Insights into the cycling stability of manganese-based zinc-ion batteries: from energy storage mechanisms to capacity fluctuation and optimization strategies

Abstract

Manganese-based materials are considered as one of the most promising cathodes in zinc-ion batteries (ZIBs) for large-scale energy storage applications owing to their cost-effectiveness, natural availability, low toxicity, multivalent states, high operation voltage, and satisfactory capacity. However, their intricate energy storage mechanisms coupled with unsatisfactory cycling stability hinder their commercial applications. Previous reviews have primarily focused on optimization strategies for achieving high capacity and fast reaction kinetics, while overlooking capacity fluctuation and lacking a systematic discussion on strategies to enhance the cycling stability of these materials. Thus, in this review, the energy storage mechanisms of manganese-based ZIBs with different structures are systematically elucidated and summarized. Next, the capacity fluctuation in manganese-based ZIBs, including capacity activation, degradation, and dynamic evolution in the whole cycle calendar are comprehensively analyzed. Finally, the constructive optimization strategies based on the reaction chemistry of one-electron and two-electron transfers for achieving durable cycling performance in manganese-based ZIBs are proposed.

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1 Introduction

The rapidly increasing energy consumption and environmental issues make it urgent to utilize large-scale electrical energy storage (EES) systems to store intermittent but renewable energy, such as solar energy, wind, and tidal energy.^1–3 Among the various EES systems, lithium-ion batteries (LIBs) have been widely used for dozens of years owing to their high gravimetric energy density and excellent cycling performance.^4,5 However, the high price of lithium and its limited resources coupled with safety and environmental concerns for the use of flammable organic electrolytes hinder the application of LIBs as large-scale EES systems.^6,7

Fortunately, the emerging aqueous rechargeable batteries, particularly aqueous zinc-ion batteries (ZIBs) hold great promise for large-scale EES because of the low cost, natural abundance, high stability, low electrochemical potential (−0.763 V vs. the NHE) and high theoretical capacity (5855 mA h cm⁻³ and 820 mA h g⁻¹) of Zn metal coupled with the high safety and high ionic conductivity of aqueous electrolytes.^8–12 In general, the energy storage performance of ZIBs is mainly related to the cathode material, Zn anode, electrolyte, and separator.¹³ Recently, although studies in the literature have focused on the issues associated with the Zn anode,^14–17 the nature of cathode materials essentially dominates the electrochemical performance of ZIBs, such as capacity, working potential, cycling stability, and rate performance. However, owing to the much larger radius of hydrated Zn²⁺ (4.3 Å) than Li⁺ (0.76 Å), many cathode materials with excellent electrochemical performance in LIBs are not suitable for ZIBs.¹⁸ Consequently, the limited performance of cathode materials for ZIBs greatly impede the development of high-performance EES systems. Thus far, manganese-based materials,^19,20 vanadium-based materials,^21–23 Prussian blue and its analogues,^24–26 Chevrel phase compounds,^27,28 and organic compounds have been exploited as cathode materials for ZIBs.^29,30 Amongst them, manganese-based materials are considered the most attractive cathodes for ZIBs and have been widely and intensively investigated owing to their low cost, natural abundance, low toxicity, multivalent states, high operation voltage, and satisfactory capacity (Fig. 1).^31–33

Fig. 1 Number of publications in recent years on manganese-based ZIBs and ZIBs (data from Web of Science, December 2023).

Significant progress has been made in manganese-based ZIBs over the past decade, as depicted in Fig. 2. Nevertheless, manganese-based cathodes in ZIBs involve various and controversial energy storage mechanisms, and six major energy storage mechanisms have been successively discovered in the past ten years: (1) Zn²⁺ insertion/extraction, (2) H⁺ insertion/extraction, (3) H⁺ and Zn²⁺ co-insertion/extraction, (4) chemical conversion, (5) dissolution–deposition, and (6) hybrid storage mechanisms (Fig. 3). These complex mechanisms invariably confuse readers, particularly for recent students in this field, and thus need to be comprehensively and clearly summarized. Moreover, the unsatisfactory long-term cycling stability (particularly at a low current) of manganese-based ZIBs has severely hampered their practical application. Meanwhile, a fluctuating capacity has been common observed (including capacity activation process and capacity degradation process) during long-term cycling and its mechanism has not been clearly understood, leading to a lack of robust methods to improve their cycling stability. Although some good reviews have been published on manganese-based ZIBs to guide researchers,^34–43 these reviews mainly focused on optimization strategies for high capacity and fast reaction kinetics, seldomly mentioning the dynamic fluctuation of capacity and lacking a deep comprehension of essential reasons for capacity activation and degradation. Consequently, a comprehensive understanding of the capacity fluctuation is urgently needed to guide researchers to design manganese-based ZIBs with stable cycling performances.

Fig. 2 Progress on manganese-based ZIBs.

Fig. 3 Illustration of energy storage mechanisms for manganese-based ZIBs. (a) Zn²⁺ insertion/extraction, (b) H⁺ insertion/extraction, (c) H⁺ and Zn²⁺ co-insertion/extraction, (d) chemical conversion reaction, (e) dissolution–deposition, and (f) hybrid storage mechanism.

In this review, the energy storage mechanisms of manganese-based ZIBs with different structures are systematically clarified and summarized. More importantly, the capacity fluctuation of manganese-based ZIBs is comprehensively analyzed for the first time. We not only summarize the immediate causes of capacity activation and degradation, but also analyze the inherent dynamic evolutions (such as pH, reaction mechanisms, and active materials) that result in capacity fluctuations. Furthermore, the constructive optimization strategies based on the reaction chemistry of one-electron and two-electron transfer for achieving durable cycling performance manganese-based ZIBs are proposed, respectively. Finally, the conclusions and outlooks on the present reaction mechanisms and optimization strategies for achieving durable cycling performances based on our personal perspectives are also provided. This review provides a comprehensive overview and the research directions focusing on the recent reaction chemistry and cycling stability, which will provide a guide for future studies on manganese-based ZIBs.

2 Crystal structure of manganese-based materials

In this review, we divide manganese-based materials into four categories, including MnO₂ polymorphs, low-valence manganese oxides, metal manganate (AMn_xO_y, A = Li, Na, Zn, Mg, Ca, etc.) and other manganese-based compounds. Various atomic structures and chemical constituents result in a variety of multivalent phases (+2, +3, +4, +5, and +7) and crystal structures for manganese-based materials (Fig. 4). The crystal structures of manganese-based materials are closely related to the energy storage performance of the corresponding ZIBs and dominate the reaction chemistry of cathodes. Therefore, it is essential to first comprehend the crystal structures of manganese-based materials.

Fig. 4 Structural drawings of various manganese-based materials.

2.1 MnO₂ polymorphs

MnO₂ is one of the most popular cathodes for ZIBs, which possesses a high theoretical capacity of 308 mA h g⁻¹ or 616 mA h g⁻¹ based on one-electron or two-electron transfer, respectively. Impressively, MnO₂ has diverse polymorphs with distinct tunneled or layered structures composed of basic octahedral units (MnO₆) connected in different ways. These diverse polymorphs can be summarized based on their tunnel size, as shown in Fig. 4. ε-MnO₂ (akhtenskite) is a special polymorph, which is densely packed without any tunnel structure. Its dense packing structure is not conducive to rapid cation insertion/extraction, resulting in low electrochemical activity for ZIBs.⁴⁴ Regulating structures such as introducing defects is an efficient strategy to increase the number of active sites to improve the electrochemical performance of ε-MnO₂.⁴⁵ λ-MnO₂ (spinel) possesses a three-dimensional (3D) spinel structure with [1 × 1] tunnels, which is a metastable form of MnO₂ and can be easily obtained through acid treatment of LiMn₂O₄ at ambient temperature.⁴⁶ It has been reported that λ-MnO₂ only can deliver a poor capacity of 33.8 mA h g⁻¹,⁴⁷ showing the lowest electrochemical activity among the MnO₂ polymorphs. Its unsatisfactory capacity may result from its limited number of 3D tunnels, restricting the migration of Zn²⁺.⁴⁸ β-MnO₂ (pyrolusite) is one of the most thermodynamically stable structure, which also has narrow [1 × 1] tunnels. However, the numerous open channels of β-MnO₂ can accommodate more cation insertion/extraction than λ-MnO₂. Recent studies have demonstrated the excellent electrochemical performance of β-MnO₂,^49–52 for example, Chen et al.⁵³ reported that β-MnO₂ can deliver a remarkable reversible capacity of 225 mA h g⁻¹ and outstanding long-term cycling stability with 94% capacity retention over 2000 cycles. γ-MnO₂ (nsutite) possesses a hybrid tunnel structure with [1 × 1] and [1 × 2] tunnels, which has been successfully applied in alkaline Zn–MnO₂ batteries and is also promising in ZIBs.^54,55 δ-MnO₂ (birnessite) features a typical two-dimensional (2D) layered structure with a wide interlayer spacing of 7.0 Å, which is favorable for cation intercalation/extraction. In addition, the layered structure of δ-MnO₂ originating from its source avoids a phase transition from tunneled MnO₂ to a layered phase, which greatly reduces severe volume changes. Notably, the interlayer space of δ-MnO₂ usually contains many cations (such as K⁺ and Na⁺) and structural water, which are beneficial to stabilize the MnO₆ layers. R-MnO₂ (ramsdellite) has a moderate tunnel size of [1 × 2] and is larger than β-MnO₂ but narrower than α-MnO₂, which can be regarded as a part of γ-MnO₂. Chen et al.⁵⁶ found that R-MnO₂ uniquely shows a low manganese dissolution property compared to other MnO₂ polymorphs due to the thermodynamic feasibility and moderate volume changes of its [1 × 2] tunnels. α-MnO₂ (hollandite) possesses large [2 × 2] tunnels, which is considered an ideal material for cation intercalation/extraction. To date, α-MnO₂ exhibits one of the most comprehensive electrochemical performances among manganese-based materials and has been extensively studied by researchers.^38,44 T-MnO₂ (todorokite) has large [3 × 3] tunnels with many intercalated metal cations and water molecules, and these guest species can effectively stabilize the tunnel framework and prevent structural collapse. However, although T-MnO₂ possesses a much larger tunnel structure than α-MnO₂, its practical capacity is much lower than α-MnO₂, which can be ascribed to the pre-intercalation of too many metal cations and water molecules, reducing the number of potential Zn²⁺ insertion sites.⁵⁷

2.2 Low-valence manganese oxides

Low-valence manganese oxides, including MnO, Mn₂O₃, Mn₃O₄ and Mn₅O₈, are also important cathode materials for ZIBs. In the case of MnO, with Mn²⁺ ions possessing a coordination number of six, it displays tetrahedral or octahedral coordination, together with specific magnetic anisotropy.⁵⁸ MnO is intrinsically inactive without any tunnels for Zn²⁺ and H⁺ insertion/extraction. However, it exhibits an excellent electrochemical performance comparable to MnO₂ after the initial activation cycles,^59,60 and its activation mechanisms will be discussed in detail in Section 4.2. In the structure of Mn₂O₃, the Mn³⁺ ions are arranged in an octahedral coordination with each O ion surrounded by four Mn ions. Similar to MnO, it lacks migration channels, and thus it has been found that pristine Mn₂O₃ can only deliver a limited capacity of 78 mA h g⁻¹ in the first discharge.⁶¹ In the case of spinel Mn₃O₄ (chemical formula of Mn²⁺Mn₂³⁺O₄), Mn²⁺ and Mn³⁺ occupy the tetrahedral (4a) and octahedral (8d) sites in the intermediate distorted cubic close-pack array of oxygen anions (16h), respectively. Mn₃O₄ demonstrates thermodynamic stability, making it typically irreversible as the final discharge product.⁴³ Mn₅O₈ (chemical formula of Mn₂²⁺Mn₃⁴⁺O₈) is another layered structure manganese oxide composed of 2D octahedral sheets of [Mn₃⁴⁺O₈] with Mn²⁺ located above and below the vacant Mn⁴⁺ sites.⁶² The large interlayer space of Mn₅O₈ offers abundant active sites and allows facile ion diffusion, and it has been reported that Mn₅O₈/rGO can deliver a high reversible capacity of 260 mA h g⁻¹ at 100 mA g⁻¹ with a stable cycling performance over 1000 cycles.⁶³ However, there are few reports on the application of Mn₅O₈ as a cathode material for ZIBs, and thus more efforts should be devoted to the study of Mn₅O₈.

2.3 Metal manganate AMn_xO_y

Metal manganate AMn_xO_y (A refers to Zn, Li, Na, Mg, etc.) can also be utilized as a cathode material for ZIBs, including ZnMn₂O₄, LiMn₂O₄, MgMn₂O₄, Na_0.44MnO₂, and CoMn₂O₄. In the structure of spinel-type ZnMn₂O₄, oxygen atoms are arranged in a hexagonal close-packed configuration, forming an octahedral site. In a perfect spinel lacking cation deficiency, the Zn²⁺ ions migrate from one tetrahedral site (4a) to another by passing through an unoccupied octahedral site (8c), encountering significant electrostatic repulsion from the Mn cations in the neighboring octahedral site (8d), thereby strongly impeding Zn²⁺ diffusion.⁶⁴ Meanwhile, due to the thermodynamic stability of ZnMn₂O₄, which is commonly the final product of manganese-based ZIBs in the capacity attenuation period, the capacity degradation caused by ZnMn₂O₄ will be discussed in detail in Section 4.1.3. Defect engineering is an effective strategy to reduce the electrostatic barrier, leading to easier Zn²⁺ diffusion.³⁹ The spinel MgMn₂O₄ possesses an analogous tetragonal symmetry structure to ZnMn₂O₄. Soundharrajan et al.⁶⁵ fabricated MgMn₂O₄ as a cathode with ZnSO₄, MgSO₄, and MnSO₄ as the electrolyte, showing the reversible insertion/extraction of both Mg²⁺ and Zn²⁺. In the structure of spinel-type LiMn₂O₄, the Mn³⁺/Mn⁴⁺ ions occupy the octahedral sites (16d) within a cubic close-packed array of oxygen atoms, while the Li⁺ ions occupy the tetrahedral sites (8a) and migrate through a three-dimensional channel.⁶⁶ LiMn₂O₄ has been widely investigated in LIBs due to the inherent advantages of the manganese element and its high specific power.⁶⁷ Interestingly, LiMn₂O₄ also shows an excellent performance in ZIBs. For instance, Zhang et al.⁶⁸ reported the fabrication of a Zn/LiMn₂O₄ battery containing ZnSO₄, Li₂SO₄, and MnSO₄ electrolyte, which exhibited a high capacity of more than 300 mA h g⁻¹ with long-term cycling stability over 2000 cycles. In addition, tunnel-type Na_0.44MnO₂ with interconnected 3D S-shaped channels allows facile cation diffusion. Similar to LiMn₂O₄ and MgMn₂O₄, Na_0.44MnO₂ demonstrates highly reversible insertion/extraction of Na⁺ and Zn²⁺ in hybrid electrolytes composed of ZnSO₄ and Na₂SO₄.^69,70

2.4 Other manganese-based compounds

Compared with crystalline manganese oxides, amorphous manganese oxides with short-range ordered atomic arrangements are ignored in ZIBs due to their lack of a high manganese valency and cation host structure. However, it was demonstrated that amorphous manganese oxides with unique superiority of structural plasticity are beneficial to accommodate the volume changes caused by Zn²⁺/H⁺ intercalation/extraction. In addition, the abundant defects in amorphous manganese oxides can serve as reversible cation storage sites, giving rise to fast ion diffusion kinetics and high specific capacity.^71,72 Besides, MnS,⁷³ Mn₂SiO₄,^74,75 LaMnO₃,⁷⁶ MnSe,⁷⁷etc. have been developed as available cathode materials for ZIBs. However, most of these cathode materials exhibit unsatisfactory electrochemical performances compared to manganese oxides owing to their intrinsically poor electrochemical activity and structural stability. Therefore, more efforts are needed to improve the inherent shortcomings of these newly developed manganese-based compounds.

