Introduction

Rechargeable aqueous zinc-based (Zn-based) batteries have recently garnered considerable attention due to their safety, sustainability, and cost-effectiveness [1,2,3,4,5,6]. Aqueous Zn||MnO2 batteries, in particular, have been extensively studied since the early 1860s [7]. To unlock their full potential, advancing our understanding of electrochemical reaction mechanisms (ERMs) and achieving high energy density are crucial [8,9,10]. However, ERM diversity and the resulting conflicting views hinder further development [7, 11,12,13]. Moreover, the challenge of achieving high-energy-density pouch cells for Zn||MnO2 batteries remains a considerable obstacle to commercialization [9, 14].

According to previous studies, six typical ERMs are revealed in aqueous Zn||MnO2 batteries, contributing to the rational design of high-energy-density Zn batteries. They include (1) insertion/extraction of main Zn2+ [15,16,17], (2) primary conversion of H+ into MnOOH [18], (3) co-intercalation of H+ and Zn2+ [19,20,21,22,23], (4) main MnO2/Mn2+ redox conversion in neutral environments [24, 25], (5) combination of MnO2/Mn2+ redox conversion with H+ and Zn2+ co-intercalation in neutral environments [26], and (6) redox conversion of only MnO2/Mn2+ in strong acid environments [27, 28]. These ERMs were reported based on the experimental results and/or characterization of the charge–discharge products by employing MnO2 polymorphs as cathodes [7]. To achieve high-energy-density Zn batteries, two key factors must be considered: the areal capacity and discharge voltage of the battery. Therefore, the direction for achieving high energy density is to maximize the areal capacity and discharge voltage.

In this review, we comprehensively introduce different ERMs of aqueous Zn||MnO2 batteries based on recently reported results. Further, we discuss the developments of electrolyte materials and innovative cell configurations for achieving high-energy-density Zn batteries. In addition, we compare the performance of various Zn||MnO2 batteries to clearly state promising directions, remaining challenges, and prospective alternatives. Furthermore, we highlight various practical application scenarios of the recently reported flexible Zn||MnO2 batteries. Finally, we clarify the key scientific problems limiting the further development of Zn||MnO2 batteries and propose their solutions.

ERMs of Zn||MnO2 Batteries

Alkaline Zn–Mn batteries, with a well-established reaction mechanism, have been commercially available for a long time [29,30,31]. However, aqueous Zn||MnO2 batteries, which can operate in mildly and strongly acidic conditions, exhibit various ERMs, posing challenges for researchers and hindering the development of this battery system. Herein, we summarize the typical ERMs of aqueous Zn||MnO2 batteries in detail, including the Zn2+/H+ intercalation chemistry and MnO2/Mn2+ conversion reaction mechanism.

Zn2+/H+ Intercalation Chemistry in Mildly Acidic Electrolytes

Aqueous Zn||MnO2 batteries exhibit diverse ERMs. Zhao et al. [32] reported the dissolution and redeposition of Zn4(OH)6SO4·5H2O accompanied by the formation and dissolution of ZnMn2O4 during the charge–discharge process of an aqueous Zn||MnO2 battery (Fig. 1a), suggesting that only Zn2+ intercalation/deintercalation occurs in the MnO2 cathode. However, Pan et al. [18] found that Zn2+ cannot be inserted into the MnO2 crystal in the presence of an organic electrolyte unless water is introduced into the electrolyte during cycling (Fig. 1b), indicating that only H+ intercalation occurs in aqueous Zn||MnO2 batteries. Interestingly, Sun et al. [19] observed the phenomenon of Zn2+/H+ co-insertion; however, they first observed H+ intercalation (1.4 V) followed by Zn2+ insertion (1.2 V) based on the discharge galvanostatic intermittent titration technique results (Fig. 1c). Notably, Ma et al. [33] reported that MnO2 formed from L-ZnxMnO2 acted as the host for H+, which was observed at 1.9–1.44 V after the occurrence of Zn2+ intercalation (1.44–0.8 V). Conversely, Huang et al. [20] believed that Zn2+ insertion occurred at a discharge plateau voltage of 1.4 V during the first discharge process (ZnxMn2O4 formation) rather than H+ intercalation (occurred at 1.2 V based on monitoring XRD characteristic signal peaks of Mn2O3 and MnOOH) (Fig. 1d). In fact, the occurrence of H+/Zn2+ co-intercalation at 1.4 V and H+/Zn2+ conversion at 1.26 V are more reasonable due to observe the appearance of byproducts at these discharge plateaus. Li et al. [21] supported this assumption by detecting ZnxMnO2 + MnOOH at 1.4 V and Mn3O4 + MnO + ZnMn3O7 at 1.26 V (Fig. 1e). Obviously, unclear explanations regarding ERMs hinder the development of aqueous Zn||MnO2 batteries. Accordingly, H+/Zn2+ intercalation and conversion chemistry need to be revealed clearly in future explorations.

