The development of a safe, clean, and affordable energy storage device to tackle the environmental impact of fossil fuels has been a major objective for researchers globally (Larcher and Tarascon, 2015; Ming et al., 2019; Soundharrajan et al., 2022b). Li-ion batteries (LIBs) are well-known as the current gold standard for energy storage, especially in long-range electric vehicles (EVs), owing to their high energy and power density. (Scarfogliero et al., 2018; Chen et al., 2020; Soundharrajan et al., 2022c). To reduce the carbon footprint of EV charging, electricity generation from renewable energy sources, such as solar and wind, must be increased, and grid-scale energy-storage is required to handle the intermittent nature of renewable energy (Zhu et al., 2022). For this application, the use of low-cost and low-energy aqueous rechargeable batteries (ARBs), in which the size of the battery is not an issue, is often considered to be a compelling choice (Zeng et al., 2019; Soundharrajan et al., 2023).

ARBs with Mn deposition/dissolution reaction mechanisms have caught the eyes of energy researchers and are expected as one of the most promising energy storage host for stationary and home electronic devices. The aim of this review article is to summarise the classical evolution of Mn deposition/dissolution chemistry and debates on the resolution of the mechanism in different cathode/anode and electrolyte combinations. A general introduction to aqueous batteries and how they’ve evolved over the years is given. Then, we discuss the origin of Mn deposition and dissolution chemistry and its potential application in electrochemical energy storage devices. Further, we discuss the actual possibilities of Mn-deposition and dissolution chemistry for the capacity performance of Mn-supported cathodes for zinc-ion batteries (ZIB). Finally, the adaptation of the Mn deposition/dissolution chemistry for the formulation of different ARBs with different metal anodes and their reaction mechanism is discussed.

ARBs have been dominating the energy-storage market for more than a century after the invention of the rechargeable Pb-acid battery by Wilhelm Sinsteden and later by Gaston Plante (Kurzweil, 2010). After 170 years, the Pb-acid battery continues to be an important energy-storage device used in day-to-day applications, such as starting batteries for cars and grid storage (Shin and Choi, 2020). After the original invention of Pb-acid batteries, many rechargeable battery technologies emerged, such as the Ni||Cd, Metal||Hydride, and Ni||Zn batteries. While the discovery of MnO₂||Zn primary battery by Georges-Lionel Leclanché has been translated into commercial portable electronic devices (Lim et al., 2021; Durena and Zukuls, 2023; Feng et al., 2023).

Highly alkaline KOH electrolytes with high pH values in the range of 9–14 was initially used in the above Durena mentioned Zn-Mn batteries, and their cycle life were typically limited to <50 cycles (Kordesch et al., 1981). This urged scientists to identify an electrolyte replacement, and accordingly, in 1986, Yamamoto and Shoji (Yamamoto and Shoji, 1986) swapped the traditional alkaline electrolyte with a mildly aqueous ZnSO₄ electrolyte. This concept was later developed as a ZIB in 2011 based on the Zn²⁺ shuttling properties, and this battery is considerably different from the conversion-based alkaline Zn-Mn batteries (Xu et al., 2012). Subsequently, in 1994, Wu et al. (Li et al., 1994) demonstrated aqueous rechargeable Li-ion batteries using a LiMn₂O₄ cathode and a VO₂ anode. The battery operates based on Li⁺ shuttling between the anode and cathode, similar to that in non-aqueous LIBs. Since the discovery of the intercalation chemistry of aqueous systems, substantial advancements have resulted in several rocking-chair-type aqueous batteries, including monovalent and multivalent metal-ion batteries (Liu et al., 2018). Recently, hybrid batteries consisting of an intercalation-type cathode paired with a metal anode or intercalation anode paired with metal oxides/sulfides were documented (Soundharrajan et al., 2018; Ao et al., 2019). The recently reported new Mn-H battery driven by the Mn oxidation reaction/Mn reduction reaction (MOR/MRR), i.e., Mn deposition/dissolution chemistry, generated the potential for new energy storage devices (Chen et al., 2018). This review provides a brief overview of the development of Mn-based batteries, starting from the discovery of rechargeable MnO₂||Zn batteries, followed by the efforts to understand the insertion conversion and deposition/dissolution reaction mechanisms in Mn-based aqueous batteries. Finally, we highlight the diverse cell setups employing Mn dissolution/deposition chemistry and the strategy to enable the stable cycling of these batteries.

