A layered δ-MnO 2 nanoflake cathode with high zinc-storage capacities for eco-friendly battery applications

Unravelling the effect of crystal lattice compression on the supercapacitive performance of hydrothermally grown nanostructured hollandite α-MnO2 induced by incremental growth time

Manganese oxide (α-MnO2) nanoparticles are highly recognised for their use in supercapacitor applications. This study demonstrates the successful synthesis of flower-like and nanorods hollandite α-MnO2 by a simple one-pot hydrothermal technique at various reaction times. The synthesised nanoparticles were characterised by various physicochemical and electrochemical characterisation techniques. The influence of the various reaction times on the structural and morphological properties was evaluated by X-ray diffraction (XRD) and scanning electron microscope. XRD patterns revealed that the synthesized MnO2 nanoparticles are tetragonal structures with crystallite sizes ranging from 13.69 to 20.37 nm estimated from the Williamson-Hall method. Moreover, the functional groups and surface area were examined by Fourier transform infrared spectroscopy and Bruner-Emmert-Teller, respectively. Furthermore, the compositional elements were studied by X-ray photoemission spectroscopy and energy-dispersive X-ray spectroscopy. Finally, the electrochemical performances were studied using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The GCD characteristics revealed that the optimised α-MnO2 has a good capacitive behaviour, which predicts the potential application in energy storage. Electrochemical studies revealed that the 3 h-MnO2 sample exhibited a superior electrochemical behaviour and demonstrated a high specific capacitance of 132 F/g at a current density of 1A/g.

Se-dopant Modulated Selective Co-Insertion of H+ and Zn2+ in MnO2 for High-Capacity and Durable Aqueous Zn-Ion Batteries

MnO2 is commonly used as the cathode material for aqueous zinc-ion batteries (AZIBs). The strong Coulombic interaction between Zn ions and the MnO2 lattice causes significant lattice distortion and, combined with the Jahn-Teller effect, results in Mn2+ dissolution and structural collapse. While proton intercalation can reduce lattice distortion, it changes the electrolyte pH, producing chemically inert byproducts. These issues greatly affect the reversibility of Zn2+ intercalation/extraction, leading to significant capacity degradation of MnO2. Herein, we propose a novel method to enhance the cycling stability of δ-MnO2 through selenium doping (Se-MnO2). Our work indicates that varying the selenium doping content can regulate the intercalation ratio of H+ in MnO2, thereby suppressing the formation of ZnMn2O4 by-products. Se doping mitigates the lattice strain of MnO2 during Zn2+ intercalation/deintercalation by reducing Mn-O octahedral distortion, modifying Mn-O bond length upon Zn2+ insertion, and alleviating Mn dissolution caused by the Jahn-Teller effect. The optimized Se-MnO2 (Se concentration of 0.8 at.%) deposited on carbon nanotube demonstrates a notable capacity of 386 mAh g-1 at 0.1 A g-1, with exceptional long-term cycle stability, retaining 102 mAh g-1 capacity after 5000 cycles at 3.0 A g-1.

Homogeneous Deposition of Zinc on N-Doped Carbon Fibers Interconnected with Sn Nanoparticles for Advanced Aqueous Zinc Batteries

Currently, inhomogeneous distribution of Zn2+ on the surface of the Zn anode is still the essential reason for dendrite formation and unsatisfactory stability of zinc ion batteries. Given the merits of strong interaction between Sn and Zn, as well as a low nucleation barrier during Zn deposition, the combination of metallic Sn with carbon material is expected to improve the deposition of zinc ions and inhibit the growth of zinc dendrites by guiding the homogeneous plating/stripping of zinc on the electrode surface. In this article, zincophilic Sn nanoparticles with low nucleation barriers and strong interaction with Zn2+ were embedded into 3D N-doped carbon nanofibers using a simple electrostatic spinning technique. Accordingly, when serving as an artificial coating layer for the zinc metal anode, an ultrastable Sn@NCNFs@Zn||Sn@NCNFs@Zn symmetric cell can be achieved for over 3500 h with a low nucleation overpotential of 29.1 mV. Significantly, the full cell device assembled with the as-prepared anode and MnO2 cathode exhibits desirable electrochemical behaviors. Moreover, this simple method could be extended to other metal-carbon composites, and to ensure ease in scaling up as required. Such significant approach can provide an effective strategy for the design of high-performance zinc anodes.

Recent Progress of Low-Dimensional Metal-Organic Frameworks for Aqueous Zinc-Based Batteries

Aqueous zinc-based batteries (AZBs) are promising energy storage solutions with remarkable safety, abundant Zn reserve, cost-effectiveness, and relatively high energy density. However, AZBs still face challenges such as anode dendrite formation that reduces cycling stability and limited cathode capacity. Recently, low-dimensional metal-organic frameworks (LD MOFs) and their derivatives have emerged as promising candidates for improving the electrochemical performance of AZBs owing to their unique morphologies, high structure tunability, high surface areas, and high porosity. However, clear guidelines for developing LD MOF-based materials for high-performance AZBs are scarce. In this review, the recent progress of LD MOF-based materials for AZBs is critically examined. The typical synthesis methods and structural design strategies for improving the electrochemical performance of LD MOF-based materials for AZBs are first introduced. The recent noteworthy research achievements are systematically discussed and categorized based on their applications in different AZB components, including cathodes, anodes, separators, and electrolytes. Finally, the limitations are addressed and the future perspectives are outlined for LD MOFs and their derivatives in AZB applications. This review provides clear guidance for designing high-performance LD MOF-based materials for advanced AZBs.

