Recent development in addressing challenges and implementing strategies for manganese dioxide cathodes in aqueous zinc ion batteries

Insertion/desertion mechanism

The insertion of Zn²⁺ is the most basic reaction mechanism of MnO₂ cathodes. Possessing an ionic radius of 0.74 Å, Zn²⁺ is easily deserted/inserted during the charging/discharging process, which exists in host materials with layer or tunnel structures. Since the inception of the MnO₂/Zn cell, the primary proposed mechanism for energy storage involved the reversible insertion and extraction of Zn²⁺. Xu et al. contributed to this understanding by demonstrating that Zn²⁺ ions can indeed reversibly intercalate and deintercalate within the tunnel structure of α-MnO₂ when using electrolytes such as ZnSO₄ or Zn(NO₃)₂^[64]. α-MnO₂ is converted to ZnMn₂O₄ after Zn²⁺ insertion. The reaction equations are given as:

The crystal structure of α-MnO₂ is characterized by its large and settled tunnels, which are well-suited for accommodating guest cations, including Zn²⁺, as depicted in Figure 3A. The tunnel structures are indeed pivotal for the reversible intercalation and deintercalation of Zn²⁺, which are essential processes in the operation of ZIBs. Figure 3B provides a visual representation of the electrochemical activity of α-MnO₂ during the battery cycling. The presence of two distinct peaks near 1.3 and 1.7 V indicates the redox reactions occurring at the cathode during charging and discharging. The redox processes involving the insertion/extraction of Zn²⁺ ions from the α-MnO₂ structure are indicated by these peaks. The cycling behavior depicted in Figure 3C, where the material is evaluated in its pristine state, after Zn²⁺ ions are extracted at a potential of 1.7 V, and after Zn²⁺ ions are inserted at a potential of 1.3 V, confirms the reversible nature of these processes. Khamsanga and Alfaruqi highlight the advantages of using nanoscale δ-MnO₂, which has a shorter ion migration path than α-MnO₂. The characteristic of faster insertion and extraction of Zn²⁺ in δ-MnO₂ is indeed advantageous for enhancing the Zn²⁺ storage capacity. As shown in Figure 3D, the shorter ion migration path in δ-MnO₂ leads to a more efficient charge storage process, which is beneficial for the overall performance of ZIBs^[67,68]. Note that this mechanism exists not only in MnO₂ but also in other cathodes in AZIBs, such as vanadium-based oxides^[69], MnO^[70], Mn₂O₃^[66], organic compounds^[71], and PBAs^[42]. In 2012, Alfaruqi et al. showed that the MnO_x cathode in AZIB involves only the Zn²⁺ (de)intercalation reaction through MnO_x tunnels^[46]. This mechanism implies that Mn disproportionation reactions result in metal ion dissolution, leading to accompanying pH variations during reactions. The observation of pH changes during electrode discharge cycling, ranging from approximately 4.2 to more than 5.2, indicates the presence of a secondary or parasitic process distinct from the primary reaction. This suggests that the electrochemical behavior of MnO_x in ARZBs is more complex than initially anticipated.

Figure 3. (A) Working mechanism of AZIB; (B) The graph depicts the anodic and cathodic reactions of the AZIB separately; (C) Zn 2p core level spectra and XRD patterns of cathodic crystalline α-MnO₂; This figure is reprinted (adapted) with permission from Xu et al. Copyright (2012) John/Wiley & Sons, Inc^[64]. (D) XRD pattern of birnessite δ-MnO₂. This figure is reprinted (adapted) with permission from Alfaruqi et al. Copyright (2015) Elsevier Inc^[68].

Co-insertion/desertion mechanism

In cases where the cathode material features an open tunneling or layered structure, the electrochemical process can become more intricate than a simple Zn²⁺ ion insertion. In such instances, the reaction may also involve the participation of H^+[72] [Figure 4A]. Aqueous electrolytes in AZIBs contain a higher concentration of H⁺ than non-aqueous electrolytes^[55,73,74]. Using in situ X-ray diffraction (XRD) and ex situ transmission electron microscopy (TEM), Gao et al. confirmed the conjoined insertion and desertion of H⁺ and Zn²⁺ in α-MnO₂^[54]. H⁺ insertion brings to the MnOOH (Mn⁴⁺ to Mn³⁺). When H⁺ is inserted at the MnO₂ cathode, the environment on the electrode surface becomes OH^- rich as the H⁺ insertion, resulting in OH^- reacting with Zn²⁺ and SO₄^2- and forming zinc hydroxide sulfate ([Zn(OH)₂]₃ZnSO₄∙5H₂O) as a by-product in the electrolyte, as shown in the in-situ XRD spectra [Figure 4B]. During intercalation, where water-coordinated Zn²⁺ and Zn²⁺ ions are inserted into MnO₂, the resulting products are identified as ZnMn₃O₇∙2H₂O^[75] and ZnMn₂O₄^[76], as indicated by the XRD patterns in the corresponding charge/discharge states [Figure 4C]. This finding implies that the fluctuations in these peaks show a reversible Zn²⁺ insertion/desertion mechanism. The reaction equation for the -MnO₂/Zn cell is expressed as:

Figure 4. (A) Phase evolution of MnO₂; This figure is reprinted (adapted) with permission from Huang et al. Copyright (2019) Oahost^[72]. (B) Phase evolution of cathode; (C) Contour graph of in situ XRD data. This figure is reprinted (adapted) with permission from Gao et al. Copyright (2020) Wiley VCH^[54].