Before reviewing the MnO₂/Mn²⁺-chemistry-inspired energy storage devices in detail, the mechanistical or historical aspects of MOR/MRR must be understood (Soundharrajan et al., 2022a). Although several reports regarding modern-day battery devices inspired by MnO₂/Mn²⁺ chemistry have been published recently (Moon et al., 2021; Zheng et al., 2021; Liu et al., 2022a; Yang et al., 2022; Naresh et al., 2023; Ye et al., 2023), the observation of unique MnO₂/Mn²⁺ chemistry was first reported in 1998 in relation to the Zn/ZnSO₄/MnO₂ rechargeable cell by Kim et al. (Kim and Oh, 1998). Although the Zn/ZnSO₄/MnO₂ rechargeable cell was not coined as ZIBs by Kim et al. (Kim and Oh, 1998), the use of mild-aqueous ZnSO₄ electrolyte for Zn/MnO₂ rechargeable cells could termed as ZIBs, as per the recent nomenclature. More specifically, the authors observed the dissolution of solid-MnO₂ cathode into soluble Mn²⁺ ions during the discharge process, whereas the soluble Mn²⁺ ions were re-deposited to form solid-MnO₂ during the charging process. More importantly, the authors found that dissolved Mn²⁺ ions were not completely re-deposited during the repeated charge/discharge cycles in the ZnSO₄ electrolyte. The authors attributed this inefficiency to the nature of the conductive-carbon surface states used for electrode fabrication. To validate their claim, the authors compared the MnO₂/Mn²⁺ chemistry of different carbon sources belonging to the acetylene and furnace black categories using cyclic voltammetry (CV). Based on the comparative CV and ex-situ scanning electron microscope (SEM) outputs, they confirmed that the cathode loaded with acetylene black with negligible surface oxygen species exhibited better MnO₂/Mn²⁺ reversibility than the other carbon sources because the lack of surface oxygen species increased facile Mn²⁺ oxidation. Additionally, they demonstrated that the reversibility of MnO₂/Mn²⁺ redox can be improved through the addition of MnSO₄ to the ZnSO₄ electrolyte. Here, the presence of MnSO₄ can suppress non-faradaic Mn dissolution and the formation of ZnSO₄ deposits on the cathode surface.

Kim et al. (Kim and Oh, 1998) initially investigated the electrolyte-induced MnO₂/Mn²⁺ chemistry on a solid-state cathode used in a Zn/MnO₂ rechargeable cell. Perret et al. (Perret et al., 2011) harvested the charge supplied by the in situ reversible MnO₂/Mn²⁺ chemistry by employing electrolytic Mn as the main means of charge storage almost a decade later in 2011. More specifically, the authors constructed a hybrid supercapacitor with composite graphene as a capacitive electrode and graphene as the battery-type electrode that acted as a substrate for the in situ electrochemical deposition/dissolution of MnO₂ in 0.5 M H₂SO₄ + 0.5 M MnSO₄ electrolyte. They observed that the dissolution/deposition efficiency of MnO₂ was only 80%, which indicates that only ∼80% of the deposited MnO₂ dissolved into the electrolyte. This inefficiency resulted in capacitor breakdown after a few hundred cycles in the coin-cell setup.

Nevertheless, the use of the beaker-cell setup with a large electrolyte volume enabled a superior cycling performance of up to ∼5,000 cycles. In 2012, the same group conducted an extended investigation on the limitations of the proposed system (Perret et al., 2012). The authors confirmed that the in situ electrochemical deposition/dissolution of MnO₂ is observed between 1.08–1.18 V and 1.09–1 V, respectively.

Additionally, they observed that the reversible solid-MnO₂ conversion to soluble-Mn²⁺ (1 cycle) is observed within a short period of 300 s. Furthermore, they revealed that the proportion of electrolyte volume to electrode mass is critical for the cycling stability of the system. Most importantly, they observed that the poor electrodeposition vs. deposition efficiency of MnO₂ altered the pH of the electrolyte and reduced the Mn²⁺ concentration in the repeated charge/discharge cycles; this eventually affected the final energy and power supplied by the system. Overall, various MOR processes may occur at different potentials, depending on the pH of the electrolyte, as illustrated in Figure 1 (Li et al., 2023). To enable reversible and high-energy Mn-redox chemistry, neutral or acidic electrolytes and high-potential redox couples are preferred. Thermodynamically, MnO₂/Zn_xMnO₂ and MnO₂/MnOOH couples proceed at ∼0.5 V vs. the standard hydrogen electrode (SHE) in a neutral electrolyte and involve the intercalation of Zn²⁺ and H⁺, respectively. In reality, the transformation of MnO₂ into Zn_xMnO₂ (charging) was observed to proceed at ∼0.84 V vs. SHE in a 1 M ZnSO₄ electrolyte (Alfaruqi et al., 2015). Similarly, the reaction of MnO₂ into MnOOH was observed to occur at ∼0.86 V vs. SHE in a 1 M ZnSO₄ electrolyte (Pan et al., 2016).