In summary, the crystal structure of MnO₂ polymorphs, low-valence manganese oxides, metal manganate, and other manganese-based compounds was discussed. It has been found that the electrochemical performance of these materials in ZIBs is predominantly influenced by their crystal structure. Materials with large tunnel or layered structures generally exhibit superior insertion/extraction capabilities for Zn²⁺/H⁺, whereas those with dense and spinel-type structures, which offer fewer reaction sites for Zn²⁺/H⁺ ions, typically demonstrate inferior electrochemical performances. Table 1 provides a compilation of the previously reported electrochemical performances of manganese-based materials in ZIBs.

Table 1 Summary of the electrochemical performance of manganese-based materials in ZIBs

Cathodes	Electrolyte	Voltage window	Specific capacity (current density)	Cycling performance	Ref.
α-MnO2	2 M ZnSO4 + 0.1 M MnSO4	1–1.8 V	285 mA h g^−1 (1/3C)	92% capacity retention over 5000 cycles at 5C	78
β-MnO2	3 M Zn(CF3SO3)2 + 0.1 M Mn(CF3SO3)2	0.8–1.9 V	225 mA h g^−1 (0.65C)	94% capacity retention over 2000 cycles at 6.5C	53
γ-MnO2	1 M ZnSO4	1–1.8 V	285 mA h g^−1 (0.05 mA cm^−2)	63% capacity retention over 40 cycles at 0.5 mA cm^−2	54
δ-MnO2	1 M Zn(TFSI)2 + 0.1 M Mn(TFSI)2	1–1.8 V	238.8 mA h g^−1 (0.2C)	93% capacity retention over 4000 cycles at 20C	79
ε-MnO2	3 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	312 mA h g^−1 (0.1 A g^−1)	80.1% capacity retention over 1000 cycles at 1 A g^−1	45
λ-MnO2	0.2 M ZnSO4 + 0.1 M MnSO4	0.8–1.9 V	250 mA h g^−1 (0.02 A g^−1)	115 mA h g^−1 over 500 cycles at 1 A g^−1	80
R-MnO2	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.85 V	168 mA h g^−1 (0.1 A g^−1)	118.3 mA h g^−1 over 500 cycles at 0.3 A g^−1	56
T-MnO2	1 M ZnSO4	0.7–2.0 V	108 mA h g^−1 (1/2C)	83% capacity retention over 50 cycles at 50 mA g^−1	57
MnO	2 M ZnSO4	1–1.9 V	330 mA h g^−1 (0.1 A g^−1)	80.7% capacity retention over 300 cycles at 0.3 A g^−1	81
Mn2O3	2 M ZnSO4 + 0.2 M MnSO4	1.0–1.8 V	292 mA h g^−1 (0.2C)	89% capacity retention over 3000 cycles at 3.08 A g^−1	82
Mn3O4	2 M ZnSO4	0.8–1.9 V	85.6 mA h g^−1 (0.2 A g^−1)	106.1 mA h g^−1 over 300 cycles at 0.5 A g^−1	83
Mn8O5/rGO	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.9 V	260 mA h g^−1 (0.1 A g^−1)	98.8% capacity retention over 1000 cycles at 0.5 A g^−1	63
ZnMn2O4	1 M ZnSO4 + 0.05 M MnSO4	0.8–1.9 V	70.2 mA h g^−1 (3.2 A g^−1)	106.5 mA h g^−1 over 300 cycles at 0.1 A g^−1	84
MgMn2O4	1 M ZnSO4 + 1 M MgSO4 + 0.1 M MnSO4	0.5–1.9 V	247 mA h g^−1 (0.05 A g^−1)	80% capacity retention over 500 cycles at 0.5 A g^−1	65
LiMn2O4	1 M ZnSO4 + 1 M Li2SO4 + 0.1 M MnSO4	0.8–1.85 V	300 mA h g^−1 (0.1 A g^−1)	93.5% capacity retention over 500 cycles at 1 A g^−1	68
CoMn2O4	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.9 V	232.3 mA h g^−1 (0.1 A g^−1)	—	85
MnCo2O4	2 M ZnSO4 + 0.5 M MnSO4	0.8–1.8 V	595.3 mA h g^−1 (0.05 A g^−1)	85% capacity retention over 250 cycles at 0.2 A g^−1	86
Na0.44MnO2	3 M ZnSO4 + 0.1 M MnSO4	1.0–1.8 V	301.3 mA h g^−1 (0.1 A g^−1)	69.3% capacity retention over 800 cycles at 1 A g^−1	87
MnS	2 M ZnSO4 + 0.1 M MnSO4	0.8–2 V	335.7 mA h g^−1 (0.3 A g^−1)	104 mA h g^−1 over 4000 cycles at 3 A g^−1	73
Mn2SiO4	1 M ZnSO4 + 0.1 M MnSO4	1.0–1.8 V	154 mA h g^−1 (0.1 A g^−1)	55% capacity retention over 40 cycles at 0.1 A g^−1	74
LaMnO3	2 M ZnSO4 + 0.2 M MnSO4	1.0–1.8 V	226 mA h g^−1 (0.1 A g^−1)	113 mA h g^−1 over 1000 cycles at 0.5 A g^−1	76

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3 Energy storage mechanisms

The energy storage mechanisms of manganese-based ZIBs are very complicated and controversial due to the various crystal structures of manganese-based materials and the intricate environment of the aqueous electrolyte in comparison to LIBs with insertion–alloying–conversion reactions.⁸⁸ In addition, the reaction of Zn²⁺ deposition to generate large flake-like Zn₄SO₄(OH)₆·xH₂O (ZHS) on the cathode surface during discharge makes the determination of the real reactions more difficult. Undoubtedly, numerous researchers have devoted significant efforts to studying the real reaction mechanisms for a long time. Accordingly, to date, the storage mechanisms of manganese-based ZIBs can be divided into six categories, as presented here.

3.1 Zn²⁺ insertion/extraction mechanism

The Zn²⁺ insertion/extraction mechanism is the earliest accepted mechanism, which was first systematically proven by Kang et al.⁸⁹ They concluded that the Zn²⁺ from the ZnSO₄ or Zn(NO₃)₂ aqueous electrolyte can be reversibly intercalated into the tunnels of α-MnO₂, and the anodic zinc can reversibly undergo dissolution/deposition upon discharging and charging (Fig. 5a). The reactions are summarized as below:

Cathode: Zn²⁺ + 2e⁻ + 2α-MnO₂ ↔ ZnMn₂O₄ (1)

Anode: Zn ↔ Zn²⁺ + 2e⁻ (2)

Fig. 5 Various energy storage mechanisms of manganese-based ZIBs. (a) Schematic of the chemistry of a zinc-ion battery. Reproduced with permission.⁸⁹ Copyright 2012, Wiley-VCH. (b) Schematic illustrating the mechanism of zinc intercalation in α-MnO₂. Reproduced with permission.⁹⁰ Copyright 2014, Springer Nature. (c) Schematic illustration of the reaction pathway of Zn-insertion in a γ-MnO₂ cathode. Reproduced with permission.⁵⁴ Copyright 2015, the American Chemical Society. (d) STEM-EDS mappings of the elemental distributions of Mn, O and Zn in the MnO₂ electrode in the discharged state during the first cycle. Reproduced with permission.⁷⁸ Copyright 2016, Springer Nature. (e) Atomic scale [001] and projections of MnO₂ demonstrating essentially ‘empty’ tunnels without presence of heavy cations. Reproduced with permission.⁹⁴ Copyright 2022, Springer Nature. (f) Schematic of the ZSH-assisted deposition–dissolution reaction model. Reproduced with permission.⁹⁵ Copyright 2022, Wiley-VCH.

Oh et al.⁹⁰ demonstrated the intercalation mechanism of Zn²⁺ in α-MnO₂ during discharge, involving an electrochemical-induced reversible phase transition of α-MnO₂ from a tunneled to layered structure. This transition is induced by the dissolution of manganese from α-MnO₂ during the discharge process, resulting in the formation of layered Zn-birnessite (11 Å). The original tunneled structure is restored through the incorporation of Mn²⁺ ions back into the layers of Zn-birnessite during charging (Fig. 5b). The reaction can be depicted as follows:

Mn⁴⁺(s) + e⁻ → Mn³⁺(s) (3)

2Mn³⁺(s) → Mn⁴⁺(s) + Mn²⁺(aq) (4)

Mn²⁺(aq) → Mn⁴⁺(s) + 2e⁻ (5)

Later, the intercalation of Zn²⁺ ions into α-MnO₂ was rectified, resulting in the formation of buserite (11 Å) and the previously observed Zn-birnessite is attributed to the dehydration of the original buserite structure.⁹¹ Owing to the narrow [1 × 1] tunnels in β-MnO₂, it has been suggested to be unfavorable for Zn intercalation. Interestingly, Chen et al.⁵³ revealed that tunnel-type β-MnO₂ undergoes a phase transition to layer-type Zn-buserite during the initial discharge process, followed by the reversible insertion/extraction of Zn²⁺ ions in the layered structure. The phase transition of MnO₂ during charge/discharge was deeply investigated by Alfaruqi et al.,⁵⁴ revealing a complex multiple-phase transformation process for γ-MnO₂. It was proposed that the original tunnel-type γ-MnO₂ undergoes a structural transformation to form a spinel-type Mn(III) phase (ZnMn₂O₄), as well as two new intermediary Mn(II) phases, namely tunnel-type γ-Zn_xMnO₂ and layered L-Zn_yMnO₂. These phases with multi-oxidation states coexist after complete electrochemical Zn-insertion (Fig. 5c). In addition, the presence of cation-defects in ZnMn₂O₄ spinel permits reversible Zn²⁺ insertion/extraction reactions, and the Mn deficiency in ZnMn₂O₄ can effectively reduce the electrostatic barrier for Zn²⁺ diffusion, resulting in the enhanced mobility of Zn²⁺ ions, and consequently faster electrode reaction kinetics.⁶⁴ However, it is worth noting that while the mechanism of Zn²⁺ insertion/extraction was initially demonstrated by researchers, some of the early analyses may have been misleading due to the absence of advanced characterization techniques and limited understanding of ZIBs.

3.2 Chemical conversion reaction mechanism

Different from Zn²⁺ insertion in the MnO₂ host, which is responsible for energy storage, the chemical conversion reaction mechanism is a new discovery, which was first proposed by Pan et al.⁷⁸ During the discharge process, MnO₂ undergoes a reaction with H⁺ derived from H₂O to form MnOOH. To achieve charge neutrality in the system, the remaining OH⁻ ions react with ZnSO₄ and H₂O in the aqueous electrolyte, resulting in the formation of large flake-like ZnSO₄[Zn(OH)₂]₃·xH₂O. The XRD pattern of the MnOOH phase and STEM-EDS mappings of the elemental distributions of Mn, O, and Zn in the discharged state strongly demonstrate the reaction chemistry between H⁺ and MnO₂ in the cathode (Fig. 5d). In the absence of H⁺, organic Zn²⁺-based electrolytes exhibit limited capacity; however, upon the addition of H₂O, the Zn/α-MnO₂ battery demonstrates electrochemical behavior similar to that observed in aqueous electrolyte, thereby providing further evidence for the occurrence of a chemical conversion reaction. The reaction chemistry can be formulated as follows:

Cathode: H₂O ↔ H⁺ + OH⁻ (6)

MnO₂ + H⁺ + e⁻ ↔ MnOOH (7)

Although ample evidence has indicated the occurrence of chemical conversion reactions leading to the transformation of MnO₂ into MnOOH, the operando XRD study unequivocally confirmed the absence of a crystalline MnOOH phase. It is worth noting that ex situ analysis methods, such as highly crystalline ZHS, may introduce external interferences and occasionally mistake the formation of MnOOH.^92,93 Therefore, more advanced in situ characterization methods are required to further validate the mechanism of the chemical conversion reaction.

3.3 H⁺ insertion/extraction mechanism

Different from the chemical conversion between MnO₂ and MnOOH revealed by Pan et al., a view of H⁺ intercalation in α-MnO₂ tunnels was proved by combining atomic-scale electron microscopy, electrochemistry and atomistic simulations.⁹⁴ The in situ XRD analysis revealed the absence of new phases except for the reversible generation and disappearance of ZHS. However, during discharge, the slight shifts in the peaks of α-MnO₂ towards lower angles indicate cation insertion. Down-tunnel STEM imaging demonstrated severe distortion of the tunnels near the surface region of MnO₂ nanowires, strongly supporting cation intercalation. Nevertheless, EDS mapping showed the absence of Zn element in MnO₂ nanowires, suggesting that Zn²⁺ insertion in the tunnels is unlikely. Furthermore, atomic-scale imaging clearly indicated the presence of unoccupied tunnels without any heavy cations, providing crucial evidence against Zn²⁺ insertion in the MnO₂ lattice (Fig. 5e). The computational results further support the favorability of H⁺ insertion over Zn²⁺. The reaction chemistry can be denoted as follows:

α-MnO₂ + H⁺ + e⁻ ↔ α-H₁MnO₂ (11)

3.4 H⁺ and Zn²⁺ co-insertion/extraction mechanism

The H⁺ and Zn²⁺ insertion/extraction mechanism is widely accepted and has been reported in numerous studies. Considering the existence of both H⁺ and Zn²⁺ cations in the electrolyte, Wang et al.⁹⁶ were the first to demonstrate that MnO₂ undergoes successive H⁺ and Zn²⁺ insertion processes. The reaction kinetics at different depths of discharge, as observed through GITT and EIS, indicated that the reaction on the first plateau occurs much faster than on the second plateau due to the smaller size of H⁺ compared to Zn²⁺. Furthermore, even without ZnSO₄ in the electrolyte, there was still evidence of H⁺ insertion reactions occurring, with the first sloping plateau being attributed solely to H⁺ insertion. Additionally, discharge down to 1.3 V and 1.0 V resulted in the formation of MnOOH and ZnMn₂O₄ respectively, further confirming the subsequent insertion of H⁺ and Zn²⁺. Subsequently, the study conducted by Jin et al.⁷⁹ also provided compelling evidence to substantiate the co-insertion mechanism. However, this investigation revealed that the intercalation of Zn²⁺ in MnO₂ is governed by a non-diffusion-controlled process, followed by H⁺ conversion reaction, rather than the consequent insertion of H⁺ and Zn²⁺. These two distinct conclusions confuse researchers in terms of which cation reacts with MnO₂ firstly and should be disclosed by further evidence.