Seed-Assisted Reversible Dissolution/Deposition of MnO2 for Long-Cyclic and Green Aqueous Zinc-Ion Batteries

Manganese oxide (MnO2) based aqueous zinc-ion batteries (AZIBs) are considered to be a promising battery for grid-scale energy storage. However, they usually suffer from the great challenge of capacity attenuation due to Mn dissolution and irreversible structural transformation. Herein, full use of the shortcomings is made to design high-performance cathode-free AZIBs. Manganese-based Prussian blue analog (Mn-PBA) is selected as a seed layer to provide a stable MnO2 electrodeposition surface. Thanks to the large specific surface area and manganophilic nature of Mn-PBA, the deposition/dissolution kinetics between Mn2+ and MnO2 are significantly enhanced. Systematic studies revealed the mechanism of MnO2 deposition-dissolution related to the reversible transformation of manganese oxide hydroxide and zinc hydroxide sulfate hydrate. Based on this, the developed cathode-free AZIBs exhibit outstanding rate performance (with a specific capacity of 273.7 mAh g-1 at 1 A g-1) and extraordinary cycle stability (maintaining a specific capacity of 52.3 mAh g-1 after 50 000 cycles at 20 A g-1). Furthermore, the AZIBs with non-toxic, biocompatible materials can be directly discarded after use, without causing pollution to the environment, which is expected to help achieve the sustainable development goals.

Inducing Mn defects within MnTiO3 cathode for aqueous zinc-ion batteries

Layered manganese-based cathode materials are considered as one of the promising cathodes benefit from inherent low manufacturing cost, non-toxic and high safety in aqueous zinc-ion batteries (AZIBs). However, the sluggish reaction kinetics within layered cathodes is inevitable due to the poor electrical/ionic conductivity. Herein, MnTiO3 is reported as a new cathode material for AZIBs and in-situ induced Mn-defect within MnTiO3 during the first charging is desirable to improve the reaction kinetics to a great extent. Additionally, DFT calculations further demonstrate that MnTiO3 with manganese defects exhibits a uniform charge distribution at the defect sites, enhancing the attraction towards H+ and Zn2+ ions. Furthermore, it performs good cycling stability which can obtain 115 mA h g-1 even at 400 mA g-1 after 450 cycles and the discharge capacity reaches up to 233.8 mAh/g at 100 mA g-1 when Mn-defect MnTiO3 was employed as the cathode. This research could provide a new method for the development and mechanism research of cathode materials for AZIBs.

Structure regulation and synchrotron radiation investigation of cathode materials for aqueous Zn-ion batteries

In view of the advantages of low cost, environmental sustainability, and high safety, aqueous Zn-ion batteries (AZIBs) are widely expected to hold significant promise and increasingly infiltrate various applications in the near future. The development of AZIBs closely relates to the properties of cathode materials, which depend on their structures and corresponding dynamic evolution processes. Synchrotron radiation light sources, with their rich advanced experimental methods, serve as a comprehensive characterization platform capable of elucidating the intricate microstructure of cathode materials for AZIBs. In this review, we initially examine available cathode materials and discuss effective strategies for structural regulation to boost the storage capability of Zn2+. We then explore the synchrotron radiation techniques for investigating the microstructure of the designed materials, particularly through in situ synchrotron radiation techniques that can track the dynamic evolution process of the structures. Finally, the summary and future prospects for the further development of cathode materials of AZIBs and advanced synchrotron radiation techniques are discussed.

Superior Cyclic Stability and Capacitive Performance of Cation- and Water Molecule-Preintercalated δ-MnO2/h-WO3 Nanostructures as Supercapacitor Electrodes

The large number of active sites in the layered structure of δ-MnO2 with considerable interlayer spacing makes it an excellent candidate for ion storage. Unfortunately, the δ-MnO2-based electrode has not yet attained the exceptional storage potential that it should demonstrate because of disappointing structural deterioration during periodic charging and discharging. Here, we represent that stable Na ion storage in δ-MnO2 may be triggered by the preintercalation of K ions and water molecules. Furthermore, the sluggish reaction kinetics and poor electrical conductivity of preintercalated δ-MnO2 layers are overcome by the incorporation of h-WO3 in the preintercalated δ-MnO2 to form novel composite electrodes. The composites contain mixed valence metals, which provide a great number of active sites along with improved redox activity, while maintaining a fast ion transfer efficiency to enhance the pseudocapacitance performance. Based on our research, the composite prepared from preintercalated δ-MnO2 with 5 wt % h-WO3 provides a specific capacitance of up to 363.8 F g-1 at a current density of 1.5 A g-1 and an improved energy density (32.3 W h kg-1) along with an ∼14% increase in capacity upon cycling up to 5000 cycles. Hence, the interaction between the preintercalated δ-MnO2 and h-WO3 nanorods results in satisfactory energy storage performance due to the defect-rich structure, high conductivity, superior stability, and lower charge transfer resistance. This research has the potential to pave the way for a new class of hybrid supercapacitors that could fill the energy gap between chemical batteries and ideal capacitors.

Recent development of manganese dioxide-based materials as zinc-ion battery cathode

The development of advanced cathode materials for zinc-ion batteries (ZIBs) is a critical step in building large-scale green energy conversion and storage systems in the future. Manganese dioxide is one of the most well-studied cathode materials for zinc-ion batteries due to its wide range of crystal forms, cost-effectiveness, and well-established synthesis processes. This review describes the recent research progress of manganese dioxide-based ZIBs, and the reaction mechanism, electrochemical performance, and challenges of manganese dioxide-based ZIBs materials are systematically introduced. Optimization strategies for high-performance manganese dioxide-based materials for ZIBs with different crystal forms, nanostructures, morphologies, and compositions are discussed. Finally, the current challenges and future research directions of manganese dioxide-based cathodes in ZIBs are envisaged.

