Recent Development of Mn-based Oxides as Zinc-Ion Battery Cathode

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.

In situ growth of Mn3O4 nanoparticles on accordion-like Ti3C2Tx MXene for advanced aqueous Zn-Ion batteries

Manganese-based cathodes are competitive candidates for state-of-the-art aqueous zinc-ion batteries (AZIBs) because of their easy preparation method, sufficient nature reserve, and environmental friendliness. However, their poor cycle stability and low rate performance have prevented them from practical applications. In this study, Mn3O4 nanoparticles were formed in situ on the surface and between the interlayers of Ti3C2Tx MXene, which was pretreated by the intercalation of K+ ions. Ti3C2Tx MXene not only provides abundant active sites and high conductivity but also hinders the structural damage of Mn3O4 during charging and discharging. Benefiting from the well-designed K-Ti3C2@Mn3O4 structure, the battery equipped with the K-Ti3C2@Mn3O4 cathode achieved a maximum specific capacity of 312 mAh/g at a current density of 0.3 A/g and carried a specific capacity of approximately 120 mAh/g at a current density of 1 A/g, which remained stable for approximately 500 cycles. The performance surpasses that of most reported Mn3O4-based cathodes. This study pioneers a new approach for building better cathode materials for AZIBs.

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

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

Preparation of Spherical δ-MnO2 Nanoflowers by One-Step Coprecipitation Method as Electrode Material for Supercapacitor

Spherical δ-MnO2 nanoflower materials were synthesized via a facile one-step coprecipitation method through adjusting the molar ratio of KMnO4 to MnSO4. The influence of the molar ratio of the reactants on the crystal structure, morphology, and electrochemical performances was investigated. At a molar ratio of 3.3 for KMnO4 to MnSO4, the spherical δ-MnO2 nanoflowers composed of nanosheets with the highest specific surface area (228.0 m2 g-1) were obtained as electrode materials. In the conventional three-electrode system using 1 M Na2SO4 as an electrolyte, the specific capacitance of the spherical δ-MnO2 nanoflowers reached 172.3 F g-1 at a current density of 1 A g-1. Moreover, even after 5000 cycles at a current density of 5 A g-1, the GCD curves remained essentially unchanged, and the specific capacitance still retained 86.50% of the maximum value. The kinetics of the electrode reaction were preliminarily studied through the linear potential sweep technique to observe diffusion-controlled contribution toward total capacitance. For the spherical δ-MnO2 nanoflower electrode material, diffusion-controlled contribution accounted for 65.1% at low scan rates and still remained significant at high scan rates (100 mV s-1), indicating excellent utilization efficiency of the bulk phase. The as-fabricated asymmetric supercapacitor HFC-7//MnO2-3.3-ASC presented a prominent specific energy of 16.5 Wh kg-1 at the specific power of 450 W kg-1. Even when the specific power reached 9.0 kW kg-1, the energy density still retained 9.5 Wh kg-1.

Activating the MnS0.5Se0.5 Microspheres as High-Performance Cathode Materials for Aqueous Zinc-Ion Batteries: Insight into In Situ Electrooxidation Behavior and Energy Storage Mechanisms

Manganese-based materials are regarded as the most prospective cathode materials because of their high natural abundance, low toxicity, and high specific capacity. Nevertheless, the low conductivity, poor cycling performance, and controversial energy storage mechanisms hinder their practical application. Here, the MnS0.5Se0.5 microspheres are synthesized by a two-step hydrothermal approach and employed as cathode materials for aqueous zinc-ion batteries (AZIBs) for the first time. Interestingly, in-depth ex situ tests and electrochemical kinetic analyses reveal that MnS0.5Se0.5 is first irreversibly converted into low-crystallinity ZnMnO3 and MnOx by in situ electrooxidation (MnS0.5Se0.5-EOP) during the first charging process, and then a reversible co-insertion/extraction of H+/Zn2+ occurs in the as-obtained MnS0.5Se0.5-EOP electrode during the subsequent discharging and charging processes. Benefiting from the increased surface area, shortened ion transport path, and stable lamellar microsphere structure, the MnS0.5Se0.5-EOP electrodes deliver high reversible capacity (272.8 mAh g-1 at 0.1 A g-1), excellent rate capability (91.8 mAh g-1 at 2 A g-1), and satisfactory cyclic stability (82.1% capacity retention after 500 cycles at 1 A g-1). This study not only provides a powerful impetus for developing new types of manganese-based chalcogenides, but also puts forward a novel perspective for exploring the intrinsic mechanisms of in situ electrooxidation behavior.