Furthermore, Bi et al. demonstrated that (Zn(OH)₂)₃(ZnSO₄)(H₂O)₄ by-product exists on the cathode surface during H⁺ insertion^[77]. After the by-products were dissolved in a mild acid, Zn 2p signals were found using X-ray photoelectron spectroscopy (XPS). The Zn²⁺ and H⁺ co-insertion/desertion is confirmed by the aforementioned results.

Conversion reaction mechanism

Other than the insertion/desertion type of charge storage mechanisms, Pan et al. proposed a chemical conversion reaction mechanism characterized by a chemical interaction between α-MnO₂ and MnOOH^[65]. This mechanism is distinct from the traditional intercalation/deintercalation process and is based on the chemical transformation of the cathode material during discharging^[65]. Upon complete discharge, protons originating from the water molecules in the electrolyte can react with α-MnO₂, producing MnOOH. During the discharge phase, hydroxide ions (OH^-) can engage in the reaction with ZnSO₄ and H₂O to generate a complex compound, ZnSO₄[Zn(OH)₂]₃·xH₂O [Figure 5A], which renders the system electrically neutral. In addition to α-MnO₂, β-MnO₂ was also reported to exhibit this charge storage mechanism, and its cycling reaction route is shown in Figure 5B^[78]. During the initial discharge phase, the transition from Mn³⁺ to Mn²⁺ occurs as protons react with β-MnO₂ to form MnOOH. The reaction process in the cathode involves the conversion and deposition of MnOOH and Mn²⁺ as ε-MnO₂ in the first charge. In later cycles, the MnOOH can be found, resulting from the reaction of the deposited MnO₂ with protons. During discharge, a portion of the MnOOH partially dissolves into Mn²⁺, generating Zn₄SO₄(OH)₆∙5H₂O (ZHS). During the charging process, the Mn²⁺ present in MnO₂ and ZHS is no longer detected. This cathode reaction process can be succinctly given as:

Figure 5. (A) The reaction path of β-MnO₂; (B) The XRD patterns of α-MnO₂; This figure is reprinted (adapted) with permission from Pan et al. Copyright (2016) Nature Research^[65]. (C) In situ pH changes of β-MnO₂/Zn. This figure is reprinted (adapted) with permission from Liu et al. Copyright (2021) Elsevier^[78].

The pH value shown in Figure 5C increases during discharging and decreases during the charge phase. This supports the involvement of H⁺ in the electrode reaction process. In the chemical conversion reaction mechanism, new materials are formed, unlike the first two ion-inserted energy storage mechanisms. In this process, hydrogen ions are the only ions inserted/extracted into the cathode material, while Zn²⁺ is primarily converted to ZHS and does not participate directly in the charging/discharging reactions. The energy density and cycle life of batteries are influenced by the types and amounts of conversion-type cathodes and the category and structure of substrates^[79]. Note that the crystal phase and electrolyte differences in manganese oxide necessitate a thorough and precise analysis of the reaction mechanism.

Dissolution-deposition mechanism

The dissolution-deposition process involves the electrical charge transfer between the electrodes, propelled by an electrical potential gradient. This process occurs under both kinetic and thermodynamic constraints, and it allows for the reversible movement of charges between the deposited electrode materials, facilitating the transformation of electrodeposition products back into soluble forms^[80]. Liang et al. designed a MnO₂/Zn battery based on the deposition-dissolution mechanism, with an electrolyte containing 0.3 mol/L ZnSO₄ and 0.3 mol/L MnSO₄^[81]. The new full battery design operates at a voltage approximately 0.55 V higher than traditional MnO₂-based aqueous batteries, attributed to the reaction of the deposition-dissolution mechanism.