Oxygen vacancies in MnOx regulating reaction kinetics for aqueous zinc-ion batteries

MnO2 cathode materials have presented challenges due to their poor conductivity, unstable structure, and sluggish diffusion kinetics for aqueous zinc-ion batteries (AZIBs). In this study, a nanostructured MnOx cathode material was synthesized using an acid etching method, Which introduced abundant Mn(III) sites, resulting in the formation of numerous oxygen vacancies. Comprehensive characterizations revealed that these oxygen vacancies facilitated the reversible adsorption/desorption of Zn2+ ions and promoted efficient electron transfer. In addition, the designed mesoporous structure offered ample active sites and shortened the diffusion path for Zn2+ and H+ ions. Consequently, the nanosized MnOx cathode exhibited enhanced reaction kinetics, achieving a considerable reversible specific capacity of 388.7 mAh/g at 0.1 A/g and superior durability with 72.0% capacity retention over 2000 cycles at 3.0 A/g. The material delivered a maximum energy density of 639.7 Wh kg-1 at 159.94 W kg-1. Furthermore, a systematic analysis of the zinc storage mechanism was performed. This work demonstrates that engineering oxygen vacancies with nanostructure regulation provides valuable insights into optimizing MnO2 cathode materials for AZIBs.

Inhibition of phase transition from δ-MnO2 to α-MnO2 by Mo-doping and the application of Mo-doped MnO2 in aqueous zinc-ion batteries

MnO2 is an oxide with many crystalline phases and is often used as a cathode material for aqueous zinc-ion batteries. However, its poor electrical conductivity and structural instability limit its further application. In the present work, Mo-doped MnO2 microflowers are successfully prepared by a facile hydrothermal method. Interestingly, it is found that the doping of Mo inhibits the phase transition from δ-MnO2 to α-MnO2, which may be related to the low crystallinity of Mo doped MnO2. Compared with undoped MnO2, Mo-doped MnO2 maintains two-dimensional morphology with a large specific surface area and mesoporous structure. In addition, the electronic conductivity and reversibility of Zn2+ insertion/extraction are improved in Mo doped MnO2. Therefore, Mo-doped MnO2 exhibits high reversible capacity and long cycling stability. For example, a high reversible capacity of 72.6 mA h g-1 can be achieved at a current density of 2000 mA g-1 after 2500 cycles.

Utilization of 2D materials in aqueous zinc ion batteries for safe energy storage devices

Aqueous rechargeable battery has been an intense topic of research recently due to the significant safety issues of conventional Li-ion batteries (LIBs). Amongst the various candidates of aqueous batteries, aqueous zinc ion batteries (AZIBs) hold great promise as a next generation safe energy storage device due to its low cost, abundance in nature, low toxicity, environmental friendliness, low redox potential, and high theoretical capacity. Yet, the promise has not been realized due to their limitations, such as lower capacity compared to traditional LIB, dendrite growth, detrimental degradation of electrode materials structure as ions intercalate/de-intercalate, and gas evolution/corrosion at the electrodes, which remains a significant challenge. To address the challenges, various 2D materials with different physiochemical characteristics have been utilized. This review explores fundamental physiochemical characteristics of widely used 2D materials in AZIBs, including graphene, MoS2, MXenes, 2D metal organic framework, 2D covalent organic framework, and 2D transition metal oxides, and how their characteristics have been utilized or modified to address the challenges in AZIBs. The review also provides insights and perspectives on how 2D materials can help to realize the full potential of AZIBs for next-generation safe and reliable energy storage devices.

Manganese vacancies and tunnel pillars synergistically improve the electrochemical performance of MnO2 in aqueous Zn ion batteries

High-oxidation niobium was used for the first time in manganese dioxide cation doping to reduce the diffusion resistance of zinc ions, in order to improve its kinetic and electrochemical properties. The results show that using a simple hydrothermal process, all niobium ions were doped into the manganese dioxide lattice. As niobium(v) was incorporated into the [2 × 2] tunnel of α-MnO2, it induced manganese vacancies, which reduced the diffusion resistance of Zn2+ in manganese dioxide, improving the migration kinetics. It acted as a tunnel pillar, avoiding the collapse of the tunnel structure during the repeated insertion/extraction of the Zn2+ process, and prevented a rapid degradation of the cycling performance. In particular, the sample with the Nb/Mn molar ratio of 0.003 exhibited the best kinetic reversibility and rate performance. After 400 cycles at 1C, the capacity retention of Nb-doped MnO2 significantly increased to 89%, which was only 55% for the undoped sample. Meanwhile, at a power density of 400 W kg-1, it presented the highest energy density of 765 W h kg-1 due to the existing doping of metal ions.

Recent advances in cellulosic materials for aqueous zinc-ion batteries: An overview

Aqueous zinc-ion batteries (AZIBs), with the merits of high safety, environmental friendliness, abundant resources, and competitive energy density are recognized as a promising secondary battery technology and are anticipated to be a great alternative to organic lithium-ion batteries (LIBs). However, the commercial application of AZIBs is severely hindered by intractable issues, including high desolvation barrier, sluggish ion transport kinetics, growth of zinc dendrite, and side reactions. Nowadays, cellulosic materials are frequently employed in the fabrication of advanced AZIBs, because of the intrinsically excellent hydrophilicity, strong mechanical strength, sufficient active groups, and unexhaustible production. In this paper, we start from reviewing the success and dilemma of organic LIBs, followed by introducing the next-generation power source of AZIBs. After summarizing the features of cellulose with great potential in advanced AZIBs, we comprehensively and logically analyze the applications and superiorities of cellulosic materials in AZIBs electrodes, separators, electrolytes, and binders with an in-depth perspective. Finally, a clear outlook is delivered for future development of cellulose in AZIBs. Hopefully, this review can offer a smooth avenue for future direction of AZIBs by means of cellulosic material design and structure optimization.