Fig. 1
figure 1

a Charge–discharge profile of an aqueous Zn||MnO2 battery and the corresponding scanning electron microscopy images. Adopted with permission from Ref. [32]. Copyright 2018, Royal Society of Chemistry. b Charge–discharge profiles of an aqueous Zn||MnO2 battery assembling by different electrolytes. Adopted with permission from Ref. [18]. Copyright 2016, Springer Nature. c Discharge galvanostatic intermittent titration technique profiles. Adopted with permission from Ref. [19]. Copyright 2017, American Chemical Society. d Schematic of the phase transformation of a cathode. Adopted with permission from Ref. [20]. Copyright 2019, Springer Nature. e, Schematic of the crystal structures and redox reactions. Adopted with permission from Ref. [21]. Copyright 2019, American Chemical Society

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Reaction Mechanism for MnO2/Mn2+ Conversion

In addition to exploring genuine conditions for H+/Zn2+ intercalation and conversion chemistry, new ERMs for MnO2/Mn2+ conversion offer the potential for obtaining high-performance aqueous Zn||MnO2 batteries. We discuss the MnO2/Mn2+ conversion chemistry in neutral or mildly acidic environments and strong acidic conditions separately. By doing so, we aim to better understand the involved chemical processes and identify optimal conditions for achieving high battery performance.

Neutral and Mildly Acidic Environments

The low discharge plateau (< 1.5 V) of the Zn||MnO2 battery involves the redox conversion of MnO2/Mn2+, complicating the understanding of electrode reaction mechanisms. Some studies have suggested that the primary reaction mechanism is the MnO2/Mn2+ dissolution–deposition reaction, while the classical cation intercalation/deintercalation mechanism plays a negligible role (Fig. 2a) [24]. This suggestion is explained by the fact that MnO2 reacts with active H2O, leading to subsequent appearance of Zn4SO4(OH)6·4H2O (ZHS) in the discharge process. Furthermore, the newly generated ZHS is converted to birnessite-MnO2 with the help of Mn2+ in the electrolyte in the charging process. Additionally, Bao et al. [34, 35] further elaborated on the role of ZHS in aqueous Zn||MnO2 batteries. In detail, the main reversible capacity and improved cycle performance of this battery resulted from the conversion process occurring between ZSH and ZnxMnO(OH)2 nanosheets at a sweeping voltage of > 1.6 V. Here, MnO2 initiated ZSH formation but contributed negligibly to the apparent capacity.

Fig. 2
figure 2

a Schematic of electrochemical reaction mechanism in a Zn||MnO2 battery. Adopted with permission from Ref. [24]. Copyright 2020, Elsevier. b Schematic of an aqueous Zn||MnO2 battery in an acetate-based electrolyte and c the corresponding charge–discharge profiles of an aqueous Zn||MnO2 battery in an acetate-based electrolyte. Adopted with permission from Ref. [25]. Copyright 2020, Wiley-VCH Verlag

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In an acetate-based electrolyte, aqueous Zn||MnO2 batteries exhibited typical MnO2/Mn2+ conversion chemistry, but a discharge plateau was obtained at approximately 1.4 V (Fig. 2b) [25]. According to the Nernst equation, the novel acetate-based electrolyte comprising 0.5 mol/L ZnCl2, 0.5 mol/L Mn(Ac)2, 2 mol/L KCl, and 1.75 mol/L HAc enhanced the discharge plateau of an aqueous Zn||MnO2 battery to approximately 1.5 V because of the small decrease in pH compared to neutral electrolytes [36]. This enhancement is because MnO2/Mn2+ conversion chemistry occurs at relatively low potentials in a mildly acidic environment. Additionally, with a further decrease in pH, the discharge plateau of the aqueous Zn||MnO2 battery increased slightly.