In 2020, Guo et al. found that the insertion/desertion of Zn²⁺ and H⁺ occurred at the same time as the reaction, as illustrated in Figure 6A^[82]. The discharge process of the first cycle with α-MnO₂ or δ-MnO₂ as the host material involves a series of reactions that produce Mn²⁺ and OH^- ions. These ions then react with the surrounding ZnSO₄ in the electrolyte to form ZHS, which consumes a significant amount of H₂O. The depletion of H₂O in the vicinity of MnO₂ suppresses the subsequent dissolution of MnO₂, increasing the Mn²⁺ content in the electrolyte and pH. On the other hand, the newly formed ZHS reacts with Mn²⁺ during the charge process of the first cycle to produce birnessite-MnO₂, which substitutes for the original MnO₂ as the host material. This dissolution-deposition mechanism is a key process that dominates the energy storage process and significantly contributes to the specific capacity of the battery. In subsequent cycles, this dissolution-deposition mechanism continues to recur, with birnessite-MnO₂ acting as the host material. The intercalation/de-intercalation reaction of Zn²⁺ ions into and out of the MnO₂ structure is considered less critical in the overall energy storage process. In their study, Kankanallu et al. investigated the chemical, morphological, and structural alterations that occur in β-MnO₂ due to the dissolution-deposition process^[83]. A multimodal approach was employed, integrating operando X-ray diffraction and absorption spectroscopy and X-ray microscopy. Figure 6B and C highlights that the main capacity during the first discharge cycle comes from the dissolution of MnO₂ into Mn²⁺ ions. In parallel, the deposition of ZHS takes place in close proximity to the dissolution site, forming a layered structure on the MnO₂ surface. Upon charging, two distinct Zn-Mn complex phases are deposited within the electrode. One is an amorphous phase that can be reversibly formed, while the other is a crystalline phase that is not reversible. Li et al. studied a redox chemical reaction involving MnO₂/Mn²⁺ and interfacial regulation^[84]. The study observed that the discharge initially caused the dissolution of MnO₂ due to H⁺ conversion, consequently increasing the pH value and Mn²⁺ concentration in the electrolyte. Following this, the formation of ZHS occurred, with the process being largely governed by intricate dissolution and deposition reactions. As the charging phase progressed, the concurrent deposition of Zn²⁺ and Mn²⁺ created a Zn-rich birnessite phase, ZnxMn₃O₇, while simultaneously causing the dissolution of ZHS. Subsequently, this was followed by the conversion of Zn_xMn₃O₇ to Mn₇O₁₃ through Zn²⁺ desertion and Mn²⁺ deposition. In addition, there is a reverse phase transition where the simultaneous insertion of Zn²⁺ and H⁺ ions induces the Zn_xMn₃O₇ dissolution, generating ZHS. Continued insertion of H⁺ ions results in the further dissolution of Zn_xMn₃O₇ until the final formation of ZHS is achieved^[84].

Figure 6. (A) Energy storage mechanism in MnO₂/Zn batteries. This figure is reprinted (adapted) with permission from Guo et al. Copyright (2020) Elsevier^[82]. (B) Mn K-edge ex situ XAS; (C) Operando X-ray diffraction results. This figure is reprinted (adapted) with permission from Kankanallu et al Copyright (2023) RSC Publishing.^[83].

The proposal of the deposition-dissolution mechanism has laid the theoretical foundation for the next-generation MnO₂/Zn batteries. Unlike the ongoing debate over the dual- or single-electron transfer in conventional mildly acidic ARZBs, the Mn deposition-dissolution involves a dual electron transfer via the Mn²⁺/Mn⁴⁺ redox reaction. Consequently, the theoretical capacity of MnO₂/Zn batteries has doubled, reaching a promising value of 616 mAh g^-1.

The charge storage mechanism of MnO₂-based AZIBs has been a subject of debate since its early development. While various reaction routes have been proposed, several critical factors continue to pose challenges. Understanding the charge storage mechanism of AZIBs is still ongoing, although it is established that the process is not merely a straightforward insertion/desertion mechanism. It is important to note that the exact function of Mn²⁺ additives in electrochemical processes, especially concerning intercalation and/or conversion reactions, is not yet fully understood. Researchers are still exploring how these additives influence the reaction mechanisms and the overall performance of the electrochemical systems. Further investigation is required to elucidate the specific effects of Mn²⁺ on the electrochemical behavior. Unlike the study conducted by Lee, which employed in situ pH analysis to investigate the correlation between electrolyte pH and the formation of ZBS [Figure 7A]^[85], there is limited comprehensive research on the structural and phase changes and the influence of pH during the charge and the discharge cycles of the MnO₂ positive electrode in AZIBs^[72]. Also, the impact of Mn²⁺ additives on the fluctuations of pH and the precise function of ZBS remains ambiguous, mainly due to the variations in the concentrations of additives and salts employed in the reported configurations. Excess additives can hinder ZBS formation, which, however, can be significantly influenced by factors such as the changing electrolyte pH (according to the Nernst equation: $$\Delta E=E c-E a=\left(E_{c}^{\theta}-E_{a}^{\theta}\right)+\frac{R T}{n F} \ln \left(\frac{C_{H^{+}}^{4}}{C_{M n^{2+}}}\right)$$. The reduction in H⁺ concentration decreases the activation energy needed for Mn deposition. Increasing pH leads to the formation of a large amount of inert “dead Mn”, causing capacity decay and the generation of electrolytic MnO₂ [Figure 7B]^[86,87]. The claim that MnOOH acts as an intermediate or final discharge product in the presence of Mn²⁺ additives, which would imply a proton conversion reaction, is indeed a subject of contention. Although this claim is supported by ex situ characterizations, where the presence of MnOOH has been detected, it has not been confirmed by in situ analyses, underscoring the need for further clarification.

Figure 7. (A) Schematic representation of the proposed electrochemical reaction in a typical MnO₂/Zn aqueous battery. (B) The development of storage capacity and the mechanisms involved in energy storage during cycles that are influenced by variations in pH levels. This figure is reprinted (adapted) with permission from Yang et al. Copyright (2023) Wiley-VCH^[88]. (C) Schematic galvanostatic discharge profiles. This figure is reprinted (adapted) with permission from Sambandam et al. Copyright (2022) Elsevier Inc^[89].

