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Energy Mater 2022;2:200040. 10.20517/energymater.2022.56 © The Author(s) 2022.
Open Access Review

Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences

1School of Flexible Electronics (Future Technologies), Nanjing Technology University, Nanjing 211816, Jiangsu, China.

2College of Intelligent Science and Control Engineering, Jinling Institute of Technology, Nanjing 211169, Jiangsu, China.

3School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, Gansu, China.

#Authors contributed equally.

*Correspondence to: Prof./Dr. Jinyuan Zhou, School of Physical Science and Technology, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, Gansu, China. E-mail: zhoujy@lzu.edu.cn ; Prof./Dr. Gengzhi Sun, School of Flexible Electronics (Future Technologies), Nanjing Technology University, 5 Xin Mofan Road, Nanjing 211816, Jiangsu, China. E-mail: iamgzsun@njtech.edu.cn

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    Abstract

    Energy storage devices, e.g., supercapacitors (SCs) and zinc-ion batteries (ZIBs), based on aqueous electrolytes, have the advantages of rapid ion diffusion, environmental benignness, high safety and low cost. Generally, SCs provide excellent power density with the capability of fast charge/discharge, while ZIBs offer high energy density by storing more charge per unit weight/volume. Although the charge storage mechanisms are considered different, manganese dioxide (MnO2) has proven to be an appropriate electrode material for both SCs and ZIBs because of its unique characteristics, including polymorphic forms, tunable structures and designable morphologies. Herein, the design of MnO2-based materials for SCs and ZIBs is comprehensively reviewed. In particular, we compare the similarities and differences in utilizing MnO2-based materials as active materials for SCs and ZIBs by highlighting their corresponding charge storage mechanisms. We then introduce a few commonly adopted strategies for tuning the physicochemical properties of MnO2 and their specific merits. Finally, we discuss the future perspectives of MnO2 for SC and ZIB applications regarding the investigation of charge storage mechanisms, materials design and the enhancement of electrochemical performance.

    INTRODUCTION

    In recent decades, electrochemical energy storage devices have been widely applied as power systems for a variety of applications ranging from portable electronics and electric vehicles to smart electric grids[1-5]. Due to their advantages of high energy density and long cycling lifespan without a memory effect, lithium-ion batteries (LIBs) are the most successful products, with the 2019 Nobel Prize in Chemistry awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino[6-11]. However, safety issues, such as fires and explosions, are always major concerns regarding the utilization of LIBs since their lithium salt-containing organic electrolytes are highly flammable[12-15]. To address this issue, one viable strategy is to develop aqueous-based devices, which are considered as promising alternatives due to their high safety and low cost[16-22].

    Presently, two types of aqueous-based devices, namely, supercapacitors (SCs) and zinc-ion batteries (ZIBs), as schematically illustrated in Figure 1A and B, have attracted tremendous attention owing to their unique features[20,23-29]. Typically, SCs exhibit ultralong cyclic stability, fast charge/discharge rates and high power density, thus explaining their broad applications in urban public transportation, aerospace, the military, and so on[30-33]. The charges are stored in SC materials based on two mechanisms: (1) adsorption/desorption of electrolyte ions on the electrode surface, excluding any redox reactions (electrochemical double-layer capacitance, EDLC); and (2) fast and reversible redox reactions on the electrode materials (pseudocapacitance). In contrast, ZIBs are considered promising candidates for grid-scale safe energy storage due to their low redox potential (-0.76 V vs. NHE), high theoretical capacity (820 mAh g-1) due to the metal Zn anode, intrinsic nonflammability, non-toxicity and high ionic conductivity of the aqueous electrolytes and low cost[34-38].

    Figure 1. Schematic illustrations of (A) SC and (B) ZIB devices. (C) publication numbers of different materials, including MXenes, TMDs, carbon, polymers and TMOs (MnO2 and RuO2), for SCs in the period from 2011 to 2022. (D) publication numbers of different cathode materials, such as TMOs (VOx and MnO2), PBAs, polymers and other materials (including carbonaceous materials, TMDs and MXenes), for ZIBs in the period from 2013 to 2022. All data were obtained from the Web of Science. EDLC: electrochemical double-layer capacitance; SCs: supercapacitors; ZIBs: zinc-ion batteries.

    In contrast to SCs, electrolyte ions (Zn2+) are inserted and extracted from the electrode materials, usually accompanied with phase transitions. Undoubtedly, the performance of both devices severely relies on their electrode materials[39-41]. Based on the statistics summarized in Figure 1C and D, it is very interesting to find that the commonly used electrode materials for SCs, including nanocarbon, conducting polymers, transition metal oxides (RuO2 and MnO2), transition metal dichalcogenides (TMDs) and transition metal carbides/nitrides (MXenes), are also suitable for ZIBs. Among these materials, the most studied one is manganese dioxide (MnO2) because of its natural abundance, non-toxicity, wide potential window and high theoretical capacitance/capacity[42-47]. For example, α-MnO2 nanoneedles synthesized by a microwave-assisted method were used as SC materials and delivered 289 F g-1 at 0.5 A g-1 in 1 M Na2SO4 and maintained 88% after cycling 10,000 times[48]. A δ-MnO2-based cathode for ZIBs showed a capacity of 200 mAh g-1 at 0.1 A g-1 after full electrochemical activation[49]. However, when utilized as electrode materials for SCs and ZIBs, MnO2 typically faces similar problems, including high electrical resistance, cycling instability, slow electrode kinetics, and so on. Moreover, the charge storage mechanisms of MnO2 in SCs and ZIBs are still under debate. In particular, based on the literature, complicated processes are involved when MnO2 is applied in ZIBs, whereas pseudocapacitive behavior is understood in SCs. Although several articles have been published summarizing the applications of MnO2 either in SCs or ZIBs, a comprehensive and comparative review to elucidate the similarity and difference in materials design for both devices is essential[50-53]. Herein, we focus on the recent advances in the design and utilization of MnO2 in aqueous-based SCs and ZIBs. In particular, we describe the respective charge storage mechanisms, highlight the materials design principles and provide a direct comparison and perspectives for future endeavors regarding MnO2-based SCs and ZIBs.

    CRYSTAL STRUCTURE

    The commonly utilized MnO2 polymorphs in SCs and ZIBs can be classified into α-MnO2, β-MnO2, δ-MnO2, γ-MnO2 and amorphous MnO2 [Figure 2] by arranging the MnO6 octahedral units either viaedge-sharing, corner-sharing or a combination of two[26]. In particular, in α-MnO2 [Figure 2A], MnO6 octahedral double chains are connected in a corner-sharing manner to form 2 × 2 tunnels (4.6 × 4.6 Å). In β-MnO2 [Figure 2B], single chains assembled by MnO6 octahedra are interlinked in a corner-sharing manner to form 1 × 1 tunnels (2.3 × 2.3 Å). In γ-MnO2, both 1 × 1 and 1 × 2 tunnels coexist and distribute randomly [Figure 2C]. Alternatively, in δ-MnO2, two-dimensional (2D) sheets are built by sharing MnO6 octahedral edges [Figure 2D] and then stacked into laminar structures with an interlayer spacing of ~7 Å[23]. It is known that the intercalated cations or water molecules between the sheets play a critical role in stabilizing the layered structure[54]. In comparison, amorphous MnO2 possesses a highly disordered structure [Figure 2E] with higher strength, lower hardness, larger surface area and even superior structural stability[55,56].