At high redox potentials of ∼1.22 V vs. SHE, MnO₂/Mn²⁺ dissolution/deposition reaction proceeded in acidic electrolytes. Nevertheless, MnO₂ deposition was observed at ∼0.84 V vs. SHE in a 2 M ZnSO₄/0.24 M MnSO₄ electrolyte (Zhao et al., 2018). To increase the redox potential further, Li et al. (Li et al., 2023) recently planned a strategy to promote Mn³⁺/Mn²⁺redox by increasing the acidity of the electrolyte beyond that of commonly reported electrolytes. In a 3 M H₂SO₄/0.5 M MnSO₄/0.1 M SnSO₄/0.05 M Na₄P₂O₇ electrolyte, they observed Mn²⁺ to Mn³⁺ oxidation at a high potential of ∼1.67 V vs. SHE, which is slightly higher than the thermodynamic redox potential of ∼1.5 V vs. SHE. In another study, Mn³⁺/Mn²⁺ redox was reported by Wu et al. (Wu et al., 2022), who employed a catholyte containing 0.1 M MnSO₄, 1 M H₂SO₄, and 1 M HCl, in which the presence of HCl promoted the formation of soluble Mn³⁺ in the form of [MnCl₄(H₂O)₂]. Therefore, by summarizing the origin and evolution of this unique Mn deposition/dissolution chemistry, we understood the challenges and strategies for future research directions.

Intercalation chemistry has been accepted worldwide after the successful commercialization of LIBs. Thus, Zn/ZnSO₄/α-MnO₂ rechargeable cells were established as rocking-chair-type batteries by Xu et al. (Xu et al., 2012) based on the Zn²⁺-shuttling-mechanism model in the MnO₂ positive electrode, which indicated that the succeeding Mn-type cathodes in mild aqueous Zn cells can be categorized as intercalation cathodes for ZIBs. After the authors coined the term ZIB, an increasing number of studies have been published in this field.

Various polymorphs of MnO_2, including β-MnO₂, δ-MnO₂, λ-MnO₂, γ-MnO₂, and ε-MnO₂ have been documented as ZIBs that exhibit the Zn²⁺ shuttling mechanism; however, the electrochemical cycling stabilities of these ZIBs are unconvincing (Lee et al., 2013; Yuan et al., 2014; Islam et al., 2017; Mathew et al., 2020; Soundharrajan et al., 2020). Similar to other MnO₂-based rechargeable systems exhibiting common intercalation mechanisms, the Jahn-teller-distortion-induced Mn²⁺ dissolution was predicted to be the main reason for the downfall of ZIBs containing Zn/MnO₂ polymorphs, which resulted in irreversible structural transformations, in mild aqueous electrolytes. Hence, to overcome this issue, the pre-inclusion of Mn²⁺ ions in the form of MnSO₄ in the ZnSO₄ electrolyte to maintain the ambient equilibrium conditions suggested by Pan et al. (Pan et al., 2016). When employing this strategy, the authors expected that MnO₂ was converted into MnOOH via the reversible H⁺ intercalation/de-intercalation mechanism. Various research groups proposed various reaction chemistries in the following years concerning Mn²⁺-comprising ZnSO₄ aqueous electrolytes for ZIBs. Based on this aspect, a detailed analysis of the electrochemical mechanism in MnO₂ cathodes was comprehensively summarized by Sambandam et al. (Sambandam et al., 2022) and Yang et al. (Yang et al., 2023).

Initially, Sun et al. (Sun et al., 2017) proposed a distinctive H⁺/Zn²⁺ co-intercalation mechanism in Zn/MnO₂ systems. Interestingly, Li et al. (Li et al., 2019) planned the probability of intercalation-induced chemical conversions. More specifically, Lv et al. (Lv et al., 2022) proposed that during discharging, H⁺/Zn²⁺ intercalation proceeds first, followed by the disproportionation of Mn³⁺, resulting in the formation of Mn²⁺. Whereas, Huang et al. (Huang Y. et al., 2019) reported a complex Zn-storage mechanism involving the participation of Zn²⁺, H⁺, and Mn²⁺ in Zn/MnO₂-based ZIBs in an Mn²⁺-containing ZnSO₄ electrolyte. The complex reaction mechanism comprising intercalation, conversion, and Mn²⁺ oxidation proposed by the authors further added to the bulletin of reaction mechanisms for Zn/MnO₂-based ZIBs in an Mn²⁺-containing ZnSO₄ electrolyte. Thus, in addition to the Zn²⁺ shuttling mechanism, several reaction mechanisms have been proposed for ZIBs in an Mn²⁺-comprising ZnSO₄ aqueous electrolyte.