The electrochemical performance of MnO_x cathodes in AZIBs is influenced by the crystallographic structure of the MnO_x polymorphs. Table 3 lists the various forms and crystal structures of MnO₂, highlighting the different tunnel structures important for ion insertion and extraction. Among the tunnel-type cathodes, such as α-MnO₂, β-MnO₂, todorokite-type MnO₂, and γ-MnO₂, are particularly noteworthy. Additionally, δ-MnO₂, λ-MnO₂, ZnMn₂O₄, and Mn₃O₄ have also been reported to show moderate performance in ARZBs. Despite numerous studies on the specific crystalline types of MnO_x in AZIBs, there is a gap in understanding the relationship between the crystal structure of MnO_x polymorphs and their corresponding performances^[87,94], where few studies have been reported. The electrochemical performance of various manganese oxide polymorphs is known to decrease under specific electrochemical conditions, with the order of performance being α > δ > γ > λ > β^[95]. It is anticipated that α- and γ-MnO₂ cathodes will exhibit capacity-driven reactions in Regions I and II. Furthermore, the charge storage mechanism for MnO₂ cathodes is dynamic throughout their lifespan, as shown in Figure 7C. The pH level of the electrolyte can induce progressive alterations in Region II, where the primary process is cathodic dissolution, influencing the associated capacity contribution. To validate these assumptions and fully understand the relations amidst crystal structure and electrochemical performance, comprehensive electrochemical analysis is required. This analysis would involve systematic studies that explore the effects of crystal structure, electrolyte composition, and electrochemical conditions on the performance of MnO_x cathodes in zinc batteries, which is critical for optimizing the materials and electrolytes for improved battery performance.

Nanostructure design

The advanced capability of nanostructured electrode materials to improve the storage and discharge of foreign ions is achieved by reducing the diffusion distance for electron and ion transfer, thereby enhancing the diffusion dynamics. Such enhancement is ascribed to (a) the increased surface area-to-volume ratio, which affords more active sites to electrochemical reactions; and (b) the unique structural characteristics in the nanoscale for an electrode. Nanostructure can be classified into four categories: (a) zero-dimensional (0D); (b) 1D; (c) 2D; and (d) hierarchical structure.

Consequently, due to their distinct surface and structural properties, nanomaterials of varying diameters display a range of performances^[96]. Nanomaterials (not only MnO₂ nanomaterials) with 0D structures, such as those with particle sizes below 100 nm, offer a short pathway for ion diffusion and a broad interface with the electrolyte^[97,98]. This enables them to offer distinct advantages; for instance, Wei et al. showed the creation of α-MnO₂ with a surface area of 208 m² g^-1 and a substantial initial discharge capacity of 234 mA h g^-1[47]. This was achieved using spherical nanoparticles and cylindrical nanorods, which were synthesized through co-precipitation. Additionally, 1D structured materials not only facilitate rapid ion diffuseness in the radial direction but also promote swift electron transport along the 1D axis^[99]. Alfaruqi et al. proved this advantage by creating MnO₂ nanorods with a huge specific surface area of 153 m²/g and an amazing first discharge specific capacity of 323 mA h/g using a straightforward solvent-free manufacturing technique^[100]. That highlighted the potential of 1D structured materials in battery applications^[100]. These unique structural characteristics provide distinct advantages over 0D structured nanoparticles. Furthermore, 2D nanomaterials, such as layered MnO₂ nanosheets, boast unique advantages over their 0D and 1D counterparts thanks to their expanded interlayer spacing, high surface-to-volume ratio, and atomic-level thickness. These features provide 2D nanomaterials with a greater number of active sites and remarkable mechanical flexibility, which has been evidenced in the work of Choi^[101], who synthesized layered MnO₂ nanosheets delivering a large discharge capacity of 350 mA h/g. These findings highlight the diverse performance and potential of different dimensional nanomaterials in various applications.

In addition to nanostructured MnO₂, nanocomposite was prepared to boost the performance for MnO₂ cathodes. Li et al. demonstrated the synthesis of a 3D core-shell structured layered MnO₂ on N-doped carbon nanowires for use as a cathode material [Figure 8A and B]^[102]. Despite the similar shapes and two pairs of distinct redox peaks observed in the cyclic voltammetry (CV) curves of both samples [Figure 8C], the δ-MnO₂ nanosheets on N-doped carbon nanowires (MnO₂@NC) cathode demonstrated a superior peak current density, which is a sign of enhanced electrochemical storage capacity. This phenomenon was further confirmed by the higher capacity of the MnO₂@NC cathode compared to the MnO₂ nanosheets grown on bare carbon nanofibers (MnO₂@CC) cathode, as shown in Figure 8D. The superior electrochemical performance of the MnO₂@NC cathode is attributed to the enhanced conductivity of the N-doped carbon nanowires. MnO₂@NC cathode material exhibits a higher capacitance than MnO₂@CC cathode material (325 mAh g^-1 at 100 mA g^-1). Moreover, MnO₂@NC demonstrates outstanding rate performance and stable cycling, maintaining around 95% of its original capacity after 2,500 cycles at a current density of 2A/g. Zhang et al. studied the integration of MnO₂ nanosheets onto a skeleton of N-doped porous carbon nanosheets to enhance the reactivity and electrical conductivity of the cathode material^[103]. Through a comprehensive investigation into the mechanisms responsible for the improved performance of the MnO₂ composite electrode in batteries, they found that the N-doped carbon played a pivotal role in lowering the valence state of Mn⁴⁺ ions within the MnO₂ structure. This reduction in valence state was instrumental in mitigating the electrostatic repulsion between Zn²⁺/H⁺ during the embedding process, thus facilitating a higher degree of cation insertion and accelerating ion migration within the electrode.