3.5 Dissolution/deposition mechanism

The dissolution of Mn²⁺ caused by Jahn–Teller distortion is commonly considered the reason for capacity attenuation.^42,44,97 Thus, to alleviate the issue of Mn dissolution, Mn²⁺ additives have been introduced to shift the disproportionation reaction 2Mn³⁺ ↔ Mn²⁺ + Mn⁴⁺.⁷⁸ However, Oh et al.⁹⁸ discovered that Mn²⁺ dissolved in the discharge state can be electro-oxidized back to MnO₂ through a reversible dissolution/deposition process of ZHS on the electrode surface, which is triggered by pH changes in the electrolyte. This phenomenon has been observed in MnO₂-based flow batteries with acidic electrolytes.⁹⁹

Cathode: 3MnO₂ + 8Zn²⁺ + 2SO₄²⁻ + 16H₂O + 6e⁻ → 3Mn²⁺ + 2Zn₄(OH)₆(SO₄)·5H₂O↓ (12)

Anode: 3Zn → 3Zn²⁺ + 6e⁻ (13)

Overall: 3MnO₂ + 3Zn + 5Zn²⁺ + 2SO₄²⁻ + 16H₂O ↔ 3Mn²⁺ + 2Zn₄(OH)₆(SO₄)·5H₂O↓ (14)

The dissolution/deposition mechanism was also confirmed by Liang and coworkers.¹⁰⁰ They concluded that the dissolution and deposition of MnO₂ contribute major capacity in the energy storage process. During the initial discharge, the dissolution of the original MnO₂ turns it to Mn²⁺ in the electrolyte, and in the subsequent charging process, Mn²⁺ is deposited on the electrode surface as birnessite-MnO₂ rather than the original MnO₂. In the subsequent cycles, birnessite-MnO₂ and Mn²⁺ are reversibly dissolved/deposited during charge/discharge.

Subsequent cycles: 3 birnessite-MnO₂ + 6H₂O + 6e⁻ ↔ 3Mn²⁺ + 12OH⁻ (17)

Side reaction: 12OH⁻ + 2SO₄²⁻ + 8Zn²⁺ + 8H₂O ↔ 2Zn₄(OH)₆SO₄·4H₂O (18)

Wu et al.¹⁰¹ further verified the Mn dissolution/deposition faradaic mechanism that governs the electrochemistry by providing direct evidence through operando, spatiotemporal-resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO₂ battery. By simultaneously visualizing and quantifying the distribution of Mn species in the electrolyte, they observed the formation of aqueous Mn species during discharge and depletion upon charge.

Different from the MnO₂/Mn²⁺ dissolution/deposition reaction, Chen et al.⁹⁵ reported a zinc sulfate hydroxide-assisted dissolution/deposition reaction for aqueous Zn–Mn batteries. They found that the role of the α-MnO₂ cathode is to consume H⁺ driving the generation of ZHS and the release of Mn²⁺ in the electrolyte provides an Mn source for the next charge deposition, with α-MnO₂ contributing little capacity in the subsequent cycles. The energy conversion occurs by the reversible dissolution/deposition between the layered Zn_xMnO(OH)₂ and ZHS (Fig. 5f). According to this energy storage mechanism, it is suggested that ZSH plays an essential role as an active material rather than being solely a byproduct affecting the cycling performance, contrary to previous literature reports.^43,100,102 Based on this understanding of ZSH-assisted deposition/dissolution reactions, successful designs of Zn–ZnO, Zn–MgO, and Zn–CaO batteries have been achieved. The energy storage mechanism is summarized as follows:

Charge: Zn₄(OH)₆SO₄·4H₂O + Mn²⁺ → Zn_xMnO(OH)₂ + 4H⁺ + SO₄²⁻ + 2e⁻ (19)

Discharge: Zn_xMnO(OH)₂ + 4H⁺ + 2e⁻ → Zn²⁺ + Mn²⁺ + 3H₂O (20)

6OH⁻ + SO₄²⁻ + 4Zn²⁺ + 4H₂O → Zn₄(OH)₆SO₄·4H₂O (21)

3.6 Hybrid storage mechanism

In recent years, an increasing number of reports has demonstrated that the energy storage mechanisms in Zn–Mn cells exhibit a hybrid storage mechanism rather than a singular mechanism. For example, in contrast to the previous simple phase transition, Xu et al.¹⁰³ revealed a complex phase evolution involving both intercalation and conversion reactions. During the initial discharge process, α-MnO₂ undergoes Zn²⁺ intercalation at around 1.4 V, resulting in the formation of α-Zn_xMnO₂. Subsequently, an H⁺ conversion reaction occurs between 1.0 and 1.3 V, leading to the generation of MnOOH and Mn₂O₃ in the cathode, thereby increasing the electrolyte pH value and producing a byproduct known as ZnSO₄·3Zn(OH)₂·5H₂O (BZSP) (Fig. 6a). The reaction can be described as follows:

MnO₂ + xZn²⁺ + 2xe⁻ → Zn_xMnO₂ (22)

4Zn²⁺ + SO₄²⁻ + 5H₂O + 6OH⁻ → ZnSO₄·3Zn(OH)₂·5H₂O (23)

MnO₂ + H⁺ + e⁻ → MnOOH (24)

2MnO₂ + 2H⁺ + 2e⁻ → Mn₂O₃ (25)

Fig. 6 Hybrid storage mechanism. XRD patterns of the α-MnO₂ cathode at different states: (a) discharged to 1.4 V in the first cycle, (b) fully charged state in the first cycle, and (c) after 100 cycles. (a)–(c) Reproduced with permission.¹⁰³ Copyright 2019, Springer Nature. (d) Schematic representation and charge storage mechanism of an MnO₂–Zn ion battery in 1 M ZnSO₄ and 1 M MnSO₄ electrolyte. Reproduced with permission.¹⁰⁴ Copyright 2019, Wiley-VCH. Schematic illustration of the different contributions to the charge storage mechanism of m-α-Mn₂O₃ during (e) first discharge and charge process and (f) second discharge process. (e) and (f) Reproduced with permission.¹⁰⁵ Copyright 2021, Wiley-VCH.

During the initial charging process, α-Zn_xMnO₂, MnOOH, and Mn₂O₃ revert to α-MnO₂ by the release of Zn²⁺ and H⁺, while BZSP reacts with Mn²⁺ to form ZnMn₃O₇·3H₂O (Fig. 6b). This reaction can be described as follows:

3(ZnSO₄·3Zn(OH)₂·5H₂O) + 3Mn²⁺ + 8e⁻ → ZnMn₃O₇ + 3ZnSO₄ + 18OH⁻ + 8Zn²⁺ + 12H₂O (26)

In the subsequent cycles, Zn²⁺ undergoes reversible insertion/extraction in/from the α-MnO₂, α-Zn_xMnO₂, and ZnMn₃O₇·3H₂O hosts. The participation of Mn²⁺ leads to the formation of new phases including Zn₂Mn₃O₈ and ZnMn₂O₄ through reversible conversion (Fig. 6c). The reaction can be described as follows:

Zn_xMnO₂ + (0.5 − x)Zn²⁺ + (1 − 2x)e⁻ → 0.5ZnMn₂O₄ (27)

Zn₂Mn₃O₈ + Mn²⁺ → 2ZnMn₂O₄ (28)

Qiao et al.¹⁰⁴ reported the occurrence of multi-redox reactions during the discharge process, with three distinct discharge regions corresponding to three different reaction mechanisms. The region between 2–1.7 V results from the dissolution of MnO₂, while the region between 1.7–1.4 V is responsible for H⁺ insertion into the residual MnO₂, and the region between 1.4–0.8 V is dominated by Zn²⁺ insertion in the residual MnO₂ (Fig. 6d), as confirmed by ex situ XRD and XPS measurements. Furthermore, the optimized addition 0.1 M H₂SO₄ significantly enhanced the energy density and maximized the electrolysis efficiency for two-electron reactions. The reaction can be written as follows:

Cathode: MnO₂ + 4H⁺ + 2e⁻ ↔ Mn²⁺ + 2H₂O (29)

MnO₂ + H⁺ + e⁻ ↔ MnOOH (30)

MnO₂ + 0.5Zn²⁺ + e⁻ ↔ Zn_0.5MnO₂ (31)

Anode: Zn²⁺ + 2e⁻ ↔ Zn (32)

The H⁺ conversion and Mn²⁺ dissolution/deposition mechanism of β-MnO₂ was demonstrated by Liu et al.⁵⁰ In the initial discharge cycle, β-MnO₂ undergoes a transformation to MnOOH and Mn²⁺, while during charge, Mn²⁺ deposition and MnOOH conversion lead to the formation of ε-MnO₂. In subsequent cycles, the deposited ε-MnO₂ reacts with H⁺ to generate MnOOH, which partially dissolves to form Mn²⁺, accompanied by the formation of ZHS during the discharge process. Upon charging, Mn²⁺ is redeposited into the previously deposited ε-MnO₂, resulting in the disappearance of ZHS. The chemical equation can be written as follows:

Cathode: MnO₂ + H⁺ + e⁻ ↔ MnOOH (33)

MnOOH + 3H⁺ + 3e⁻ ↔ Mn²⁺ + H₂O (34)

4Zn²⁺ + SO₄²⁻ + 8H₂O ↔ Zn₄(OH)₆SO₄·5H₂O + 6H⁺ (35)

Anode: Zn ↔ Zn²⁺ + 2e⁻ (36)

The energy storage mechanism of α-Mn₂O₃ was systematically and comprehensively investigated by Ma et al.¹⁰⁵ They demonstrated that α-Mn₂O₃ is unsuitable for H⁺ insertion. In fact, α-Mn₂O₃ undergoes an irreversible phase transition to layer-type L-Zn_xMnO₂ by electrochemical reaction with H₂O and Zn²⁺, accompanied by the dissolution of Mn²⁺ in the electrolyte. This process leads to the formation of OH⁻, ultimately generating ZHS in the first discharge process. During the first charge process, Zn²⁺ extraction from L-Zn_xMnO₂ forms L-Zn_x–yMnO₂, while Mn²⁺ is oxidized to Mn(IV), resulting in the deposition of birnessite-MnO₂ on the outer surface of the electrode (Fig. 6e). The ZHS fades due to the formation of H⁺ by deposition of Mn²⁺. In the second discharge process between 1.9–1.44 V, H⁺ insertion in MnO₂ (formed from L-Zn_xMnO₂ and Mn²⁺) leads to the generation of MnOOH. In the range of 1.44–0.8 V, Zn²⁺ inserts in L-Zn_x–yMnO₂, together with formation of ZSH, and it should be noted that the residual α-Mn₂O₃ will continue undergoing an irreversible phase transition in the range of 1.3–0.8 V until it completely transforms into L-Zn_xMnO₂ and Mn²⁺, as shown in Fig. 6f.

In summary, the storage mechanisms of manganese-based ZIBs including Zn²⁺ insertion/extraction, chemical conversion reaction, H⁺ insertion/extraction, H⁺ and Zn²⁺ co-insertion/extraction, dissolution/deposition and hybrid storage mechanisms have been extensively investigated by researchers. However, although significant advancements have been made in determining the storage mechanisms of manganese-based ZIBs over the past decade, their intricate reaction chemistry is still under debate, hindering a consensus. Moreover, the underlying causes of different storage mechanisms and some abnormal phenomena such as the unusual “turning point” of the GCD curve have not been reasonably explained, resulting in obscure optimization strategies based on the storage mechanism and further impeding their practical application. Therefore, more efforts are urgently needed to determine the real reaction chemistry of manganese-based ZIBs during charge/discharge.

4 The mechanisms of capacity fluctuation

Although numerous studies have demonstrated the long-term cycling performance of manganese-based ZIBs at high C-rates (≥5C),^56,79,102 it should be noted that this excellent performance is primarily attributed to capacitive processes dominating the capacity contribution rather than the typical diffusion-controlled charge storage mechanisms for batteries.¹⁸ Given the ongoing controversy regarding the primary causes of battery failure, there is a lack of robust methods to enhance the cycling stability of manganese-based ZIBs, particularly at a low current. Besides the crucial challenges encountered by Zn anodes such as dendrite growth, passivation, corrosion and hydrogen evolution,^106–108 on the cathode side, issues such as Mn dissolution, dramatic volume changes, and the irreversible consumption of Mn²⁺ lead to the generation of electrochemically inactive materials, which can result in severe performance degradation and even battery failure.^109–111 Furthermore, the phenomenon of capacity activation, which commonly occurs during the initial cycles, has received limited attention from researchers. In general, the long-term cycling behavior of manganese-based ZIBs consists of an activation period, a stabilization period, and ultimately a decay period. However, previous review articles failed to address the underlying reasons for the dynamic capacity fluctuation from capacity activation to capacity attenuation, resulting in inadequate guidance and references for enhancing the cycling stability of manganese-based ZIBs. Thus, the mechanisms of capacity degradation, capacity activation, and dynamic capacity fluctuation are discussed in the following sections.

4.1 Capacity degradation mechanisms

4.1.1 Jahn–Teller distortion. Most manganese-based materials consist of basic octahedral units (MnO₆) interconnected through corner and/or edge sharing, with the Mn cation located at the center of each MnO₆ octahedron adopting a six-oxide coordination. Under the influence of an electric field, the 3d orbitals of the Mn cation in the MnO₆ octahedra split into two degenerate orbitals, namely e_g and t_2g. Due to the lower formation energy of the triply degenerate t_2g orbitals (d_xy, d_xz and d_yz) compared to the doubly degenerate e_g orbitals (d_z² and d_x²–y²), the three electrons in the 3d orbital in Mn⁴⁺ will occupy the d_xy, d_xz and d_yz orbitals, respectively, forming a stable structure. However, once Mn⁴⁺ gains an electron through the reduction process to form Mn³⁺, in addition to occupying the d_xy, d_xz and d_yz orbitals, the remaining electron in the 3d orbital of Mn³⁺ will occupy the d_z² orbital in e_g. The asymmetry of the electron distribution between the d_z² and d_x²–y² orbitals in Mn³⁺ leads to the distortion of the MnO₆ geometry from octahedral to tetragonal symmetry by lowering the overall energy.^34,112 Consequently, this distortion induces elongation of two axial Mn–O bonds and contraction of four equatorial Mn–O bonds (Fig. 7a). These changes can be observed through in situ Raman spectroscopy as a shift towards higher wavenumbers for the peak corresponding to the Mn–O stretching vibrations during the discharge processes.¹¹²

Fig. 7 Mechanisms of capacity degradation. (a) Mechanism of Jahn–Teller distortion. Reproduced with permission.³⁴ Copyright 2023, the Royal Society of Chemistry. (b) Manganese dissolution caused by the unstable or disordered surface. Reproduced with permission.¹¹³ Copyright 2020, Elsevier. (c) Large volumetric change induced by phase transition. Reproduced with permission.¹¹⁶ Copyright 2020, Elsevier. (d) Electrode pulverization induced by large volumetric change. Reproduced with permission.⁵³ Copyright 2017, Springer Nature. (e) Kinetic properties of the redox reactions in Zn/MnO₂ cells. Reproduced with permission.¹¹⁷ Copyright 2019, the American Chemical Society. (f) Galvanostatic charge/discharge curve of ZnMn₂O₄. Reproduced with permission.⁴⁹ Copyright 2019, Elsevier. (g) Relationship between pH values and the potentials of Mn²⁺ deposition based on the Nernst equation. Reproduced with permission.¹¹⁹ Copyright 2023, Wiley-VCH. (h) Total Gibbs energy differences for two possible reaction paths. Reproduced with permission.¹²⁰ Copyright 2022, the Royal Society of Chemistry. (i) TEM image of the MnO₂ electrode with 100 cycles. Reproduced with permission.⁹⁴ Copyright 2022, Springer Nature. (j) Schematic illustration of the crystal structural transformation. Reproduced with permission.¹⁰² Copyright 2021, Elsevier.