Regulating the kinetics of zinc-ion migration in spinel ZnMn2O4 through iron doping boosted aqueous zinc-ion storage performance

Spinel ZnMn₂O₄ with a three-dimensional channel structure is one of the important cathode materials for aqueous zinc ions batteries (AZIBs). However, like other manganese-based materials, spinel ZnMn₂O₄ also has problems such as poor conductivity, slow reaction kinetics and structural instability under long cycles. Herein, ZnMn₂O₄ mesoporous hollow microspheres with metal ion doping were prepared by a simple spray pyrolysis method and applied to the cathode of aqueous zinc ion battery. Cation doping not only introduces defects, changes the electronic structure of the material, improves its conductivity, structural stability, and reaction kinetics, but also weakens the dissolution of Mn²⁺. The optimized 0.1 % Fe-doped ZnMn₂O₄ (0.1% Fe-ZnMn₂O₄) has a capacity of 186.8 mAh g^-1 after 250 charge-discharge cycles at 0.5 A g^-1 and the discharge specific capacity reaches 121.5 mAh g^-1 after 1200 long cycles at 1.0 A g^-1. The theoretical calculation results show that doping causes the change of electronic state structure, accelerates the electron transfer rate, and improves the electrochemical performance and stability of the material.

Amorphous K-Buserite Microspheres for High-Performance Aqueous Zn-Ion Batteries and Hybrid Supercapacitors

Aqueous Zn-ion batteries (AZIBs) and Zn-ion hybrid supercapacitors (AZHSCs) are considered promising energy-storage alternatives to Li-ion batteries due to the attractive merits of low-price and high-safety. However, the lack of suitable cathode materials always hinders their large-scale application. Herein, amorphous K-buserite microspheres (denoted as K-MnO_x ) are reported as cathode materials for both AZIBs and AZHSCs, and the energy-storage mechanism is systematically revealed. It is found that K-MnO_x is composed of rich amorphous K-buserite units, which can irreversibly be transformed into amorphous Zn-buserite units in the first discharge cycle. Innovatively, the transformed Zn-buserite acts as active materials in the following cycles and is highly active/stable for fast Zn-diffusion and superhigh pseudocapacitance, enabling the achievement of high-efficiency energy storage. In the AZIBs, K-MnO_x delivers 306 mAh g^-1 after 100 cycles at 0.1 A g^-1 with 102% capacity retention, while in the AZHSCs, it shows 515.0/116.0 F g^-1 at 0.15/20.0 A g^-1 with 92.9% capacitance retention at 5.0 A g^-1 after 20 000 cycles. Besides, the power/energy density of AZHSCs device can reach up to 16.94 kW kg^-1 (at 20 A g^-1 )/206.7 Wh kg^-1 (at 0.15 A g^-1 ). This work may provide some references for designing next-generation aqueous energy-storage devices with high energy/power density.

Sodium Ion Pre-Intercalation of δ-MnO2 Nanosheets for High Energy Density Aqueous Zinc-Ion Batteries

With the merits of low cost, environmental friendliness and rich resources, manganese dioxide is considered to be a promising cathode material for aqueous zinc-ion batteries (AZIBs). However, its low ion diffusion and structural instability greatly limit its practical application. Hence, we developed an ion pre-intercalation strategy based on a simple water bath method to grow in situ δ-MnO₂ nanosheets on flexible carbon cloth substrate (MnO₂), while pre-intercalated Na⁺ in the interlayer of δ-MnO₂ nanosheets (Na-MnO₂), which effectively enlarges the layer spacing and enhances the conductivity of Na-MnO₂. The prepared Na-MnO₂//Zn battery obtained a fairly high capacity of 251 mAh g^-1 at a current density of 2 A g^-1, a satisfactory cycle life (62.5% of its initial capacity after 500 cycles) and favorable rate capability (96 mAh g^-1 at 8 A g^-1). Furthermore, this study revealed that the pre-intercalation engineering of alkaline cations is an effective method to boost the properties of δ-MnO₂ zinc storage and provides new insights into the construction of high energy density flexible electrodes.

Nanostructured Conducting Polymers and Their Applications in Energy Storage Devices

Due to the energy requirements for various human activities, and the need for a substantial change in the energy matrix, it is important to research and design new materials that allow the availability of appropriate technologies. In this sense, together with proposals that advocate a reduction in the conversion, storage, and feeding of clean energies, such as fuel cells and electrochemical capacitors energy consumption, there is an approach that is based on the development of better applications for and batteries. An alternative to commonly used inorganic materials is conducting polymers (CP). Strategies based on the formation of composite materials and nanostructures allow outstanding performances in electrochemical energy storage devices such as those mentioned. Particularly, the nanostructuring of CP stands out because, in the last two decades, there has been an important evolution in the design of various types of nanostructures, with a strong focus on their synergistic combination with other types of materials. This bibliographic compilation reviews state of the art in this area, with a special focus on how nanostructured CP would contribute to the search for new materials for the development of energy storage devices, based mainly on the morphology they present and on their versatility to be combined with other materials, which allows notable improvements in aspects such as reduction in ionic diffusion trajectories and electronic transport, optimization of spaces for ion penetration, a greater number of electrochemically active sites and better stability in charge/discharge cycles.

