Negatively charged nanoporous membrane for a dendrite-free alkaline zinc-based flow battery with long cycle life

High-voltage and dendrite-free zinc-iodine flow battery

Zn-I2 flow batteries, with a standard voltage of 1.29 V based on the redox potential gap between the Zn2+-negolyte (-0.76 vs. SHE) and I2-posolyte (0.53 vs. SHE), are gaining attention for their safety, sustainability, and environmental-friendliness. However, the significant growth of Zn dendrites and the formation of dead Zn generally prevent them from being cycled at high current density (>80 mA cm-2). In addition, the crossover of Zn2+ across cation-exchange-membrane also limits their cycle stability. Herein, we propose a chelated Zn(P2O7)26- (donated as Zn(PPi)26-) negolyte, which facilitates dendrite-free Zn plating and effectively prevents Zn2+ crossover. Remarkably, the utilization of chelated Zn(PPi)26- as a negolyte shifts the Zn2+/Zn plating/stripping potential to -1.08 V (vs. SHE), increasing cell voltage to 1.61 V. Such high voltage Zn-I2 flow battery shows a promising stability over 250 cycles at a high current density of 200 mA cm-2, and a high power density up to 606.5 mW cm-2.

The Critical Analysis of Membranes toward Sustainable and Efficient Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFB) are a promising technology for large-scale storage of electrical energy, combining safety, high capacity, ease of scalability, and prolonged durability; features which have triggered their early commercial implementation. Furthering the deployment of VRFB technologies requires addressing challenges associated to a pivotal component: the membrane. Examples include vanadium crossover, insufficient conductivity, escalated costs, and sustainability concerns related to the widespread adoption of perfluoroalkyl-based membranes, e.g., perfluorosulfonic acid (PFSA). Herein, recent advances in high-performance and sustainable membranes for VRFB, offering insights into prospective research directions to overcome these challenges, are reviewed. The analysis reveals the disparities and trade-offs between performance advances enabled by PFSA membranes and composites, and the lack of sustainability in their final applications. The potential of PFSA-free membranes and present strategies to enhance their performance are discussed. This study delves into vital membrane parameters to enhance battery performance, suggesting protocols and design strategies to achieve high-performance and sustainable VRFB membranes.

Tin Hybrid Flow Batteries with Ultrahigh Areal Capacities Enabled by Double Gradients

Tin-based hybrid flow batteries have demonstrated dendrite-free morphology and superior performance in terms of cycle life and energy density. However, the quick accumulation of electrodeposits near the electrode/membrane interface blocks the ion transport pathway during the charging of the battery, resulting to a very limited areal capacity (especially at high current density) that significantly hinders its deployment in long-duration storage applications. Herein, a conductivity-activity dual-gradient design is disclosed by electrically passivating the carbon felt near the membrane/electrode interface and chemically activating the carbon felt near the electrode/current collector interface. In consequence, the tin metals are preferentially plated at the region near electrode/current collector, preventing the ion transport pathway from being easily blocked. The resultant gradient electrode demonstrated an unprecedentedly high areal capacity of 268 mAh cm-2 at a current density of as high as 80 mA cm-2. Numerical modeling and experimental characterizations show that the dual-gradient electrode differs from conventional electrodes with regard to their reaction current density distribution and electrodeposit distribution during charging. This work demonstrates a new design strategy of 3D electrodes for hybrid flow batteries to induce a desirable distribution of electrodeposits and achieve a high areal capacity at commercially relevant current densities.

Porous Ceramic Metal-Based Flow Battery Composite Membrane

In metal-based flow battery, membranes significantly impact energy conversion efficiency and security. Unfortunately, damages to the membrane occur due to gradual accumulation of metal dendrites, causing short circuits and shortening cycle life. Herein, we developed a rigid hierarchical porous ceramic flow battery composite membrane with a sub-10-nm-thick polyelectrolyte coating to achieve high ion selectivity and conductivity, to restrain dendrite, and to realize long cycle life and high areal capacity. An aqueous zinc-iron flow battery prepared using this membrane achieved an outstanding energy efficiency of >80%, exhibiting excellent long-term stability (over 1000 h) and extremely high areal capacity (260 mAh cm-2). Low-field nuclear magnetic resonance (NMR) spectroscopy, small-angle X-ray scattering, in situ infrared spectroscopy, solid-state NMR analysis, and nano-computed tomography revealed that the rigid hierarchical pore structures and numerous hydrogen bonding networks in the membrane contributed to the stable operation and superior battery performance. This study contributes to the development of next-generation metal-based flow battery membranes for energy and power generation.