Recent Advances in Graphene-Based Materials for Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) are a promising alternative for large-scale energy storage due to their advantages of environmental protection, low cost, and intrinsic safety. However, the utilization of their full potential is still hindered by the sluggish electrode reaction kinetics, poor structural stability, severe Zn dendrite growth, and narrow electrochemical stability window of the whole battery. Graphene-based materials with excellent physicochemical properties hold great promise for addressing the above challenges foe ZIBs. In this review, the energy storage mechanisms and challenges faced by ZIBs are first discussed. Key issues and recent progress in design strategies for graphene-based materials in optimizing the electrochemical performance of ZIBs (anode, cathode, electrolyte, separator and current collector) are then discussed. Finally, some potential challenges and future research directions of graphene-based materials in high-performance ZIBs are proposed for practical applications.

A silver and manganese dioxide composite with oxygen vacancies as a high-performance cathode material for aqueous zinc-ion batteries

Aqueous zinc ion batteries (AZIBs) are regarded as a promising alternative for energy storage due to their safety, cost-effectiveness and environmental friendliness. Manganese dioxide is considered a promising cathode material for energy storage because of its abundant reserves and high energy density. However, its inherent low electronic conductivity and limited cycling performance due to structural instability hinder its further development. Herein, a silver and manganese dioxide composite (Ag@MnO2) enriched with oxygen vacancies was prepared by a simple liquid-phase reduction method. The introduction of silver particles facilitates the improvement of electrical conductivity, and the incorporation of oxygen vacancies helps change the surface properties of manganese dioxide, providing additional active sites for ion transport, enhancing the overall electrochemical kinetics, and further improving the battery performance. As a result, the Ag@MnO2 cathode exhibits an astonishingly high capacity of 353 mAh g-1 at a current density of 0.1 A g-1 and a capacity retention of 78% after 1500 cycles. Additionally, electrochemical and structural analyses have revealed that the Ag@MnO2 cathode undergoes a reversible and stable process of H+ and Zn2+ insertion/extraction.

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.

Recent Advancements of Graphene-Based Materials for Zinc-Based Batteries: Beyond Lithium-Ion Batteries

Graphene-based materials (GBMs) possess a unique set of properties including tunable interlayer channels, high specific surface area, and good electrical conductivity characteristics, making it a promising material of choice for making electrode in rechargeable batteries. Lithium-ion batteries (LIBs) currently dominate the commercial rechargeable battery market, but their further development has been hampered by limited lithium resources, high lithium costs, and organic electrolyte safety concerns. From the performance, safety, and cost aspects, zinc-based rechargeable batteries have become a promising alternative of rechargeable batteries. This review highlights recent advancements and development of a variety of graphene derivative-based materials and its composites, with a focus on their potential applications in rechargeable batteries such as LIBs, zinc-air batteries (ZABs), zinc-ion batteries (ZIBs), and zinc-iodine batteries (Zn-I2 Bs). Finally, there is an outlook on the challenges and future directions of this great potential research field.

Recent Progress of the Cathode Material Design for Aqueous Zn-Organic Batteries

Aqueous zinc-ion batteries (ZIBs) have emerged as the most promising candidate for large-scale energy storage due to their inherent safety, environmental friendliness, and cost-effectiveness. Simultaneously, the utilization of organic electrode materials with renewable resources, environmental compatibility, and diverse structures has sparked a surge in research and development of aqueous Zn-organic batteries (ZOBs). A comprehensive review is warranted to systematically present recent advancements in design principles, synthesis techniques, energy storage mechanisms, and zinc-ion storage performance of organic cathodes. In this review article, we comprehensively summarize the energy storage mechanisms employed by aqueous ZOBs. Subsequently, we categorize organic cathode materials into small-molecule compounds and high-molecular polymers respectively. Novel polymer materials such as conjugated polymers (CPs), conjugated microporous polymers (CMPs), and covalent organic frameworks (COFs) are highlighted with an overview of molecular design strategies and structural optimization based on organic cathode materials aimed at enhancing the performance of aqueous ZOBs. Finally, we discuss the challenges faced by aqueous ZOBs along with future prospects to offer insights into their practical applications.