    Figure 2. Crystal structures of (A) α-, (B) β-, (C) γ-, (D) δ- and (E) amorphous MnO2. Reproduced with permission[26]. Copyright 2019, Royal Society of Chemistry.

    SUPERCAPACITORS

    Charge storage mechanisms

    MnO2 is considered a pseudocapacitive material[57]. Traditionally, it is believed that the capacitance of MnO2 originates from surface redox reactions with a theoretical value of ~110 μF cm-2 based on a Brunauer-Emmett-Teller surface (10 to 180 m2 g-1)[58]. Nevertheless, a much higher capacitance (over 300 F g-1) was obtained experimentally in a neutral electrolyte, suggesting extra contributions from other electrochemical mechanisms beyond the surface redox reactions[59]. Zhang et al. confirmed the capacitance from the insertion/extraction of Na+ in δ-MnO2 by assembling a sodium-ion capacitor [Figure 3A][60]. The interlayer spacing of layered δ-MnO2 was expanded to ~7 Å after activation during the first discharge. Moreover, the contribution of diffusion-controlled capacitance was determined to be as high as 64.4% of the total charge storage at 0.1 mV s-1 (0-1.2 V vs. SCE), indicating that the dominant charge storage depends on Na+ insertion/extraction. In another work, Chen et al. investigated the charge storage mechanism of Zn2+pre-intercalated α-MnO2 (ZnxMnO2) nanowires in a blended aqueous electrolyte (2 M ZnSO4 and 0.4 M MnSO4)[61].Ex-situ Inductive Coupled Plasma (ICP) analysis showed that the Zn/Mn molar ratio in ZnxMnO2 varied from 0.029 to 0.596 during the first three discharge processes, corresponding to the reversible insertion/extraction of Zn2+ [Figure 3B]. The reaction mechanism follows Equation (1):

    Figure 3. (A) Intercalation/deintercalation of Na+ using NaClO4 as the electrolyte. Reproduced with permission[60]. Copyright 2018, Elsevier. (B) charge storage mechanism of α-MnO2-based SC using ZnSO4 and MnSO4 as the electrolyte. Reproduced with permission[61]. Copyright 2020, Wiley-VCH. (C) redox reactions of MnO2 in acidic, neutral and alkaline electrolytes, respectively. Reproduced with permission[63]. Copyright 2019, Elsevier.

    ZnxMnO2 - 2(x-y)e- ↔ ZnyMnO2 + (x-y)Zn2+     (1)

    The electrochemical behavior of MnO2 is found to be heavily dependent on the pH of the electrolyte [Figure 3C][62,63]. In a neutral electrolyte, the generated Mn3+ ions involve a slow process of disproportionation following Equation (2), leading to Mn2+ loss and continuous capacity decay[64,65]. Furthermore, MnO2 structures experience volume expansion, thereby loosening the electrical contacts between MnO2 particles, increasing the series resistance and lowering the capacitance over time[66,67]. These issues compromise the power density and cycle life of MnO2 directly[63,68]. In an acidic electrolyte, except for the redox reaction between H+ and MnO2, excessive H+ induces the further reduction of MnOOH to Mn2+, leading to the irreversible dissolution of MnO2 Equations (3) and (4)[63]. In an alkaline electrolyte, hydroxyl ions (OH-) react with MnO2 at the surface, generating a layer of insulative Mn(OH)2, which prevents the inner active materials from being exposed to the electrolyte ions. The overall electrochemical reactions in an alkaline electrolyte follow Equations (5)-(9) with a narrow voltage window of < 0.7 V[69].

    In a neutral electrolyte:

    2Mn3+ ↔ Mn4+ + Mn2+(aq)    (2)

    In an acidic electrolyte:

    MnO2 + H+ + e- → MnOOH    (3)

    MnOOH + 3H+ + e- → Mn2+(aq) + 2H2O    (4)

    In an alkaline electrolyte:

    MnO2 + H2O + e- → MnOOH + OH-     (5)

    MnOOH + H2O + e- → Mn(OH)2 + OH-     (6)

    3Mn(OH)2 ↔ Mn3O4·2H2O + 2H+ + 2e-     (7)

    Mn3O4·2H2O + OH- ↔ 2MnOOH + Mn(OH)3 + e-     (8)

    4MnOOH + 2Mn(OH)3 + 6OH- ↔ 6MnO2·5H2O + 3H2O + 6e-     (9)

    Overall, two main charge storage mechanisms are involved for MnO2 when adopted in aqueous SCs: (i) surface redox reactions Equation (10); and (ii) intercalation/deintercalation of cations Equation (11):

    (MnO2)surface + M+ + e- ↔ (MnOOM)surface     (10)

    (MnO2)bulk + M+ + e- ↔ (MnOOM)bulk     (11)

    Performance enhancements

    Nanostructure design

    Enhancing the ion diffusion kinetics via nanostructural design is considered a feasible method to improve the capacitance of MnO2[70-77]. Xiong et al. prepared interlayer expanded MnO2 (0.93 nm) with intercalated tetramethylammonium ions (TMA+), following a two-step cation exchange [Figure 4A][78]. Density functional theory (DFT) calculations suggested that the expanded interlayer can weaken the interactions between the negatively charged MnO2 and K+ due to the decreased diffusion energy barrier, thereby accelerating ion diffusion during charge/discharge. A specific capacitance of 160 F g-1 was obtained for the restacked MnO2 in aqueous K2SO4 electrolytes at 0.2 A g-1, while at 10 A g-1, 70% capacitance was retained (110 F g-1). In contrast, in the cases of K- and H-MnO2 nanobelts, only 50% and 55% capacitance retentions were obtained, respectively, as the current density increased from 0.2 to 10 A g-1 [Figure 4B]. Moreover, the long-term cycling performance of the restacked MnO2 nanosheets was evaluated at 5 A g-1 for 5000 cycles with 100% retention.

    Figure 4. (A) Synthetic process of restacked MnO2. (B) rate capability in K2SO4 electrolyte at current densities from 0.2 to 10 A g-1. Reproduced with permission[78]. Copyright 2017, American Chemical Society. (C) structural evolution of Mn3O4 during electrochemical oxidation. (D) cyclic performance of Na0.5MnO2 nanowall arrays (NWAs) in different potential windows. Reproduced with permission[84]. Copyright 2017, Wiley-VCH.

    Foreign ion/molecular pre-insertion

    Since the charge storage capacity of MnO2 can be enhanced by the Mn4+/Mn3+ redox pair with cation adsorption (or intercalation)[79-81], it is speculated that the pre-insertion of cations in MnO2 could improve the specific capacitance by enhancing ion diffusion[81-83]. Following this guidance, Jabeen et al. prepared birnessite Na0.5MnO2 arrays on a carbon cloth by the electrochemical conversion of Mn3O4 in 10 M Na2SO4 via cyclic scanning at 10 mV s-1 within the potential window of 0 and 1.3 V (vs. Ag/AgCl) for 500 cycles [Figure 4C][84]. The incorporation of Na+ not only upgraded the specific capacitance but also widened the workable potential window to 1.3 V. The redox reaction of the Mn3+/Mn4+ couple was verified by ex-situX-ray photoelectron spectroscopy, while the newly emerged peaks at 0.96 V were attributed to the insertion and extraction of residual Na+. The obtained Na0.5MnO2 electrode showed an obviously improved capacitance of 366 F g-1 without compromising its cycling stability [Figure 4D]. Asymmetric SCs were constructed using Na0.5MnO2 and Fe3O4 as the cathode and anode, respectively. The asymmetric SC using 1 M Na2SO4as the electrolyte exhibited a cell voltage of 2.6 V, a capacitance of 88 F g-1 and an energy density of 81 Wh kg-1.