In particular, Beoun Lee et al. (Lee et al., 2016) claimed that only the reversible Mn²⁺ dissolution/MnO₂ deposition from the cathode material in a ZnSO₄ electrolyte and not the conventional Zn²⁺ (de)intercalation chemistry is the primary capacity donor (Eq 1). 3 M n O 2 + 8 Z n 2 + + 2 S O 4 2 − + 16 H 2 O + 6 e − ↔ 3 M n 2 + + 2 Z n 4 O H 6 S O 4 ∙ 5 H 2 O (1)

More specifically, Mn²⁺ dissolves during the discharge process along with the in situ precipitation of Zn₄(OH)₆ (SO₄).5H₂O (ZHS), whereas the dissolved Mn²⁺ ions are re-deposited as MnO₂ during re-charging, and ZHS dissolves into the electrolyte in the Zn/α-MnO₂ ZIBs constructed by the author. The authors verified the reaction chemistry via in situ pH monitoring, by which they observed the reversible variation in pH during the charge/discharge process driven by the Mn²⁺ dissolution/MnO₂ deposition chemistry inside Zn/MnO₂ ZIBs. Additionally, from the in situ X-ray diffraction (XRD) analysis, the authors did not find any evidence of Zn²⁺ intercalation, whereas the reversible formation/dissolution of ZHS was a exceeding observation. The formation of ZHS was reported as an inevitable parasitic reaction in ZIBs containing a ZnSO₄ electrolyte in the following years, irrespective of the choice of cathode materials (Islam et al., 2017; Sambandam et al., 2018a; 2018b).

The addition of Mn²⁺ ions was primarily claimed to be a structural stabilizer in the early versions of ZIBs; however, the electrochemical activity of Mn²⁺ ions contributing to the energy output is accepted in modern-day ZIBs. Beoun Lee et al. (Lee et al., 2016) claimed that MnO₂/Mn²⁺ chemistry was the primary capacity contributor in Mn-based ZIBs in a ZnSO₄ electrolyte. In the following years, collective classical (de)intercalation and MnO₂/Mn²⁺ chemistry were challenged as the capacity contributors in Mn-based ZIBs in an Mn²⁺-containing ZnSO₄ electrolyte. For example, in 2018, Zhao et al. (Zhao et al., 2018) reported that ZHS critically influences Zn/MnO₂-based ZIBs in an Mn²⁺-containing ZnSO₄ electrolyte. To verify this, they directly used ZHS as the cathode material to emulate Zn/MnO₂-based ZIBs. Interestingly, the authors found that the Mn²⁺ ions consume ZHS in the 2 M ZnSO₄ + 0.24 M MnSO₄ electrolyte during the first charging process, generating MnO₂ in situ, as per the following reaction (Eq 2). 3 M n 2 + + 2 Z n 4 O H 6 S O 4 ∙ 5 H 2 O ↔ 3 M n O 2 + 2 S O 4 2 − + 8 Z n 2 + + 16 H 2 O + 6 e − (2)

Although the authors did not account for the availability of additional Mn²⁺ ions in the electrolyte to encourage in situ MnO₂ deposition during subsequent cycles, they claimed that in situ MnO₂ deposition was followed by conventional intercalation mechanisms in the following charge/discharge cycles in Zn/MnO₂-based ZIBs, i.e., the two-step reversible redox reactions of Mn²⁺ shown in Eqs. 3, 4. 6 M n 2 + + 4 Z n 4 O H 6 S O 4 ∙ 5 H 2 O ↔ 3 Z n M n 2 O 4 + 4 S O 4 2 − + 32 H 2 O + 13 Z n 2 + + 6 e − (3) 7 Z n M n 2 O 4 + 5 + 2 x H 2 O ↔ Z n x M n 7 O 13 + y ∙ 5 H 2 O + 7 − 2 x Z n 2 + + 4 x H + + 6 e − (4)

Although they observed the in situ deposition of MnO₂ from the ZHS cathode in the first cycle, they did not consider the reversibility of the in situ deposition of MnO₂ despite observing the in situ ZHS formation/dissolution during subsequent cycles. More specifically, Guo et al. (Guo et al., 2020) proposed that reversible Mn²⁺ dissolution/MnO₂ deposition is the dominant capacity supplier in the Zn/MnO₂-based ZIBs with a Mn²⁺-containing ZnSO₄ electrolyte. They observed that the H⁺/Zn²⁺ (de)intercalation reaction contributed the least to the capacity supplied by Zn/MnO₂-based ZIBs. With the help of sophisticated synchrotron X-ray experiments, including XRD, X-ray absorption spectroscopy, X-ray nano-tomography, and X-ray fluorescence microscopy, Kankanallu et al. (Kankanallu et al., 2023) further confirmed that the (de)intercalation process is absent in Zn/MnO₂ batteries. They explained that two types of Zn-Mn complexes, i.e., an amorphous Zn-Mn complex and a poorly crystalline ZnMn₂O₄ phase, are formed in the amorphous ZHS precipitate. The former can be reversibly dissolved in the electrolyte, contributing to the discharge capacity together with the dissolution of MnO₂, whereas the latter is found to be irreversible. The authors attributed the capacity fade to the build-up of the irreversible ZnMn₂O₄ phase upon cycling. Mn deposition/dissolution chemistry with zinc metal as anode are listed in (Table 1).