It has been widely acknowledged that the Jahn–Teller distortion during discharge leads to the disproportionation of Mn³⁺ (2Mn³⁺ → Mn²⁺ + Mn⁴⁺), resulting in the release of Mn²⁺ and causing severe dissolution issues. A consensus has been reached regarding the detrimental impact of inevitable Mn dissolution on the rapid capacity decay observed in manganese-based ZIBs, which is attributed to the loss of active cathode materials for reactions. Also, it has been unveiled that approximately 1/3 of the total manganese on the electrode dissolves in the electrolyte after the full discharge.⁹⁰ Once the outermost manganese atoms dissolve in the electrolyte, the subsequent dissolution of subsurface atoms occurs through a chain reaction, leading to structural collapse (Fig. 7b).¹¹³ Moreover, the irreversible phase transformations, considerable strain, and unstable interface triggered by the Jahn–Teller distortion lead to structural degradation, eventually resulting in capacity degradation. In short, during the cycling process of manganese-based ZIBs, the presence of Jahn–Teller distortion poses a significant challenge given that it leads to crystal structure deterioration and the gradual loss of active materials, resulting in a rapid performance degradation.

4.1.2 Dramatic volume changes. Previous studies have demonstrated that manganese-based LIBs undergo large volume changes due to the shuttle of Li⁺ caused by lattice expansion or contraction.¹¹⁴ Similarly, repetitive insertion/extraction of hydrated H⁺/Zn²⁺ ions will cause significant volume changes in manganese-based ZIBs. In comparison with Li⁺ ions (0.76 Å), the larger hydrated radius of Zn²⁺ ions (4.3 Å) will destabilize and weaken the crystal framework, resulting in irreversible volumetric alterations and structural damage. In addition, the strong electrostatic force between Zn²⁺ ions and the host materials results in aggravated volume expansion and sluggish Zn²⁺ diffusion.¹¹⁵ Furthermore, the irreversible phase transformation from a tunnel to layer or spinel structure during the charge/discharge process can lead to drastic volume changes, causing pulverization, cracking, and collapse of the active materials (Fig. 7c and d).^34,53,116 Once structural collapse occurs, some of the active materials will lose electrical contact with the current collector, resulting in the formation of dead Mn. The dramatic volume changes not only compromise the structure stability and lead to the loss of active materials, but also reduce the electrochemical activity of manganese-based cathode materials, resulting in capacity fading and poor rate performance. Yang et al.¹¹⁷ conducted a comprehensive investigation combining experimental and theoretical approaches to elucidate the rapid capacity degradation observed in Zn/MnO₂ batteries, corresponding to the irreversible phase transformations occurring at the discharge plateaus of around 1.26 V, which are considered kinetically limiting reactions (Fig. 7e). The conversion reactions induce a large volume change and lead to the formation of electrochemically inactive ZHS. Thus, by increasing the lower cutoff voltage from 1.0 to 1.3 V, the kinetically limited irreversible reactions can be directly eradicated. As a result, when charged/discharged in the potential range of 1.3–1.8 V at a low current density of 1C, stable capacity retention over 150 cycles was achieved.

4.1.3 Irreversible consumption of Mn²⁺ to generate inactive manganese compounds. The essential reason for the capacity fading of manganese-based cathodes upon cycling is the loss of active materials. During the period of capacity decay, it is commonly observed that the original active cathode materials exhibit reduced crystallinity with the formation of electrochemically inactive manganese compounds on the cathode, such as ZnMn₂O₄, Mn₃O₄, and ZnMn₃O₇. Consequently, many reports suggested that the formation of an inert manganese compound phase is responsible for the ultimate performance attenuation.^43,49,93

Taking hetaerolite ZnMn₂O₄ as a representative example, Zn²⁺ migration in the perfect spinel ZnMn₂O₄ is difficult due to their strong electrostatic repulsion. Additionally, ZnMn₂O₄ exhibits high resistivity, which is six orders of magnitude higher than that of MnO₂,¹¹⁸ resulting in poor electrode conductivity. Consequently, when ZnMn₂O₄ was used as a cathode material, only an ultralow discharge capacity of ∼32.6 mA h g⁻¹ could be delivered at 0.05C (Fig. 7f), manifesting its strong electrochemical inactivity.⁴⁹ Moreover, once a few of irreversible ZnMn₂O₄ nanoparticles are accumulated on the cathode surface, a mass of nucleation sites will be produced, inducing the formation of more ZnMn₂O₄ nanoparticles. The coverage of abundant ZnMn₂O₄ nanoparticles on the electrode would impede cation diffusion and the charge transport of the inner active materials. Unfortunately, these ZnMn₂O₄ nanoparticles cannot be electrochemically decomposed during the charging process as long as they are formed and attached to the electrode, which can be a crucial reason accounting for the capacity degradation of manganese-based cathodes after long-term cycling.

There are two primary mechanisms responsible for the formation of inactive ZnMn₂O₄ nanoparticles, where one is attributed to the conversion of tunnel or layered-Zn_xMn₂O₄ due to the higher thermodynamic stability of spinel ZnMn₂O₄, as shown in the following process:

MnO₂ + xZn²⁺ + 2xe⁻ → Ζn_xMnO₂ (37)

Ζn_xMnO₂ → λ-ΖnMnO₂ + 2(x − 1)Zn²⁺ + (4x − 2)e⁻ (38)

The other involves an electrodeposition reaction from an Mn²⁺-containing electrolyte, which can be denoted as:

2Mn²⁺ + Zn²⁺ + 8OH⁻ → ZnMn₂O₄ + 4H₂O + 2e⁻, E = 1.320 V vs. Zn/Zn²⁺ (39)

The electrodeposition potential of Mn²⁺ to form MnO₂ is known to be 1.869 V (vs. Zn/Zn²⁺),¹²¹ which is higher than that of ZnMn₂O₄ (Fig. 7g). In addition, the Gibbs energy of path 1 is lower than that of path 2 (Fig. 7h). Moreover, theoretical calculation indicates a lower formation energy compared to MnO₂.¹²⁰ These pieces of evidences indicate that the generation of ZnMn₂O₄ is favored by electrodeposition. Based on the aforementioned analyses and equation, it is evident that the concentration of Mn²⁺ will eventually decrease significantly due to the continuous formation of an irreversible ZnMn₂O₄ phase, leading to loss of active materials based on the Mn²⁺ electrodeposition reaction.

The capacity decay resulting from H⁺ insertion/extraction was elucidated by Yuan et al.⁹⁴ With an increase in the cycle number, MnO₂ nanowires gradually dissolved, and some tiny needle-like nanograins were discovered after 50 cycles, which were identified as the Zn–Mn–O phase. Notably, the number of Zn–Mn–O nanograins kept increasing with an increase in the number of cycles, and almost all the nanowires were converted to Zn–Mn–O after 100 cycles, as shown in Fig. 7i. Meanwhile, the concentration of Mn²⁺ kept increasing within 50 cycles due to the Mn³⁺ disproportionation reaction, which is consistent with the TEM results. However, the Mn³⁺ disproportionation reaction was largely but not totally reversible, leading to the gradual loss of active MnO₂, and the accumulation of dissolved Mn²⁺ was thermodynamically favored to form the irreversible Zn–Mn–O phase due to the reprecipitation Mn²⁺, resulting in a rapid decrease in Mn²⁺ concentration and capacity. In addition, by disassembling an in situ cell after 5000 cycles, Wei et al.¹²² found that the capacity fading during long-term cycling was attributed to the irreversible consumption of Mn²⁺ in the electrolyte, where Mn²⁺ is irreversible converted to the woodruffite phase (ZnMn₃O₇·2H₂O) with poor electrochemical activity, leading to a performance decay based on the capacity contribution from the dissolution/deposition of Zn-birnessite. Liang et al.¹⁰² concluded that the in situ electrodeposition of MnO₂ from pre-added Mn²⁺ during the charging process should be a side reaction, where Mn²⁺ in the electrolyte is consumed to form ε-MnO₂. However, ε-MnO₂ will irreversibly convert to the inactive λ-ZnMn₂O₄, leading to capacity decay (Fig. 7j), which was also confirmed by Xia and coworkers.¹²³ In addition, the depletion of the Mn²⁺ electrolyte to generate irreversible λ-ZnMn₂O₄ will accelerate the disproportionation reaction, thereby triggering the dissolution of more original active MnO₂. Moreover, it was found that pre-adding a large amount of Mn²⁺ did not improve the cycle stability; in contrast, excess Mn²⁺ in the electrolyte promoted the conversion of Mn²⁺ to the ZnMn₂O₄ phase, leading to severe capacity fading and even battery failure.¹²⁴

4.2 Capacity activation mechanisms

4.2.1 Electrodeposition of Mn²⁺. It is recognized that the Mn²⁺ additive in the electrolyte plays an important role in achieving a sustainable cycling performance for manganese-based ZIBs. Early studies have found that the pre-added Mn²⁺ can effectively suppress the dissolution of the cathode by alleviating the disproportionation of Mn³⁺ (2Mn³⁺ → Mn²⁺ + Mn⁴⁺) caused by Jahn–Teller distortion.⁷⁸ Nevertheless, according to the Nernst eqn (40), a high pH value can reduce the oxidation and deposition potential of Mn²⁺, and recent investigations have concluded that Mn²⁺ in a high pH environment created by ZSH can be oxidized at a low charging potential.⁹⁵ It has even been reported that active MnO₂ can be electrodeposited on the substrate from the MnSO₄-based electrolyte, with an electrodeposition plateau at around 1.73 V. Also, the in situ formed MnO₂ can be directly used for subsequent discharge/charge with the typical GCD curve (Fig. 8a).¹¹⁷ In electrolyte containing 2 M ZnSO₄ and 0.1 M MnSO₄, when the charge cut-off voltage was 1.7 V, no new diffraction peaks could be detected in the XRD pattern in the fully charged state; meanwhile, the cycling performance showed fast capacity fading. However, when the charge cut-off voltage increased to 1.8 or 1.9 V, a new set of peaks emerged in the fully charged state, which can be well indexed to ε-MnO₂ (Fig. 8b), accompanied by an increase in capacity with an increase in the number of cycles.¹²³ These findings suggest that the Mn²⁺ ions in the electrolyte can be electro-oxidized at a high potential, and the increased capacity is attributed to the newly formed MnO₂.

Fig. 8 Mechanisms of capacity activation. (a) Electrodeposition of MnO₂. Reproduced with permission.¹¹⁷ Copyright 2019, the American Chemical Society. (b) XRD patterns of the MnO₂/CC electrodes in the first charge state with different charge cut-off voltages. Reproduced with permission.¹²³ Copyright 2020, Elsevier. (c) Effect of the Mn²⁺ additive concentration in the electrolyte on the cycling performance. Reproduced with permission.¹²⁵ Copyright 2019, Springer Nature. (d). Capacity activation of the ZnMn₂O₄/Zn cell. (e) Electrochemical reactions inside the ZnMn₂O₄/Zn cell. (d and e) Reproduced with permission.¹²⁶ Copyright 2020, Elsevier. (f) Diagram of energy storage mechanism for the MnO cathode of ZIBs. Reproduced with permission.⁸¹ Copyright 2019, Elsevier. (g) Cycling performance of the Zn/Mn₃O₄ battery. Reproduced with permission.⁸³ Copyright 2018, Elsevier. (h) Elemental analysis of dissolved Mn²⁺ ions in 2 M ZnSO₄ electrolyte during cycling. (i) HRTEM of 1st fully charged state. (h and i) Reproduced with permission.⁶⁰ Copyright 2020, Elsevier. (j) Schematic diagram of Zn²⁺ storage mechanism of Mn₂O₃. Reproduced with permission.¹²⁷ Copyright 2023, Wiley-VCH.

The identification of electro-oxidized compounds with a nanosheet morphology is challenging due to their low crystallinity and complex components in the bulk cathode. In the literature, ε-MnO₂,^102,123 birnessite,^56,122 vernadite,^95,128 and their Zn compounds have been reported for electro-oxidized manganese oxides. But the newly formed MnO_x on the electrode surface caused by re-oxidation of Mn²⁺ compensating the capacity decay has reached a consensus. It has been reported that about 18.9% of the capacity is attributed to the contribution of pre-added Mn²⁺.¹²⁹ However, the concentration of the Mn²⁺ additive has a significant influence on the electrodeposited MnO_x. In an electrolyte with no Mn²⁺ additive, the capacity of MnO₂ decayed rapidly; however, with an increasing concentration of Mn²⁺ additive, the cycling stability gradually improves (Fig. 8c). Nevertheless, when the additive concentration reached 0.2 M, an increase in capacity was observed with cycle number due to the deposition of a large amount of aggregated nanosheets on the electrode surface.¹²⁵ These aggregated nanosheets augmented the active mass at the cathode, and consequently enhanced the cycling performance. Furthermore, Kim et al.¹²⁶ provided a deep comprehension of the role played by pre-added Mn²⁺ in the exceptional performance of a ZnMn₂O₄ cathode (Fig. 8d). They proposed that the additional capacity comes from two parts, where one is based on the quasi-reversible in situ electro-deposition/dissolution of MnO_x from pre-added Mn²⁺, and the other is based on the reversible insertion/extraction of Zn²⁺ from undissolved MnO_x (Fig. 8e), which may be generated from the incomplete dissolution process due to the gradual pH changes during the electrochemical cycling process.