In-Situ Construction of V2 O5 Nanosheet/Nitrogen-Doped Carbon Nanosheet Heterostructures with Interfacial C─O Bridging Bonds as the Cathode Material for Zn Ion Batteries

Layered oxides are widely used as the electrode materials for metal ion batteries. However, for large radius size ions, such as Zn2+ and Al3+ , the tightly stacked layers and poor electrical conductivity of layered oxides result in restricted number of active sites and sluggish reaction kinetics. In this work, a facile in-situ construction strategy is provided to synthesize layered oxide nanosheets/nitrogen-doped carbon nanosheet (NC) heterostructure, which shows larger interlayer spacing and better electrical conductivity than the layered oxides. As a result, the Zn2+ ion diffusion inside the interlayer gallery is greatly enhanced and the storage sites inside the gallery can be better used. Meanwhile, the NC layers and oxide nanosheets are bridged by the C─O bonds to form a stable structure, which contributes to a better cycling stability than the pure layered oxides. The optimal V2 O5 @NC-400 cathode shows a capacity of 467 mA h g-1 at 0.1 A g-1 for 300 cycles, and long-term cyclic stability of 4000 cycles at 5 A g-1 with a capacity retention of 92%. All these performance parameters are among the best for vanadium oxide-based cathode materials.

Zn-ion Batteries: Charge Storing Mechanism and Development Challenges

Improving the energy share of renewable energy technologies is the only solution to reduce greenhouse gas emissions and air pollution. The high-performing green battery energy storage technologies are critical for storing energy to address the intermittent nature of renewable energy resources. In recent years, aqueous batteries, particularly Zn-ion batteries (ZIBs), have achieved and shown great potential for stationary energy storage systems owing to their low cost and safer operation. However, the practical applications of the ZIBs have significantly been impeded due to the gap between the breakthroughs achieved in academic research and industrial developments. The present review discusses the ZIB's advantages, possibilities, and shortcomings for stationary energy storage systems. The Review begins with a brief introduction to the ZIBs and their charge storage mechanisms based on the structural properties of cathode materials. The scientific and technical challenges that obstruct the commercialization of the ZIBs are discussed in detail concerning their impact on accelerating the utilization of the ZIBs for real-life applications. The final section highlights the outlook on research in this flourishing field.

Inside-out regulation of MnO toward fast reaction kinetics in aqueous zinc ion batteries

An "inside-out regulation" strategy is proposed to improve the Zn2+ storage of MnO by Ni doping into the lattice and graphene wrapping outside the nanoparticles. The as-prepared Ni-MnO@rGO exhibits 112 mA h g-1 at 2.0 A g-1 over 800 cycles, due to the improved transport of electrons and ions from the synergistical function of intrinsic doping and external graphene encapsulation.

Layered porous Mn0.18V2O5@C with manganese and carbon provided by a metal-organic framework precursor as a cathode material for aqueous zinc-ion batteries