Strong Acidic Environment

The potential for MnO2/Mn2+ conversion depends on the pH value of the electrolyte. Chao et al. [27] revealed that adding 0.1 mol/L H2SO4 to 1 mol/L ZnSO4 + 1 mol/L MnSO4 electrolyte solution resulted in a discharge plateau of 1.95 V based on MnO2/Mn2+ conversion during chronoamperometric charging at 2.2 V vs. Zn/Zn2+ (Fig. 3a). The electrolytic Zn||MnO2 battery exhibited a superior rate performance of up to 60 mA/cm2 (Fig. 3b) and cycling stability for 1800 cycles at 30 mA/cm2 (Fig. 3c). Furthermore, Chuai et al. [28] modified the strong acid electrolyte of 0.1 mol/L H2SO4 + 1 mol/L ZnSO4 + 1 mol/L MnSO4 by introducing 0.07 mmol/L polyvinylpyrrolidone as a cationic accelerator (CA). The assembled Zn||MnO2 battery demonstrated an energy density of 50 W h/m2 over 2000 cycles (Fig. 3d, e) because of efficient cation migration in the electrolyte and effective charge transfer at the electrode–electrolyte interface, facilitated by CA introduction. H+ concentration considerably influences the potential of MnO2/Mn2+ conversion. Although the high discharge plateau (1.95 V) of the aqueous Zn||MnO2 battery was achieved under strong acid conditions, anode corrosion limits the further development of this system. Therefore, developing high-voltage MnO2/Mn2+ conversion reactions in neutral or mildly acidic environments is urgently needed.

Fig. 3
figure 3

a Galvanostatic discharge curves and b the rate capability at various rates and c long-cycle performance of an aqueous Zn||MnO2 battery in a modified strong acid electrolyte. Adopted with permission from Ref. [27]. Copyright 2019, Wiley-VCH Verlag. d Long-cycle performance of an aqueous Zn||MnO2 battery in a modified strong acid electrolyte. Adopted with permission from Ref. [28]. Copyright 2022, Wiley-VCH Verlag

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Zn2+/H+ Intercalation Chemistry and MnO2/Mn2+ Conversion

The development of high-voltage Zn||MnO2 batteries in neutral and mildly acidic environments can promote their practical application. On the one hand, these conditions help realize high discharge plateaus, resulting in high energy densities. On the other hand, it can alleviate the issue of anode corrosion in strong acids. Shen et al. [26] calculated the critical concentration of Mn2+ in an electrolyte for tuning the triple point (MnO2–MnOOH–Mn2+) from low to high pH values based on thermodynamic calculations. As shown in Fig. 4c, 0.005 mol/L Mn2+ allowed for MnO2/Mn2+ conversion. The cyclic voltammetry curve of the Zn||MnO2 battery using 0.005 mol/L Mn2+ exhibited excellent redox peaks corresponding to MnO2/Mn2+ conversion (Fig. 4b). Note that the redox potential for MnO2/Mn2+ conversion in a neutral environment exhibited a large variation, namely, from 1.4 to 1.8 V (vs. Zn/Zn2+), which could be attributed to differences in Mn2+ activity in different electrolytes. Furthermore, H+ increases the activity of Mn2+ and the reaction potential of Mn2+/MnO2 redox reactions. In neutral electrolytes, by tuning the activity of Mn2+ (e.g., coordination with water molecules or additives, desolvation, and so on), the reaction potential for MnO2/Mn2+ conversion can also tuned. Additionally, the rate performance of the Zn||MnO2 battery using 0.005 mol/L Mn2+ displayed a distinct 1.75 V discharge plateau resulting from MnO2/Mn2+ conversion (Fig. 4c). In addition, the cyclic stability of Zn||MnO2 batteries over 3000 cycles demonstrated considerable potential for practical application. However, the low areal capacity and harsh reaction conditions of this strategy challenge its further development. To overcome these limitations, future research should focus on exploring new electrolytes for achieving a large-scale, high-voltage aqueous Zn||MnO2 battery in neutral or mildly acidic environments.

Fig. 4
figure 4

Adopted with permission from Ref. [26]. Copyright 2021, Wiley-VCH Verlag

a E–pH diagram of MnO2 for different Mn2+ concentrations. b Cyclic voltammetry curves of Zn||MnO2 batteries performed at 1 mV/s in electrolytes with MnSO4 concentrations of 0, 0.005, 0.05, and 0.1 mol/L. c Charge–discharge curves at different current densities. d Cycle stability of Zn||MnO2 batteries.

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High-Energy-Density Zn||MnO2 Batteries

The development of high-energy-density Zn||MnO2 batteries is crucial for their commercialization. To achieve this goal, two key factors must be realized: high areal capacity and discharge voltage plateau.