Aqueous Rechargeable Zn-Iodine Batteries: Issues, Strategies and Perspectives

The static aqueous rechargeable Zn-Iodine batteries (ARZiBs) have been studied extensively because of their low-cost, high-safety, moderate voltage output, and other unique merits. Nonetheless, the poor electrical conductivity and thermodynamic instability of the iodine cathode, the complicated conversion mechanism, and the severe interfacial reactions at the Zn anode side induce their low operability and unsatisfactory cycling stability. This review first clarifies the typical configuration of ARZiBs with a focus on the energy storage mechanism and uncovers the issues of the ARZiBs from a fundamental point of view. After that, it categorizes the recent optimization strategies into cathode fabrication, electrolyte modulation, and separator/anode modification; and summarizes and highlights the achieved progress of these strategies in advanced ARZiBs. Given that the ARZiBs are still at an early stage, the future research outlook is provided, which hopefully may guide the rational design of advanced ARZiBs.

An Innovative Concept of Membrane-Free Redox Flow Batteries with Near-Zero Contact Distance Between Electrodes

Given that the ion-exchange membrane takes up more than 30% of redox flow battery (RFB) cost, considerable cost reduction is anticipated with the membrane-free design. However, eliminating the membrane/separator would expose the membrane-free RFBs to a higher risk of short-circuits, and the dendrite growth may aggravate this issue. The current strategy based on expanding distances between electrodes is proposed to address short-circuit issues. Nevertheless, this approach would decrease the energy efficiency (EE) and could not restrain dendrite growth fundamentally. Herein, an inexpensive and electron-insulating boron nitride nanosheets (BNNSs)-Nylon hybrid interlayer (BN/Nylon) is developed for general membrane-free RFBs to achieve "near-zero distance" contact between electrodes. And the Lewis acid sites (B atoms) in BNNS can interact with the Lewis base anions in electrolytes, enabling a reduced Pb2+concentration gradient. Additionally, the ultrahigh thermal conductivity and mechanical strength of BNNSs promote the uniform plating/stripping process of Pb and PbO2. Compared with conventional soluble lead RFBs, introducing BN/Nylon interlayers boosts EE by ≈38.2% at 25 mA cm-2, and extends the cycle life to 100 cycles. This innovative strategy premieres the application of the BN/Nylon interlayer concept, offering a novel perspective for the development of general membrane-free RFBs.

Extending the Cycle Lifetime of Solid-State Zinc-Air Batteries by Arranging Stable Zinc Species Channels

The solid-state zinc-air batteries have attracted extensive attention due to their high theoretical energy density, high safety, and the compact structure. In this work, a novel hydrogel solid-state electrolyte was developed that was equipped with an interpenetrating network of zinc polyacrylate (PAZn) and polyacrylamide (PAM). At the same time, a cyclodextrin derivative with sulfonate groups was introduced as an additive. From the design of anionic groups in the network, effective and stable channels for zinc species have been established. The unique structure of the additives regulates the uniform deposition of zinc. After using this solid-state electrolyte, the cycle lifetime of solid-state zinc-air batteries assembled have been significantly extended. The byproducts were greatly suppressed and generated the smooth zinc electrode surface after the charge-discharge cycling.

Functional materials for aqueous redox flow batteries: merits and applications

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, offering vast potential for large-scale applications. Their unique configuration allows energy and power to be decoupled, making them highly scalable and flexible in design. Aqueous RFBs stand out as the most promising technologies, primarily due to their inexpensive supporting electrolytes and high safety. For aqueous RFBs, there has been a skyrocketing increase in studies focusing on the development of advanced functional materials that offer exceptional merits. They include redox-active materials with high solubility and stability, electrodes with excellent mechanical and chemical stability, and membranes with high ion selectivity and conductivity. This review summarizes the types of aqueous RFBs currently studied, providing an outline of the merits needed for functional materials from a practical perspective. We discuss design principles for redox-active candidates that can exhibit excellent performance, ranging from inorganic to organic active materials, and summarize the development of and need for electrode and membrane materials. Additionally, we analyze the mechanisms that cause battery performance decay from intrinsic features to external influences. We also describe current research priorities and development trends, concluding with a summary of future development directions for functional materials with valuable insights for practical applications.

Redox Targeting-based Neutral Aqueous Flow Battery with High Energy Density and Low Cost