2D Mesoporous Zincophilic Sieve for High-Rate Sulfur-Based Aqueous Zinc Batteries

Sulfur-based aqueous zinc batteries (SZBs) attract increasing interest due to their integrated high capacity, competitive energy density, and low cost. However, the hardly reported anodic polarization seriously deteriorates the lifespan and energy density of SZBs at a high current density. Here, we develop an integrated acid-assisted confined self-assembly method (ACSA) to elaborate a two-dimensional (2D) mesoporous zincophilic sieve (2DZS) as the kinetic interface. The as-prepared 2DZS interface presents a unique 2D nanosheet morphology with abundant zincophilic sites, hydrophobic properties, and small-sized mesopores. Therefore, the 2DZS interface plays a bifunctional role in reducing the nucleation and plateau overpotential: (a) accelerating the Zn²⁺ diffusion kinetics through the opened zincophilic channels and (b) inhibiting the kinetic competition of hydrogen evolution and dendrite growth via the significant solvation-sheath sieving effect. Therefore, the anodic polarization is reduced to 48 mV at 20 mA cm^-2, and the full-battery polarization is reduced to 42% of an unmodified SZB. As a result, an ultrahigh energy density of 866 Wh kg_sulfur^-1 at 1 A g^-1 and a long lifespan of 10,000 cycles at a high rate of 8 A g^-1 are achieved.

Complementary Operando Electrochemical Quartz Crystal Microbalance and UV/Vis Spectroscopic Studies: Acetate Effects on Zinc-Manganese Batteries

Zinc-ion batteries, in which zinc ions and protons do intercalation and de-intercalation during battery cycling with various proposed mechanisms under debate, have been studied. Recently, electrolytic zinc-manganese batteries, exhibiting the pure dissolution-deposition behavior with a large charge capacity, have been accomplished through using electrolytes with Lewis acid. However, the complicated chemical environment and mixed products hinder the investigation though it is crucial to understand the detailed mechanism. Here, cyclic voltammetry coupled electrochemical quartz crystal microbalance (EQCM) and ultraviolet-visible spectrophotometry (UV-Vis) are respectively, for the very first time, used to study the transition from zinc-ion batteries to zinc electrolytic batteries by the continuous addition of acetate ions. These complementary techniques operando trace the mass and the composition evolution. The observed formation and dissolution of zinc hydroxide sulfate (ZHS) and manganese oxides evince the effect of acetate ions on zinc-manganese batteries from an alternative perspective. Both the amount of acetate and the pH value have large impacts on the capacity and Coulombic efficiency of the MnO₂ electrode, and thus they should be optimized when constructing a full zinc-manganese battery with high rate capability and reversibility.

Amorphous Heterostructure Derived from Divalent Manganese Borate for Ultrastable and Ultrafast Aqueous Zinc Ion Storage

Aqueous zinc-manganese (Zn-Mn) batteries have promising potential in large-scale energy storage applications since they are highly safe, environment-friendly, and low-cost. However, the practicality of Mn-based materials is plagued by their structural collapse and uncertain energy storage mechanism upon cycling. Herein, this work designs an amorphous manganese borate (a-MnBOx ) material via disordered coordination to alleviate the above issues and improve the electrochemical performance of Zn-Mn batteries. The unique physicochemical characteristic of a-MnBOx enables the inner a-MnBOx to serve as a robust framework in the initial energy storage process. Additionally, the amorphous manganese dioxide, amorphous Znx MnO(OH)2 , and Zn4 SO4 (OH)6 ·4H2 O active components form on the surface of a-MnBOx during the charge/discharge process. The detailed in situ/ex situ characterization demonstrates that the heterostructure of the inner a-MnBOx and surface multicomponent phases endows two energy storage modes (Zn2+ /H+ intercalation/deintercalation process and reversible conversion mechanism between the Znx MnO(OH)2 and Zn4 SO4 (OH)6 ·4H2 O) phases). Therefore, the obtained Zn//a-MnBOx battery exhibits a high specific capacity of 360.4 mAh g-1 , a high energy density of 484.2 Wh kg-1 , and impressive cycling stability (97.0% capacity retention after 10 000 cycles). This finding on a-MnBOx with a dual-energy storage mechanism provides new opportunities for developing high-performance aqueous Zn-Mn batteries.

A design of MnO-CNT@C3N4 cathodes for high-performance aqueous zinc-ion batteries

Manganese oxides have been regarded as one of the most promising candidates in rechargeable aqueous zinc ion batteries due to their high specific capacity, high operating voltage, low cost and no-toxicity. Nevertheless, the grievous dissolution of manganese and the sluggish Zn²⁺ ions diffusion kinetics deteriorate the long cycling stability and the rate performance. Herein, we propose a combination of hydrothermal and thermal treatment strategy to design a MnO-CNT@C₃N₄ composite cathode material where MnO cubes are coated by carbon nanotubes (CNTs) and C₃N₄. Owing to the enhanced conductivity by CNTs and the alleviation of the dissolution of Mn²⁺ from the active material by C₃N₄, the optimized MnO-CNT@C₃N₄ exhibits an excellent rate performance (101 mAh g^-1 at a large current density of 3 A g^-1) and a high capacity (209 mAh g^-1 at a current density of 0.8 A g^-1), which is much better than its MnO counterpart. The energy storge mechanism of MnO-CNT@C₃N₄ is confirmed to be the co-insertion of H⁺/Zn²⁺. The present work provides a viable strategy for the design of advanced cathodes for high-performance zinc ion batteries.