Figure 8. (A) Schematic representation of preparation process for MnO₂@NC electrode; (B) SEM images of MnO₂@CC; (C) CV curves at 0.5 mV s^-1; (D) GCD curves at 0.1 A g^-1[102]. This figure is reprinted (adapted) with permission from Li et al. Copyright (2022) Wiley^[102].

Interlayer adjustment

The impact of unstable electrode material structure on AZIBs during reaction can be improved by adjusting the interlayer spacing [Figure 9A]^[101]. Nam et al. explored the effect of structural water content on the capability of layered MnO₂ (cw-MnO₂). Their research indicated a direct correlation between the water content in cw-MnO₂ and the battery capacity [Figure 9B]^[101]. Specifically, they found that cw-MnO₂ with an optimal water molecule content of 0.94 exhibited superior capacity, reaching up to 350 mA h g^-1. This finding was further supported by the data presented in Figure 9C-E, which clearly demonstrated the enhanced performance of the cw-MnO₂ with the ideal amount of structural water. This finding indicates that suitable water content and interlamellar spacing can help achieve a satisfactory cycle performance by preventing manganese dissolution and preserving the integrity of the electrode structure during cycling. However, the optimal quantity of structural water remains uncertain. Determining how the layer spacing of Mn-based materials and their electrochemical performance relate to the quantity of structural water is crucial. The presence of structural water increases interlayer spacing, shielding electrostatic interactions and thereby facilitating foreign cation diffusion and stabilizing the host structure. However, an elevated content of structural water promotes rapid Zn²⁺ diffusion, which compromises structural stability and adversely affects the electrochemical performance.

Figure 9. (A) Zn²⁺-Intercalated structures and relative energies; (B) TGA profiles of pristine. Discharge-charge voltage profiles at various current densities for (C) pristine cw-MnO, (D) cw-MnO₂-100, and (E) cw-MnO₂-300; This figure is reprinted (adapted) with permission from Nam et al. Copyright (2019) RSC Publishing^[101]. (F) Discharge-charge curves for the second cycle (G) differential discharge-charge capacity curves. This figure is reprinted (adapted) with permission from Liu et al. Copyright (2019) Royal Society of Chemistry^[104].

Furthermore, as a pillar to support the structure, guest species can be pre-intercalated into the cathode material of a tunnel or layered construction. Liu et al. delved into the effects of pre-intercalated K⁺ ions on the electrochemical properties of tunnel-structured MnO₂-graded nanotubes, denoted as α-K_0.19MnO₂^[104]. For comparison, α-K_0.07MnO₂ nanotubes were prepared. The investigation demonstrated that the shape and crystal structure of α-K_0.07MnO₂ nanotubes did not change following the removal of K⁺. Although the discharging/charging plateaus of both materials were comparable in slope [Figure 9F], the α-K_0.19MnO₂ cathode showed a lower overpotential. Furthermore, as shown in Figure 9G, it exhibited a higher capacity, reaching 270 mAh g^-1. Furthermore, the redox potential of the α-K_0.19MnO₂ cathode material is lower during insertion/extraction of Zn²⁺ and H⁺ ions. This implies that the reversible insertion/extraction of H⁺ and Zn²⁺ into the MnO₂ is facilitated by the presence of K⁺ ions.

Besides, nucleating agents can be added in the preparation stage of MnO₂ to improve the layer spacing. Hong et al. reduced the energy barrier of MnO₂ crystal growth, nucleation and refinement by adding a polyethylene glycol (PEG) nucleating agent in the preparation stage of MnO₂, which made PEG-regulated δ-MnO₂ (PR-δ-MnO₂) have smaller average grain diameter, higher specific surface area and water-rich interlayer^[105]. Compared to the original δ-MnO₂, PR-δ-MnO₂ has a greater capacity (295 mAh g^-1), better magnification, and increased cycle stability.

Defect engineering

Heteroatom doping

Defect introduction can involve using specific methods to create vacancies in the positive electrode material, thereby inducing structural defects. It can also be achieved by doping impurity atoms into the positive electrode material to create heterogeneous atomic defects or by replacing specific elements in Mn-based composite materials to enhance the electrochemical potential of the positive electrode material.