Neutral aqueous flow batteries with common traits of the redox flow batteries, such as the independence of energy and power, scalability and operational flexibility, and additional merits of outstanding safety and low corrosivity show great promise for storing massive electrical energy from solar and wind energy. Particularly, the ferricyanide/ferrocyanide ([Fe(CN)6 ]3-/4- ) couple has been intensively employed as redox mediator to store energy in the catholyte ascribed to its abundance, low corrosivity, remarkable redox reversibility and stability. However, the low energy density arising from poor solubility of [Fe(CN)6 ]3-/4- restricts their commercial applications for energy storage systems. In this study, the practical energy density of a [Fe(CN)6 ]3-/4- -based catholyte is significantly boosted from 10.5 to 92.8 Wh L-1 by combining the counter-ion effect and the single-molecule redox-targeting (SMRT) reactions between [Fe(CN)6 ]3-/4- and Prussian blue (Fe4 [Fe(CN)6 ]3 , PB)/Prussian white (PW). Paired with concentrated K2 S anolyte, we demonstrate a neutral aqueous SMRT-based PB-Fe/S flow battery with ultra-long lifespan over 7000 cycles (4500 h) and ultra-low chemical cost of electrolytes in the cell as 19.26 $ kWh-1 . Remarkably, under the influences of SMRT reactions in the presence of PB granules in the catholyte, the capacity after 7000 cycles of the PB-Fe/S flow battery is 181.8 % of the initial capacity without PB.

Low-cost Zinc-Iron Flow Batteries for Long-Term and Large-Scale Energy Storage

Aqueous flow batteries are considered very suitable for large-scale energy storage due to their high safety, long cycle life, and independent design of power and capacity. Especially, zinc-iron flow batteries have significant advantages such as low price, non-toxicity, and stability compared with other aqueous flow batteries. Significant technological progress has been made in zinc-iron flow batteries in recent years. Numerous energy storage power stations have been built worldwide using zinc-iron flow battery technology. This review first introduces the developing history. Then, we summarize the critical problems and the recent development of zinc-iron flow batteries from electrode materials and structures, membranes manufacture, electrolyte modification, and stack and system application. Finally, we forecast the development direction of the zinc-iron flow battery technology for large-scale energy storage.

Exploring the Mechanism of Single-Crystal MnO2 as Cathodes for Zinc Ion Batteries

MnO2 has the advantages of low cost and abundant resources, so it is considered to be an important electrode material in zinc ion batteries. However, its practical application is still challenged by easy collapse and capacity loss. In this paper, a stable single crystal β-MnO2 nanorod cathode material was prepared. When used as ZIBs cathode material, single crystal β-MnO2 has high ionic diffusion kinetics and calculability. In this paper, we prepared single-crystal MnO2 through hydrothermal nanotechnology. By leveraging the benefits of the single-crystal structure, we optimized the structural stability, ion conductivity, surface reactions, and phase control of the cathode material, resulting in improved battery performance and cycle life. In the fabricated single-crystal MnO2 aqueous zinc-ion battery, the elimination of internal crystal faces in MnO2 leads to ordered lattice arrangement, enabling a more direct and unobstructed diffusion path for H+ ions within the lattice. This significantly enhances the ion conductivity of the cathode material, promoting the rate and efficiency of the battery's charge and discharge processes. Therefore, single-crystal MnO2 exhibits excellent cycling performance for zinc-ion storage in ZIBs, achieving a high specific capacity of 224.7 mA h g-1 after 250 cycles under a current density of 0.3 A g-1 , while maintaining a Coulombic efficiency of 99.58 %.

Thermo-Osmosis in Charged Nanochannels: Effects of Surface Charge and Ionic Strength

Thermo-osmosis refers to fluid migration due to the temperature gradient. The mechanistic understanding of thermo-osmosis in charged nano-porous media is still incomplete, while it is important for several environmental and energy applications, such as low-grade waste heat recovery, wastewater recovery, fuel cells, and nuclear waste storage. This paper presents results from a series of molecular dynamics simulations of thermo-osmosis in charged silica nanochannels that advance the understanding of the phenomenon. Simulations with pure water and water with dissolved NaCl are considered. First, the effect of surface charge on the sign and magnitude of the thermo-osmotic coefficient is quantified. This effect was found to be mainly linked to the structural modifications of an aqueous electrical double layer (EDL) caused by the nanoconfinement and surface charges. In addition, the results illustrate that the surface charges reduce the self-diffusivity and thermo-osmosis of interfacial liquid. The thermo-osmosis was found to change direction when the surface charge density exceeds -0.03C · m^-2. It was found that the thermo-osmotic flow and self-diffusivity increase with the concentration of NaCl. The fluxes of solvent and solute are decoupled by considering the Ludwig-Soret effect of NaCl ions to identify the main mechanisms controlling the behavior. In addition to the advance in microscopic quantification and mechanistic understanding of thermo-osmosis, the work provides approaches to investigate a broader category of coupled heat and mass transfer problems in nanoscale space.