Mo-Pre-Intercalated MnO2 Cathode with Highly Stable Layered Structure and Expanded Interlayer Spacing for Aqueous Zn-Ion Batteries

Although manganese-based oxides possess high voltage and low cost, the sluggish reaction kinetics and poor structural stability hinder their applications in aqueous rechargeable Zn-ion batteries (ZIBs). Herein, a molybdenum (Mo) pre-intercalation strategy is proposed to solve the above issues of δ-MnO₂. The pre-intercalated Mo dopants, acting as the interlayer pillars, can not only expand the interlayer spacing but also reinforce the layered structure of δ-MnO₂, finally achieving enhanced reaction kinetics and superb cycling stability during carrier (de)intercalation. Moreover, oxygen defects, introduced due to Mo-pre-intercalation, play a critical role in the fast reaction kinetics and capacity improvement of the Mo-pre-intercalated δ-MnO₂ (Mo-MnO₂) cathode. Therefore, the Mo-MnO₂ cathode displays a high energy density of 451 Wh kg^-1 (based on cathode mass), excellent rate capability, and admirable long-term cycling performance with a high capacity of 159 mAhg^-1 at 1.0 A g^-1 after 1000 cycles. In addition, the energy storage mechanism of Zn²⁺/H⁺ stepwise reversible (de)intercalation is also revealed by ex situ experiments. This work provides an insightful guide for boosting the electrochemical performance of Mn-based oxide cathodes for ZIBs.

Aging-Responsive Phase Transition of VOOH to V10O24·nH2O vs Zn2+ Storage Performance as a Rechargeable Aqueous Zn-Ion Battery Cathode

Vanadium oxyhydroxide has been recently investigated as a starting material to synthesize different phases of vanadium oxides by electrochemical or thermal conversion and has been used as an aqueous zinc-ion battery (AZIB) cathode. However, the low-valent vanadium oxides have poor phase stability under ambient conditions. So far, there is no study on understanding the phase evolution of such low-valent vanadium oxides and their effect on the electrochemical performance toward hosting the Zn²⁺ ions. The primary goal of the work is to develop a high-performance AZIB cathode, and the highlight of the current work is the insight into the auto-oxidation-induced phase transition of VOOH to V₁₀O₂₄·nH₂O under ambient conditions and Zn²⁺ intercalation behavior thereon as an aqueous zinc-ion battery cathode. Herein, we demonstrate that hydrothermally synthesized VOOH undergoes a phase transition to V₁₀O₂₄·nH₂O during both the electrochemical cycling and aerial aging over 38-45 days. However, continued aging till 150 days at room temperature in an open atmosphere exhibited an increased interlayer water content in the V₁₀O₂₄·nH₂O, which was associated with a morphological change with different surface area/porosity characteristics and notably reduced charge transfer/diffusion resistance as an aqueous zinc-ion battery cathode. Although the fresh VOOH cathode had impressive specific capacity at rate performance, (326 mAh/g capacity at 0.1 A/g current and 104 mAh/g capacity at 4 A/g current) the cathode suffered from a continuous capacity decay. Interestingly, the aged VOOH electrodes showed gradually decreasing specific capacity with aging at low current and however followed the reverse order at high current. At a comparable specific power of ∼64-66 W/kg, the fresh VOOH and aged VOOH after 60, 120, and 150 days of aging showed the respective energy densities of 208.3, 281.2, 269.2, and 240.6 Wh/kg. Among all the VOOH materials, the 150 day-aged VOOH cathode exhibited the highest energy density at a power density beyond 1000 W/kg. Thanks to the improved kinetics, the 150 day-aged VOOH cathode delivered a considerable energy density of 39.7 Wh/kg with a high specific power of 4466 W/kg. Also, it showed excellent cycling performance with only 0.002% capacity loss per cycle over 20 300 cycles at 10 A/g.

Suppressed Layered-to-Spinel Phase Transition in δ-MnO2 via van der Waals Interaction for Highly Stable Zn/MnO2 Batteries

Although birnessite-type manganese dioxide (δ-MnO₂ ) with a large interlayer spacing (≈7 Å) is a promising cathode candidate for aqueous Zn/MnO₂ batteries, the poor structural stability associated with Zn²⁺ intercalation/deintercalation limits its further practical application. Herein, δ-MnO₂ ultrathin nanosheets are coupled with reduced graphene oxide (rGO) via van der Waals (vdW) self-assembly in a vacuum freeze-drying process. It is interesting to find that the presence of vdW interaction between δ-MnO₂ and rGO can effectively suppress the layered-to-spinel phase transition in δ-MnO₂ during cycling. As a result, the coupled δ-MnO₂ /rGO hybrid cathode with a sandwich-like heterostructure exhibits remarkable cycle performance with 80.1% capacity retained after 3000 cycles at 2.0 A g^-1 . The first principle calculations demonstrate that the strong interfacial interaction between δ-MnO₂ and rGO results in improved electron transfer and strengthened layered structure for δ-MnO₂ . This work establishes a viable strategy to mitigate the adverse layered-to-spinel phase transition in layered manganese oxide in aqueous energy storage systems.

Comparative Review on the Aqueous Zinc-Ion Batteries (AZIBs) and Flexible Zinc-Ion Batteries (FZIBs)

Lithium-ion batteries (LIBs) have been considered an easily accessible battery technology because of their low weight, cheapness, etc. Unfortunately, they have significant drawbacks, such as flammability and scarcity of lithium. Since the components of zinc-ion batteries are nonflammable, nontoxic, and cheap, AZIBs could be a suitable replacement for LIBs. In this article, the advantages and drawbacks of AZIBs over other energy storage devices are briefly discussed. This review focused on the cathode materials and electrolytes for AZIBs. In addition, we discussed the approaches to improve the electrochemical performance of zinc batteries. Here, we also discussed the polymer gel electrolytes and the electrodes for flexible zinc-ion batteries (FZIBs). Moreover, we have outlined the importance of temperature and additives in a flexible zinc-ion battery. Finally, we have discussed anode materials for both AZIBs and FZIBs. This review has summarized the advantages and disadvantages of AZIBs and FZIBs for future applications in commercial battery technology.