    Defect engineering

    In addition to ion diffusion, the high electrical resistance of MnO2 needs to be lowered to achieve rapid electron transport during the redox reactions[85-87]. Defect engineering (heteroatom doping and the introduction of oxygen vacancies) has been demonstrated to be an effective strategy for improving the conductivity of MnO2[88-96]. Kang et al. designed a thick Au-doped MnO2 film (1.35 μm) by electrochemically depositing MnO2 and sputtering Au alternately to adjust the electronic structure of MnO2 [Figure 5A][97]. Au atoms distributed in the lattice of MnO2 with a total doping level of 9.9 at.% act as electron donors to induce a new state in the bandgap (~1.0 eV), thereby enhancing the overall conductivity, which is beneficial to the kinetics of the electrode reaction [Figure 5B]. As shown in Figure 5C, the specific capacitance of the Au-doped MnO2 is much higher than that of pristine MnO2 (2.5 eV). With increasing Au-doping level, the specific capacitance first increases and then decreases slightly. As a result, the Au-doped MnO2 film achieved a gravimetric capacitance (Cg) as high as 626 F g-1 tested in 2 M Li2SO4 at 5 mV s-1 and superior stability over 15000 charge/discharge cycles [Figure 5D]. In addition, substituting Mn with heterogeneous atoms (such as Co, Ni, Al, Fe, Ag and Au) in MnO2 tends to change the electronic structure of MnO2 via electron donation. For example, Wang et al. synthesized a series of interlinked Fe-doped MnO2 nanostructures via a hydrothermal method[98]. It was confirmed that the incorporation of Fe atoms effectively prevented the collapse of MnO2 crystals during protonation, thereby prolonging the service life of the device. The optimized sample with a mass loading of ~5 mg cm-2 exhibited a specific capacitance of 267.0 F g-1 at 0.1 A g-1 with 68.6% retention at 1 A g-1 and excellent cycling stability over 2000 cycles at 2 A g-1 (~100% retention).

    Figure 5. (A) Fabrication and (B) first-principle calculations of Au-doped MnO2. Differential charge densities of (left) Au-substituted and (right) Au-interstitial MnO2. Green indicates a loss of electrons and pink represents a gain of electrons. (C) specific capacitance of Au/MnO2 at different Au sputtering times. (D) cycling stability of pure MnO2 and Au-doped MnO2 electrodes with the same thickness of ~1.35 μm. Reproduced with permission[97]. Copyright 2013, Wiley-VCH. (E) fabrication of amorphous MnO2@MWCNT fibers. (F) CV of amorphous MnO2@MWCNT fiber electrode. (G) cycling and bending stability of fiber SC at 1 A cm-3. Reproduced with permission[100]. Copyright 2017, American Chemical Society.

    Alternatively, Peng et al. hydrothermally built an oxygen vacancy-rich MnO2-x/reduced graphene oxide (rGO) composite in tetrahydrofuran using manganese carbonyl (Mn(CO)5) and GO as the precursors[99]. Benefiting from the good electronic conduction in the interconnected rGO networks and high redox activity of the partially reduced MnO2 nanoparticles, the vacancy-rich MnO2-x/rGO film showed a high specific capacitance of 675.5 F g-1(0.5 A g-1) in a 0.5 mol L-1 Na2SO4 solution and retained 96.1% of the capacitance after 10,000 cycles. Amorphous MnO2 with abundant vacancies and ion transport channels has also been considered as a promising electrode material for SCs. As a typical work, Shi et al. fabricated hybrid fiber electrodes by anchoring amorphous MnO2 on well-aligned multiwall carbon nanotube (MWCNT) sheets, followed by twisting [Figure 5E][100]. The MWCNT sheets, which were drawn from a spinnable array, showed excellent electron conductivity that facilitated the redox reaction of amorphous MnO2. The direct chemical deposition of amorphous MnO2 was carried out at 80 °C using acidic KMnO4 and MWCNTs as the precursors via Equations (12) and (13):

    4MnO4- + 3C + H2O → 4MnO2 + CO32- + 2HCO3-     (12)

    4MnO4- + 4H+ → 4MnO2 + 3O2 + 2H2O    (13)

    Benefiting from the abundant ion transport channels and the fast electron transport of the MWCNTs, the as-prepared amorphous MnO2@MWCNT fibers exhibited a capacitance of 60.8 F cm-3 at 0.2 A cm-3 with the calculated contribution of MnO2 as high as 615.2 F g-1 and excellent rate performance retaining 44.1 F cm-3 at 20 A cm-3 [Figure 5F]. The symmetric SC made of amorphous MnO2@MWCNT fibers delivered a volumetric capacitance (CV) of 10.9 F cm-3 at 0.1 A cm-3 and retained 6.9 F cm-3 at 5 A cm-3, together with an EV of 1.5 mWh cm-3 at the PV of 0.05 W cm-3. In addition, the fiber SC also presented outstanding stability over 15,000 charge/discharge cycles and mechanical robustness for 5000 bending/unbending cycles [Figure 5G].

    Hybridization

    Combining the merits of two materials via hybridization is highly desired for improving the performance of MnO2[101-104]. Zhu et al. developed a core-shell structure by depositing δ-MnO2 nanosheets on the outer surface of β-MnO2 via two-step hydrothermal reactions using MnOOH nanowires as the self-sacrificial template and KMnO4 as the oxidant [Figure 6A][105]. It was found from their experiments that the content of Mn3+ played a critical role in improving the electrical conductivity of MnO2 due to the double-exchange interaction forming Mn3+-O-Mn4+. The as-prepared MnO2 with a Mn3+/Mn4+ ratio of 0.69 exhibited a Cg of 306 F g-1 at 0.25 A g-1, retaining 226 F g-1 at 64 A g-1 and 207 F g-1 after cycling at 2 A g-1 for 3000 times. Liu et al. prepared a three-dimensional (3D) mesoporous architecture composed of MnO2/polyaniline (PANI) networks via the top-down exfoliation of δ-MnO2, the electrostatic attraction between the MnO2 nanosheets and aniline monomer and the chemical polymerization in the presence of (NH4)2S2O8[Figure 6B][106]. The hybrid exhibited a high conductivity of 0.08 S cm-1, which was slightly lower than that of neat PANI (0.09 S cm-1), due to the electrical interconnections formed by PANI. The as-obtained MnO2/PANI composite presented a high Cg of 762 F g-1 at 1 A g-1, retaining 587 F g-1 at 10 A g-1 and 578 F g-1 at 5 A g-1 over 8000 charge/discharge cycles.

    Figure 6. (A) Synthesis of β-MnO2/parallel birnessite core/shell nanorod and a diagram illustrating the enhanced utilization of Mn. Reproduced with permission[105]. Copyright 2018, American Chemical Society. (B) preparation of 3D network composed of MnO2 sheets and PANI chains. Reproduced with permission[106]. Copyright 2017, American Chemical Society. (C) fabrication of Ti3C2Tx/MnO2 NW composite paper. Reproduced with permission[107]. Copyright 2018, Wiley-VCH. (D) fabrication of GQD/MnO2 heterostructures. Reproduced with permission[113]. Copyright 2018, Wiley-VCH. GQDs: graphene quantum dots.