In addition to the direct usage of solid-MnO₂ as an active material for Zn/MnO₂ batteries involving charge-storage mechanisms with partial deposition/dissolution chemistry, cathode-free Zn/MnO₂ batteries driven by the direct participation of deposition/dissolution chemistry have recently attracted attention from energy researchers.

In cathode-free Zn/MnO₂ batteries, conducting carbon can be used as a cathode instead of solid MnO₂, with a metallic Zn anode. During charging, MnO₂ (Eq 5) and Zn deposition (Eq 6) proceed on the cathode and anode, respectively (Liu et al., 2020b; Aguilar et al., 2022). M n a q 2 + + 2 H 2 O → ε − M n O 2 s + 2 e − + 4 H a q + (5) Z n a q 2 + + 2 e − → Z n s (6)

However, cathode-free Zn/MnO₂ batteries lose substantial capacity owing to the incomplete dissolution of MnO₂ (under high areal capacities) and the poor electronic conductivity of the deposited MnO₂.

Notably, the electrolyte is crucial in promoting the reversible Mn²⁺/MnO₂ reaction to ensure the long life of a Zn/MnO₂ battery. Recently, considerable research efforts have been targeted toward improving the reversibility of Mn²⁺/MnO₂ reactions in Zn/MnO₂ batteries. For example, Xinhua Zheng et al. (Zheng et al., 2021) used Br as a redox mediator to enhance the chemical dissolution of over-deposited MnO₂, thereby preserving the reversibility of Mn²⁺/MnO₂ deposition/dissolution.

The authors validated that the presence of Br⁻ in the electrolyte is directly involved in the dissolution of over-deposited MnO₂ on carbon felt based on the following reaction: Eqs. 7, 8 M n O 2 + 4 H + + 3 B r − ↔ M n 2 + + B r 3 − + 2 H 2 O (7) B r 3 − + 2 e − ↔ 3 B r − (8)

The authors claimed that 1/3 of the MnO₂/Br₂-Zn battery capacity mainly originated from the electrochemical conversion of Br^3- to Br⁻ via a redox reaction. Furthermore, from the ex-situ XRD analysis, the Br mediator was found to enhance the reversibility of MnO₂/Mn²⁺ reactions in the MnO₂/Br₂-Zn battery (Figure 2). The designed MnO₂/Br-Zn battery revealed a high discharge voltage of 1.98 V with an aerial capacity of 5.8 mAh cm^-2. More importantly, the MnO₂/Br-Zn battery exhibited long-term stability with 100% capacity retention for 600 cycles at a 20 C rate. Furthermore, the authors scaled up a MnO₂/Br₂-Zn battery to obtain a large capacity of 1,200 mAh and a potential energy density of 32.4 Wh kg^-1. The imperative highlight of this study is the practically feasible low-energy cost under 15 US$ kWh^-1, which is highly beneficial for the large-scale energy-storage market.

Lei et al. Lei et al. (2021) used a redox mediator to facilitate MnO₂ dissolution and enhanced cyclic stability by recovering the lost capacity from exfoliated MnO₂ layers. The authors used iodine as a redox mediator (0.1 M KI), as it has a suitable potential (0.536 V vs. SHE) with high reversibility and fast kinetics. They reported the working mechanism of the KI mediator for facilitating MnO₂ dissolution as follows (Lei et al., 2021): I 2 + 2 e − → 2 I − (9) 3 M n O 2 + 3 I − + 4 H + → M n 2 + + I 3 − + 2 H 2 O (10) I 3 − + 2 e − → 3 I − (11)