High Areal Capacity

Electrolyte Designs

Two major challenges hinder the practical implementation of Zn||MnO2 batteries: the underutilization of high-loading materials at the cathode side and Zn dendrite growth at the anode side. To address these issues, researchers have turned to electrolyte engineering. Zhao et al. [37] developed and fabricated a two-dimensional fluorinated-porous covalent organic framework (FCOF) film as a protective layer on the Zn surface. The strong interaction between fluorine (F) in FCOF and Zn reduces the surface energy of the Zn (002) crystal plane, enabling its preferential growth during the electrodeposition process. Consequently, Zn deposits exhibited a horizontally arranged platelet morphology with the preferred (002) orientations (Fig. 5a). The pouch cell assembled using a modified Zn anode demonstrated an areal capacity of > 0.5 mA h/cm2 for 250 cycles (Fig. 5b). Furthermore, Li et al. [38] designed an amphiphilic hydrogel electrolyte that effectively used cathode materials and regulated Zn (002) plate deposits (Fig. 5c). At a high loading of MnO2 of 17.3 mg/cm2, the utilization rate of cathode materials remained high at 62.7% with a remarkable peak areal capacity of approximately 1.40 mA h/cm2 at 3.3 C (Fig. 5d). Hence, the assembled Zn||MnO2 battery exhibited an areal capacity of > 2 mA h/cm2 for 200 cycles (Fig. 5e). Notably, with an increase in the MnO2 loading mass, cathode material utilization and cycle stability decreased rapidly. In general, a Zn||MnO2 battery will achieve thousands of cycles and 100% utilization of cathode materials when the loading mass ranges between 0 and 3 mg/cm2. Therefore, this performance level does not meet the standard of commercialization. When mass loading reached > 16.8 mg/cm2, cycle stability and cathode utilization rapidly decreased to < 200 cycles and < 62.7%, indicating the need to improve these factors further in future studies employing high-loading mass. In addition, introducing redox mediators, such as I− [39, 40] or Br− [41], promoted MnO2 dissolution, achieving high areal capacity. Lei et al. [39] proposed that when KI was added to the electrolyte, it formed I3−, activating inactive MnO2 (Fig. 5f). The assembled Zn||MnO2 battery exhibited an areal capacity of > 4 mA h/cm2 for 100 cycles (Fig. 5g).

Fig. 5
figure 5

a Schematic of the preferred orientations of Zn deposits. b Schematic of a flexible transparent battery assembly. Adopted with permission from Ref. [37]. Copyright 2021, Springer Nature. c Targeted permeation of cathodes and horizontally controlled Zn deposition using amphiphilic hydrogel electrolytes with brush-like structures in a water environment. d Expected and actual areal capacities when loading different masses of MnO2. e Cycle performance of Zn||MnO2 batteries assembled using an amphiphilic hydrogel electrolyte with 16.8 mg/cm2 of MnO2 loading mass at 3 mA/cm2. Adopted with permission from Ref. [38]. Copyright 2023, Elsevier. f Schematic of a Zn–manganese battery and a conceptual diagram of KI promoting the MnO2 dissolution process. g Cycling performance of aqueous Zn||MnO2 batteries. Adopted with permission from Ref. [39]. Copyright 2021, Royal Society of Chemistry

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New Configuration

In addition to electrolyte engineering, innovative cell configurations have also enhanced the electrochemical performance of aqueous Zn||MnO2 batteries. Yang et al. [42] developed sustainable high-energy aqueous Zn||MnO2 batteries by exploiting stress-governed metal electrodeposition and fast Zn2+ diffusivity. Figure 6a shows schematics of an entire Zn-based battery under external mechanical stress, leading to improved electrochemical performance. Specifically, its half-cell delivered an initial coulombic efficiency (CE) of 99.86% and an average CE of 99.97% at an aggressive current density of 20.0 mA/cm2 and areal capacity of 4.0 mA h/cm2, with negligible Zn loss during repeated cycles (Fig. 6b). Furthermore, full cells exhibited a specific capacity of 300 mA h/g even when loading 52 mg/cm2 of MnO2, demonstrating the substantial benefits of stress (Fig. 6c). In another novel configuration of Zn||MnO2 batteries, the repeated addition of new electrolytes activated inactive MnO2, promoting their long-term cycling performance (Fig. 6d) [43].