Besides the contribution of pre-added Mn²⁺ to the increased capacity, the dissolved Mn²⁺ from the original cathode material also leads to performance activation. For instance, it has been observed that MnO, which was previously considered inactive for Zn²⁺ storage due to its lack of tunnels, exhibits a competitive electrochemical performance comparable to MnO₂ after undergoing an activation process. Chen et al.¹³⁰ attributed the activation of MnO to its spontaneous dissolution in ZnSO₄ electrolyte, which can be denoted as eqn (41). The spontaneous dissolution behavior of MnO provides adequate Mn²⁺ for the ZSH-assisted deposition/dissolution reaction and results in performance activation, where the energy storage mechanism can be described by eqn (19)–(21). Besides, it was found that there is a positive correlation between capacity and rest time of MnO, manifesting that the dissolution of Mn plays a key role in the performance activation.

MnO + 2H⁺ → Mn²⁺ + H₂O (41)

4.2.2 Electrochemical oxidation in the charging process. Additionally, it has been reported that the inert MnO can be activated by electrochemical oxidation in the charging process, leading to the formation of active MnO_x (x > 2), which serves as the active phase for the reversible insertion/extraction in the following cycles. Kang et al.⁸¹ claimed that the electrochemical oxidation of MnO into porous layered-type MnO₂ nanosheets in the initial charging process is responsible for MnO activation. Combined with the phase, morphology evolution and increased Mn valence (3.9), it is evident that the surface of the MnO particles is partially oxidized into layered-type MnO₂ nanosheets, serving as electro-active sites for subsequent energy storage (Fig. 8f). Sun et al.¹³¹ further confirmed this activation transition process, wherein MnO undergoes sequential oxidization to form layered-type MnO₂ in the initial few cycles, resulting in performance activation. In addition, Mn₃O₄, which is electrochemically inactive in alkaline Zn–MnO₂ batteries, it has also been reported to exhibit a good electrochemical performance in ZIBs.⁸³ An ex situ XRD analysis revealed the formation of Mn₅O₈ and birnessite after the first charging process. As cycling proceeded, the peak intensity of Mn₃O₄ weakened, while the peaks corresponding to birnessite became much stronger, indicating a transformation from Mn₃O₄ to birnessite. It should be noted that Mn₅O₈ is an intermediate product resulting from the incomplete electrochemical oxidation reaction and will finally transfer to birnessite after further electrochemical oxidation. The gradual generation of both the Mn₅O₈ intermediate and birnessite through the progressive electrochemical oxidation of Mn₃O₄ contributes to a gradual increase in capacity during the initial tens of cycles (Fig. 8g).

4.2.3 Mn-defects. Besides spontaneous dissolution and electrochemical oxidation, which contribute to the performance activation of MnO, in situ-generated Mn-defects also play a significant role in comprehending its performance activation. Zhou et al.⁶⁰ first reported that the induction of Mn defects through an in situ electrochemical process can achieve the activation of MnO (Fig. 8h and i). The electrochemical extraction of Mn²⁺ from the MnO host during charging provides more accessible channels for Zn²⁺ insertion/extraction. DFT calculations further confirmed the electrochemical activation of MnO caused by Mn defects, given that the differential charge density implies that electrons easily accumulate around the Mn defects to form a strong electrostatic field, which is highly favorable for Zn²⁺ adsorption. However, the uniform charge distribution of pristine MnO may cause severe destruction to the MnO framework upon Zn²⁺ insertion. The activation of MnO through charge-induced Mn dissolution was also verified by Zhao et al.⁵⁹ They concluded that the cycling performance of MnO is strongly related to the concentration of pre-added Mn²⁺, where insufficient amounts of Mn²⁺ in the electrolyte can lead to severe structural deterioration due to the greater dissolution of Mn, while an excess amount may over-suppress its dissolution. In addition, the Mn₂O₃/CoO heterointerface with poor initial discharge capacity was found to be activated by the Mn-defects in the charging process.¹²⁷ The distortion of Mn₂O₃ due to Zn²⁺ insertion in the initial cycles will lead to the easy release of Mn atoms from the framework during charging, resulting in the formation of Mn vacancies (Fig. 8j). Furthermore, a strong internal electric field is generated at the interface of the heterostructure due to the homogeneous charge distribution of Mn₂O₃/CoO heterointerface, which will induce strong electrostatic fields at the Mn species for electron rearrangement. Owing to the similar ionic radius of Zn²⁺ and Mn²⁺, the Mn dissolved from the host structure acts as a sacrificial site, providing more insertion sites and channels for Zn²⁺ insertion, leading to performance activation.

4.3 Dynamic capacity fluctuation for long-term cycling

4.3.1 The evolution of active materials. In conjunction with the analysis of cycling performance, ex situ XRD results, and morphology evolutions, Chen and colleges concluded that the evolution of the active materials is responsible for capacity fluctuation.⁵⁶ As shown in Fig. 9a, it is noticeable that the cycling performance of MnO₂ shows an obviously capacity fluctuation with an arched trend. The capacity increased rapidly during the initial cycles but subsequently decayed rapidly after reaching the peak value. The ex situ XRD patterns show that the diffraction peak intensity of MnO₂ gradually decreased in the initial cycles, manifesting the continuous dissolution of MnO₂ with an increase in the number of cycles. Moreover, numerous newly deposited birnessite nanosheets with low crystallinity were found on the electrode surface, indicating that the dissolved Mn²⁺ from the original MnO₂ was converted to birnessite during charging. Birnessite with specific nanosheet structures is electrochemical active, which can provide more reaction sites than the original MnO₂. Additionally, the deposition of birnessite compensates for the capacity loss caused by MnO₂ dissolution, resulting in performance activation. A similar activation mechanism of MnO was investigated simultaneously by Sun et al.¹³² However, with a further increase in the number of cycles, the birnessite nanosheets faded and the ZnMn₂O₄/Mn₃O₄ phase emerged. Unfortunately, both ZnMn₂O₄ and Mn₃O₄ are electrochemically inactive due to their inaccessible pathways for Zn²⁺/H⁺ insertion/extraction, resulting in limited capacity delivery. As the intensity of the signals for the ZnMn₂O₄/Mn₃O₄ phase continued to increase, a noticeable capacity degradation was observed during the subsequent cycling. In addition, owing to the competition between activation towards birnessite and degradation towards ZnMn₂O₄/Mn₃O₄, a transitional period with a relatively stable capacity existed between the activation and decay processes. However, once most of the original MnO₂ was converted to birnessite, severe capacity attenuation occurred (Fig. 9b). Moreover, it should be noted that the fluctuation tendency is closely associated with the current density, where with an increase in current density, MnO₂ exhibited enhanced cycling stability. This can be attributed to the mild dissolution issue of the original MnO₂, followed by the slow deposition and conversion reaction rate under high currents.

Fig. 9 Evolution of the active materials during long-term cycling. (a) Cycling performance at different specific currents of β-MnO₂ electrode. (b) Schematic illustration of the transformation process and reaction mechanism in each step of MnO₂ during long-term cycling. (a) and (b) Reproduced with permission.⁵⁶ Copyright 2022, Elsevier.

4.3.2 The evolution of capacity contribution ratio from Mn²⁺. Chao et al.¹²⁰ proposed a competitive capacity evolution (Mn-CCE) protocol to elucidate the capacity fluctuation behavior of manganese-based ZIBs. Based on the fluctuation characteristics and capacity contribution ratio from Mn²⁺ (CfM), the cycle curve can be divided into four distinct regions, as illustrated in Fig. 10a. Firstly, region I exhibits a rapid capacity increase at the beginning and ends with a slower growth rate; next, region II presents a mild ascending trend of capacity after region I; then, region III displays an evident rising trend of capacity, which ends with the peak value; finally, region IV demonstrates a quick downward trend in capacity. The magnitude of the variation in Zn²⁺ within region I was found to be significantly higher than that of Mn²⁺ during charge/discharge, with a calculated CfM value of less than 5%. This suggests that Zn²⁺ insertion/extraction plays a dominant role in the capacity contribution within region I. Therefore, the observed increase in capacity in this region can be attributed to electrode wetting. In region II, CfM slightly increases and the deposition of ZnMn₂O₄ is quasi-reversible due to the low pH environment. The capacity contribution mainly originates from the reversible intercalation/deintercalation of H⁺/Zn²⁺, resulting in a relatively flat cycle curve. Instead, CfM significantly increases with a rapidly decrease in the concentration of Mn²⁺ in region III, manifesting that Mn²⁺ dominates the reaction chemistry rather than H⁺/Zn²⁺ insertion/extraction, leading to an obvious increase in capacity. Besides, Qiu et al.¹²³ found that properly increasing the pre-added Mn²⁺ can expand the trend of capacity increase and achieve a higher maximum capacity value. The irreversible deposition of ZnMn₂O₄ nanoparticles provides nucleation sites for subsequent Mn²⁺ deposition, thereby inducing the accumulation of more ZnMn₂O₄ nanoparticles on the electrode surface, giving rise to a fast increase in CfM. However, the abundant accumulation of ZnMn₂O₄ nanoparticles on the cathode surface significantly increases the resistance and restricts the ion/charge transfer between the electrolyte and inner MnO₂, causing rapid capacity fading in region IV. In brief, the different capacity contributions originating from cation insertion/extraction and Mn²⁺ conversion during long-term cycling result in a dynamic performance evolution.

Fig. 10 Evolution of CfM and pH during long-term cycling. (a) Mn-based competitive capacity evolution protocol. Reproduced with permission.¹²⁰ Copyright 2022, The Royal Society of Chemistry. (b) Evolution of capacity and charge storage mechanisms during cycles associated with pH change. (c) Dominant reactions determined by the pH environment. (d) Summary of potential reasons for gradual pH increase. (e) Illustration of the H⁺/Zn²⁺/Mn²⁺-dominated electrochemical process at different pH environments. (b)–(e) Reproduced with permission.¹¹⁹ Copyright 2023, Wiley-VCH.

4.3.3 The evolution of pH. Moreover, new insight into the pH evolution in near-neutral pH electrolytes rationally makes the underlying cause for capacity fluctuation clear (Fig. 10b and c).¹¹⁹ Owing to the unavoidable occurrence of the hydrogen evolution reaction,¹³³ irreversible H⁺ extraction reaction, Mn³⁺ disproportionation reaction and backward hydrolysis reaction,^134,135 complete restoration of the initial pH state within a single cycle becomes unattainable, giving rise to a continuous increase in pH in the whole cycle calendar (Fig. 10d). The deposition potential of Mn²⁺ is high in the initial cycles due to the relatively low pH, wherein cation intercalation reactions dominate this period and result in a relatively stable evolution curve of capacity. As the pH continues to increase, the formation of deposited manganese oxide becomes more easily triggered with the dominant process changing to Mn deposition, leading to a rapid increase in capacity. As the cycle proceeds, the dissolution of manganese oxide becomes harder due to the high pH value and the formation of abundant irreversible manganese oxide consumes a large amount of H⁺, which causes a significant increase in pH, ultimately depleting the Mn²⁺ electrolyte and resulting in capacity attenuation (Fig. 10e). Consequently, the capacity fluctuation of manganese-based ZIBs with rapid increase and decay can be attributed to pH evolution in near-neutral pH electrolytes.

In summary, the mechanisms of capacity attenuation, capacity activation and dynamic capacity fluctuation in manganese-based ZIBs were comprehensively discussed. The inevitable dissolution of Mn, dramatic volume changes, and irreversible consumption of Mn²⁺ lead to capacity attenuation. Capacity activation is attributed to electrolyte Mn²⁺ electrodeposition, electrochemical oxidation and induced Mn-defects of low-valence manganese oxides during the charging process. Also, the evolution of the active materials, capacity contribution ratio from Mn²⁺, and pH results in dynamic capacity fluctuation.

5 Strategies for improving cycling stability

Undoubtedly, the capacity fluctuation poses a significant challenge for the commercial application of manganese-based ZIBs. Although converting Mn²⁺ to manganese oxides can provide additional capacity, resulting in an increase in capacity, this “fake” high-performance is transitory and the capacity will rapidly decay due to the generation of “dead” Mn. Consequently, enhanced cycling stability can be achieved by mitigating Mn dissolution and Mn²⁺ deposition, thereby enabling the dominant capacity contribution from Zn²⁺/H⁺ insertion/extraction. Additionally, improving the reversibility of the dissolution/deposition reaction represents an alternative approach to enhance the cycling stability. Thus, strategies aimed at optimizing the cycling stability should focus on either promoting one-electron transfer in the Mn⁴⁺ ↔ Mn³⁺ reaction or facilitating two-electron transfer in the Mn⁴⁺ ↔ Mn²⁺ reaction.

5.1 Strategies based on one-electron transfer reaction

5.1.1 Rational composite design. Rationally compositing conductive materials or transition metal oxides not only significantly enhances the electronic conductivity and ionic diffusion efficiency of semi-conductive manganese-based materials, but importantly improves their structural stability and mitigates side reactions during cycling. The enhanced structural stability primarily originates from the synergistic effects generated by two stable interfaces (the interface between the outer component and electrolyte, as well as the interface between components) through chemical bonding or physical isolation.

The multilayered rGO coated on MnO₂ nanowires with a thickness of 5 nm, as reported by Mai et al.,¹³⁶ was found to effectively alleviate direct contact between the internal active MnO₂ and the electrolyte. In pure ZnSO₄ electrolyte, it was observed that the concentration of Mn²⁺ significantly decreased during the discharge process after rGO coating, indicating the effective inhibition of the Mn dissolution issue. Additionally, the coating of polypyrrole (PPy) on MnO₂ also proved to suppress Mn dissolution, where PPy not only serves as a protective layer but also forms an Mn–N bond at the interface between MnO₂ and PPy (Fig. 11a), resulting in cycle performance of 500 cycles without capacity fading.¹¹³ Recently, Zhu et al. successfully fabricated a hydrophobic and uniform amino-propyl phosphonic acid (AEPA) coating layer with a few nanometer thickness on the surface of MnO₂. It was found that AEPA can form strong chemical bonds with MnO₂, effectively inhibiting the dissolution of Mn by creating a barrier between the electrolyte and cathode.¹³⁷ Yolk–shell structured K-birnessite@mesoporous carbon nanospheres (KMOH@C) with abundant void space between the KMOH core and carbon shell were synthesized by Zhai et al.¹³⁸ The mass of internal voids between the KMOH core and the carbon skeleton could efficiently restrain the volume changes, aggregation, and further pulverization of the KMOH nanosheets during the charging/discharging (Fig. 11b).¹¹⁶ As a result, KMOH@C showed excellent long-term cycle stability over 6000 cycles at 3 A g⁻¹. Also, graphite nanosheets, which possess superior mechanical properties, were demonstrated to enhance the stress of MnO₂ and suppress the further expansion and extrusion during the phase change process (Fig. 11c).