During the discharging process, I⁻ chemically reacts with residual MnO₂ or exfoliated MnO₂ layers to form Mn²⁺ and produces I₃ ⁻. Subsequently, I₃ ⁻ is electrochemically reduced to I⁻, which continues to consume the remaining dead MnO₂. This process prevents the accumulation of dead MnO₂ and the insertion of H⁺, thereby increasing the Coulombic efficiency and capacity of the battery. The authors confirmed the reaction between I⁻ and MnO₂ (Equation 10) using ultraviolet-visible spectroscopy. The two peaks at 289 nm and 351 nm in the spectra correspond to I₃ ⁻. The triiodide crossover (I₃ ⁻) was reported to be retarded due to the formation of solid I₂, which does not react with MnO₂, during the charging process. Generally, the mediator reaction is driven by the lower redox potential of the mediator than that of Mn²⁺/MnO₂. The author explained this via charge-discharge voltage profiles, in which at a discharge potential of >1.45 V, regular MnO₂ dissolves to form Mn²⁺ until the potential reaches the reduction potential of I₂ to I⁻. At potentials <1.45 V, the mediator reaction occurs according to Eq 9, and this process continues until all the MnO₂ is consumed. By deploying this strategy to the Zn-Mn battery, the authors achieved stable cyclic stability of over 400 cycles at 2.5 mA/cm². Further, they designed Zn-Mn redox-flow batteries and achieved stability of 225 cycles at 15 mA/cm² and more than 50 cycles at a high areal capacity of 50 mA/cm², which is the highest among those of the reported Zn-Mn batteries. A similar approach to improve performance through the addition of a redox mediator was reported by Liu et al. (Liu et al., 2022b). They demonstrated that the addition of CrCl₃ to the electrolyte may reduce suspended MnO₂. Therefore, their Zn-MnO₂ cell with a modified electrolyte could deliver a stable cycling performance for 500 cycles, delivering ∼0.4 mAh cm^-2 at 10 mA cm^-2, whereas the cell without CrCl₃ stopped working after <100 cycles.

In another report, Liu et al. (Liu et al., 2021b) studied the reaction mechanism of the Zn-MnO₂ battery and its relationship with the pH of the electrolyte by substituting the H₂SO₄ additive with CH₃COOH. They suggested that a dual mechanism occurs in the Zn-MnO₂ battery, which includes 1) dissolution and deposition of MnO₂ and 2) (de)intercalation of H⁺ and Zn²⁺ into residual MnO₂. They conducted X-ray photoelectron spectroscopy (XPS) analysis to confirm the presence of a dual Zn²⁺ and H⁺ intercalation mechanism in the Zn-MnO₂ battery. Under low pH, dissolution and deposition of MnO₂ from the electrolyte is the primary reaction and not the (de)intercalation process. The drawback of using an H₂SO₄ additive is that its strong acidity causes corrosion and H₂ evolution reactions at the Zn anode.

Additionally, further intercalation of H⁺ increases the pH of the electrolyte, retarding the dissolution of MnO₂ during the discharge process. The buffering effect of the added CH₃COOH reduces the fluctuation phenomenon and, therefore, increases cyclic stability. They assembled a soft package battery using an acidic CH₃COOH electrolyte with an Al plastic film and achieved a long lifetime of 2000 cycles with a Coulombic efficiency of 99.6%. The beneficial use of weak acids was additionally demonstrated by Mickaël et al. (Mateos et al., 2020). They conducted an in-depth quantitative spectro-electrochemical analysis of MnO₂ thin-films in unbuffered and buffered aqueous electrolytes of different natures, compositions, and pH values. The study demonstrated that nearly two-electron storage capacity of MnO₂ is accessible when a weak Bronsted acid, such as [Zn(H₂O)₆]²⁺ or [Mn(H₂O)₆]²⁺ complexes, is present at a sufficiently high concentration. Moreover, reversible MnO₂/Mn²⁺ dissolution and deposition can also be achieved.

Similarly, Zhong et al. (Zhong et al., 2022) highlighted the beneficial effects of the acetate ion (Ac⁻) as an additive. They proposed that dissolution, deposition, and (de)intercalation contributed to the charge-storage mechanism in their Zn/MnO₂ cell, and the use of Ac⁻ increased the proportion of the reaction, which resulted in an increase in cell capacity from 221.1 mAh g^-1 to 339.5 mAh g^-1. Accordingly, buffered electrolytes loaded with Mn²⁺ can yield high gravimetric capacity and stable voltages at the MnO₂ electrodes. A high gravimetric capacity of 450 mAh g⁻¹ is obtained at a high rate of 1.6 A g⁻¹, with a Coulombic efficiency close to 100% and an MnO₂ utilization of 84%. This study revealed another interesting mechanism in mild aqueous electrolytes and clarified the benefit of buffered electrolytes for maximizing the performance of ZIBs (Mateos et al., 2020).