Advances in the regulation of kinetics of cathodic H+/Zn2+ interfacial transport in aqueous Zn/MnO2 electrochemistry

Rechargeable aqueous Zn-MnO₂ energy storage systems have attracted extensive attention owing to their high theoretical capacity and non-flammable mild aqueous electrolytes. Nevertheless, the complicated reaction mechanism of a MnO₂-based cathode severely restricts its further development. Therefore, it is crucial to clarify the kinetics of H⁺/Zn²⁺ interfacial transport in the MnO₂ cathode for realizing controllable regulation of interfacial ion transport and then realizing high capacity and long lifespan. Recently, based on different reaction mechanisms, various strategies have been employed to improve the performance of aqueous Zn/MnO₂ cells, such as surface modifications and structural engineering. Herein, we systematically summarize the recent advances in the modulation of interfacial H⁺/Zn²⁺ transport and related redox kinetics to effectively improve the electrochemical responses. Furthermore, the challenges of designing novel MnO₂ cathodes have also been prospected in detail to provide possible guidelines for the development of Zn/MnO₂ batteries.

Van der Waals Interaction-Driven Self-Assembly of V2O5 Nanoplates and MXene for High-Performing Zinc-Ion Batteries by Suppressing Vanadium Dissolution

Aqueous zinc-ion batteries (AZIBs) are attractive energy storage devices that benefit from improved safety and negligible environmental impact. The V₂O₅-based cathodes are highly promising, but the dissolution of vanadium is one of the major challenges in realizing their stable performance in AZIBs. Herein, we design a Ti₃C₂T_x MXene layer on the surface of V₂O₅ nanoplates (VPMX) through a van der Waals self-assembly approach for suppressing vanadium dissolution during an electrochemical process for greatly boosting the zinc-ion storage performance. Unlike conventional V₂O₅/C composites, we demonstrate that the VPMX hybrids offer three distinguishable features for achieving high-performance AZIBs: (i) the MXene layer on cathode surface maintains structural integrity and suppresses V dissolution; (ii) the heterointerface between V₂O₅ and MXene enables improved host electrochemical kinetics; (iii) reduced electrostatic repulsion exists among host layers owing to the lubricating water molecules in the VPMX cathode, facilitating interfacial Zn²⁺ diffusion. As a result, the as-made VPMX cathode shows a long-term cycling stability over 5000 cycles, surpassing other reported V₂O₅-based materials. Especially, we find that the heterointerface between V₂O₅ and MXene and lubricated water molecules in the host can achieve an enhanced rate capability (243.6 mAh g^-1 at 5.0 A g^-1) for AZIBs.

Suppressing the Exacerbated Hydrogen Evolution of Porous Zn Anode with an Artificial Solid-Electrolyte Interphase Layer

Rechargeable Zn batteries are widely studied as aqueous, safe, and environmentally friendly alternatives to Li-ion batteries. The 3D porous Zn anode has been extensively reported for suppressing Zn dendrite growth and accelerating the electrode kinetics. However, we demonstrate herein that the undesirable hydrogen evolution reaction (HER) is also exacerbated for porous Zn electrode. Therefore, a polytetrafluoroethylene (PTFE) coating is further applied on the porous Zn serving as the artificial solid-electrolyte interphase (SEI), which is demonstrated to effectively inhibit the hydrogen evolution and maintain the Zn plating kinetics. By utilizing the synergistic effects of the porous morphology and artificial SEI layer, better performances are obtained over porous Zn or bare Zn foil, including dendrite-free Zn plating/stripping up to 2000 h at 2 mA cm^-2 and extended cycling in the Zn||V₂O₅ cell. This work suggests two complementary strategies for achieving simultaneously dendrite-free and side-reaction-suppressed Zn batteries.

Material Design and Energy Storage Mechanism of Mn-Based Cathodes for Aqueous Zinc-Ion Batteries

Mn-based cathodes have been widely explored for aqueous zinc-ion batteries (ZIBs), by virtue of their high theoretical capacity and low cost. However, Mn-based cathodes suffer from poor rate capability and cycling performance. Researchers have presented various approaches to address these issues. Therefore, these endeavors scattered in various directions (e. g., designing electrode structures, defect engineering and optimizing electrolytes) are necessary to be connected through a systematic review. Hence, we comprehensively overview Mn-based cathode materials for ZIBs from the aspects of phase compositions, electrochemical behaviors and energy storage mechanisms, and try to build internal relations between these factors. Modification strategies of Mn-based cathodes are then introduced. Furthermore, this review also provides some new perspectives on future efforts toward high-energy and long-life Mn-based cathodes for ZIBs.

Recycling of Zinc-Carbon Batteries into MnO/ZnO/C to Fabricate Sustainable Cathodes for Rechargeable Zinc-Ion Batteries

Acidic zinc-carbon dry batteries have been widely used in life because of their low cost. However, a great quantity of used batteries is discarded as refuse, which not only wastes resources but also leads to environmental contamination. To reuse spent batteries on a large scale, this study concerns a simple, effective, and sustainable strategy to turn them into MnO/ZnO/C composites. After a conventional leaching treatment followed by pyrolysis, the rust cathode materials can be reduced to MnO/ZnO/C. When serving as a rechargeable zinc-ion battery cathode, this electrode provides a maximum reversible capacity of around 362 mAh g^-1 _MnO ) and a rate capability of 191 mAh g^-1 _MnO at a high current rate of 1.20 A g^-1 . Furthermore, ZnO gradually dissolves in the electrolyte with the increase of discharge cycles, replenishing the Zn²⁺ content in the electrolyte and further enhancing cycling stability (98.02 % after 500 cycles). The device also exhibits a remarkable energy density of 336.37 Wh kg^-1 , low self-discharge rate, and can efficiently power a LED panel. This strategy offers an economical and facile route to convert zinc-carbon battery waste into useful materials for aqueous rechargeable zinc ion batteries.