Liang et al. have investigated a novel approach to enhance the electrochemical performance of electrodes by altering their surface characteristics and the associated reaction kinetics^[114]. They found that incorporating graphite nanosheets (GNS) with high hydrophobicity and minimal oxygen defects can effectively impede the rate-limiting step of Mn²⁺ adsorption. This manipulation of the surface properties creates free-standing MnO₂ electrodes that exhibit remarkable stability, particularly during high depth of discharge (DOD) cycling and when subjected to multiple charge-discharge cycles. Post-cycling analysis of these electrodes revealed that the occurrence of undesired side reactions was significantly delayed and reduced. This finding challenges the conventional belief that extending the effective cycle life of a cell could be achieved solely by inhibiting the in situ electrodeposition of MnO₂. The electrodes developed through this strategy demonstrate a significantly extended cycling life, surpassing 600 cycles, while preserving the high C-rate stability that is characteristic of α-MnO₂. The outstanding performance of the electrodes validates the kinetic inhibition strategy proposed by the researchers. This strategy not only improves the cycling stability of MnO₂/Zn batteries but also offers a new avenue for developing rechargeable batteries with long cycle lives. Hu et al. improved the electrochemical performance by depositing highly conductive Co₉S₈ on Ni foams (NF) to form a layered core-shell structure, as shown in Figure 10A, which provided a greater number of electrochemically active sites^[115]. This method essentially involves first synthesizing MnO₂ on NF and then heat treating it at 400 °C to form bean sprout-like MnO₂, as shown in Figure 10B. Next, a simple hydrothermal method was used to uniformly grow MnO₂@Co₃O₄ nanowires on MnO₂/NF. Subsequently, an amino exchange reaction was carried out on Co₃O₄/MnO₂/NF to form a MnO₂@Co₃O₄ core-shell structure, and finally, MnO₂@Co₉S₈ was synthesized through a vulcanization reaction. The number of electrochemically reactive active sites and conductivity of the electrode material were increased by coating the MnO₂ arrays with highly conductive Co₉S₈. This enables the MnO₂@Co₉S₈/NF electrode to have a high specific capacitance that is 3.4 and 10.1 times greater than that of the Co₃O₄@MnO₂ and pure MnO₂ electrodes, respectively. It achieves an impressive area capacitance of 1,540 mC cm^-2 and an energy density of 346.5 mWh cm^-2. Additionally, it maintains a capacity retention of 97.5% after 36,000 cycles, showcasing its excellent stability. By achieving high efficiency, stability, and long-life operation, these methods provide new concepts and solutions for AZIBs. This is anticipated to encourage the advancement and use of ZIB technology with more environmentally beneficial outcomes.

Figure 10. (A) Schematic representation of preparation process for MnO₂@Co₉S₈/NF; (B) Transmission mapping images of the core-shell structured MnO₂@Co₉S₈/NF. This figure is reprinted (adapted) with permission from Hu et al. Copyright (2021) Pergamon^[115].

Surface modification

Surface coatings offer a range of functions and properties that make them essential in various industrial applications. One primary function is to protect against corrosion, wear, oxidation, and pollution. This safeguards the underlying materials and substrates, prolonging their lifespan. In this regard, surface coating provides an effective method for optimizing manganese dioxide cathode materials for AZIBs. It can enhance the stability of manganese dioxide. By forming a protective film on the surface of MnO₂, it can prevent unnecessary reactions with the external environment. This protective film can block the entry of oxygen, moisture, and other harmful substances. Also, surface coating methods can effectively inhibit the dissolution of the cathode material and shield the electrostatic interaction between guest ions^[91,116].

Carbon materials are frequently used as coatings for MnO₂ cathode materials in batteries. The cycle stability of the electrode is improved by this coating process, which is especially efficient in declining the dissolution of manganese ions from the cathode. A straightforward and economical technique has been devised by Islam et al. to generate α-MnO₂@C or carbon-coated nanoparticles, which function very well as cathodes in AZIBs^[117]. The synthesis process uses maleic acid (C₄H₄O₄) as the carbon source to create a gel, which is then subjected to annealing at a relatively low temperature of 270 °C^[117]. TEM analysis confirmed the presence of a uniform carbon network interspersed among the α-MnO₂ nanoparticles. This carbon coating not only enhanced the structural integrity of the nanoparticles but also contributed to improved electrochemical performance. The α-MnO₂@C composite exhibited a remarkable initial discharge capacity of 272 mAh g^-1 at a current density of 66 mA g^-1, which is a significant improvement over the pristine α-MnO₂, which had a discharge capacity of 213 mAh g^-1 under the same conditions. Moreover, the α-MnO₂@C composite demonstrated superior cycling stability compared to the original α-MnO₂. This enhanced stability is likely due to the protective effect of the carbon coating, which mitigates the dissolution of manganese ions during cycling, thereby preserving the integrity of the cathode material and maintaining its capacity over extended charge-discharge cycles^[117]. In the CV curves presented in Figure 11A, a comparison between the pristine α-MnO₂ electrode and the α-MnO₂@C electrode reveals notable differences. The α-MnO₂@C electrode exhibits a more pronounced peak intensity and a larger enclosed area in the high voltage region, which signifies greater electrochemical activity. The peaks at 1.57 and 1.6 V are distinctly visible for the α-MnO₂@C and α-MnO₂ electrodes, respectively. These observations suggest that the carbon coating has a positive effect on the electrochemical performance of α-MnO₂, enhancing its capacity and stability. Graphene, another carbon-based material, has also been used to modify MnO₂ cathodes. Wu et al. have demonstrated that graphene coating can play a role similar to carbon coating. It can increase the capacity of the battery cathode material and contribute to its stability^[118]. The performance of the cathode material after surface engineering modification, as shown in Figure 11B and C, is superior to that of other AZIB cathode materials. This indicates that graphene coating is a valid strategy for enhancing the electrochemical properties of MnO₂ cathodes, leading to improved battery performance.