At present, vanadium-based cathodes for aqueous zinc-ion batteries (AZIBs) are limited by their slow reaction kinetics, poor electrical conductivity, and low capacity retention. To overcome these problems, here, we design a layered porous Mn0.18V2O5@C as the cathode material for AZIBs using a manganese-containing metal-organic framework as a template through a simple solvothermal method. Such an electrode delivers an excellent specific capacity (380 mA h g-1 at 0.1 A g-1) accompanied by superior cycling stability (about 85% capacity retention for 2000 cycles at 6 A g-1). The excellent electrochemical performance of Mn0.18V2O5@C is ascribed to the improved interface activity including smooth zinc ion transport, abundant ion reaction active sites and accelerated charge transfer resulting from the coordination of the porous structure, doped conductive carbon, and the stable channel structure derived from the pillar effect of doping manganese ions, preventing a premature collapse of the electrode structure. It is also revealed by structural evolution analysis that the residual zinc ions also combine with the original Mn0.18V2O5 to form a ZnxMnyV2O5 phase, which serves as an additional structural pillar and in the meantime, also participates in the following cycles. These favorable electrochemical results suggest that Mn0.18V2O5@C is a suitable cathode material for AZIBs.

Mechanism of Chalcophanite Nucleation in Zinc Hydroxide Sulfate Cathodes in Aqueous Zinc Batteries

Aqueous Zn-ion batteries with MnO2-based cathodes have seen significant attention owing to their high theoretical capacities, safety, and low cost; however, much debate remains regarding the reaction mechanism that dominates energy storage. In this work, we report our electron microscopy study of cathodes containing zinc hydroxide sulfate (Zn4SO4(OH)6·xH2O, ZHS) together with carbon nanotubes cycled in electrolytes containing ZnSO4 with varied amounts of MnSO4 incorporated. The primary Mn-containing phase is formed in situ in the cathode during cycling, where a dissolution-deposition reaction is identified between ZHS and chalcophanite (ZnMn3O7·3H2O). Mechanistic details of this reaction, in which the chalcophanite nucleates then separates from the ZHS flakes as the ZHS dissolves while acting as the primary Zn source for the reaction, are revealed using surface sensitive methods. These findings indicate the reaction is local to the ZHS flakes, providing new insight toward the importance of ZHS and the cathode microstructure.

Polymorphism and Multi-Component Crystal Formation of GABA and Gabapentin

This study exploits the polymorphism and multi-component crystal formation of γ-amino butanoic acid (GABA) and its pharmaceutically active derivative, gabapentin. Two polymorphs of GABA and both polymorphs of gabapentin are structurally revisited, together with gabapentin monohydrate. Hereby, GABA form II is only accessible under special conditions using additives, whereas gabapentin converts to the monohydrate even in the presence of trace amounts of water. Different accessibilities and phase stabilities of these phases are still not fully clarified. Thus, indicators of phase stability are discussed involving intermolecular interactions, molecular conformations, and crystallization environment. Calculated lattice energy differences for polymorphs reveal their similar stability. Quantification of the hydrogen bond strengths with the atoms-in-molecules (AIM) model in conjunction with non-covalent interaction (NCI) plots also shows similar hydrogen bond binding energy values for all polymorphs. We demonstrate that differences in the interacting modes, in an interplay with the intermolecular repulsion, allow the formation of the desired phase under different crystallization environments. Salts and co-crystals of GABA and gabapentin with fumaric as well as succinic acid further serve as models to highlight how strongly HBs act as the motif-directing force in the solid-phase GABA-analogs. Six novel multi-component entities were synthesized, and structural and computational analysis was performed: GABA fumarate (2:1); two gabapentin fumarates (2:1) and (1:1); two GABA succinates (2:1) and (1:1); and a gabapentin:succinic acid co-crystal. Energetically highly attractive carboxyl/carboxylate interaction overcomes other factors and dominates the multi-component phase formation. Decisive commonalities in the crystallization behavior of zwitterionic GABA-derivatives are discussed, which show how they can and should be understood as a whole for possible related future products.

Methods for Characterizing Intercalation in Aqueous Zinc Ion Battery Cathodes: A Review

Aqueous zinc ion batteries have gained research attention as a safer, economical and more environmentally friendly alternative to lithium-ion batteries. Similar to lithium batteries, intercalation processes play an important role in the charge storage behaviour of aqueous zinc ion batteries, with the pre-intercalation of guest species in the cathode being also employed as a strategy to improve battery performance. In view of this, proving hypothesized mechanisms of intercalation, as well as rigorously characterizing intercalation processes in aqueous zinc ion batteries is crucial to achieve advances in battery performance. This review aims to evaluate the range of techniques commonly used to characterize intercalation in aqueous zinc ion battery cathodes, providing a perspective on the approaches that can be utilized to rigorously understand such intercalation processes.