Fig. 6
figure 6

Adopted with permission from Ref. [43]. Copyright 2021, Royal Society of Chemistry

a Schematic of an entire Zn-based battery under mechanical strength. b Coulombic efficiency (CE) profiles with the initial and average CE values for Zn electrodeposited on a carbon paper current collector. c High-capacity cycle performance of Zn||MnO2 coin cells (MnO2 loading: 52 mg/cm2). Adopted with permission from Ref. [42]. Copyright 2023, Royal Society of Chemistry. d Electrochemical performance of rescued Ba2+-pillared δ-MnO2 at a current density of 0.3 A/g with rescue for five recycles.

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High Voltage

Improving the high discharge plateau of aqueous Zn||MnO2 batteries is crucial for developing high-energy-density cells. However, MnO2/Mn2+ conversion heavily depends on proton concentrations, causing corrosion issues in the Zn anode. To address this challenge, a strategy is urgently needed to facilitate the interaction of protons with the cathode without affecting the Zn anode. To address this issue, Mateos et al. [44] first proposed the use of weak Brønsted acids to achieve a highly reversible conversion of MnO2/Mn2+ in a mild environment. This approach was further developed by Liu et al. [45], who created a phosphate proton reservoir that only provided protons to the MnO2 cathode without affecting the Zn anode (Fig. 7a). This innovative design yielded a high discharge plateau of 1.75 V (Fig. 7b) and maintained cycling stability over 3000 cycles (Fig. 7c). In addition to electrolyte innovations, exploiting new cell configurations is another important development direction for commercializing aqueous Zn||MnO2 batteries. Cui et al. [46, 47] designed a proton-shuttle-shielding and hydrophobic-ion-conducting membrane to separate the neutral anode and acidic cathode, resulting in a hybrid Zn||MnO2 battery, exhibiting a high discharge plateau of 2.05 V and an areal capacity of 18 mA h/cm2 over 160 cycles (Fig. 7d, e). Similarly, Zhong et al. [48] employed ion-selective membranes to develop an innovative cell configuration, resulting in a higher discharge plateau of 2.83 V than all previous reports (Fig. 7g). These advancements in cell configurations have considerably improved the performance of aqueous Zn||MnO2 batteries. Moreover, as shown in Table 1, we compare the performance of various aqueous Zn||MnO2 batteries with high areal capacity and voltage [25, 27, 28, 37,38,39, 42, 45, 47,48,49,50,51,52,53,54,55,56].

Fig. 7
figure 7

Adopted with permission from Ref. [48]. Copyright 2020, Springer Nature

a Schematic of a phosphate proton reservoir in aqueous Zn||MnO2 batteries. b Electrochemical performance of aqueous Zn||MnO2 cells. c Cycling stability of an aqueous Zn||MnO2 battery. Adopted with permission from Ref. [45]. Copyright 2022, American Chemical Society. d Discharge profiles of an aqueous Zn||MnO2 battery. e Long-term cycling performance of an aqueous Zn||MnO2 battery at 20 mA/cm2 and a charge capacity of 18 mA h/cm2. Adopted with permission from Ref. [47]. Copyright 2022, American Chemical Society. f Schematic of the cell structure and chemical reactions at the cathode and anode. g Discharge curves for DZMB at various discharge current densities.

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Table 1 Performance comparison of aqueous Zn||MnO2 batteries
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Others

Flexibility in constructing Zn||MnO2 batteries is crucial for their use in various real-world scenarios. To address this issue, researchers have explored the use of flexible hydrogel electrolytes to assemble flexible cells [57,58,59,60,61,62]. Li et al. [63] proposed a double-network strategy to achieve excellent compression performance in acidic, alkaline, and mild environments (Fig. 8a). The resulting Zn||MnO2 batteries exhibited exceptional electrochemical performance, even under 50% compression (Fig. 8b). In addition, An et al. [64] developed a freestanding, lightweight, and zincophilic MXene/nanoporous oxide heterostructure-engineered separator for fabricating flexible Zn||MnO2 batteries (Fig. 8c). The flexible full cell with a modified separator showed stable cycling stability with a high capacity of 224.0 mA h/g and a high-capacity retention of 99.03% (Fig. 8d). Furthermore, Zn||MnO2 microbatteries (MBs) were fully developed by integrating flexible electrodes and hydrogel electrolytes. However, the key issue with the MBs is the flexibility of current collectors. Wang et al. [65] designed and fabricated a new flexible current collector based on Au/Ni (Fig. 8e). MBs were successfully fabricated by electrodepositing active electrode materials (e.g., Zn and MnO2) on the surface of Au/Ni and the coating of a hydrogel electrolyte between them (Fig. 8f). It is also very crucial to develop flexible Zn||MnO2 batteries in the future by employing different schemes, including modified hydrogel electrolytes or innovative electrode designations (e.g., new binder design [66], electrode material innovation [67], or optimized Zn anodes [68, 69]).