Fig. 11 Strategy for rational composite design based on one-electron transfer reaction. (a) Optimal configuration of the disordered MnO₂ surface covered by PPy. Reproduced with permission.¹¹³ Copyright 2020, Elsevier. (b) Schematic illustration of the Zn-ion/electron transport in KMOH@C with oxygen vacancies. Reproduced with permission.¹³⁸ Copyright 2021, Elsevier. (c) Schematic illustration of the volume change in MnO₂ and MnO₂/graphite. Reproduced with permission.¹¹⁶ Copyright 2020, Elsevier. (d) Illustration of the Mn valence state distribution in ZMO QD@C. (e) Mechanism for the structural stability of ZMO QD@C. (f) Long-term cycle test. (d)–(f) Reproduced with permission.¹³⁹ Copyright 2022, Wiley-VCH. (g) Cycling performance of hetero-structured Mn₂O₃–ZnMn₂O₄ hollow octahedrons. Reproduced with permission.¹⁴⁶ Copyright 2021, Wiley-VCH. (h) Schematic diagram showing the effects of bismuth in manganese-based ZIBs. Reproduced with permission.¹⁴⁷ Copyright 2021, the American Chemical Society. (i) Contact angle tests of BiMOF-CP and CP samples. (j) Element analysis of deposited Mn²⁺ in 2 M ZnSO₄/0.2 M MnSO₄ aqueous electrolyte during cycling of BiMOF-CP and CP. (k) Cycling performance of BiMOF-CP and CP. (i)–(k) Reproduced with permission.¹⁴⁸ Copyright 2023, Wiley-VCH.

ZnMn₂O₄ quantum dots (ZMO QD@C) were synthesized within a porous carbon framework through in situ electrochemically inducing Mn-MIL-100-derived Mn₃O₄ quantum dots and carbon composite, as reported by Niu et al.¹³⁹ In comparison to ZMO-bulk, the presence of a new peak at 1025 cm⁻¹ for ZMO QD@C is attributed to the formation of Mn–O–C bonds. The XPS spectra further confirmed the existence of Mn–O–C bonds, with new peaks observed at 531.9 eV for O 1s and 286.3 eV for C 1s in ZMO QD@C, respectively. Moreover, the analysis of the Mn valence states revealed that Mn predominantly exists as Mn(IV) at the interface between ZMO QDs and the carbon matrix, while being present as Mn(III) within the interior of ZMO QDs (Fig. 11d). The formation of stable Mn–O–C bonds at the interface effectively mitigates the disproportionation reactions and dissolution processes occurring on the surface of ZMO QDs (Fig. 11e). In addition, the establishment of a stable interface can also prevent direct contact between the electrolyte and the interior ZMO, thereby suppressing the dissolution of Mn³⁺ within the interior of ZMO QDs during discharge. Consequently, even in pure ZnSO₄ electrolyte, a super stable cycling performance can be realized (Fig. 11f). Additionally, it has been reported that the formation of an Mn–O–Ce bond in the MnO₂@CeO₂ composite can also ameliorate the disproportionation reaction and enhance the cycling stability.¹⁴⁰ Compositing with 2D layered MXenes not only enhances the charge transfer kinetics, but also mitigates issues such as volume expansion/contraction, structure dissolution, and side reactions of MnO₂.^141–145 For example, Yan et al.¹⁴¹ demonstrated that the incorporation of 3D Ti₃C₂T_x@MnO₂ microflowers with superior structural integrity resulted in good cycle stability with capacity retention of 90.6% over 2000 cycles at 0.5 A g⁻¹, which is much better than pure MnO₂ with a capacity retention only 21.4%. Hetero-structured Mn₂O₃–ZnMn₂O₄ hollow octahedrons with superior structural stability were designed via a two-step metal–organic framework template strategy.¹⁴⁶ The synergistic effects of the distinct composition and hollow heterostructure induced very stable Zn²⁺ storage, and the unique hollow octahedral structure could be well maintained even after 2000 cycles (Fig. 11g).

Gou et al.¹⁴⁷ demonstrated that the MnO₂–Bi₂O₃ composite undergoes a competitive reaction during the cycling process, resulting in the formation of Bi₂Mn₄O₁₀ instead of electrochemically inactive ZnMn₂O₄ due to its lower formation energy, as shown in eqn (42) and (43). This leads to improved cycling performance (Fig. 11h). Thus, to avoid Mn²⁺ electrodeposition and its further transformation to electrochemically inactive λ-ZnMn₂O₄, a kinetic inhibition strategy was employed by incorporating graphite nanosheets with limited surface defects in MnO₂ electrodes. The hydrophobicity and smooth surface of the graphite nanosheets effectively suppressed the rate-determining Mn²⁺ adsorption during nucleation, thereby ensuring a long-term cycling performance even at a high depth of discharge.¹⁰²

Bi₂O₃ + 6H⁺ → 2Bi³⁺ + 3H₂O (42)

2Bi³⁺ + 4MnO₂ + 2H₂O + 2e⁻ → Bi₂Mn₄O₁₀ + 4H⁺ (43)

Instead of blocking Mn²⁺ deposition, pyridine-3,5-dicarboxylate (Bi-PYDC) with a specific poorly hydrophilic and manganophilic nature (Fig. 11i), was reported to regulate the Mn²⁺ deposition reaction in cathode-free ZIBs.¹⁴⁸ As depicted in Fig. 11j, the concentration of Mn²⁺ rapidly decreased after the first cycle in the Zn/CP system. The fast deposition rate of CP induced the rapid generation of MnO₂ and inert phase, resulting in an unstable cycling performance. In contrast, the Zn/Bi-PYDC system maintained a relatively high and steady Mn²⁺ concentration due the fewer nucleation sites for Mn²⁺ deposition. This indicates a moderate deposition rate of Mn²⁺ for Bi-PYDC, which effectively prevented the excessive depletion of Mn²⁺ in the electrolyte and maintained an equilibrium reaction between dissolution and deposition. Moreover, a certain amount of Mn²⁺ in the electrolyte could effectively suppress the disproportionation reaction of deposited MnO₂. Consequently, the cathode-free ZIBs showed outstanding long-term cycling stability over 10 000 cycles at 1 mA cm⁻² (Fig. 11k).

5.1.2 Pre-intercalation. The pre-intercalation of crystal water, ions, and organic species in manganese-based materials is an effective strategy for reinforcing the inherent structural stability. These intercalated guest species can form chemical bonds with basic polyhedron or act as pillars to stabilize the host framework. For example, Liang and coworkers claimed that K⁺ pre-intercalated in the [2 × 2] tunnels of α-MnO₂ enables the formation of K–O bonds with MnO₆ octahedra,¹⁴⁹ which can stabilize the host framework and suppress the manganese dissolution to a great extent during cycling (Fig. 12a and b). Through atomic imaging, Yuan et al.¹⁵⁰ directly observed that Co²⁺ was intercalated in the tunnel space of α-MnO₂ [2 × 2] (Fig. 12c). The low binding energy (−6.62 eV) exhibited by Co²⁺ intercalated in tunnel MnO₂ implies the possibility of the formation a Co–O bond between Co²⁺ and the surrounding O atoms, thereby enhancing the inherent stability of the Mn–O octahedron and effectively inhibiting the dissolution of Mn (Fig. 12d). The formation of metal–O bonds was further confirmed by pre-intercalated Al³⁺ in α-MnO₂.¹⁵¹In situ Raman spectroscopy revealed a highly reversible phase transformation between α-MnO₂ and layered birnessite, and the peak located at 280 cm⁻¹ is attributed to Al–O bonds. Despite the phase transition inducing α-MnO₂ to form layered birnessite during discharge, the signal of Al–O bonds persisted, indicating that the consistent presence of Al–O bonds not only stabilized the tunnel structure of α-MnO₂ but also alleviated the structural deformation and dissolution of the layered birnessite (Fig. 12e).

Fig. 12 Strategy for pre-intercalation based on one-electron transfer reaction. (a) Element analysis of dissolved Mn²⁺ in 2 M ZnSO₄ aqueous electrolyte during cycling of KMO and α-MnO₂. (b) Schematic of the incorporation of K⁺ ions stabilizing the Mn polyhedrons. (a) and (b) Reproduced with permission.¹⁴⁹ Copyright 2019, Wiley-VCH. (c) [001] down-tunnel imaging showing the atomic structure of α-MnO₂ with Co²⁺ and K⁺ occupying the tunnel center position. (d) Schematic of the incorporation of Co²⁺ ions stabilizing the Mn polyhedrons. (c) and (d) Reproduced with permission.¹⁵⁰ Copyright 2023, Wiley-VCH. (e) Structure diagrams of pristine MnO₂ and Al-intercalated MnO₂. Reproduced with permission.¹⁵¹ Copyright 2021, Elsevier. (f) Schematic illustration of the synthetic process of Cu-intercalated MnO₂ electrode. Reproduced with permission.¹⁵² Copyright 2022, Elsevier. (g) Long-term cycling stability of Zn-δ-NMOH battery and Zn-δ-NMOH-500 battery. Reproduced with permission.¹⁵⁴ Copyright 2019, the American Chemical Society. (h) Schematic illustration of PANI-intercalated MnO₂ nanolayers. Reproduced with permission.¹⁵⁵ Copyright 2018, Springer Nature. (i) HAADF-STEM images of and PVP–MnO₂. (j) Selective proton Grotthuss intercalation of interlayer configuration optimization. (k) Cycling stability of PVP–MnO₂. (i)–(k) Reproduced with permission.¹⁵⁶ Copyright 2023, Wiley-VCH.

The application of the pre-intercalation strategy in δ-MnO₂ is considered promising due to its much larger interlamellar spacing (about 0.7 nm) compared to tunnel-type MnO₂. Cu²⁺ intercalation in δ-MnO₂ was reported by Zhang et al. (Fig. 12f).¹⁵² DFT calculations indicated that the pre-intercalation of Cu²⁺ can effectively enhance the structural stability of δ-MnO₂, which is attributed to the strong ionic bonds with oxygen atoms. The pre-intercalation of alkali ions play an important role in reinforcing the structural stability of δ-MnO₂, where Na⁺ or K⁺ acts as pillars stabilizing the layered structures. This led to a minimal loss capacity of 3% between the 20th and 100th cycles at 1/3C, and long cycling stability with 90% retention after 1000 cycles at 10C for the K_0.27MnO₂·0.54H₂O cathode, as reported by Simon et al.¹⁵³

The effect of crystal water on manganese-based materials cannot be neglected, where the critical role of crystal water in layered MnO₂ was explored in SIBs previously.^157,158 It was found that crystal water not only acts as interlayer pillars through hydrogen bonding with the MnO₆ layers to suppresses Mn dissolution, but also exhibits a charge shielding effect that mitigates the strong electrostatic interactions between the cations and the host framework, thereby enhancing the ion diffusion kinetics. As reported by Zhi et al.,¹⁵⁴ Na⁺ and crystal water pre-intercalated in the δ-MnO₂ interlayer space (δ-NMOH), which acted as pillars to stabilize the layered structures. Benefitting from Na⁺ and crystal water pre-intercalation, δ-NMOH showed an ultra-high capacity retention of 98% and well-retained layered structures over 10 000 cycles. However, when δ-NMOH was thermally treated at 500 °C to remove the crystal water, it showed a poor electrochemical performance compared with δ-NMOH due to the loss of a lubricant effect for cation insertion/extraction and the insufficient remaining Na⁺ ions to stabilize the host structure (Fig. 12g). Lou et al.¹⁵⁹ devised a method to pre-intercalate ammonium ions in layered δ-MnO₂ through an ion–exchange reaction between K⁺ ions and ammonium ions. These intercalated ammonium ions act as interlayer pillars, expanding the lattice spacing of δ-MnO₂ and forming stable hydrogen-bond networks, thereby alleviating the Jahn–Teller effect and demonstrating superior cycling stability over 10 000 cycles. Interestingly, anions were also reported to intercalate in MnO₂ for enhanced cycle performance. For example, Zhang et al.¹⁶⁰ successfully introduced PO₄³⁻ in the MnO₂ layer with rich oxygen defects via a simple phosphorization process.

Besides ions and crystal water, polymers have also been introduced as stabilizers for δ-MnO₂. A one-step inorganic/organic interface reaction was utilized to synthesize polyaniline-intercalated δ-MnO₂ (Fig. 12h), which was proven to be effective in preventing an irreversible phase transformation and structure collapse of δ-MnO₂ during the repeated hydrated Zn²⁺/H⁺ intercalation/de-intercalation process, leading to a stable cycle performance even at a low current of 0.2 A g⁻¹.¹⁵⁵ In addition, it was reported that pre-intercalated poly(3,4-ethylenedioxythiophene) (PEDOT) with an enlarged interlayer spacing acts as a structural pillar, which is beneficial to stabilize the layered structure of MnO₂.¹⁶¹ Recently, polyvinylpyrrolidone (PVP) pre-intercalation in MnO₂ (PVP–MnO₂) with a hybrid superlattice was reported by Bai et al. (Fig. 12i).¹⁵⁶ PVP–MnO₂ exhibited preferential H⁺ Grotthuss intercalation behavior and effectively mitigated structural collapse by impeding the transport of [Zn(H₂O)₆]²⁺ ions due to their larger radius and stronger coulombic interaction with the host material (Fig. 12j). Consequently, this resulted in outstanding cycle stability with nearly 100% capacity retention over 20 000 cycles at 10 A g⁻¹ (Fig. 12k).

5.1.3 Heteroatom doping. Heteroatom doping is considered an effective strategy for regulating the chemical nature and geometrical configuration of manganese-based materials. On the one hand, the incorporation of heteroatoms not only facilitates ion transfer but also modifies the electronic structure to enhance the conductivity, ultimately improving the reaction kinetics and electrochemical performance. On the other hand, heteroatom doping can modulate the strength of Mn–O bonds or form chemical bonds with Mn atoms to enhance the crystal structure stability.^162,163

Metal ion doping has been commonly employed to improve the cycling stability of manganese-based ZIBs. Also, various metal ions have been demonstrated as available dopants. For instance, it was reported that the substitution of Ti and the resulting oxygen vacancy can create a charge depletion zone, leading to the formation of a built-in electric field. The presence of oxygen vacancy can modify the internal electric field and improve the electrostatic stability by compensating for the nonzero dipole moment.¹⁷⁰ As a result, this leads to a significantly reduced charge-transfer impedance and diffusion rate for both Zn²⁺ and H⁺, facilitating their intercalation within the MnO₂ tunnel structure, and ultimately contributing to an outstanding long-term cycling performance.¹⁷¹ The incorporation of Ni²⁺ in Mn₂O₃ was demonstrated to reduce the formation energy of the Mn–O bond (Fig. 13a), which could effectively stabilize the Mn–O bond and alleviate the dissolution of manganese.¹⁶⁴ This resulted in a remarkable capacity retention of 85.6% over 2500 cycles at 1 A g⁻¹ for Ni-doped Mn₂O₃. Multivalent cobalt-doped Mn₃O₄ was successfully synthesized by Wang et al.,¹⁶⁵ wherein the doped Co exhibited various valence states and played multiple roles. Specifically, Co²⁺ acts as a “structural pillar” to stabilize the framework of the phase change product δ-MnO₂, while the Co⁴⁺ in the layer enhances the conductivity of Mn⁴⁺ and contributes to its high specific capacity. Importantly, doping with Co²⁺ and Co³⁺ can effectively suppress the Jahn–Teller distortion and facilitate ion diffusion (Fig. 13b). The doping of high-valent Mo⁶⁺ has been reported to result in an elevated oxidation state of Mn and a reduction in oxygen defects, thereby enhancing the structural stability and inhibiting the Jahn–Teller effect during discharge/charge processes.¹⁷² Analogously, the substitution of Fe³⁺ at the Mn site can alleviate the collapse of the MnO₂ crystal structure caused by Jahn–Teller distortion due to the increase in the mean valence of manganese.¹⁷³

Fig. 13 Strategy for heteroatom doping based on one-electron transfer reaction. (a) Calculated formation energy (in eV) of Mn₂O₃ and Ni-doped Mn₂O₃. Reproduced with permission.¹⁶⁴ Copyright 2021, Wiley-VCH. (b) Effect of various Co ions in phase change product. Reproduced with permission.¹⁶⁵ Copyright 2020, Wiley-VCH. (c) Mass content of dissolved Mn element in 2 M ZnSO₄ aqueous electrolyte during cycling. (d) Cycling performance of N-KMO at a current density of 1 A g⁻¹. (c) and (d) Reproduced with permission.¹⁶⁶ Copyright 2022, Wiley-VCH. (e) Schematic diagram of enhanced reaction feasibility of F-MnO₂. Reproduced with permission.¹⁶⁷ Copyright 2021, Elsevier. (f) Charge density difference distribution diagrams of ZMO and S-ZMO. Reproduced with permission.¹⁶⁸ Copyright 2023, Elsevier. (g) Electron density difference of K, Fe-ZMO and ZMO. Reproduced with permission.¹⁶⁹ Copyright 2022, Elsevier.

