbon-based materials or metal oxide materials. By doing so, ion-selective channels are constructed to inhibit the dissolution of Mn and reduce the internal interface resistance when MnO₂ serves as the cathode of AZIBs. The fundamental advantages of these strategies reside in suppressing side reactions, accelerating ion transport, and stabilizing the interface structure, offering a crucial approach to overcome the bottlenecks of cycle life and rate performance in AZIBs. As depicted in Figure 10A, manganese-based Prussian blue analog (Mn-PBA) is chosen as a seed layer to offer a stable surface for MnO₂ electrodeposition^[136]. Due to the substantial specific surface area of Mn-PBA, the deposition/dissolution kinetics between Mn²⁺ and MnO₂ are notably enhanced. In Figure 10B, the inherent zwitterionic groups on sulfobetaine chains can offer distinct ion migration channels for positive and negative ions, which considerably promotes electrolyte ionic transport^[137]. As illustrated in Figure 10C, this insight can only be gained after isolating the electrochemical reactions on the surface of MnO₂ and eliminating any MnO₂ deposits on the metal collector within the electrochemical cell^[138]. As shown in Figure 10D, acesulfame potassium, serving as a co-solute with strong polarity, is incorporated into a conventional low-concentration aqueous electrolyte. This incorporation enhances the ionic conductivity of the electrolyte and accelerates the desolvation process of zinc ions at the electrode/electrolyte interface^[139]. Figure 10E presents a comprehensive MnO₂/Mn²⁺ redox chemistry, which is put forward to account for the abnormal electrochemical behavior of the β-MnO₂ cathode. This suggests that a reversible dissolution/deposition process of layered manganese oxides takes place subsequent to the initial MnO₂ dissolution reaction^[100]. It reveals a new perspective for studying the working mechanism of AZIBs.

Figure 10. (A) Schematic illustration of ZnMn-PBA as a seed layer to assist MnO₂ deposition-dissolution for cathode-free AZIBs. Reprinted with permission^[136]. Copyright 2024, John Wiley and Sons; (B) Schematic of the fabrication of the ZSC-gel matrix. Reprinted with permission^[137]. Copyright 2020, John Wiley and Sons; (C) Transformation of the electrochemical interface at a dynamic δ-MnO₂ cathode. Reprinted with permission^[138]. Copyright 2021, John Wiley and Sons; (D) Schematic diagram of the zinc anode status. Reprinted with permission^[139]. Copyright 2025, John Wiley and Sons; (E) The schematic diagram of the proposed charge storage mechanism of the β-MnO₂ cathode. Reprinted with permission^[100]. Copyright 2022, Elsevier. AZIBs: Aqueous zinc-ion batteries.

CONCLUSIONS AND FURTHER PROSPECTS

MnO₂, as a cathode material for AZIBs, has garnered significant attention owing to abundant manganese reserves, low cost, and high reversible capacity. As illustrated in Figure 11, with the advancement of MnO₂ research, the ion storage mechanisms of MnO₂ as a cathode in AZIBs during cycling primarily include the conversion reaction mechanism, Zn²⁺ intercalation mechanism, dissolution-deposition mechanism, and H⁺/Zn²⁺ co-intercalation mechanism. However, MnO₂ suffers from issues such as manganese dissolution, irreversible structural transformation, side reactions, and the Jahn-Teller effect during cycling, which restrict its long-term stability. To address these challenges, various modification strategies have been proposed, including structural control, defect engineering, morphological design, and electrolyte optimization. Element doping adjusts the lattice structure, regulates the crystal framework, improves ion diffusion pathways, enhances structural stability, and mitigates capacity fading caused by phase transitions. Surface modification of MnO₂ particles using carbon materials or conductive polymers can significantly suppress manganese dissolution, increase electrical conductivity, and enhance the cycling stability of MnO₂. Moreover, combining MnO₂ with conductive materials to construct a three-dimensional conductive network improves the electron and ion transport capabilities of the electrode and alleviates volume expansion. Introducing oxygen vacancies or lattice defects enhances the electronic conductivity and Zn²⁺ storage capacity of the material. Stabilizing the MnO₂ surface with high-concentration electrolytes or additives inhibits side reactions and extends the cycle life.

Figure 11. Summary of the ion storage mechanisms, optimization strategies, and prospects of MnO₂ for future high-performance AZIBs. AZIBs: Aqueous zinc-ion batteries.