Fig. 8
figure 8

Adopted with permission from Ref. [65]. Copyright 2023, Elsevier

a Transmission electron microscopy images of hydrogels consisting of polyacrylamide and betaine methacrylate sulfonate. b Cycle performance under compression. Adopted with permission from Ref. [63]. Copyright 2023, Wiley-VCH Verlag. c Schematics of a flexible Zn||MnO2 battery and d its cycle performance. Adopted with permission from Ref. [64]. Copyright 2022, American Chemical Society. e Fabrication process of two-dimensional metal patterns transformed from a three-dimensional-printed stamp. f Fabrication of a Zn||MnO2 microbattery on a filter paper.

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Challenges and Perspectives

Based on this review, aqueous Zn||MnO2 batteries have exhibited promising electrochemical properties and revealed their electrochemical reaction mechanism, accompanied by the achievements of ideal specific/areal capacity, cyclic stability, and a high discharge plateau. Despite these advancements, the practical application of aqueous Zn||MnO2 batteries has not yet been achieved because of several challenges and issues. To develop commercial aqueous Zn||MnO2 batteries that can be fabricated on a large scale, the following challenges must be addressed:

  1. (1)

    ERM clarity. Diverse ERMs in aqueous Zn||MnO2 batteries have caused controversy and hindered their commercialization. ERMs involve Mn2+, Zn2+, and H+ cations, with MnO2/Mn2+ redox conversion dependent on proton concentration. However, the complex interplay between Zn2+ intercalation and H+ conversion chemistry complicates ERMs. To clarify ERMs, it is essential to separate MnO2/Mn2+ conversion, Zn2+ intercalation, and H+ conversion chemistry. By artificially fixing MnO2/Mn2+ conversion at high voltage (> 1.5 V), the ERM of Zn2+ intercalation and/or H+ conversion can be revealed at low voltage (< 1.5 V). Nevertheless, ERMs at low voltage remain unclear, and exploring these mechanisms is crucial for the rapid development of aqueous Zn||MnO2 batteries.

  2. (2)

    High-areal-capacity and high-voltage Zn||MnO2 batteries. To commercialize Zn||MnO2 batteries, improving their energy density is crucial and can be achieved by enhancing their high areal capacity and voltage. Current challenges in this aspect include the insufficient use of high-loading cathode materials and Zn dendrite growth at the anode. To address these issues, researchers must optimize various measures, such as electrolyte engineering and innovative cell configurations, for improving cathode utilization and inhibiting dendrite growth. Moreover, MnO2/Mn2+ conversion in strong acid environments occurs at relatively high voltage (vs. Zn2+/Zn), resulting in Zn anode corrosion. Therefore, the development of a neutral or mildly acid electrolyte that can activate MnO2/Mn2+ conversion at a high voltage is essential.

Conclusion

The development of aqueous Zn||MnO2 batteries has been hindered by a lack of deep understanding of their ERMs. Despite substantial progress, their real reaction mechanisms remain unclear. Suppose the reaction potentials for MnO2/Mn2+ conversion (> 1.75 V vs. Zn2+/Zn), Zn2+ intercalation (approximately 1.40 V vs. Zn2+/Zn), and H+ intercalation/conversion (approximately 1.20 V vs. Zn2+/Zn) can be fixed at a specific reaction potential. Then, their natural reaction mechanism will be revealed. In addition, high energy density and superior stability are prerequisites for aqueous Zn||MnO2 battery commercialization. Therefore, research efforts must be intensified to develop high-areal-capacity and high-voltage aqueous Zn||MnO2 batteries with excellent electrochemical features. To yield high areal capacity, a profound exploration of the relationship between the mass loading of cathode materials and cycle stability or utilization is highly important. Development in this area should focus on fully utilizing the high-loading mass of MnO2 and reducing the degree of irreversible reactions. For high voltage, developing the high reaction potential of MnO2/Mn2+ conversion (> 1.75 V vs. Zn2+/Zn) in neutral electrolytes is challenging. The key factor for achieving high voltage of MnO2/Mn2+ conversion is to tune Mn2+ activity. Overall, these advancements will enable their use in future electric vehicle and stationary storage applications.