    Recently, a hybrid paper electrode with high flexibility was proposed by Zhou et al. through vacuum infiltrating a homogeneous suspension of MXene (Ti3C2Tx) nanosheets and MnO2 nanowires [Figure 6C][107]. The high conductivity of the 2D MXene sheets (over 8000 S cm-1) facilitated the electron transport of MnO2 during charge/discharge and prevented the aggregation of MnO2, thus ensuring the accessibility of the electrolyte ions. The optimized performance was obtained with a Ti3C2Tx/MnO2 mass ratio of six, delivering a Ca (areal capacitance) of 205 mF cm-2 (corresponding to a CV of 1025 F cm-3), 98% retention after cycling at 0.2 mA cm-2 for 10000 times and good bending/unbending robustness. Carbonaceous materials were also adopted to improve the electrochemical performance of MnO2 electrodes[89,108-112]. Jia et al.reported that graphene quantum dots (GQDs) were adopted using a plasma enhanced chemical vapor deposition process for the modification of MnO2 nanosheets via Mn-O-C bonds [Figure 6D][113]. An electric field was generated at the interface between the GQDs (~5.2 eV) and MnO2(~4.4 eV) owing to their different work functions, thereby providing a barrier for electronic transmission and enabling the extraction of free electrons from MnO2 and the subsequent accumulation at the edges of the GQDs until the Fermi levels were aligned. Consequently, the MnO2/GQD heterostructures demonstrated an expanded potential window of 1.3 V (vs. Hg/HgCl2) in 1 M Na2SO4, a Cg of 1170 F g-1 at 5 mV s-1 and good stability with 92.7% capacitance retained after 10,000 cycles.

    ZINC-ION BATTERIES

    Charge storage mechanisms

    Distinct from those described for SCs above, the charge storage mechanisms of MnO2 in aqueous ZIBs are relatively complicated, possibly involving the insertion/extraction of Zn2+, the co-insertion/extraction of H+ and Zn2+ and the reversible dissolution-deposition of MnO2/Mn2+[50]. Typically, although MnO2 with tunnel structures that can accommodate Zn2+, such as α-MnO2 (2 × 2 tunnels), β-MnO2 (1 × 1 tunnels) and γ-MnO2 (1 × 2 and 1 × 1 tunnels), are considered as promising cathodes for ZIBs, they suffer from irreversible phase conversions to layered structure, spinel structure (ZnMn2O4) or both during discharge/charge, as shown in Figure 7A[114,115]. Alfaruqi et al. reported the application of a hydrothermally synthesized α-MnO2 cathode for rechargeable ZIBs using a ZnSO4 aqueous solution as the electrolyte within a cell voltage of 1.0-1.8 V[116]. X-ray diffraction (XRD) and ex-situ synchrotron X-ray absorption spectroscopy measurements confirmed the reversible insertion and extraction of Zn2+ accompanied by the formation and decomposition of ZnMn2O4. Alternatively, Lee et al. proposed a reversible and electrochemically triggered phase conversion between α-MnO2 and layered Zn-birnessite (or Zn-buserite) upon the intercalation/deintercalation of Zn2+[117]. The phase conversion mechanism during Zn2+ insertion was ascribed to the partial dissolution of MnO2 in the electrolyte because of the Jahn-Teller effect. With the insertion of Zn2+, Mn4+ is reduced to Mn3+ Equation (14) and the gray bridge-like double chains of the Mn3+O6 units are gradually destroyed because of Mn2+ dissolution Equation (15), thereby forming a Zn-birnessite structure. Upon recharging, the dissolved Mn2+ can intercalate back and bridge the layers to tunnels with α-MnO2 completely recovered Equation (16)[114]:

    Figure 7. (A) Phase transition between Zn-birnessite and α-MnO2. Reproduced with permission[114]. Copyright 2018, Wiley-VCH. (B and C) reaction pathway of Zn insertion in prepared γ-MnO2 cathode. Reproduced with permission[118]. Copyright 2015, American Chemical Society. (D) Zn2+ intercalation/deintercalation mechanism for orthorhombic MnO2. Reproduced with permission[120]. Copyright 2021, Royal Society of Chemistry.

    Mn4+(s) + e- → Mn3+(s)    (14)

    2Mn3+(s) → Mn4+(s) + Mn2+(aq)     (15)

    Mn2+(aq) → Mn4+(s) + 2e-     (16)

    Distinctly, using ex-situ and synchrotron XRD and in-situ X-ray absorption near edge structure, Alfaruqi et al. unveiled that orthorhombic γ-MnO2 underwent a phase transformation upon the insertion of Zn2+ [Figure 7B] following Equations (17) and (18)[118]. During discharge, the oxidation state of Mn in γ-MnO2 was reduced from Mn4+ to Mn3+/Mn2+, accompanied with a structural transformation from an orthorhombic Mn4+ phase to a spinel-type Mn3+ phase (ZnMn2O4) and two intermediary soluble Mn2+ phases, namely, γ-ZnxMnO2 (tunnel type) and L-ZnyMnO2 (layered type), as a result of the electrochemical Zn intercalation [Figure 7C]. In fact, γ-MnO2 is composed of R-MnO2 (ramsdellite) and β-MnO2 (pyrolusite) phases in a slightly distorted hcp array of the oxygen anion sublattice. In contrast, the ZnMn2O4 and L-ZnyMnO2 phases share a cubic close-packing (ccp) array of the oxygen network. The intercalation of Zn2+ in γ-MnO2 led to an anisotropic expansion of the orthorhombic unit cell, transforming the hcp oxygen sublattice to a ccp structure[119]. It was noted that a small hump corresponding to the high intensity line of the spinel-type ZnMn2O4 phase (2θ of ~32°) appeared to be retained even after complete charging. This behavior clearly suggests that the spinel phase may not completely revert to the orthorhombic phase and most likely may contribute to some of the capacity loss observed during extended cycling.

    Zn2+ + 2e- + 2MnO2 → Zn2MnO4     (17)

    nZn2+ + 2xe- + MnO2 → ZnnMnO2     (18)

    δ-MnO2 with a typical layered structure has also been directly utilized as a cathode material for ZIBs. Li et al. investigated the electrochemical storage mechanism of a -MnO2 cathode using ex-situ XRD, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy[120]. The results, as schematically shown in Figure 7D, indicated that at the first discharge stage, birnessite Znx+yMnO2 was formed during the intercalation of Zn2+ into -MnO2 and then some Zn2+ ions were retained in the MnO2 interlayer channel (ZnxMnO2) in the subsequent charge/discharge. Therefore, the storage behavior was described as the interlayer diffusion of Zn2+ without any observations regarding the changes in crystal structure.