Thin Film Composite Membranes with Regulated Crossover and Water Migration for Long-Life Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are promising for large-scale long-duration energy storage owing to their inherent safety, decoupled power and energy, high efficiency, and longevity. Membranes constitute an important component that affects mass transport processes in RFBs, including ion transport, redox-species crossover, and the net volumetric transfer of supporting electrolytes. Hydrophilic microporous polymers, such as polymers of intrinsic microporosity (PIM), are demonstrated as next-generation ion-selective membranes in RFBs. However, the crossover of redox species and water migration through membranes are remaining challenges for battery longevity. Here, a facile strategy is reported for regulating mass transport and enhancing battery cycling stability by employing thin film composite (TFC) membranes prepared from a PIM polymer with optimized selective-layer thickness. Integration of these PIM-based TFC membranes with a variety of redox chemistries allows for the screening of suitable RFB systems that display high compatibility between membrane and redox couples, affording long-life operation with minimal capacity fade. Thickness optimization of TFC membranes further improves cycling performance and significantly restricts water transfer in selected RFB systems.

A High Voltage Aqueous Zinc-Vanadium Redox Flow Battery with Bimodal Tin and Copper Clusters by a Continuous-Flow Electrometallic Synthesis

Aqueous zinc-based redox flow batteries are promising large-scale energy storage applications due to their low cost, high safety, and environmental friendliness. However, the zinc dendritic growth has depressed the cycle performance, stability, and efficiency, hindering the commercialization of the zinc-based redox flow batteries. We fabricate the carbon felt modified with bimodal sized tin and copper clusters (SCCF) with the electrometallic synthesis in a continuous-flow cell. The SCCF electrode provides a larger zinc nucleation area and lower overpotential than pristine carbon felt, which is ascribed to the well-controlled interfacial interaction of bimodal tin and copper particle clusters by suppressing unwanted alloy formation. The zinc symmetric flow battery and the zinc-based hybrid redox flow battery show the improved zinc plating and stripping efficiency. The SCCF electrode exhibits 75% improved cycling stability compared to the pristine carbon felt electrode in the zinc symmetric flow battery. Notably, the high-voltage aqueous zinc-vanadium redox flow battery demonstrates a high average cell voltage of 2.31 V at 40 mA cm^-2, showing a Coulombic efficiency of 99.9% and an energy efficiency of 87.6% for 100 cycles. We introduce a facile strategy to suppress the zinc dendritic growth, enhancing the performance of the zinc-based redox flow batteries.

Functionalized Nanocomposite Gel Polymer Electrolyte with Strong Alkaline-Tolerance and High Zinc Anode Stability for Ultralong-Life Flexible Zinc-Air Batteries

Increasing pursuit of next-generation wearable electronics has put forward the demand of reliable energy devices with high flexibility, durability, and enhanced electrochemical performances. Flexible aqueous zinc-air batteries (FAZABs) have attracted great interests owing to the high energy density, safety, and environmental benignity, for which quasi-solid-state gel polymer electrolytes (QSGPEs) are state-of-the-art electrolytes with high ionic conductivity, flexibility, and resistance to leakage problems of traditional liquid electrolytes. Compared to commonly used PVA-KOH electrolyte with poor electrolyte retention capability and cycling stability, a new type of sulfonate functionalized nanocomposite QSGPE is applied in FAZABs with high ionic conductivity, strong alkaline tolerance, and high zinc anode stability. Notably, the existence of (1) strong anionic sulfonate groups of QSGPEs, contributing to the exposure of preferred Zn (002) plane that is more resistant to zinc dendrite formation, and (2) nano-attapulgite electrolyte additives, beneficial for the enhancement of ionic conductivity, electrolyte uptake, and retention capability, endows a ultralong cycling life of 450 h for the fabricated FAZAB. Furthermore, flexible energy belts and knittable energy wires fabricated with a series/parallel unit of several FAZABs can be used to power various wearable electronics.

Application of Poly(ether sulfone)-Based Membranes in Clean Energy Technology

Poly(ether sulfone) (PES) is a kind of polymer materials with excellent electrical insulation and acid/alkali stability. PES can be operated at high temperature continuously for a long time and still maintain excellent property stability in the environments with rapidly changed temperature, namely, great thermostability. Moreover, PES has low molding shrinkage, good dimensional stability and excellent film-forming characteristics. Compared with inorganic membranes, PES-based membranes have lower cost, which have received more attention and wide recognition in the field of clean energy technologies in recent years, such as flow batteries, fuel cells, water treatment, and gas separation. Therefore, this review summarizes the research status and prospect of the utilization of PES-based membranes in clean energy fields, in order to further promote their development and application.