Figure 11. (A) First cycle and (inset) second cycle. This figure is reprinted (adapted) with permission from Islam et al. Copyright (2017) Elsevier^[117]. (B) Cycling performances of MGS; (C) the Ragone plot (based on the weight of cathode material) of the MGS cell; This figure is reprinted (adapted) with permission from Wu et al. Copyright (2018) Wiley-VCH^[118]. (D) Cycling stability of MnO₂ and PANI@MnO₂ electrodes after 3000cycles; (E) Scheme of the synthetic procedure for PANI@MnO₂ composite; (F) Schematic illustration of the preparation process of MnO₂/rGO/PANI. This figure is reprinted (adapted) with permission from Mao et al. Copyright (2019) Springer^[122].

Conductive polymers, for instance, polyaniline (PANI)^[119], polypyrrole^[120], and polythiophene^[121], are excellent candidates for coating materials in battery technology due to their high conductivity and ability to prevent the dissolution of active substances in cathode materials. The application of this coating has the potential to significantly enhance battery performance by increasing the stability of the cathode material^[122]. Bao et al. have synthesized PANI@MnO₂ composite materials and compared the stability of α-MnO₂ and PANI@MnO₂ electrodes in a 1 mol/L Na₂SO₄ aqueous solution, as illustrated in Figure 11D and E^[123]. After 3,000 cycles, the α-MnO₂ electrode retained 75% of its specific capacitance, while the PANI@MnO₂ composite exhibited a higher retention rate of 90%. The enhanced stability of the PANI@MnO₂ composite is indeed due to the strong molecular interactions between the amine groups of PANI and the hydroxyl groups of MnO₂. These interactions, along with the uniform coating of PANI on the surface of MnO₂, are crucial for the electrochemical stability of the composite material. The uniform coating of PANI on MnO₂, together with the robust molecular interactions between the amine groups of PANI and the hydroxyl groups of MnO₂, contributes to the enhanced stability of the composite material^[119]. This coating effectively inhibits the dissolution of MnO₂ and the degradation of PANI, leading to superior cycling stability when compared to the individual components.

Surface engineering has proven to be a powerful tool in exploiting advanced composite materials for energy storage applications. By carefully designing the surface composition and structure, it is possible to achieve a synergistic effect that enhances the performance of the material. In the case of Mn-based electrodes, surface engineering can address the issue of manganese dissolution and improve electrical conductivity. Mao et al. have demonstrated this concept with the development of a composite aerogel electrode material (MnO₂/rGO/PANI) [Figure 11F]^[122]. This material is prepared by coating PANI onto manganese dioxide (MnO₂) and then combined with reduced graphene oxide (rGO). The resulting composite, when used in a (MnO₂/rGO/PANI)/Zn battery, demonstrates a substantial capacity of 241.1 mAh g^-1 and maintains 82.7% of its initial capacity after 600 cycles. The MnO₂/rGO aerogel has a compact and dense structure, with MnO₂ spheres encapsulated by rGO nanosheets and evenly dispersed throughout the graphene network. The N 1s spectrum, with its three distinct peaks at 399.5, 400.4 and 401.1 eV, indicates the presence of amine groups, polarons, and positively charged nitrogen species, respectively. These groups are essential for the conductivity of PANI, as they facilitate electron transfer within the polymer. The presence of these groups in the composite material suggests that the PANI coating significantly enhances the conductivity of the MnO₂/rGO aerogel^[124-126]. The method offers a practical approach to overcoming the limited conductivity of MnO₂. Kamenskii et al. have engineered a composite material by integrating poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) into the MnO₂ framework^[127]. This composite material, when used to coat MnO₂, results in a denser and smoother surface, particularly for the MnO₂/PEDOT:PSS electrode, compared to the bare MnO₂ electrode. The modification of PEDOT:PSS not only expands the surface area but also boosts the conductivity of the electrode, which makes it easier for the charge to be transported on the electrode surface. This results in a notable improvement in the capacity of the electrode and its ability to maintain its performance over multiple cycles. The MnO₂/PEDOT electrode demonstrates a remarkable specific capacity of 238 mAh g^-1, with an impressive capacity retention rate of 99%. In contrast, the pristine δ-MnO₂ cathode experiences a 10% decrease in capacity, whereas the modified electrodes show no capacity fading after stabilization. The structure stability is enhanced by the composite structure produced by combining organic compounds with manganese dioxide, making it more resistant to structural collapse and reducing adverse electrochemical performance caused by phase change^[122]. This has been confirmed by the scanning electron microscopy (SEM) and ex-situ XPS analysis. Simultaneously, it improves the interaction between components. These advantages make such composite materials widely applicable in electrochemical energy storage devices such as ZIBs.