The aforementioned modification strategies are mutually complementary, offering a multidimensional solution for the efficient application of MnO₂ cathode materials in AZIBs. These strategies significantly enhance the cycle stability and rate capability of MnO₂, thereby enabling its more efficient utilization. Regarding the current limitations of MnO₂ as a cathode for AZIBs, future research directions will primarily focus on the following aspects:

Development of multifunctional materials: MnO₂ is constrained by inherent drawbacks, such as poor electronic conductivity, inferior rate capability, and inadequate cycling stability, thus highlighting the critical need for developing multifunctional materials. Integrating MnO₂ with conductive materials (e.g., graphene, carbon nanotubes) to fabricate multifunctional composites enhances the electrical conductivity of MnO₂, increases the specific surface area of the material, and offers more active sites, thereby boosting its electrochemical reaction efficiency during Zn²⁺ insertion/extraction. Furthermore, surface modification techniques can enhance the conductivity and structural stability of MnO₂, leading to improved overall battery performance. Controlling the particle size, morphology, and structure of MnO₂ via nanotechnology increases its specific surface area, provides more active sites, and improves Zn²⁺ intercalation/deintercalation efficiency. Notably, engineering MnO₂ into a 3D network structure helps mitigate volume expansion during Zn²⁺ insertion and reduces the risk of electrode structural rupture. Through these modification strategies, the electrochemical performance of MnO₂ can be significantly enhanced, particularly its stability under long-term cycling and performance at high current densities.

Electrode/electrolyte interface optimization: The electrolyte/electrode interface in zinc-ion batteries is critical to the long-term stability and safety of AZIBs. The issues of zinc dendrite formation and interfacial instability can be addressed by designing appropriate electrolytes and interface layers. Electrolyte additives, such as organic acids and zinc salts, can modulate the deposition behavior of metallic zinc, suppress the uneven deposition of Zn²⁺, and inhibit dendrite growth. Furthermore, introducing a thin, conductive interlayer (e.g., conductive polymers, inorganic/organic composites) between the MnO₂ cathode and zinc anode can effectively facilitate ion and electron transport, reduce side reactions at the electrode/electrolyte interface, and prevent corrosion between MnO₂ and metallic zinc. Additionally, constructing a protective layer on the electrode surface using highly conductive polymers (e.g., polyaniline) or conductive carbon materials not only enhances interfacial stability but also promotes the smooth intercalation of Zn²⁺ during cycling, inhibits dendrite growth, and contributes to the development of stable AZIBs.

In-depth investigation of the zinc storage mechanism: Advanced characterization techniques, such as in-situ synchrotron radiation X-ray diffraction, are employed to explore the dynamic changes in microstructure, morphology, and composition of MnO₂ materials during Zn²⁺ insertion and adsorption. Density functional theory (DFT) calculations are utilized to determine the optimal configuration of Zn²⁺ intercalated into MnO₂. Moreover, the structural transformation, electrode/electrolyte interface stability, ion transport mechanisms, and characteristics of the cathode electrolyte interphase (CEI) film of MnO₂ during cycling processes are systematically studied. The principles governing ion transport, interface stability rules, and criteria for structural stability are elaborated. Additionally, the correlation between microstructure and cycling performance remains to be clarified, and the storage behavior of sodium ions is analyzed. The integration of theoretical and experimental approaches provides crucial insights for optimizing MnO₂ materials, offers theoretical support for enhancing their efficiency and durability in energy storage devices, and guides structural optimization in the development of MnO₂-based composite materials.

Environmental friendliness and sustainable development: With the growing demand for AZIBs, the recycling and reuse of spent batteries have become increasingly critical. Recovering and reusing manganese from waste zinc-based batteries can not only reduce the cost of raw material procurement but also mitigate resource scarcity and environmental pollution. Although zinc-ion batteries pose lower environmental risks compared to lithium-ion batteries, substantial research efforts are still required for their waste treatment and recycling during large-scale application. The development of efficient recycling technologies enables the effective recovery of elements such as manganese and zinc from batteries, thereby reducing reliance on natural resources and enhancing the sustainability of the energy storage industry. Furthermore, the recovered manganese can be utilized to synthesize new MnO₂ materials for battery cathodes, forming a closed-loop recycling system to improve resource recycling efficiency. This process not only minimizes the environmental impact of waste batteries but also reduces the demand for natural ores, thus further promoting the green development of aqueous zinc-ion batteries.

High-performance AZIBs hold great promise for large-scale grid energy storage applications. It is crucial to continuously optimize the performance of AZIBs, further enhance their energy density, long-cycle stability, cost-effectiveness, and recyclability, and advance their commercialization. However, to date, in the context of Zn//MnO₂ batteries, critical challenges remain, including the ambiguity surrounding the internal mechanisms of intercalation-deintercalation and dissolution-deposition, as well as the failure to achieve high energy density. Addressing how to effectively integrate these two mechanisms, exploring the development of high-performance manganese-based electrodes, and fabricating practical aqueous zinc-ion batteries with high energy density and excellent cycling stability thus demands more in-depth investigation.