Recent Advances in the Unconventional Design of Electrochemical Energy Storage and Conversion Devices

As the world works to move away from traditional energy sources, effective efficient energy storage devices have become a key factor for success. The emergence of unconventional electrochemical energy storage devices, including hybrid batteries, hybrid redox flow cells and bacterial batteries, is part of the solution. These alternative electrochemical cell configurations provide materials and operating condition flexibility while offering high-energy conversion efficiency and modularity of design-to-design devices. The power of these diverse devices ranges from a few milliwatts to several megawatts. Manufacturing durable electronic and point-of-care devices is possible due to the development of all-solid-state batteries with efficient electrodes for long cycling and high energy density. New batteries made of earth-abundant metal ions are approaching the capacity of lithium-ion batteries. Costs are being reduced with the advent of flow batteries with engineered redox molecules for high energy density and membrane-free power generating electrochemical cells, which utilize liquid dynamics and interfaces (solid, liquid, and gaseous) for electrolyte separation. These batteries support electrode regeneration strategies for chemical and bio-batteries reducing battery energy costs. Other batteries have different benefits, e.g., carbon-neutral Li-CO2 batteries consume CO2 and generate power, offering dual-purpose energy storage and carbon sequestration. This work considers the recent technological advances of energy storage devices. Their transition from conventional to unconventional battery designs is examined to identify operational flexibilities, overall energy storage/conversion efficiency and application compatibility. Finally, a list of facilities for large-scale deployment of major electrochemical energy storage routes is provided.

Graphical abstract

A solid-to-solid metallic conversion electrochemistry toward 91% zinc utilization for sustainable aqueous batteries

The diffusion-limited aggregation (DLA) of metal ion (Mⁿ⁺) during the repeated solid-to-liquid (StoL) plating and liquid-to-solid (LtoS) stripping processes intensifies fatal dendrite growth of the metallic anodes. Here, we report a new solid-to-solid (StoS) conversion electrochemistry to inhibit dendrites and improve the utilization ratio of metals. In this StoS strategy, reversible conversion reactions between sparingly soluble carbonates (Zn or Cu) and their corresponding metals have been identified at the electrode/electrolyte interface. Molecular dynamics simulations confirm the superiority of the StoS process with accelerated anion transport, which eliminates the DLA and dendrites in the conventional LtoS/StoL processes. As proof of concept, 2ZnCO₃·3Zn(OH)₂ exhibits a high zinc utilization of ca. 95.7% in the asymmetry cell and 91.3% in a 2ZnCO₃·3Zn(OH)₂ || Ni-based full cell with 80% capacity retention over 2000 cycles. Furthermore, the designed 1-Ah pouch cell device can operate stably with 500 cycles, delivering a satisfactory total energy density of 135 Wh kg^-1.

Bio-Derived and Cost-Effective Membranes with High Selectivity for Redox Flow Batteries Based on Host-Guest Chemistry

Redox flow batteries (RFBs) stand out as a promising energy storage system to solve the grid interconnection problems of renewable energy. Membranes play a critical role in regulating the performance of RFBs, and the selectivity is commonly controlled via either size exclusion or Donnan exclusion. Membranes typically account for 40% of the stack cost of RFBs, and it is essential to develop cost-effective membranes with high selectivity to achieve widespread application. Here, a type of membrane composed of highly abundant materials derived in nature, based on a scalable fabrication process, is reported. Moreover, high selectivity is achieved attributed to the host-guest interactions between membranes and redox species, which effectively alleviate the crossover of redox-active molecules. By incorporating starch into a chitosan matrix for zinc-iodine RFBs, the highly selective recognition of starch and chitosan (host) toward triiodide (guest) builds a "wall" to block the triiodide-based active materials, meanwhile, the conducting properties of such a membrane are not compromised. The proof-of-concept battery delivers a Coulombic efficiency of 98.6% and energy efficiency of 77.4% at a current density of 80 mA cm^-2 , showing the promise of such a novel and cost-effective membrane design beyond traditional selectivity chemistry.

Emerging chemistries and molecular designs for flow batteries

Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.

Development of efficient aqueous organic redox flow batteries using ion-sieving sulfonated polymer membranes

Redox flow batteries using aqueous organic-based electrolytes are promising candidates for developing cost-effective grid-scale energy storage devices. However, a significant drawback of these batteries is the cross-mixing of active species through the membrane, which causes battery performance degradation. To overcome this issue, here we report size-selective ion-exchange membranes prepared by sulfonation of a spirobifluorene-based microporous polymer and demonstrate their efficient ion sieving functions in flow batteries. The spirobifluorene unit allows control over the degree of sulfonation to optimize the transport of cations, whilst the microporous structure inhibits the crossover of organic molecules via molecular sieving. Furthermore, the enhanced membrane selectivity mitigates the crossover-induced capacity decay whilst maintaining good ionic conductivity for aqueous electrolyte solution at pH 9, where the redox-active organic molecules show long-term stability. We also prove the boosting effect of the membranes on the energy efficiency and peak power density of the aqueous redox flow battery, which shows stable operation for about 120 h (i.e., 2100 charge-discharge cycles at 100 mA cm⁻²) in a laboratory-scale cell.