To prevent the irreversible side reaction involving Mn²⁺ dissolution, a pre-treatment with an artificial electrode microskin (EMS) was proposed and applied on the surface of an MnO₂ cathode (Liu et al., 2020b). The bionic EMS composed of C–H groups containing phenylalanine and tyrosine amino acids can effectively confine Mn²⁺ dissolution via a physical barrier and chemical adsorption. Therefore, the dissolved Mn²⁺ species are localized on the cathode surface owing to van der Waals forces, hydrogen bonding, and/or ionic bonding. Interestingly, the assembled aqueous Zn//δ-MnO₂ battery with a 10.1-μm microskin displayed a discharge capacity of 415 mAh g⁻¹ at 20 mA g⁻¹. The cell exhibited relatively good capacity retention at low current densities but struggled at medium and high-rate operations. Notably, although adding an extra layer on the electrode surface can form a protective layer, the layer may partially passivate the movement of host ions (protons or Zn²⁺), thereby reducing reaction rates. The results demonstrated a highly fluctuated cycling process at high reaction rates; however, the material strategies and cell components need further improvement.

Widening the potential window is the main challenge in achieving feasible ZIBs. The potential window is limited by the aqueous electrolyte in which the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can proceed on the cathode and anode, respectively. Although the use of acidic electrolytes inhibits the OER on the cathode, the presence of H⁺ promotes HER on the Zn anode (Xie et al., 2020). In theory, the concept of employing acidic and alkaline electrolytes separated by a neutral medium significantly extends the operating range up to 2.83 V, achieving a possible energy density of 1,621.7 Wh kg^-1 in Zn-MnO₂ batteries (Zhong et al., 2020). However, the H⁺ present in the catholyte may still migrate to the anolyte, resulting in irreversible HER on the anode. One strategy to suppress the impact of H⁺ migration to the anolyte is to employ a proton trapping agent (PTA), as demonstrated by Sun. et al. (Sun et al., 2022). PTAs are a category of salts containing weak acidic ions, such as H₂PO₄ ⁻, C₆H₅O₇ ^3-, and Ac⁻, that are capable of trapping H⁺, as shown in Figure 3. Sun. et al. (Sun et al., 2022) suggested that CH₃OO⁻ has the best ability to suppress HER, enabling the reversible plating and stripping of the Zn anode. They increased the lifetime of their Zn/MnO₂ cell 5-fold to 400 cycles by using 1 M MnSO₄ + 0.15 M H₂SO₄ + 0.1 M NaCl as the catholyte and 0.8 M ZnSO₄ + 0.2 M Zn(COOH)₂ + 0.1 M NaCl as the anolyte. The concept of electrolyte buffering can be applied by adding an electrolyte additive and by introducing a cathode additive that can attract protons during the MnO₂ deposition process (Eq 12). Using the cathode additive approach, Aguilar et al. (Aguilar et al., 2022) prepared a cathode composite consisting of 10 wt% Mg(OH)₂ and 90 wt% carbon. Mg(OH)₂ buffered the electrolyte by consuming protons during discharging (Eq. 12) and releasing them during charging (Eq 13). This approach suppressed the corrosion of the Zn anode during charging and promoted MnO₂ dissolution during discharging. Consequently, a stable cell performance with a discharge capacity of 412 mAh g^-1 for 180 cycles was achieved (Aguilar et al., 2022). M g O H 2 s + 2 H a q + + 4 H 2 O ↔ M g H 2 O 6 2 +   p K s = 11   in pure water (12) M g H 2 O 6 2 + ↔ M g O H H 2 O 5 + + H a q +   p K a 1 = 11.4   in pure water (13)

The use of Ac⁻ in a slightly different manner was reported by Chen et al. (Chen et al., 2021). They proposed an interesting approach of using an oversaturated gel electrolyte (OSGE) prepared by dissolving Zn(CH₃CO₂)₂.2H₂O and CH₃CO₂K in deionized water with polyacrylic acid at 75 ^°C. This gel electrolyte provides an excellent solvation sheath, which is stable during the high-voltage operation of ZIBs. The OSGE-based Zn-MnO₂ battery retained 304 mAh g^-1 capacity at 1 C with a limit in the range of 0.8–2.0 V after more than 100 cycles. The system could operate at high current densities of 10 C and achieved 82.7% capacity retention after 2000 cycles. Moreover, the system showed highly stable Zn deposition at room temperature while operating up to 2.0 V.

The high viscosity of the gel electrolyte demonstrates excellent compatibility with ZIBs, generating possibilities for new solid-electrolyte strategies. In a recent report, a novel tri-layer hydrogel electrolyte was prepared via multiple injection and molding steps to form three layers of polyacrylamide (PAM)/ZnSO₄, alkaline sodium polyacrylate (PAA-Na)/ZnSO₄, and PAM/ZnSO₄/H⁺ (Shen Z. et al., 2021). The two-electron redox of MnO₂/Mn²⁺ and anode protection are crucial for maintaining high battery performance.