Electrolyte regulation

Electrolyte regulation is one of the most efficient methods to reduce the dissolution of cathode materials during cycling, and it plays an important role in solving the anode reversibility problem^[128]. Using a ZnSO₄ aqueous solution as the electrolyte in AZIBs is indeed common due to its favorable electrochemical properties^[129]. However, it is important to recognize that cells utilizing ZnSO₄ electrolytes often require an extended activation period to reach their maximum specific capacity. Furthermore, the cycling stability of AZIBs with ZnSO₄ electrolytes can be suboptimal [Figure 12A]^[130]. Therefore, strategies from the angle of electrolytes are adopted, such as replacing ZnSO₄ electrolytes with Zn(CF₃SO₃)₂, synthesizing new zinc electrolytes, and employing additives. However, so far, little has been done on new zinc salts for MnO₂ cathodes.

Figure 12. (A) MnO₂/Zn battery chemistry. This figure is reprinted (adapted) with permission from Zhang et al. Copyright (2017) Springer Science and Business Media LLC^[130]. (B) CV scanning performance of MnO₂ electrodes with and without 0.1 M MnSO₄ additive in 2 M ZnSO₄ aqueous electrolyte. (C) Rate performance (using an electrolyte with a MnSO₄ additive). This figure is reprinted (adapted) with permission from Pan et al. Copyright (2016) Nature Research^[65].

Adding additives can effectively inhibit Mn²⁺ dissolution, regulate the ion distribution to obtain a dendrite-free Zn anode, stabilize the electrode/electrolyte interface, and improve the adaptability to extreme environmental conditions^[131]. Regulating the electrolyte composition can effectively modulate the redox potential and mitigate side reactions, thereby controlling the formation of undesirable by-products. Treating the dissolution of the cathode material as a reversible process, one can leverage Le Chatelier's principle to optimize the salt concentration of the electrolyte. This strategy aims to increase the concentration of specific cations within the electrolyte, thus declining the dissolution of these cations from the cathode material^[132]. It has been particularly effective in the context of Mn-based cathode materials. Pan et al. have conducted research on the MnO₂/Zn battery using a mild ZnSO₄ electrolyte, where they observed an initial specific capacity of 210 and 255 mA h/g at 0.2 C in the first two cycles. However, they noted a rapid decrease in capacity after these initial cycles^[65]. To mitigate this issue, they introduced MnSO₄ into the ZnSO₄ electrolyte, which effectively reduced the dissolution of α-MnO₂ by altering the dissolution equilibrium of Mn²⁺ from the α-MnO₂ electrodes. The CV curves in Figure 12B and C demonstrate that adding MnSO₄ did not disrupt the redox reaction of the MnO₂ electrode. Instead, it significantly improved the utilization rate of the MnO₂ active materials. The capacity of the electrodes was significantly improved, reaching 285 mAh g^-1 at a C/3 rate and 260 mAh g^-1 at a 1C rate. Furthermore, the addition of MnSO₄ to the electrolyte of MnO₂/Zn batteries has been found to notably enhance their long-term cycling stability. These batteries maintained an impressive capacity retention of 92% after 5,000 cycles at a demanding 5C current rate. Employing this technique on various cathode materials, including NaV₃O₈, C₁₂Fe₂N₁₂Zn₃, and Co₃O₄, can lead to comparable enhancements in performance. Employing high-concentration electrolytes (HCEs) is a proven method to reduce cathode dissolution, primarily due to the limited availability of free water molecules that can be saturated within the electrolyte solution^[133,134]. This strategy has been shown to be particularly effective in preventing the leaching of cations from cathode materials such as V₂O₅ and Mn₂O₃ when ZnCl₂-based HCEs are utilized. Using HCEs can significantly enhance the stability and longevity of these materials in battery applications^[135,136]. Zhang et al. discovered that a Zn(CF₃SO₃)₂ electrolyte with additional Mn(CF₃SO₃)₂ performs better in inhibiting Mn2+ dissolution than a ZnSO₄ electrolyte with MnSO₄^[130].

Gel electrolytes have been greatly developed for flexible AZIBs^[137], which enhances the excellent cycle durability of batteries^[138-140]. Further, the incorporation of solid or gel-based electrolytes in battery technology has been demonstrated to mitigate the formation of Zn dendrites^[141,142]. Solid or gel electrolytes, characterized by a reduced water content and a strong adsorption affinity, can effectively prevent the manganese dissolution, contributing to achieving long-term cyclability^[138].

Based on the above, the mainstream approaches to boosting the performance of MnO₂-based cathode materials have been systematically summarized [Table 4]. A primary method to achieve enhancement involves amalgamating MnO₂ into nanocomposite materials, which capitalizes on the synergistic effects of the various components to yield outstanding electrochemical performance. Elaboration on the crystal structure of MnO₂ itself, such as augmenting defect density and refining the nanostructure, can result in the stabilized crystal structure with enlarged interlayer spacing and the amplified surface area with more active sites. Also, surface modifications can effectuate alterations in the surface properties of MnO₂ and further speed up the kinetics of the electrochemical reaction. Further, regulating the solvent in the electrolyte, salt ratio and concentration and incorporating additives can not only enhance electrochemical performance but alter the reaction routes.