Aqueous organic redox flow batteries are promising for grid-scale energy storage, although their practical application is still limited. Here, the authors report highly ion-conductive and selective polymer membranes, which boost the battery’s efficiency and stability, offering cost-effective electricity storage.

Anion Concentration Gradient-Assisted Construction of a Solid-Electrolyte Interphase for a Stable Zinc Metal Anode at High Rates

Coulombic efficiency (CE) and cycle life of metal anodes (lithium, sodium, zinc) are limited by dendritic growth and side reactions in rechargeable metal batteries. Here, we proposed a concept for constructing an anion concentration gradient (ACG)-assisted solid-electrolyte interphase (SEI) for ultrahigh ionic conductivity on metal anodes, in which the SEI layer is fabricated through an in situ chemical reaction of the sulfonic acid polymer and zinc (Zn) metal. Owing to the driving force of the sulfonate concentration gradient and high bulky sulfonate concentration, a promoted Zn²⁺ ionic conductivity and inhibited anion diffusion in the SEI layer are realized, resulting in a significant suppression of dendrite growth and side reaction. The presence of ACG-SEI on the Zn metal enables stable Zn plating/stripping over 2000 h at a high current density of 20 mA cm^-2 and a capacity of 5 mAh cm^-2 in Zn/Zn symmetric cells, and moreover an improved cycling stability is also observed in Zn/MnO₂ full cells and Zn/AC supercapacitors. The SEI layer containing anion concentration gradients for stable cycling of a metal anode sheds a new light on the fundamental understanding of cation plating/stripping on metal electrodes and technical advances of rechargeable metal batteries with remarkable performance under practical conditions.

Recent Advances of Aqueous Rechargeable Zinc-Iodine Batteries: Challenges, Solutions, and Prospects

Aqueous rechargeable zinc-iodine batteries (ZIBs), including zinc-iodine redox flow batteries and static ZIBs, are promising candidates for future grid-scale electrochemical energy storage. They are safe with great theoretical capacity, high energy, and power density. Nevertheless, to make aqueous rechargeable ZIBs practically feasible, there are quite a few hurdles that need to be overcome, including self-discharge, sluggish kinetics, low energy density, and instability of Zn metal anodes. This article first reviews the electrochemistry in aqueous rechargeable ZIBs, including the flow and static battery configurations and their electrode reactions. Then the authors discuss the fundamental questions of ZIBs and highlight the key strategies and recent accomplishments in tackling the challenges. Last, they share their thoughts on the future research development in aqueous rechargeable ZIBs.

Magnetic zinc-air batteries for storing wind and solar energy

With the consensus on carbon peak and neutrality around the globe, renewables, especially wind and solar PV will grow fast. Correspondingly, the batteries for renewables would be scheduled to meet the requirements of performance, lifetime, cost, safety, and environment. Rechargeable zinc-air battery is a promising candidate for energy storage. However, the lifetime and power density of zinc-air batteries remain unresolved. Here we propose a concept of magnetic zinc-air batteries to achieve the demand of the next generation energy storage. Firstly, an external magnetic field can effectively inhibit dendrite growth of the zinc depositing layer and expel H₂ or O₂ bubbles away from the electrode’s surface, extending the battery life. Secondly, magnetic fields can promote electrons, ions, and O₂ transfer, enhancing power density of zinc-air batteries. Lastly, four schemes to generate magnetic fields for zinc-air batteries are exhibited to fulfill battery energy storage demand of high performance and long service life.

Ion Sieve: Tailoring Zn2+ Desolvation Kinetics and Flux toward Dendrite-Free Metallic Zinc Anodes

Tip-induced dendrites on metallic zinc anodes (MZAs) fundamentally deteriorate the rechargeability of aqueous Zn metal batteries (ZMBs). Herein, an intriguing ion sieve (IS) consisting of 3D intertwined bacterial cellulose, deposited on the surface of MZAs (Zn@IS) through an in situ self-assembly route, is first presented to be effective in inhibiting dendrite-growth on MZAs. Experimental analyses together with theoretical calculations suggested that the IS coating can facilitate the desolvation of [Zn(H₂O)₆]²⁺ clusters via a strong interplay with Zn ions, weaken hydrogen evolution reaction of MZAs, and homogenize the ion flux with the abundant nanopores serving as ion tunnels, synergistically enabling dendrite-free Zn deposition on the Zn@IS anodes. Consequently, a lifespan up to 3000 h at a cutoff capacity of 0.25 mA h cm^-2 was observed in a Zn@IS∥Zn@IS symmetric cell. In terms of application, pairing with a carbon-nanotube@MnO₂ cathode as an example, the full ZMBs acquired enhanced rechargeability with much higher capacity retention over 73.3% after 3000 cycles compared to the counterpart with pristine MZA (21%).