The battery exhibits a high capacity of 516 mAh g^-1 at 50 mA g^-1 and excellent capacity retention of 93% at 5 A g^-1 after 5,000 cycles. Moreover, owing to the flexible nature of the hydrogel, this solid electrolyte demonstrated outstanding mechanical stability while maintaining more than 99.4% capacity. This allowed the assembly of the fibrous Zn/MnO₂ battery with excellent flexibility, demonstrating the potential for use in wearable electronics.

A unique approach to improve the reversibility of the MnO₂/Mn²⁺ redox reaction through the regulation of the upper cut-off voltage (UCV) was proposed by Liu et al. (Liu et al., 2023). By increasing the UCV to 2.2 V, followed by a constant-voltage holding step during the first charge, the deposited MnO₂ participated in the two-electron reduction reaction from MnO₂ to Mn²⁺. This activation originated from the contraction and formation of oxygen vacancies in the original MnO₂ cathode, forming weakly bonded MnO₂ deposits. The authors demonstrated that the modified charging protocol (2.2 V + hold vs. 1.8 V) in the Zn/MnO₂ cell with a 3 M ZnSO₄ electrolyte without Mn²⁺ increased capacity retention from 31.5% to 94.6% after 300 cycles. To protect the Zn anode from being corroded, Deng et al. (Deng et al., 2023) employed the concept of a nanomicellar electrolyte by adding methyl-urea (Mu) molecules to ZnSO₄/MnSO₄ electrolytes. The addition of Mu divided the aqueous solvent environment into hydrophilic and hydrophobic regions in which cations and anions were encapsulated by nanomicelles, as illustrated in Figure 4. The participation of Mu molecules in the solvation of Mn²⁺ and Zn²⁺ expels water molecules and decreases the desolvation energy barrier. Moreover, the nanomicelles enable a controllable release of Mn²⁺ and Zn²⁺, facilitating uniform Zn deposition, as illustrated in Figure 4B.

The deposited Zn is further protected from water molecules by the in-situ-formed interphase layer comprising Zn_x (Mu)_ySO₄. nH₂O. With the use of 7 M Mu in a 2 M ZnSO₄ + 0.5 M MnSO₄ electrolyte, they operated the Zn/MnO₂ battery for 800 cycles with 100% capacity retention at 0.5 mAh cm^-2, whereas the cells without the Mu additive failed after 200 cycles. The authors also claimed that the improved cell has an average discharge voltage of 1.87 V, and they estimated that the battery has a specific energy of ∼800 Wh kg^-1 at the material level.

Overall, previous research on ZIBs has claimed that the capacity performance of Mn-based cathodes is mainly due to the active material of the cathode. As discussed in this section, more recent studies have confirmed that the dissolved Mn²⁺ ions, either from the cathode or in the form of electrolyte addition, are the main contributors to the capacity of the Mn deposition and dissolution in ZIBs. The contribution of Mn deposition/dissolution to the capacity performance of Mn-based cathodes for ZIBs is therefore very imperative. However, the lack of a comprehensive and practical protocol for differentiating capacity hinders the true capacity contribution when using manganese-based oxides as cathodes in ZIBs. Still, there is a strong interest in utilizing the use of Mn deposition/dissolution as a way to increase the capacity output of ZIBs has been overlooked by researchers. However, the consequences are amplified by the complexity of the reaction mechanism and the large volume expansion due to the uncontrollable gas evolution induced by the pH change associated with the addition of Mn²⁺.

Rechargeable aqueous Mn batteries are promising systems for storing energy sustainably owing to their safety, reasonably high voltage, and low-cost. Initially, the use of mild acidic electrolytes allowed aqueous Mn batteries to be rechargeable. Since then, our understanding of the working mechanism of rechargeable aqueous Mn batteries has progressed significantly. Rational electrolyte design enables the utilization of the high-potential MnO₂/Mn²⁺ redox couple, allowing for high-energy storage. The use of electrolyte additives can promote facile reaction kinetics and stable cycling performance. To obtain robust performances of rechargeable aqueous Mn batteries based on MnO₂/Mn²⁺ redox, suitable redox couples on the anode have been sought.

This includes metal dissolution/deposition (Sn, Cu, Bi, and Al), Pb/PbSO₄ redox, Na⁺ adsorption/desorption, and hydrogen evolution/reduction reaction. Thus, overall, the underlying reaction mechanism in the aqueous batteries involving MnO₂/Mn²⁺ redox couple associated with the different anode environments is schematically illustrated in Figure 6.

1. In view of the complexity of the redox reaction that takes place in both the deposition and dissolution voltage ranges of the aqueous batteries, the electrochemical stability window must be analyzed, and its true potential can only be realized at high arial mass loadings. Since the battery is charged or discharged with a high probability of accumulation of by-products, there is a serious challenge with the reversibility of charge or discharge capacity and the volume expansion associated with battery failure problems.