Nonmetal element doping is also important for enhancing the cycling stability of manganese-based ZIBs. For example, Lou et al.¹⁶⁶ constructed N-doped KMn₈O₁₆ (N-KMO) with abundant oxygen vacancies and large specific surface area. It was found that N-doping effectively inhibited the dissolution of Mn²⁺ caused by the Jahn–Teller distortion, as confirmed by the ICP results (Fig. 13c), thereby resulting in an impressive capacity retention of 91% over 2500 cycles (Fig. 13d). Moreover, the introduction of N-doping facilitated the formation of Mn–N bonds, thereby promoting interfacial dynamics in manganese-based materials and effectively mitigating manganese dissolution to ensure structural integrity.^174,175 In addition, N-doped ZnMn₂O₄ (N-ZMO) is conductive to the release of electrostatic forces caused by charge changes. The reduced electrostatic forces during Zn²⁺ insertion alleviate the volume expansion and structural collapse of N-ZMO.¹⁷⁶ Owing to the extremely high electronegativity and electron absorption of fluorine, the fluorine doping can promote robust bonding with the surrounding atoms. The dispersed doped fluorine in F-MnO₂ acts as a pin to anchor the lattice framework, preventing the excessive dissolution of the manganese-based framework in the discharge process. Although the partially dissolved manganese leads to Mn vacancy in the lattice, the “pinning effect” of fluorine protects against lattice collapse. During the charging process with manganese deposition, Mn²⁺ preferentially deposits into metastable coordination environments associated with Mn vacancy defects, thus facilitating the restoration of the original crystal structure (Fig. 13e).¹⁶⁷ The sulfur-doping proposed by Zhao et al. aimed to enhance the structural stability of MnO₂. This improvement was attributed to the substitution of the S anions, which have lower electronegativity than the O anions in the crystal structure. Consequently, this substitution reduced the diffusion barrier for Zn²⁺ ions across the cathode and prevented the formation of thermodynamically stable ZnMn₂O₄.¹⁷⁷ Additionally, Yang et al.¹⁶⁸ demonstrated that S-doping could effectively modulate the electrical structure of ZnMn₂O₄ by inducing the non-uniform distribution of charge distribution. The accumulation of electrons near the Mn atom signifies strong interactions between Mn and O (Mn–O) as well as between Mn and S (Mn–S) (Fig. 13f), thereby improving the structural stability of ZnMn₂O₄.

Dual heteroatom doping with distinct functionalities represents a novel approach to achieve a stable cycling performance in manganese-based ZIBs.^169,178 For example, Shao et al.¹⁶⁹ reported the preparation of K, Fe co-doped ZnMn₂O₄ (K, Fe-ZMO) as a cathode for stable ZIBs. In the structure of K, Fe-ZMO, K occupies partial Zn sites, while Fe occupies partial Mn sites. The dual doping strategy promotes strong interactions between the Fe atoms and O atoms as well as the Mn atoms and O atoms (Fig. 13g). Meanwhile, the lower formation energy of K, Fe-ZMO ensures structural stability and low manganese dissolution. However, it is important to note that the performance of batteries is intricately linked to both the doping sites and content of dopants. Not all forms of doping are beneficial; in fact, some can be detrimental. By employing controllable favorable heteroatom doping, significant enhancements in intrinsic reaction kinetics can be achieved, resulting in an increase in capacity and energy power density, as well as improved cycling life.¹⁷⁹

5.2 Strategies based on two-electron transfer reaction

The two-electron transfer reaction can eliminate the shortcomings associated with the insertion mechanism, such as detrimental Mn dissolution, dramatic volume changes and complex phase transformation. Furthermore, batteries employing the deposition/dissolution mechanism exhibit enhanced capacity compared to those based on the insertion mechanism. However, achieving a stable cycling performance in manganese-based ZIBs using the deposition/dissolution mechanism still faces challenges due to the insufficient Mn²⁺ absorption sites and competing reactions of Mn²⁺/Mn³⁺ during charging, as well as the incomplete dissolution of MnO₂ caused by its low electrical conductivity. The strategies for improving the reversibility of two-electron transfer reaction are discussed in the following sections.

5.2.1 Rational composite design. A porous carbon matrix-encapsulated MnO (MnO@PC) nanocomposite demonstrated the enhanced dissolution of MnO. The hierarchical meso-microporous carbon matrix possesses a large specific surface area, facilitating electrolyte infiltration and providing a negatively charged surface, which promotes the adsorption of cations. This enables better utilization of Zn²⁺ and Mn²⁺ in the electrolyte for ZSH deposition and the reversible deposition/dissolution process of Zn_xMnO(OH)₂.¹³⁰ The composite of negatively charged Zn–Mn MOF-derived porous carbon and MnO (MnO/MZ) exhibited strong adsorption of Mn²⁺ owing to the strong electrostatic interactions between the negatively charged MnO/MZ and positively charged Mn²⁺, which significantly promoted the dissolution/deposition reaction of Mn²⁺/MnO₂. As depicted in Fig. 14a, compared to MnO₂, Super P, and monometallic Mn-MOF-derived MnO/M, MnO/MZ demonstrated the highest negatively charged zeta potential in the same mild pH environment. Moreover, the similarly lower zeta potentials observed for Super P and MnO/M suggest that Zn plays a crucial role in generating negative charge, which is possibly attributed to the vaporization of positively charged Zn during calcination. The ICP results further confirmed the superior Mn²⁺ absorption ability of MnO/MZ (Fig. 14b), leading to a significant decrease in Mn²⁺ concentration after soaking in MnSO₄ solution. Despite the continuous dissolution of MnO during cycling, the dissolved Mn²⁺ can effectively undergo electro-oxidation and back deposition to form MnO₂, resulting in performance activation. Once all the MnO is completely dissolved, the highly reversible reaction between MnO₂ and Mn²⁺ dominates the capacity contribution, thereby ensuring excellent cycling stability over 11 000 cycles.¹³²

Fig. 14 Strategy for rational composite design based on two-electron transfer reaction. (a) Zeta potentials of MnO/MZ, MnO/M, Super P carbon, and MnO₂. (b) Adsorption of Mn²⁺ by MnO/MZ, MnO/M, Super P carbon, and MnO₂. (a) and (b) Reproduced with permission.¹³² Copyright 2022, Wiley-VCH. (c) MESP distributions of MnO₂ and MnO₂/MoO₃. (d) DFT calculations on the formation of structural water (left) and release of Mn (right) and their relative energy diagrams. (e) Schematic illustration and long-term cycling of MnO₂/MoO₃. (c)–(e) Reproduced with permission.¹⁸⁰ Copyright 2023, the American Chemical Society.

Liu and colleagues found that both the dissolution of MnO₂ and back-deposition of Mn²⁺ can be facilitated by compositing with MoO₃.¹⁸⁰ After introducing MoO₃, the Mn–O bond strength was weakened, leading to the formation of more oxygen vacancies by reacting with the oxygen atoms in MnO₂. In addition, compared to pure MnO₂, MnO₂/MoO₃ possessed more structural water during the whole discharge process, which resulted from the proton interactions with the lattice oxygen and the lower energy barrier for Mn release, as confirmed by the DFT results (Fig. 14d). Therefore, the inherent structural stability of MnO₂ was weakened and the Mn release and dissolution process were promoted. Moreover, MnO₂/MoO₃ exhibited a negative charge potential, while MnO₂ possessed a very high positive charge potential (Fig. 14c). This indicates that MnO₂/MoO₃ is beneficial to attract Mn²⁺ for deposition. Consequently, due to the high reversibility of the dissolution and deposition reaction, MnO₂/MoO₃ demonstrated a stable cycling performance with 92.6% capacity retention over 300 cycles at 0.1 A g⁻¹ and 80.1% capacity retention over 16 000 cycles at 1 A g⁻¹ in pure ZnSO₄ electrolyte (Fig. 14e). Besides, organic molecules were demonstrated to improve the reversibility of the dissolution/deposition reaction. For instance, poly(1,5-diaminoanthraquinone) (PDAAQ) plays a key role on the regulation of the Mn²⁺ concentration, which induced a long-term cycling life exceeding 2000 cycles.¹⁸¹

5.2.2 Electrolyte regulation. A strongly acidic environment undoubtedly promotes the MnO₂/Mn²⁺ reaction according to the Nernst equation; however, it severely corrodes the Zn metal anode. Zn(CH₃COO)₂ is a typical near-neutral electrolyte, which can alleviate the corrosion issues and enhance the coulombic efficiency for Zn plating/stripping. During the discharge process, the protonation of the O atoms on the cathode surface leads to the formation of OH* intermediates, and the continuous reactions between OH* and proton induce the formation and desorption of H₂O, resulting in cathode dissolution. Due to the high polarizability and high electronegativity of the CH₃COO⁻ groups, they strongly adsorb on the surface Mn sites,^182,183 effectively weakening the binding of H₂O on MnO₂ and reducing the barriers for MnO₂ dissolution. The highly reversible Mn⁴⁺/Mn²⁺ redox reaction not only increased the working potential (Fig. 15a–c), but also ensured long-term cycling stability over 4000 cycles with a high capacity of 556 mA h g⁻¹.¹⁸⁴ The promotion of the dissolution/deposition reaction chemistry by acetate ions was also confirmed by Zhong et al.¹⁸⁵ The co-deposition of Zn(H₂PO₄)₂ with MnO₂ at the cathode was demonstrated to function as a local proton reservoir, facilitating the dissolution of MnO₂, as reported by Liu et al.¹⁸⁶ During the discharge, Zn(H₂PO₄)₂ deprotonates to form Zn₃(PO₄)₂ and provides protons to promote the MnO₂/Mn²⁺ dissolution reaction. During the charge process, Zn₃(PO₄)₂ is reversibly protonated to Zn(H₂PO₄)₂, restoring protons at the cathode (Fig. 15d), as indicated by eqn (44) and (45). Therefore, a high discharge voltage of 1.75 V was achieved with an enhanced reversible two-electron transfer reaction (Fig. 15e), as well as alleviating the corrosion of the Zn anode. In addition, pH buffer is ideal to achieve highly reversible MnO₂/Mn²⁺ reaction in mild aqueous electrolyte, as demonstrated by Balland et al.¹⁸⁷

MnO₂ + 4H⁺ + 2e⁻ ⇌ Mn²⁺ + 2H₂O (44)

Zn(H₂PO₄)₂ + 2Zn²⁺ + 4H₂O ⇌ Zn₃(PO₄)₂·4H₂O + 4H⁺ (45)

Fig. 15 Strategy for electrolyte regulation based on two-electron transfer reaction. (a) Galvanostatic discharge curves of a Zn/MnO₂ battery in sulfate-based, (b) sulfonate-based, and (c) acetate-based aqueous electrolytes. (a)–(c) Reproduced with permission.¹⁸⁴ Copyright 2020, Wiley-VCH. (d) Schematic illustration of Zn(H₂PO₄)₂ for facilitating MnO₂ dissolution. (e) Long-term cycling performance of a Zn–MnO₂ cell with ZnCl₂/Mn(H₂PO₄)₂ electrolyte. (d) and (e) Reproduced with permission.¹⁸⁶ Copyright 2022, the American Chemical Society. (f) Diagram of the electrochemical behavior of urea-based eutectic electrolyte at the cathode interface. Reproduced with permission.¹⁸⁸ Copyright 2023, Oxford University Press.

The inevitable co-deposition of Zn²⁺ and Mn²⁺ to form an irreversible ZMO product will lead to poor cycling stability in manganese-based ZIBs. Through the modulation of the Zn²⁺-solvation shell to reduce the Zn²⁺ deposition at the interface, the reversibility of the dissolution/deposition reaction can be improved. For example, Hu et al.¹⁸⁸ constructed a quasi-eutectic electrolyte composed of 2 M Zn(OTf)₂ + 4 M urea + 0.25 M MnSO₄. It was found that the binding energy of the Mn²⁺-solvation shell with urea was very weak, while the binding energy of the Zn²⁺-solvation shell was strong. The significant difference in the Mn²⁺/Zn²⁺ ionic solvation structure led to different mass transfer between Zn²⁺ and Mn²⁺ at the cathode interface (Fig. 15f). The high desolvation energy of Zn²⁺ makes Zn²⁺ co-deposition with Mn²⁺ at the cathode interface difficult, thereby restricting the mass transfer of Zn²⁺. When the electrolyte contains urea, ZMO cannot be discovered at the cathode during charging and discharging. In contrast, without urea additive, the inert ZnMn₃O₇ phase is obviously detected during charging and discharging. Moreover, with the addition of urea, the valence states of Mn in the deposition increased obviously, thus giving rise to a higher capacity and better reversibility. In addition, a cationic accelerator strategy (CA) by adding poly(vinylpyrrolidone) was proposed to regulate the cationic solvation structures, as reported by Chuai et al.¹⁸⁹ Theoretical calculations and experimental results demonstrated the effective promotion of the rapid migration of Zn²⁺ and Mn²⁺ in the electrolyte by CA, facilitating sufficient charge transfer at electrode–electrolyte interface. This beneficially contributed to the reversible electrochemical deposition/dissolution chemistry of cathodic MnO₂/Mn²⁺ and anodic Zn/Zn²⁺, leading long-term cycling stability for over 2000 cycles. The concentration of pre-added Mn²⁺ significantly influenced the long-term cycling performance. Shen et al.¹⁹⁰ demonstrated that adjusting the Mn²⁺ concentration to a critical range can recycle Mn²⁺ from MnOOH disproportionation, enabling the reversible redox conversion between MnO₂ and Mn²⁺. As a result, an ultra-long lifespan of 16 000 cycles without noticeable capacity degradation was achieved.