Dendrite-Free Zinc-Based Battery with High Areal Capacity via the Region-Induced Deposition Effect of Turing Membrane

Zinc-based batteries are promising for use as energy storage devices owing to their low cost and high energy density. However, zinc chemistry commonly encounters serious dendrite issues, especially at high areal capacities and current densities, limiting their application. Herein, we propose a novel membrane featuring ordered undulating stripes called "Turing patterns", which can effectively suppress zinc dendrites and improve ion conductivity. The crests and troughs in the Turing membrane can effectively adjust the Zn(OH)₄^2- distribution and provide more zinc deposition space. The coordinated Cu ions during membrane formation can interact with Zn(OH)₄^2-, further smoothing zinc deposition. Even at a high current density of 80 mA·cm^-2, the Turing membrane enables an alkaline zinc-iron flow battery (AZIFB) to work stably with an ultrahigh areal capacity of 160 mA·h·cm^-2 for approximately 110 cycles, showing an energy efficiency of 90.10%, which is by far the highest value ever reported among zinc-based batteries with such a high current density. This paper provides valid access to zinc-based batteries with high areal capacities based on membrane design and promotes their advancement.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Act in contravention: a non-planar coupled electrode design utilizing "tip effect" for ultra-high areal capacity, long cycle life zinc-based batteries

Aqueous zinc-based batteries (ZBBs) have great potential as commercial energy storage devices. However, the poor cycling stability of zinc anode under high areal capacity limits their further application. Herein, a coupled non-planar electrode design achieved by the tailored flat-top pyramid carbon felt (TCF) is proposed for ZBBs, which can effectively increase the zinc deposition sites, adjust the deposition morphology, optimize the current and electrolyte flow velocity distribution and provide necessary space for zinc plating. Interestingly, by utilizing "tip effect", the coupled TCFs enable precise control of the zinc dendrite growth position, effectively reducing the risk of short circuit. Based on such coupled TCFs, zinc-iodine flow batteries can deliver an ultra-high areal capacity of 240 mAh cm^-2 and a superb cycling stability over 300 cycles (areal capacity of 160 mAh cm^-2) at a high current density of 40 mA cm^-2. Therefore, we provide an effective strategy for high areal capacity zinc anode design, which may promote the development of high energy density and long cycle life ZBBs.

Electrokinetic-Driven Fast Ion Delivery for Reversible Aqueous Zinc Metal Batteries with High Capacity

Aqueous zinc (Zn) metal batteries (ZMBs) are considered a promising candidate for grid-scale energy storage due to their freedom from fire hazards. However, a limited reversibility of Zn metal electrode caused by dendritic Zn growth has hindered the advent of high-capacity Zn metal batteries (>4 mAh cm^-2 ). Herein, it is reported that fast electrokinetic Zn-ion transport extremely improves the Zn metal reversibility. It is revealed that a negatively charged porous layer (NPL) provides the electrokinetic Zn-ion transport by surface conduction, and consequently impedes the depletion of Zn-ion on electrode surface as indicated by numerical simulations and overlimiting current behavior. Due to the quick Zn-ion delivery, a dendrite-free and densely packed Zn metal deposit is accommodated inside its pores. With the introduction of the NPL, the cycling stability of Zn symmetric cell is enhanced by 21 times at 10 mA cm^-2 /10 mAh cm^-2 . Average Coulombic efficiency of 99.6% is achieved over 500 cycles for electrodeposition/stripping at 30 mA cm^-2 /5 mAh cm^-2 on NPL-Cu electrode. Furthermore, a high-capacity Zn/V₂ O₅ full cell with the NPL exhibits an extraordinary stability over 1000 cycles at a capacity of 4.8 mAh cm^-2 .

A Dendrite-Free Tin Anode for High-Energy Aqueous Redox Flow Batteries

Metal-based aqueous redox flow batteries (ARFBs) such as zinc-based ARFBs have attracted remarkable attention owing to their intrinsic high energy density. However, severe dendrite issues limit their efficiency and lifespan. Here an aqueous metal anode operating between Sn(OH)₆ ^2- (stannate) and metal Sn is presented, providing a reversible four-electron transfer at -0.921 V vs standard hydrogen electrode. In strong contrast to severe Zn dendrites, the Sn(OH)₆ ^2- /Sn electrode shows smooth and dendrite-free morphology, which can be attributed to its intrinsic low-surface-energy anisotropy which facilitates isotropic crystal growth of Sn metal. By coupling with iodide/tri-iodide (I^- /I₃ ^- ), the static Sn-I cell demonstrates a stable cycling for 500 cycles (more than 2 months). In contrast, the state-of-the-art Zn anode suffers from serious dendrites and lasts less than 45 cycles (190 h) in Zn-I cells. A stable continuous flow cycling of Sn-I cell achieves a Sn areal capacity of 73.07 mAh cm^-2 at an average discharge voltage of 1.3 V for 350 h. The alkaline Sn electrode demonstrates dendrite-free morphology and superior performance in cycle life and areal capacity compared to state-of-the-art Zn metal anodes, offering a promising metal anode for high-energy ARFBs and other metal-based rechargeable aqueous batteries.