Highly efficient Ag doped δ-MnO2 decorated graphene: Comparison and application in electrochemical detection of H2O2

Cytotoxic H₂O₂ is an inevitable part of our life, even during this contemporary pandemic COVID-19. Personal protective equipment of the front line fighter against coronavirus could be sterilized easily by H₂O₂ for reuse. In this study, Ag doped δ-MnO₂ nanorods supported graphene nanocomposite (denoted as Ag@δ-MnO₂/G) was synthesized as a nonenzymatic electrochemical sensor for the sensitive detection of H₂O₂. The ternary nanocomposite has overcome the poor electrical conductivity of δ-MnO₂ and also the severe aggregation of Ag NPs. Furthermore, δ-MnO₂/G provided a rougher surface and large numbers of functional groups for doping more numbers of Ag atoms, which effectively modulate the electronic properties of the nanocomposite. As a result, electroactive surface area and electrical conductivity of Ag@δ-MnO₂/G increased remarkably as well as excellent catalytic activity observed towards H₂O₂ reduction. The modified glassy carbon electrode exhibited fast amperometric response time (<2 s) in H₂O₂ determination. The limit of detection was calculated as 68 nM in the broad linear range (0.005-90.64 mM) with high sensitivity of 104.43 µA mM^-1 cm^-2. No significant interference, long-term stability, excellent reproducibility, satisfactory repeatability, practical applicability towards food samples and wastewater proved the efficiency of the proposed sensor.

Calcium-intercalated birnessite MnO2 anchored on carbon nanotubes as high-performance cathodes for aqueous zinc-ion batteries

Aqueous Zn-ion batteries (ZIBs) show great potential in energy storage systems because of their high theoretical capacities, high safety, low cost, and environmental friendliness. The lack of suitable cathode materials for sustaining the Zn²⁺ intercalation/deintercalation severely restricts their further application. Herein, calcium-intercalated birnessite MnO₂ anchored on carbon nanotubes (CNTs) was designed as a cathode for ZIBs. The cathode material can be facilely produced by a simple one-pot reaction process. The external calcium-intercalated MnO₂ with large layer spacing affords a fast ionic migration rate and the internal CNTs serving as a structural framework endow the electrode with better electrical conductivity. Benefiting from the larger interlayer spacing and the enhanced electrical conductivity, the CNT-CaMO cathode shows a high specific capacity of 351.8 mA h g^-1 at 200 mA g^-1 and a long cycle life over 6000 cycles. Besides, the H⁺ and Zn²⁺ co-intercalation storage mechanism was confirmed by ex situ XRD, SEM, and XPS analyses. This work opens up a new way to develop aqueous ZIB cathode materials with a high reversible capacity and long cycle life.

Synergistic Optimization Strategy Involving Sandwich-like MnO2@rGO and Laponite-Modified PAM for High-Performance Zinc-Ion Batteries and Zinc Dendrite Suppression

Optimization of the cathode structure and exploration of a novel electrolyte system are important approaches for achieving high-performance zinc-ion batteries (ZIBs) and zinc dendrite suppression. Herein, a quasi-solid-state ZIB combining a sandwich-like MnO₂@rGO cathode, a laponite (Lap)-modified polyacrylamide (PAM) hydrogel electrolyte, and an electrodeposited zinc anode is designed and constructed by a synergistic optimization strategy. The MnO₂ composite prepared through the intercalation of rGO shows developed mesopores, providing accessible ion transport channels and exhibiting a high electrical conductivity. Thanks to the high dispersion of Lap nanoplates in the hydrogel and good charge-averaging effect, the Zn//PAM-5%Lap//Zn symmetrical battery exhibits a consistent low-voltage polarization of less than 60 mV within 2000 h without a short-circuit phenomenon or any over-potential rise, indicating a stable zinc peeling/plating process. The optimized quasi-solid-state ZIB delivers a high reversible capacity of 291 mA h g^-1 at a current density of 0.2 A g^-1 due to the synergistic effect of each component of ZIB. Even at a high rate of 2 A g^-1, it still maintains a high reversible capacity of 97 mA h g^-1 after 2000 cycles, indicating its excellent electrochemical performance. Furthermore, the assembled flexible battery performs excellently in terms of damage and bending resistance.

Dual metal ions and water molecular pre-intercalated δ-MnO2 spherical microflowers for aqueous zinc ion batteries

Layered δ-MnO₂ is a promising cathode material for aqueous zinc ion batteries (AZIBs) due to its high theoretical capacity, high operating voltage and low cost. However, the dissolution of MnO₂ and the disproportionation of Mn³⁺ will lead to irreversible reaction and serious structural degradation of the material during cycling process. In this work, the Al³⁺ pre-intercalated K_0.27MnO₂·0.54H₂O was prepared by a one-step hydrothermal method with citric acid as the complexing agent and weak reducing agent. Based on the pillars of bimetallic ions K⁺, Al³⁺ and water, the framework and interlayer of δ-MnO₂ is stabilized. Besides, a certain amount of Al³⁺ facilitates the increase of crystal water compared with the pure K_0.27MnO₂·0.54H₂O, which is not only conducive to promote the construction of porous and loose 3D morphology, but also beneficial to improve the stability of layered structure and accelerate the migration rate of zinc ions. Contributed to the dissolution/deposition reaction mechanism combined with H⁺/Zn²⁺ co-insertion/co-extraction mechanism, it has achieved the high capacity with the maximum reversible specific capacity of 269.5 mAh g^-1 at 0.5 A g^-1 and excellent stability with 205.8 mAh g^-1 even after 300 cycles in Zn//Al-KMO battery.