In summary, a comprehensive discussion was presented on optimization strategies aimed at improving the cycling stability in manganese-based ZIBs, and the manganese-based materials for superior long-term cycling stability in ZIBs are summarized in Table 2. The formation of stable chemical bonds through compositing, pre-intercalation and heteroatom doping can mitigate the dissolution and structural collapse of manganese-based materials. Moreover, rational composite design can effectively reduce the deposition reaction of Mn²⁺ ions, preventing rapid electrolyte Mn²⁺ consumption and the formation of an inert ZMO phase. Meanwhile, reducing the dissolution barrier of manganese, promoting Mn²⁺ adsorption, and selectively alleviating Zn²⁺ deposition on the cathode by compositing or electrolyte regulation are beneficial for Mn dissolution and Mn²⁺ redeposition. Consequently, a stable cycling performance in manganese-based ZIBs can be achieved based on the reversible Mn³⁺ ↔ Mn²⁺ reaction or Mn⁴⁺ ↔ Mn²⁺ reaction, respectively.

Table 2 Summary of the stable cycling performance of manganese-based materials in ZIBs

Reaction mechanism/electrodes	Electrolyte	Voltage window	Capacity retention (cycles/current density)	Strategies/causes	Ref.
One-electron transfer reaction				Rational composite design
α-MnO2@graphite nanosheets	2 M ZnSO4 + 0.1 M MnSO4	1–1.8 V	95% (600/1C)	Suppress Mn^2+ electrodeposition	102
			98% (3000/6C)
MnO2@CeO2	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	89.68% (1000/1 A g^−1)	Suppress Jahn–Teller distortion	140
ZnMn2O4@C	2 M ZnSO4	0.8–1.8 V	86.4% (1500/1 A g^−1)	Suppress Jahn–Teller distortion	139
δ-MnO2-C NA	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	90.4% (50 000/4 A g^−1)	Buffer volume changes	191
N-doped carbon@δ-MnO2	2 M ZnSO4 + 0.3 M MnSO4	1–1.8 V	87% (3500/2 A g^−1)	Buffer volume changes	192
α-MnO2/graphene scrolls	2 M ZnSO4 + 0.2 M MnSO4	1–1.85 V	94% (3000/3 A g^−1)	Suppress cathode dissolution	136
Mn2O3–ZnMn2O4	Zn(CF3SO3)2 + 0.1 M MnSO4	0.8–1.8 V	93.3% (2000/3 A g^−1)	Superior structural stability	146
Ti3C2Tx@MnO2	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	90.6% (2000/0.5 A g^−1)	Suppress cathode dissolution	141
N-Mn3O4/MnO	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	77.2% (2500/10 A g^−1)	Alleviate stress	193
				Pre-intercalation
K0.27MnO2·0.54H2O	2 M ZnSO4 + 0.3 M MnSO4	0.7–1.8 V	91% (1000/10C)	Suppress cathode dissolution	153
Al-intercalated MnO2	2 M ZnSO4	0.8–1.8 V	94.5% (2000/2 A g^−1)	Suppress cathode dissolution	151
Na0.55Mn2O4·1.5H2O	2 M ZnSO4 + 0.1 M MnSO4 + 0.5M Na2SO4	0.8–1.9 V	70% (10 000/6.5C)	Suppress cathode dissolution	194
Na0.46Mn2O4·1.4H2O	2 M ZnSO4 + 0.2 M MnSO4	0.95–1.85 V	98% (10 000/20C)	Suppress cathode dissolution	154
Na^+-intercalated MnO2	2 M ZnSO4 + 0.2 M MnSO4	0.9–1.8 V	93% (1000/4 A g^−1)	Suppress cathode dissolution	195
Cu-MnO2	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.9 V	90.1% (700/5 A g^−1)	Suppress cathode dissolution	152
CoxMnO2	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	84.6 (500/0.5 A g^−1)	Suppress cathode dissolution	150
NH4^+-intercalated MnO2	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	69% (10 000/4 A g^−1)	Suppress cathode dissolution	159
PANI-intercalated MnO2	2 M ZnSO4 + 0.1 M MnSO4	1–1.8 V	∼84% (5000/2 A g^−1)	Suppress cathode dissolution	155
PVP-MnO2	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	∼100% (20 000/10 A g^−1)	Suppress cathode dissolution	156
				Heteroatom doping
Al-doped MnO2	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	79% (15 000/4 A g^−1)	Suppress cathode dissolution	196
Bi-doped MnO2	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	93% (10 000/1 A g^−1)	Suppress cathode dissolution	197
Ni-doped MnO	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	91% (6000/3 A g^−1)	Enhance structure stability	198
Co-doped Mn3O4	2 M ZnSO4 + 0.2 M MnSO4	0.2–2.2 V	80% (1100/2 A g^−1)	Suppress Jahn–Teller distortion	165
N-doped KMn8O16	2 M ZnSO4 + 0.1 M MnSO4	0.8–1.8 V	91% (2500/1 A g^−1)	Suppress Jahn–Teller distortion	166
N-doped Na2Mn3O7	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	78.9% (550/2 A g^−1)	Suppress cathode dissolution	174
K, Fe-doped ZnMn2O4	2 M ZnSO4 + 0.2 M MnSO4	0.8–1.8 V	88.1% (500/1 A g^−1)	Suppress cathode dissolution	169
Two-electron transfer reaction				Rational composite design
MnO2/MoO3	3 M ZnSO4	0.2–2.2 V	92.6% (300/0.1 A g^−1)	Facilitating dissolution/deposition	180
			80.1% (16 000/1 A g^−1)
MnO/MN	3 M ZnSO4	0.6–1.9 V	80.3% (200/0.1 A g^−1)	Facilitating deposition	132
			−(11 000/2 A g^−1)
MnO/PC	3 M ZnSO4	0.8–1.9 V	93% (120/0.1 A g^−1)	Facilitating deposition/dissolution	130
			−(2000/2 A g^−1)
MnO2@PDAAQ	2 M ZnSO4 + 0.2 M MnSO4	0.2–1.85 V	−(2000/2 A g^−1)	Facilitating deposition/dissolution	181
				Electrolyte regulation
MnO2/Zn(H2PO4)2	2 M ZnCl2 + 0.07 M Mn(H2PO4)2	0.5–2.1 V	∼100% (3000/2 mA cm^−2)	Facilitating dissolution	186
MnO2	1 M ZnSO4 + 1 M MnSO4 + 0.1 M H2SO4 + 0.07 mM PVP	1–2.2 V	90% (2000/20C)	Facilitating deposition/dissolution	189
MnO2	2 M Zn(OTf)2 + 4 M urea + 0.25 M MnSO4	0.8–1.8 V	76.8% (350/0.5 A g^−1)	Restricting Zn^2+ deposition	188
MnO2	2 M ZnSO4 + 0.4 M MnSO4 + 0.8 NaAc	1–1.8 V	−(350/0.5 A g^−1)	Facilitating deposition/dissolution	185
MnO2	1 M Zn(Ac)2 + 0.4 M Mn(Ac)2	1–1.8 V	∼100% (4000/5 mA cm^−2)	Facilitating deposition/dissolution	184
MnO2@CNT	2 M ZnSO4 + 0.005 M MnSO4	1–2.4 V	∼100% (16 000/10 mA cm^−2)	Facilitating deposition	190

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6 Summary and perspectives

Manganese-based ZIBs exhibit distinctive advantages and hold great promise for large-scale energy storage applications due to their cost-effectiveness, natural availability, low toxicity, multivalent states, high operation voltage, and satisfactory capacity. However, their unsatisfactory cycle stability coupled with intricate and contentious storage mechanisms hinder their commercial applications. In this review, the energy storage mechanisms of manganese-based ZIBs with different structures were systematically clarified and summarized. Subsequently, the capacity fluctuation of manganese-based ZIBs including capacity activation, degradation, and dynamic evolution in the whole cycle calendar were comprehensively analyzed. Lastly, the constructive optimization strategies based on the reaction chemistry of one-electron or two-electron transfer for achieving a durable cycling performance in manganese-based ZIBs were put forward, respectively. Nevertheless, challenges and disputes still persist in manganese-based ZIBs. Herein, we conclude some challenges and prospects in the reaction chemistry and optimization strategies for achieving a stable cycling performance in manganese-based ZIBs, as illustrated in Fig. 16.

Fig. 16 Perspectives for achieving stable cycling performance in manganese-based ZIBs.

(i) Systematic investigation of energy storage mechanisms. Given that the storage mechanisms have been discovered successively and the quick development of advanced characterization techniques in recent years, early reports may give incorrect and incomplete analyses. For example, there have been reports suggesting the complete transformation of the tunnel phase in α-MnO₂ during discharge, followed by subsequent recovery upon charging. However, the extent of this remarkable structural reversibility raises questions due to the high energy demand associated with charge ordering during tunnel formation.¹⁹⁹ Moreover, conventional characterization of the reaction mechanisms heavily relies on interpretations based on bulk-level characterizations, which may overlook the crucial local changes occurring within cells and potentially lead to misinterpretation of the reaction mechanisms due to the presence of side products or residual electrolytes.²⁰⁰ For instance, routinely used XRD techniques may yield ambiguous data due to the formation of various phases of low symmetry, adding layers upon cycling. In addition, the specific roles of Zn²⁺ and H⁺ during charge/discharge remain ambiguous. However, employing crucial analysis methods, such as solid-state ⁶⁷Zn and ¹H (¹D) NMR-mass spectroscopy with isotope labeling can potentially facilitate a more comprehensive evaluation of the contribution of Zn²⁺ and H⁺, although this task remains challenging.⁹² Furthermore, it is worth noting that in most reports, a single electrochemical reaction process is declared in the whole cycle calendar. However, the reaction chemistry usually consists of multiple processes that dominate at different stages with a dynamic change in the Mn valence and electrolyte pH. Therefore, investigations into energy storage mechanisms based solely on initial cycles may not fully account for the complexities of the entire cycle. Consequently, the development of more advanced characterization technologies such as atomic-level characterization and in situ techniques combined with the continuous monitoring of dynamic changes in the entire cycle, will facilitate a comprehensive understanding of the energy storage mechanisms inherent to manganese-based ZIBs.

(ii) Paying attention to the influence of pre-added Mn²⁺. On the one hand, the Mn²⁺ additives would stabilize the cycling performance to some extent by compensating for the capacity loss of Mn²⁺ electrodeposition or mitigating the Jahn–Teller effect. On the other hand, the pre-added Mn²⁺ increases the complexity of reaction chemistry in manganese-based ZIBs and may obscure the electrochemical behaviors of the original cathode due to its deposition. Although extra capacity can be obtained, the accumulation of by-products such as a ZMO phase resulting from the transformation of deposited Mn²⁺ can lead to rapid capacity degradation and even battery failure. In addition, the content of Mn²⁺ in the electrolyte not only impacts the cycle stability, but also dominates the reaction rate between insertion/extraction chemistry and dissolution/deposition chemistry. Excessive Mn²⁺ in the electrolyte may facilitate the dissolution/deposition reaction, while inadequate Mn²⁺ may compromise the cycling stability. Moreover, the presence of pre-added Mn²⁺ has an influence on pH fluctuations, thereby further impacting the energy storage mechanism. The excessive addition of Mn²⁺ can effectively suppress the formation of ZHS, but may also alter the reaction mechanism by reducing the electrolyte pH.⁹² Therefore, gaining a comprehensive understanding of pre-added Mn²⁺ is important for researchers to comprehend the underlying reaction mechanisms in manganese-based ZIBs and devise effective strategies to enhance their cycle stability.

(iii) Developing new manganese-based cathodes. Developing novel manganese-based cathodes is of paramount importance for the large-scale implementation of ZIBs. Despite the fact that traditional manganese oxides have been widely investigated as cathode materials for ZIBs, ZIBs still suffer from the lack of high-performance cathode materials. Ensuring the structural stability of manganese-based materials during Zn²⁺ storage is crucial for achieving cycling stability. The search for new stable manganese-based cathodes, such as manganese-based layered double hydroxide (LDH) and high entropy oxides (HEOs), presents a promising avenue.²⁰¹ For example, HEOs (Co_0.6Ni_0.6Fe_0.6Mn_0.6Cu_0.6O₄) with long-range disorders and a fixed lattice structure can effectively alleviate the structural collapse induced by the strong electrostatic force between Zn²⁺, thereby achieving remarkable long-term stability.²⁰² Additionally, a trinary LDH with hydrogen vacancies (Ni₃Mn_0.7Fe_0.3-LDH) has been reported as a novel cathode material for ZIBs, exhibiting a reversible high capacity of up to 328 mA h g⁻¹ and demonstrating excellent cycling stability over 500 cycles with a capacity retention of 85%.²⁰³ However, their stable cycling performance still relied on the presence of pre-added Mn²⁺, which may lead to a misinterpretation of their inherent structural stability. Therefore, to realize the stable cycling performance of manganese-based ZIBs, it is imperative to not only optimize traditional manganese oxides but also dedicate more efforts towards developing new manganese-based cathodes with intrinsic structural stability.

(iv) Consideration of factors beyond the cathode. Besides rational modifications to manganese-based cathodes and electrolytes, attention should also be given beyond the cathode. The Zn anode faces challenges such as dendrite formation, passivation, corrosion, and hydrogen evolution, which significantly impede the cycling performance stability. Effective strategies for achieving Zn anodes with a long lifespan include the deposition of artificial interface layers, electrode design, and electrolyte modifications.²⁰⁴ For example, the addition of DMSO to the electrolyte has been demonstrated to facilitate the formation of a solid electrolyte interphase on the anode, effectively preventing Zn dendrite growth and water decomposition.²⁰⁵ In addition, although glass fiber separators are commonly used in ZIBs, their inherent fragility and large pores make them susceptible to penetration by Zn dendrites, leading to battery failure after repetitive plating/stripping.^206,207 Modifying traditional glass fiber separators or developing novel separators can help regulate the electric field distribution uniformly and suppress dendrite formation as well as parasitic side reactions. Besides, current collectors and binders cannot be neglected as well. Furthermore, optimizing the cut-off voltage from the perspective of electrochemical parameters is another way to enhance the cycling stability of manganese-based ZIBs, which could avoid side reactions or activate inactive materials.^117,208,209 For instance, extending the voltage cut-off to 2.2 V enables the activation of previously inactive components, facilitating a reversible MnO₂/Mn²⁺ reaction with two-electron transfer.²¹⁰ However, the investigation of cycling stability beyond the cathode remains inadequate. Therefore, to achieve the maximum cycling stability, more attention should be paid to optimizing beyond the cathode.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

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