Interfacial Designs of MXenes for Mild Aqueous Zinc-Ion Storage

Limited Li resources, high cost, and safety risks of using organic electrolytes have stimulated a strong motivation to develop non-Li aqueous batteries. Aqueous Zn-ion storage (ZIS) devices offer low-cost and high-safety solutions. However, their practical applications are at the moment restricted by their short cycle life arising mainly from irreversible electrochemical side reactions and processes at the interfaces. This review sums up the capability of using 2D MXenes to increase the reversibility at the interface, assist the charge transfer process, and thereby improve the performance of ZIS. First, they discuss the ZIS mechanism and irreversibility of typical electrode materials in mild aqueous electrolytes. Then, applications of MXenes in different ZIS components are highlighted, including as electrodes for Zn2+ intercalation, protective layers of Zn anode, hosts for Zn deposition, substrates, and separators. Finally, perspectives are put forward on further optimizing MXenes to improve the ZIS performance.

High-Voltage and Stable Manganese Hexacyanoferrate/Zinc Batteries Using Gel Electrolytes

Because of the high safety and environmental friendliness, aqueous zinc-ion batteries have gained a lot of attention in recent years. Prussian blue and its analogues are regarded as a promising cathode material of zinc-ion batteries. Manganese hexacyanoferrate is appropriate among them due to its high operating voltage, large capacity, and cheap price. However, the poor cycling stability of manganese hexacyanoferrate, mainly caused by transition metal dissolution, side reaction, and phase transition, greatly restricts its practical application. In this work, gelatin is used to limit the content of free water in the electrolyte, thus reducing the dissolution effect of transition metal manganese. The introduction of gelatin improves the durability of the Zn anode as well. The optimized MnHCF/gel-0.3/Zn battery displays a high reversible capacity (120 mAh·g^-1 at 0.1 A·g^-1), an excellent rate performance (42.7 mAh·g^-1 at 2 A·g^-1), and a good capacity retention (65% at 0.5 A·g^-1 after 1000 cycles).

Ionic-Liquid-Assisted Synthesis of Mixed-Phase Manganese Oxide Nanorods for a High-Performance Aqueous Zinc-Ion Battery

Aqueous zinc-ion batteries (ZIBs) provide a safer and cost-effective energy storage solution by utilizing nonflammable water-based electrolytes. Although many research efforts are focused on optimizing zinc anode materials, developing suitable cathode materials is still challenging. In this study, one-dimensional, mixed-phase MnO₂ nanorods are synthesized using ionic liquid (IL). Here, the IL acts as a structure-directing agent that modifies MnO₂ morphology and introduces mixed phases, as confirmed by morphological, structural, and X-ray photoelectron spectroscopy (XPS) studies. The MnO₂ nanorods developed by this method are utilized as a cathode material for ZIB application in the coin-cell configuration. As expected, Zn//MnO₂ nanorods show a significant increase in their capacity to 347 Wh kg^-1 at 100 mA g^-1, which is better than bare MnO₂ nanowires (207.1 Wh kg^-1) synthesized by the chemical precipitation method. The battery is highly rechargeable and maintains good retention of 86% of the initial capacity and 99% Coulombic efficiency after 800 cycles at 1000 mA g^-1. The ex situ XPS, X-ray diffraction, and in-depth electrochemical analysis confirm that MnO₆ octahedra experience insertion/extraction of Zn²⁺ with high reversibility. This study suggests the potential use of MnO₂ nanorods to develop high-performance and durable battery electrode materials suitable for large-scale applications.

Challenges and Strategies in the Development of Zinc-Ion Batteries

Although promising, the practical use of zinc-ion batteries (ZIBs) remains plagued with uncontrollable dendrite growth, parasitic side reactions, and the high intercalation energy of divalent Zn²⁺ ions. Hence, much work has been conducted to alleviate these issues to maximize the energy density and cyclic life of the cell. In this holistic review, the mechanisms and rationale for the stated challenges shall be summarized, followed by the corresponding strategies employed to mitigate them. Thereafter, a perspective on present research and the outlook of ZIBs would be put forth in hopes to enhance their electrochemical properties in a multipronged approach.