    The co-intercalation of H+ and Zn2+ is also an important mechanism for MnO2 cathodes in ZIBs. Liu et al. investigated the charge storage process of tunnel-structured MnO2 nanotubes pre-intercalated by K+(α-K0.19MnO2) in 3 M Zn(CF3SO3)2 and 0.2 M Mn(CF3SO3)2[121]. It was found that α-K0.19MnO2 underwent chemical conversion and cation interaction, as schematically shown in Figure 8A. The ex-situ XRD results demonstrated the reversible formation/decomposition of the Zn-buserite phase during the insertion/extraction of H+ and Zn2+ [Figure 8B and C], corresponding to the two platforms in the discharge/charge curve. Liu et al. studied the phase evolution of β-MnO2 upon electrochemical charge/discharge in 3 M ZnSO4 and 0.2 M MnSO4[122]. Ex-situ XRD suggested that the intercalation of H+ retained the crystal structure of β-MnO2 with the formation of Zn2(OH)2(SO4)(H2O)4, while the subsequent insertion of Zn2+ resulted in a partial phase transformation to spinel ZnMn2O4 [Figure 8D]. Taking the evidence from high-resolution transmission electron microscopy (HRTEM) and TEM [Figure 8E] into consideration, it was concluded that the irreversible formation of ZnMn2O4 after long-term cycling resulted in capacity fade for the β-MnO2 cathode. Li et al. revealed the storage mechanism of birnessite-type MnO2 in 2.0 M ZnSO4 and 0.5 M MnSO4 [Figure 8F][123]. When discharged from 1.9 to 1.5 V, the H+ intercalation reaction produced MnOOH Equation (19). Subsequently, with further discharge to 1.41 V, the Zn2+ insertion reaction Equation (20) led to the formation of Zn2Mn4O8·H2O. Finally, MnOOH and Zn2Mn4O8·H2O completely converted into Mn2+ and Zn4SO4(OH)6·5H2O with a Mn4+/Mn2+ two-electron pathway achieved Equations (21) and (22):

    Figure 8. (A) Co-intercalation of Zn2+ and H+ in α-K0.19MnO2. (B) voltage-time profile at 1 C and (C) corresponding XRD patterns (ex situ) of α-K0.19MnO2 during second discharge/charge cycle. The electrolyte contains Zn(CF3SO3)2 (3 M) and Mn(CF3SO3)2 (0.2 M). Reproduced with permission[121]. Copyright 2019, Royal Society of Chemistry. (D) GCD profile and corresponding XRD patterns (ex situ) of β-MnO2 at 0.05 C, suggesting the insertion/extraction of H+. (E) TEM/HRTEM observations of β-MnO2 nanofiber and schematic crystal structure (hetaerolite). Reproduced with permission[122]. Copyright 2019, Elsevier. (F) sequential insertion of H+ and Zn2+ during discharge process. Reproduced with permission[123]. Copyright 2020, Royal Society of Chemistry. (G) charge storage mechanism of MnO2 cathode involving dissolution-deposition process. Reproduced with permission[125]. Copyright 2020, Elsevier.

    MnO2 + H+ + e → MnOOH    (19)

    4MnO2 + 2Zn2+ + 4e + H2O → Zn2Mn4O8·H2O    (20)

    2MnOOH + SO42 + 4Zn2+ + 7H2O + 2e → Zn4SO4(OH)6·5H2O + 2Mn2+     (21)

    3Zn2Mn4O8·H2O + 8SO42 + 61H2O + 26Zn2+ + 12e → 8Zn4SO4(OH)6·5H2O + 12Mn2+     (22)

    In contrast, Jin et al. unveiled the successive intercalation of Zn2+ and H+, respectively, corresponding to the high and low voltage plateaus by exploring the electrochemical behavior of δ-MnO2 in a Zn(TFSI)2-based aqueous electrolyte[124]. Bulky TFSI- (vs. SO42-) can decrease the number of water molecules surrounding the Zn2+ cation and reduce the solvation effect, thus facilitating Zn2+ transport and charge transfer. Therefore, in the Zn(TFSI)2-based electrolyte, the non-diffusion-controlled mechanism dominates the first step of fast Zn2+ storage in bulk δ-MnO2 without a significant phase transition Equation (23), while the diffusion-controlled conversion reaction between H+ and MnO2 dominates the following step reactions Equations (24)-(26):

    MnO2 + xZn2+ + 2xe- ↔ ZnxMnO2 (non-diffusion controlled)    (23)

    H2O ↔ H+ + OH-     (24)

    MnO2 + H+ + e- ↔ MnOOH (diffusion controlled)    (25)

    3Zn2+ + 6OH- + Zn(TFSI)2 + xH2O ↔ Zn(TFSI)2[Zn(OH)2]3·xH2O    (26)

    Typically, the dissolution of Mn2+ and the generation/decomposition of Zn4SO4(OH)6·H2O (ZSH) observed in ZnSO4 electrolytes are regarded as side reactions for capacity fade. However, a recent study performed by Guo et al. proposed a dissolution-deposition mechanism in a Zn//MnO2 battery Equations (27)-(30) and confirmed the capacity contribution from the reversibly formed ZSH[125]. As shown in Figure 8G, α-MnO2 (or -MnO2) reacted with H2O to produce ZSH and Mn2+ in the first discharge process and then ZSH reacted with Mn2+ to form birnessite-MnO2 in the first charge process. Such a dissolution-deposition mechanism dominated the subsequent energy storage processes, with the newly formed birnessite-MnO2 as a host material contributing most of the specific capacity. In contrast, the intercalation/deintercalation of H+/Zn2+ in residual undissolved MnO2 is considered to contribute negligible capacity.

    3MnO2 + 6H2O + 6e- → 3Mn2+ + 12OH-     (27)

    3Mn2+ + 12OH- 6e- → 3birnessite-MnO2 + 6H2O    (28)

    3birnessite-MnO2 + 6H2O + 6e- ↔ 3Mn2+ + 12OH-     (29)

    12OH- + 2SO42- + 8Zn2+ + 8H2O ↔ 2Zn4(OH)6SO4·4H2O    (30)

    Performance enhancements

    Foreign ion/molecular pre-insertion

    One of the current issues for the development of Mn-based cathodes is the sluggish reaction kinetics caused by a high-energy barrier for Zn2+ migration due to the strong electrostatic interactions with the host material, as well as the serious structural transformation during cycling[64]. The incorporation of cations (e.g., K+, Na+, Zn2+ and Ca2+) in MnO2 via surface coordination has been proven to be an effective strategy for accelerating ion diffusion in tunnels or interlayer corridors, improving electrical conductivity and stabilizing the host structures[61,121,126,127]. Fang et al. synthesized potassium manganate (K0.8Mn8O16, KMO) nanorods via the intercalation of K+ into the tunnel cavities and used them as cathode materials for ZIBs[128]. It is reported that the steadily intercalated K+ via the K-O bonds in the tunnels of KMO can effectively suppress the dissolution of Mn [Figure 9A]. As a result, the KMO-based ZIB exhibited a capacity of over 300 mAh g-1 at 100 mA g-1, an energy density of 398 Wh kg-1 (based on the mass of the cathode) and impressive durability over 1000 cycles at 1 A g-1 (154 mAh g-1). In contrast, pristine α-MnO2 exhibited a significant capacity fade with only 50 mAh g-1 after 200 cycles. Anions (e.g., PO43) can also be inserted into host materials, leading to structural defects, narrow bandgaps and enhanced electrical conductivity[129]. Zhang et al. employed crosslinked vertical multilayer graphene (VMG) arrays as the skeleton for the uniform growth of MnO2 nanosheets[130]. The obtained MnO2@VMG shell/core arrays were subsequently phosphorized under an Ar flow at 200 °C in the presence of NaH2PO2·H2O to form P-MnO2-x@VMG [Figure 9B]. It was found that the phosphorization induced the intercalation of PO43- and oxygen defects in MnO2 simultaneously and expanded the interlayer spacing of (001) (from 0.68 to 0.70 nm), which facilitated ion transfer. The obtained P-MnO2-x@VMG cathode exhibited enhanced electrochemical performance in an aqueous electrolyte of 2 M ZnSO4 and 0.2 M MnSO4, delivering a capacity of 302.8 mAh g-1 at 0.5 A g-1, a high energy density of 369.5 Wh kg-1 and > 90% capacity retention after 1000 cycles at 2.0 A g-1. In comparison, MnO2@VMG provided 261.1 mAh g-1 at 0.5 A g-1 and 79.4% capacity retention after 1000 cycles at 2.0 A g-1.