Progressive activation of porous vanadium nitride microspheres with intercalation-conversion reactions toward high performance over a wide temperature range for zinc-ion batteries

Rechargeable aqueous zinc-ion batteries have great promise for becoming next-generation storage systems, although the irreversible intercalation of Zn²⁺ and sluggish reaction kinetics impede their wide application. Therefore, it is urgent to develop highly reversible zinc-ion batteries. In this work, we modulate the morphology of vanadium nitride (VN) with different molar amounts of cetyltrimethylammonium bromide (CTAB). The optimal electrode has porous architecture and excellent electrical conductivity, which can alleviate volume expansion/contraction and allow for fast ion transmission during the Zn²⁺ storage process. Furthermore, the CTAB-modified VN cathode undergoes a phase transition that provides a better framework for vanadium oxide (VO_x). With the same mass of VN and VO_x, VN provides more active material after phase conversion due to the molar mass of the N atom being less than that of the O atom, thus increasing the capacity. As expected, the cathode displays an excellent electrochemical performance of 272 mAh g^-1 at 5 A g^-1, high cycling stability up to 7000 cycles, and excellent performance over a wide temperature range. This discovery creates new possibilities for the development of high-performance multivalent ion aqueous cathodes with rapid reaction mechanisms.

Defect Engineered Ternary Spinel: An Efficient Cathode for an Aqueous Rechargeable Zinc-Ion Battery of Long-Term Cyclability

The rational defect engineering of Mn-based spinel cathode materials by metal-ion doping and vacancy creation fosters reversible intercalation/deintercalation of charge carriers and boosts the charge storage performance of an aqueous rechargeable zinc-ion battery (ZIB). Herein, we demonstrate the Zn²⁺ ion storage performance of a defect-engineered ternary spinel cathode based on Zn, Ni, and Mn. The defect engineering of ZnMn₂O₄ is achieved by Ni²⁺ doping and creating a cation (Mn³⁺ and Zn²⁺) deficiency. The engineered cathode material has cubic spinel structure in contrast to the defect-free ZnMn₂O₄. The DFT studies show that the defect engineering modifies the electronic structure and improves the electronic conductivity. An aqueous rechargeable ZIB is fabricated by using the spinel cathode, and its performance is evaluated in terms of charge-discharge cycling stability, specific capacity, and so on. The ternary spinel-based ZIB has a very long charge-discharge cycling stability with a specific capacity as high as 265 mAh g^-1 (at 100 mA g^-1). A 2-fold enhancement in the specific capacity is observed after 5000 cycles. Ni doping affords ultralong cycling stability. The self-discharge studies for a year show that the device retains 63% of the initial performance.

Insight on Cathodes Chemistry for Aqueous Zinc-Ion Batteries: From Reaction Mechanisms, Structural Engineering, and Modification Strategies

By virtue of low cost, eco-friendliness, competitive gravimetric energy density, and intrinsic safety, more and more attention has increasingly focused on aqueous zinc ion batteries (AZIBs) as a promising alternative for scalable energy storage. However, plagued by a complex interfacial process, sluggish dynamics, lability of electrodes and electrolytes, insufficient energy density, and poor cycle life heavily restrict practical applications of AZIBs, indicating that profound understandings on cathode storage chemistry are necessarily needed. Hence, this paper comprehensively summarizes recent advance in cathodes with critical insight on the energy storage mechanism. Furthermore, the issues and challenges for high-performance cathodes are meticulously explored, presenting inspiring structural engineering and modification strategies. Finally, rational evaluations on representative cathodes are rendered, suggesting the potential development direction of AZIBs.