    Figure 9. (A) Tunnel structure stabilized by K+ and cycling performance of KMO and α-MnO2 at 1 A g-1. Reproduced with permission[128]. Copyright 2019, Wiley-VCH. (B) synthesis of P-MnO2-x@VMG shell/core arrays. Reproduced with permission[130]. Copyright 2020, Wiley-VCH. (C) preparation and structural advantage of PANI-intercalated MnO2 nanolayers. Reproduced with permission[131]. Copyright 2018, Nature. (D) optimized structure of cw-MnO2 for Zn2+ intercalation and the corresponding relative energies. Reproduced with permission[54]. Copyright 2019, Royal Society of Chemistry. HT: hydrothermal method; TCS: triple-corner-sharing; KMO: K0.8Mn8O16; PP: phosphorization; VMG: vertical multilayer graphene.

    Alternatively, Huang et al. prepared mesoporous PANI-intercalated MnO2 nanosheets with a thickness of ~10 nm and expanded interlayer space of ~1.0 nm through the polymerization at the interface of the organic and aqueous phases [Figure 9C][131]. The oxidation-induced polymerization of aniline (in CCl4) and the reduction of MnO42- (in H2O) occurred simultaneously, thereby restricting the growth of MnO2 anisotropically and facilitating the layer-by-layer assembly of the 2D MnO2 and PANI. Since the guest polymer in the interlayer of the MnO2 nanosheets efficiently prevented phase transformation and strengthened the layered structure during repeated insertion/extraction of hydrated cations, a reversible discharge capacity of 280 mAh g-1 at 200 mA g-1 was achieved with 110 mAh g-1 retained even at 3 A g-1. Compared to its monovalent counterparts, Zn2+ requires high energy for desolvation at the electrode-electrolyte interface, thereby imposing an additional energy penalty for its facile intercalation[132]. In addition, the strong electrostatic interaction between Zn2+ and the host frameworks leads to the sluggish diffusion of Zn2+[82]. Nam et al. demonstrated that the interlayer crystal water can effectively screen the electrostatic interactions between Zn2+ and the host, thus facilitating Zn2+ diffusion[54]. Layered MnO2 containing crystal water in the interlayer space (cw-MnO2) was prepared through electrochemical transformation from spinel-Mn3O4 in a 1 M MgSO4 solution because the insertion of H3O+ is far more favorable over Mg2+. DFT calculations revealed that Zn2+ prefers octahedrally coordinated triple-corner-sharing (TCS) sites with three H2O molecules by forming a tridentate bond with a Zn-Mn dumbbell structure [Figure 9D]. Therefore, upon intercalation, Zn2+ ions tend to migrate together with the coordinate water molecules because of the strong Zn-H2O coordination bond, which effectively weakens the electrostatic interaction between Zn2+ and MnO2. As a result, cw-MnO2 exhibited high reversible capacities of 350 mAh g-1 at 100 mA g-1, 154 mAh g-1 at 3 A g-1 and 116 mAh g-1 after 200 cycles at 3 A g-1. A similar observation was reported by Wang et al., where H2O molecules not only stabilize the intercalated Na+ in the structure (Na0.44Mn2O4·1.5H2O) but also promote the intercalation of Zn2+ during discharge in ZIBs[133].

    Defect engineering

    Defect engineering is another widely adopted method to enhance the performance of MnO2 by tuning the electronic structure, enhancing the structural robustness and adjusting the interaction between the host and Zn2+[134,135]. Oxygen vacancies (VO) have proven their capability in gauging the adsorption/desorption of Zn2+[93]. For example, Xiong et al. proposed the use of oxygen-deficient δ-MnO2 (VO-MnO2) as cathode materials for ZIBs[136]. The storage mechanisms of Zn2+ in VO-MnO2 involve the insertion/extraction of Zn2+ into the interlayer spacing, surface redox reaction of xZn2+ + 2xe- + MnO2 → MnOOZnx and the formation of electric double layers [Figure 10A]. The calculated Gibbs free energies of Zn2+ adsorption at the vicinity sites to VO are close to thermoneutral values of ~0.05 eV [Figure 10B], suggesting a weakened bonding strength between Zn and O due to the diminished charge transfer. Therefore, more reversible adsorption/desorption of Zn2+ can be achieved for VO-MnO2 rather than pristine MnO2. As a result, VO-MnO2 in an aqueous electrolyte of 1 M ZnSO4 and 0.2 M MnSO4 delivered a specific capacity of 345 mAh g-1 without any decay after 100 cycles at 0.2 A g-1 [Figure 10C]. The Zn-ion storage capability of MnO2 can also be improved by heteroatom doping[137]. Zhao et al. hydrothermally synthesized Ni-doped MnO2 with a stoichiometry of Ni0.052K0.119Mn0.948O2-0.208H2O (NKMO)[138]. The reference sample prepared without Ni(NO3)2 was KMO. The presence of the Ni-induced tetragonal-orthorhombic lattice distortion can mediate the cooperative motion of H+ via the formation of hydrogen bonds similar to Grotthuss proton hopping in water [Figure 10D]. The electrochemical performance of NKMO and KMO cathodes was evaluated using a Zn plate as an anode in a 3 M ZnSO4 and 0.2 M MnSO4 aqueous electrolyte. The discharge capacity of the Zn/NKMO cell was ~303 mAh g-1, which was ~29% higher than that of Zn/KMO (~235 mAh g-1). As shown in Figure 10E, the long-term cycling stability of the Zn/KMO and Zn/NKMO cells examined at 4 C indicated that NKMO showed a slight sacrifice in capacity (73.6% for KMO vs. 71.4% for NKMO after 2000 cycles).

    Figure 10. (A) Zn2+ storage mechanism for VO-MnO2. (B) comparison of adsorption energies for Zn2+ on δ-MnO2 and VO-MnO2. (C) cycling performance of VO-MnO2 at 0.2 A g-1. Reproduced with permission[136]. Copyright 2019, Wiley-VCH. (D) Zn2+ storage in Ni-doped MnO2 regulated by TO distortion. (E) cycling performance of Ni-doped MnO2 at 4 C. Reproduced with permission[138]. Copyright 2021, Wiley-VCH. (F) fabrication of N-MnO2-x@TiC/C arrays. (G) cycling capability of MnO2-x@TiC/C and N-MnO2-x@TiC/C at 1.0 A g-1. Reproduced with permission[140]. Copyright 2019, Wiley-VCH. (H) structure of A-MnO2-δ and in-situ XRD patterns during second charge/discharge cycle. (I) cycling performance of A-MnO2-δ at 1 A g-1. Reproduced with permission[141]. Copyright 2020, Elsevier. KMO: K0.8Mn8O16; NKMO: Ni0.052K0.119Mn0.948O2-0.208H2O.

    Alternatively, Wang et al. reported a Ce-doped MnO2 nanorod-like electrode material synthesized by a hydrothermal method[139]. Cerium doping induced a structural transformation of MnO2 from its original β-phase to the α-phase, along with the appearance of a [2 × 2] tunnel structure. Its electrochemical properties were investigated using zinc as an anode, 2 M ZnSO4 and 0.1 M MnSO4 as an electrolyte, glass fiber as a separator and the prepared materials as the cathode. Compared to β-MnO2, 0.1 mmol Ce doping displayed a higher initial capacity and Coulombic efficiency of 134 mAh g-1 at 5 C and 82%. More importantly, after 100 cycles, the capacity retention of 0.1 mmol Ce doping was almost twice as high as that of β-MnO2 at such a high current density. When the rate was in the range of 1-5 C, the 0.1 mmol Ce-doped electrode had high discharge capacities, indicating a better rate capability (308 mAh g-1). Additionally, the galvanostatic intermittent titration technique (GITT) was used to calculate the chemical diffusion coefficient of Zn2+ in the two electrode materials. The diffusion coefficient of Zn2+ was improved by one to two orders of magnitude for the 0.1 mmol Ce-doped cathode than the β-MnO2 cathode. Moreover, Zhang et al. simultaneously introduced oxygen vacancies and heteroatom doping into MnO2 to form a N­doped MnO2-x (N­MnO2-x) branch on the surface TiC/C nanorod array (N­MnO2-x@TiC/C) for ZIBs [Figure 10F][140]. DFT calculations indicated that the combination of oxygen vacancies and N doping greatly improved the conductivity of MnO2 with a much narrower bandgap (0.1 eV). The electrochemical performance of N-MnO2-x@TiC/C was studied in an aqueous solution of 2 M ZnSO4 and 0.2 M MnSO4. The designed N­MnO2-x@TiC/C electrode showed improved Zn2+ storage performance with faster reaction kinetics, higher capacity (285 mAh g-1 at 0.2 A g-1) and excellent cycling performance. After 1000 cycles at 1 A g-1, the N-MnO2-x@TiC/C electrode delivered a reversible capacity of 172.7 mAh g-1 with a capacity retention of 84.7% [Figure 10G], higher than its MnO2@TiC/C counterpart with a reversible capacity of 84.8 mAh g-1 (capacity retention of 55.6%).

    Amorphous structures with disordered atomic arrangements and abundant structural defects that may lead to enhanced ion diffusion kinetics, improved capacity and alleviated volume expansion are considered to be promising cathode candidates for ZIBs[55,90]. Cai et al. demonstrated the feasibility of amorphous MnO2(A-MnO2-δ, δ was estimated to be 0.12) in ZIBs and studied the structural evolution of A-MnO2-δ in 2 M ZnSO4 and 0.2 M MnSO4[141]. In-situ XRD profiles were collected at different states in the second discharge/charge cycle [Figure 10H]. Accordingly, the Zn2+ storage in A-MnO2-δ can be divided into four stages: (I: 1.85-1.30 V) no observable peak was found, suggesting that the cation insertion in the A-MnO2-δ did not change the amorphous structure or generate a new crystalline structure; (II: 1.3-1.0 V) layered Zn4SO4(OH)6·5H2O was generated, as evidenced by a new set of diffraction peaks at 8.1°, 16.2°, 24.4°, 32.7°, 33.5°, 34.9°, 35.3°, 36.3° and 38.6° (JCPDS No. 39-0688); (III: 1.0-1.5 V) the peaks of Zn4SO4(OH)6·5H2O became weakened gradually and completely disappeared at a capacity of 157.3 mAh g-1; (IV: 1.50-1.85 V) no other peaks appeared, suggesting ion extraction from the amorphous Mn-based compound. The A-MnO2-δ cathode exhibited a reversible capacity of 147 mAh g-1 at 1 A g-1 with a capacity retention of 78% after 1000 cycles [Figure 10I].

    Hybridization

    Hybridization has been confirmed as a useful method for improving the performance of MnO2 in ZIBs[142-146]. Li et al. anchored MnO2 particles on N-doped hollow carbon spheres (NHCSs@MnO2) via a direct reaction between carbon and KMnO4 [Figure 11A][147]. The hollow porous carbon nanospheres provided large interfaces, thus ensuring the fast transport of ions/electrons. The obtained NHCSs@MnO2 exhibited improved performance for Zn2+ storage in 2 M ZnSO4 and 0.1 M MnSO4 with an excellent reversible capacity of ~206 mAh g-1 and a retention ratio of 89.5% after 200 cycles at 0.1 A g-1 [Figure 11B]. Zhang et al. used a CNT network as a conductive scaffold on which conformable MnO2 sheath and rough poly(3, 4-ethylenedioxythiophene) (PEDOT) protective layers were deposited sequentially to form a binder-free CNT/MnO2/PEDOT (CMOP) electrode[Figure 11C][148]. CNT/MnO2 (CMO) was also prepared as a control sample. The PEDOT layers prevented MnO2 from dissolution, which effectively boosted the cycling life of the battery. The electrochemical behavior of MnO2, MnO2/PEDOT, CMO and CMOP was evaluated in 2 M ZnCl2 and 0.4 M MnSO4. As demonstrated, the CMOP cathode delivered a maximum specific capacity of 306.1 mAh g-1 at 1.1 A g-1 and 176.8 mAh g-1 at 10.8 A g-1 and good long-term stability with a capacity retention of 81.3% after 2000 charge/discharge cycles [Figure 11D].

    Figure 11. (A) Synthesis of SiO2@phenol-formaldehyde resin (RF), SiO2@RF@polydopamine (PDA), NHCSs and NHCSs@MnO2 composites using tetraethoxysilane (TEOS) as Si source. (B) cyclic performance of NHCSs@MnO2 at 100 mA g-1 for 200 cycles. Reproduced with permission[147]. Copyright 2020, Elsevier. (C) 3D structure of CNT conductive networks and fabrication of CMOP cathodes. (D) long-term cycling performance of CMO and CMOP electrodes. Reproduced with permission[148]. Copyright 2019, Wiley-VCH. (E) synthesis and structural analysis of K-V2C@MnO2 and calculated absorption energies for Zn2+ on the surfaces of perfect MnO2-V2C and δ-MnO2. Reproduced with permission[149]. Copyright 2021, American Chemical Society. (F) preparation of MnO2/PPy nanorod. Reproduced with permission[150]. Copyright 2020, Elsevier. CNT: carbon nanotube; CMO: CNT/MnO2; CMOP: CNT/MnO2/PEDOT; NHCS: N-doped hollow carbon sphere; PEDOT: poly(3,4-ethylenedioxythiophene).

    Alternatively, Zhu et al. hybridized MnO2 nanosheets with a V2CTx MXene (K-V2C@MnO2) by K+ intercalation, followed by a hydrothermal growth strategy [Figure 11E][149]. It was found that the adsorbed K+ on the V2C surface facilitated the growth of K-birnessite MnO2, hydrogen bonds existed between MnO2 (O) and K-V2C (H from functional groups), and the Gibbs free energy of Zn2+ adsorption in V2C@MnO2 was lowered in comparison with pristine δ-MnO2. The performance of K-V2C@MnO2 was evaluated in 2.0 M ZnSO4 and 0.25 M MnSO4. A remarkable capacity of 408.1 mAh g-1 at 0.3 A g-1 was achieved and 119.2 mAh g-1 was retained after 10,000 cycles at 10 A g-1. Huang et al. attempted to coat a thin layer of polypyrrole (PPy) to slow the interfacial electrochemical kinetics and prevent the dissolution of MnO2 [Figure 11F][150]. Interestingly, strong Mn-N bonds were found at the interface of MnO2 and PPy. DFT calculations revealed that the energy barrier for Mn escape was increased to 6.76 eV owing to the Mn-N bond. The performance of MnO2 and MnO2/PPy was tested in 2 M ZnSO4 and 0.1 M MnSO4. In comparison with MnO2(133 mAh g-1), MnO2/PPy provided 256 mAh g-1 after 50 cycles at 100 mA g-1 without capacity fade.