Challenges and Perspectives for Doping Strategy for Manganese-Based Zinc-ion Battery Cathode

As one of the most appealing options for large-scale energy storage systems, the commercialization of aqueous zinc-ion batteries (AZIBs) has received considerable attention due to their cost effectiveness and inherent safety. A potential cathode material for the commercialization of AZIBs is the manganese-based cathode, but it suffers from poor cycle stability, owing to the Jahn–Teller effect, which leads to the dissolution of Mn in the electrolyte, as well as low electron/ion conductivity. In order to solve these problems, various strategies have been adopted to improve the stability of manganese-based cathode materials. Among those, the doping strategy has become popular, where the dopant is inserted into the intrinsic crystal structures of electrode materials, which would stabilize them and tune the electronic state of the redox center to realize high ion/electron transport. Herein, we summarize the ion doping strategy from the following aspects: (1) synthesis strategy of doped manganese-based oxides; (2) valence-dependent dopant ions in manganese-based oxides; (3) optimization mechanism of ion doping in zinc-manganese battery. Lastly, an in-depth understanding and future perspectives of ion doping strategy in electrode materials are provided for the commercialization of manganese-based zinc-ion batteries.

Metal-Organic Framework-Based Materials for Aqueous Zinc-Ion Batteries: Energy Storage Mechanism and Function

Aqueous rechargeable zinc-ion batteries (ZIBs) featuring competitive performance, low cost and high safety hold great promise for applications in grid-scale energy storage and portable electronic devices. Metal-organic frameworks (MOFs), relying on their large framework structure and abundant active sites, have been identified as promising materials in ZIBs. This review comprehensively presents the current development of MOF-based materials including MOFs and their derivatives in ZIBs, which begins with Zn storage mechanism of MOFs, followed by introduction of various types of MOF-based cathode materials (PB and PBA, Mn-based MOF, V-based MOF, conductive MOF and their derivatives), and the regulation approaches for Zn deposition behavior. The key factors and optimization strategies of MOF-based materials that affect ZIBs performance are emphasized and discussed. Finally, the challenges and further research directions of MOF-based materials for advanced zinc-ion batteries are provided.

Iron-Based NASICON-Type Na4 Fe3 (PO4 )2 (P2 O7 ) Cathode for Zinc-Ion Battery: Zn2+ /Na+ Co-Intercalation Enabling High Capacity

The development of high-performance cathode materials for aqueous zinc-ion batteries (ZIBs) based on nontoxic and earth-abundant elements remains a great challenge. This study introduces the iron-based NASICON-type material Na₄ Fe₃ (PO₄ )₂ (P₂ O7) with carbon layer (NFPP@C) as a cathode material for ZIBs. When Zn²⁺ /Na⁺ dual ion electrolyte is employed, NFPP@C shows a high capacity of 114.4 mAh g^-1 with two voltage plateaus, excellent rate capability (95 mAh g^-1 at 2 A g^-1 ), and long-term cycling stability (66.4 mAh g^-1 after 1800 cycles at 1 A g^-1 ). The outstanding electrochemical performance is ascribed to the synergistic use of NFPP@C and dual ion electrolyte. The NASICON-structure and carbon layer of NFPP@C enable fast ion and electron transport, whereas Na⁺ in the electrolyte reduces the concentration gradient between the electrode and electrolyte, and thus inhibits excessive extraction of Na⁺ from NFPP, maintaining structural stability. Moreover, Zn²⁺ /Na⁺ co-intercalation in NFPP@C brings two potential platforms and enhanced capacity.

Ultralong-Life Cathode for Aqueous Zinc-Organic Batteries via Pouring 9,10-Phenanthraquinone into Active Carbon

Organic carbonyl electrode materials have shown a great potential in various rechargeable batteries but limited by the problems of poor cycling and rate performance owing to their high solubility in aqueous electrolytes and low conductivity. To address these problems, the 9,10-phenanthraquinone (PQ)@active carbon (AC) composite fabricated by melting PQ molecules into porous AC is considered as a superstable cathode material for aqueous zinc batteries. The introduction of AC improves the structural stability and restrains the PQ dissolution in an aqueous electrolyte. As a result, the PQ@AC composite electrode delivers a reversible discharge capacity of 150.0 mA h g^-1 at a current density of 0.1 A g^-1, and it also features an unprecedented cycling performance of 36 000 cycles with a capacity retention of 96.3% at 5 A g^-1. Moreover, the Zn²⁺ and H⁺ in an aqueous electrolyte are verified to co-insert into the PQ@AC composite electrode using various ex situ characterizations and electrochemical test. This strategy provides a new avenue for organic carbonyl compounds with quinone substructures to improve their electrochemical performance of other batteries.