    SUMMARY AND OUTLOOK

    Summary

    This review focuses on the applications of MnO2 for aqueous energy storage (SCs and ZIBs) and summarizes and compares the similarities and differences of the corresponding charge storage mechanisms and the principles for materials modification [Table 1].

    Table 1

    Similarities and differences in using MnO2 as electrode material in SCs and ZIBs

    ClassificationMechanismsModification strategies
    SCs☆ Surface redox reaction
    ☆ Bulk intercalation
    (no phase transition)
    • Nanostructural design
    • Foreign ion/molecular pre-insertion
    • Defect engineering
    • Hybridization
    ZIBs☆ Insertion/extraction of Zn2+
    ☆ Co-insertion of H+ and Zn2+
    ☆ Dissolution-deposition
    (with phase transition)
    • Foreign ion/molecular pre-insertion
    • Defect engineering
    • Hybridization

    Charge storage mechanisms

    In the case of SCs, the surface redox reactions and bulk intercalation of electrolyte ions are the two dominating mechanisms for charge storage in MnO2 Equations (10) and (11). The pH of the electrolyte severely affects the electrochemical reactions during charge/discharge. In an acidic solution, an irreversible reaction occurs between H+ and MnOOH, forming dissoluble Mn2+. In an alkaline solution, a passivation layer of Mn(OH)2 is generated as a result of the reaction between MnO2 and OH-. In a neutral electrolyte, the disproportionation of Mn3+ to dissoluble Mn2+ is usually observed. Considering the chemical stability of the zinc metal anode, a near-neutral electrolyte is usually selected (e.g., ZnSO4 aqueous solution) for ZIBs. Three possible mechanisms are proposed: (i) intercalation/deintercalation of Zn2+; (ii) co-insertion/extraction of H+/Zn2+; and (iii) dissolution-deposition of MnO2/Mn2+. Although it is still under debate, the charge storage mechanism generally involves the insertion of H+ (fast kinetics) to generate MnOOH, the insertion of Zn2+ (slow kinetics) to generate Zn2MnO4 and the irreversible deposition of ZSH on the surface of electrodes. Indeed, the existing works on designing MnO2 cathodes for ZIBs are very similar to the explorations of MnO2-based SCs using near-neutral electrolytes (zinc salt-based aqueous electrolyte, e.g., ZnSO4, ZnCl2, ZnTFSI or Zn(CF3SO3)2). However, different from the monovalent cations mostly investigated in SCs, Zn2+ has a larger hydrated ionic radius, higher desolvation energy barrier and stronger electrostatic interaction with MnO2, all of which induce complex electrode reactions and structural instability in MnO2 cathodes for ZIBs.

    MnO2 modification methods

    Low electrical conductivity, structural instability and slow electrode reaction kinetics are common issues of MnO2 as electrodes for energy storage. Based on our summary, similar strategies have been adopted for MnO2 modification in both SCs and ZIBs, including foreign ion/molecular pre-insertion, defect engineering, nanostructural design and hybridization. Typically, the pre-inserted ions/molecules in MnO2 facilitate the ion intercalation/deintercalation by expanding the interlayer spaces, shield the strong electrostatic interaction between intercalated ions and MnO2 and stabilize the crystal structure of MnO2 during ion intercalation/deintercalation. The introduction of oxygen vacancies and heteroatom doping (defect engineering) are effective methods to regulate the electron structure of MnO2, which significantly affect the electron transport during the redox reaction. Rational nanostructural design contributes to the accessibility of electrolyte ions to active sites, thereby improving the capacity of the electrode. Hybridizing with high conductive carbonaceous materials (e.g., CNTs and rGO) can efficiently improve the conductivity of bare MnO2, thus boosting the kinetics of the electrode reaction.

    Outlook

    Although significant efforts have been devoted to the investigation of MnO2 for potential applications in SCs and ZIBs and there are several prospective aspects that need to be considered:

    (1) MnO2 is considered as one of the most promising candidates for SCs because of its low cost, earth abundance, high theoretical capacitance and environmental benignness. Nevertheless, the performance of MnO2 in SCs is far from satisfactory. An in-depth understanding of the underlying mechanisms and efficient strategies for performance improvements are highly required.

    (2) To date, the mechanism for Zn2+ storage is still under debate and severely depends on the testing conditions. More efforts are required by employing advanced characterization techniques.

    (3) The introduction of oxygen vacancies and heteroatoms is deemed to be an effective method to regulate the properties of MnO2 for enhancing the charge storage in both SCs and ZIBs. However, since MnO2 crystals are composed of MnO6 octahedral units, it remains necessary to quantitatively identify the concentration and position of oxygen vacancies and heteroatoms that play a critical role in governing MnO2 performance.

    (4) The pre-intercalation of cations, water molecules and polymers has been demonstrated to be an effective strategy to enhance the capacitance/capacity and stability of MnO2. The mechanisms for performance enhancement need to be further investigated.

    (5) Hybridization with conductive materials (e.g., graphene and MXenes) is considered a feasible method to efficiently improve the electron transfer/transport of MnO2. In addition, the interfacial properties between two materials are believed to contribute the performance enhancement. However, a detailed explanation of interface-dependent electrochemical behavior is still required.

    (6) Finally, more attention should be paid to the performance decay mechanisms of MnO2 in SCs and ZIBs, which are the guidelines for future materials design.

    DECLARATIONS

    Authors’ contributions

    Preparing the manuscript draft: Dai H, Zhou R

    Writing-review: Zhang Z

    Editing: Zhou J

    Funding acquisition, supervision: Sun G

    Availability of data and materials

    Not applicable.

    Financial support and sponsorship

    This work was supported by the National Natural Science Foundation of China (No. 21975123), Anhui Provincial Key R&D Program (No. 2022i01020021) and Six Talent Peaks Project in Jiangsu Province (No. XCL-024).

    Conflicts of interest

    All authors declared that there are no conflicts of interest.

    Ethical approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Copyright

    © The Author(s) 2022.

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    Cite This Article

    OAE Style

    Dai H, Zhou R, Zhang Z, Zhou J, Sun G. Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences. Energy Mater 2022;2:200040. http://dx.doi.org/10.20517/energymater.2022.56

    AMA Style

    Dai H, Zhou R, Zhang Z, Zhou J, Sun G. Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences. Energy Materials. 2022; 2(6):200040. http://dx.doi.org/10.20517/energymater.2022.56

    Chicago/Turabian Style

    Dai, Henghan, Ruicong Zhou, Zhao Zhang, Jinyuan Zhou, Gengzhi Sun. 2022. "Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences" Energy Materials. 2, no.6: 200040. http://dx.doi.org/10.20517/energymater.2022.56

    ACS Style

    Dai, H.; Zhou R.; Zhang Z.; Zhou J.; Sun G. Design of manganese dioxide for supercapacitors and zinc-ion batteries: similarities and differences. Energy Mater. 20222, 200040. http://dx.doi.org/10.20517/energymater.2022.56

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