RGO/Manganese Silicate/MOF-derived carbon Double-Sandwich-Like structure as the cathode material for aqueous rechargeable Zn-ion batteries

Aqueous rechargeable Zn-ion batteries (ARZIBs) have been attracting a great deal of attention due to their immense potential in large-scale power grid applications. It is of great significance to explore cathode material with novel designed structure and first-class performances for ARZIBs. Herein, we successfully construct a double-sandwich-like structure, MOF-derived carbon/manganese silicate/reduced graphene oxide/manganese silicate/MOF-derived carbon (denoted as rGO/MnSi/MOF-C), as the cathode material for ARZIBs. Among the double-sandwich-like structure, manganese silicate (Mn₂SiO₄, denoted as MnSi) is in the middle of internal reduced graphene oxide (rGO) and external MOF-8 derived carbon (MOF-C). This integrated rGO/MnSi/MOF-C with double-sandwich-like structure can not only avert the sluggish electronic conduction progress caused by the conventional three-phase mixture system of rGO, MnSi and MOF-C, but also display promising Zn²⁺ storing capability. As expected, in mild aqueous 2 M (mol L^-1) ZnSO₄ + 0.2 M MnSO₄ electrolyte, the initial discharge capacity of rGO/MnSi/MOF-C cathode reaches to 246 mAh·g^-1, and the peak discharge capacity reaches to 462 mAh·g^-1 at 0.1 A·g^-1. This work not only involves the novel MnSi-based cathode for ARZIBs, but also first demonstrates our assumption of constructing the double-sandwich-like structure to improve Zn²⁺ storage. Moreover, the concept "double-sandwich-like structure" provides an idea for synthesizing the integrated carbon/transition metal silicates (TMSs)/carbon structure to boost the electrochemical properties of TMSs for energy-storing devices.

A Zn-S aqueous primary battery with high energy and flat discharge plateau

We demonstrate a disposable aqueous primary battery chemistry that comprises environmentally benign materials of the sulfur cathode and Zn anode in a 1 M ZnCl₂ aqueous electrolyte. The Zn-S battery shows a high energy density of 1083.3 Wh kg^-1 for sulphur with a flat discharge voltage plateau around 0.7 V. When operating at a high mass loading of 8.3 mg cm^-2 for sulfur in the cathode, the battery exhibits a very high areal capacity of 11.4 mA h cm^-2 and areal energy of 7.7 mW h cm^-2.

Recent Advances on Spinel Zinc Manganate Cathode Materials for Zinc-Ion Batteries

Zinc metal is abundant in nature, non-toxic, harmless, and cheap. Zinc-ion batteries (ZIBs) have also emerged as the times require, which has attracted scholars' research interest. In the zinc-ion batteries, the cathode material is indispensable. Manganese oxides are widely used in electrode materials because of their various valence states (+2, +3, +4, +7). ZnMn₂ O₄ (ZMO) is a mixed metal oxide with a spinel structure similar to LiMn₂ O₄ . Due to the synergistic effect of Zn and Mn, it has the advantages of high theoretical capacity. In recent years, researchers have gradually applied ZnMn₂ O₄ to zinc ion batteries. In order to obtain high-energy-density zinc ion batteries, it is also very important to match electrolytes with a wide operating voltage window and develop a highly reversible anode. In the first instance, we investigate the research progress of spinel ZnMn₂ O₄ as a reliable candidate material for zinc ion batteries. Later on, we review the optimization and modification measures of anode and electrolyte to improve the electrochemical properties of spinel ZnMn₂ O₄ . On this basis, we propose the reasonable research direction and development prospects for this material. It is hoped that there will be a help to researchers in this field.