Development of titanium 3D mesh interlayer for enhancing the electrochemical performance of zinc-bromine flow battery

Zinc dendrite growth negatively affects zinc–bromine flow battery (ZBB) performance by causing membrane damage, inducing self-discharge. Herein, in a ZBB, a conventional polymer mesh was replaced with a titanium-based mesh interlayer; this provided additional abundant active sites for the Zn²⁺/Zn redox reaction and well-developed electrolyte flow channels, which resulted in improved reaction kinetics and suppressed Zn dendrite growth. Compared with a ZBB cell comprising a conventional polymer mesh and a carbon-based electrode, the ZBB cell using the titanium mesh interlayer and a carbon-based electrode showed significantly reduced frequency of the refreshing process, which occurs at regular cycling intervals during practical use for removing residual zinc dendrites in ZBB; also, the average energy efficiency at a current density of 40 mA cm⁻² increased by 38.5%. Moreover, the modified ZBB cell exhibited higher energy efficiency at a high current density of 80 mA cm⁻², which is an improvement of 14.7% than in case of the contemporary polymer mesh. Consequently, this study can provide helpful insights for new anode side structures including spacer mesh for developing high-performance ZBBs.

Effect of Electrolyte Additives on the Water Transfer Behavior for Alkaline Zinc-Iron Flow Batteries

Alkaline zinc-iron flow batteries (AZIFBs) are a very promising candidate for electrochemical energy storage. The electrolyte plays an important role in determining the energy density and reliability of a battery. The substantial water migration through a membrane during cycling is one of the critical issues that affect the reliability and performance of an AZIFB. In this work, it has been proven that the reason for water migration in AZIFBs is the synergetic effect of concentration gradient, different ionic strengths of negolyte and posolyte, and the electric field. To address the issue of water migration in AZIFBs, a series of additives are employed and the effects of additives on the water transfer behavior and electrochemical performance of AZIFBs are investigated in detail. The results indicate that all investigated additives can suppress water migration through a polybenzimidazole membrane because of the shrunken gap of osmotic pressure and ionic strength between negolyte and posolyte. Moreover, organic additives such as glucose can decrease battery performance because of the increased polarizability of the electrode, whereas inorganic additives such as Na₂SO₄ demonstrate no distinct effect on battery performance. Specifically, an AZIFB that employs Na₂SO₄ as an additive in the negative electrolyte can afford a Coulombic efficiency of ∼99% and a voltage efficiency of ∼88% for 120 cycles at 80 mA cm^-2, together with a good effect for inhibiting water migration behavior. This paper presents an effective way to suppress water migration and increase the reliability of AZIFBs.

Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the commercialization of ARFB energy-storage systems are highlighted.

A Metal-Organic Framework as a Multifunctional Ionic Sieve Membrane for Long-Life Aqueous Zinc-Iodide Batteries

The introduction of the redox couple of triiodide/iodide (I₃ ^- /I^- ) into aqueous rechargeable zinc batteries is a promising energy-storage resource owing to its safety and cost-effectiveness. Nevertheless, the limited lifespan of zinc-iodine (Zn-I₂ ) batteries is currently far from satisfactory owing to the uncontrolled shuttling of triiodide and unfavorable side-reactions on the Zn anode. Herein, space-resolution Raman and micro-IR spectroscopies reveal that the Zn anode suffers from corrosion induced by both water and iodine species. Then, a metal-organic framework (MOF) is exploited as an ionic sieve membrane to simultaneously resolve these problems for Zn-I₂ batteries. The multifunctional MOF membrane, first, suppresses the shuttling of I₃ ^- and restrains related parasitic side-reaction on the Zn anode. Furthermore, by regulating the electrolyte solvation structure, the MOF channels construct a unique electrolyte structure (more aggregative ion associations than in saturated electrolyte). With the concurrent improvement on both the iodine cathode and the Zn anode, Zn-I₂ batteries achieve an ultralong lifespan (>6000 cycles), high capacity retention (84.6%), and high reversibility (Coulombic efficiency: 99.65%). This work not only systematically reveals the parasitic influence of free water and iodine species to the Zn anode, but also provides an efficient strategy to develop long-life aqueous Zn-I₂ batteries.

A High-Performance Aqueous Zinc-Bromine Static Battery

The highly reversible zinc-bromine redox couple has been successfully applied in the zinc-bromine flow batteries; however, non-electroactive pump/pipe/reservoir parts and ion-selective membranes are essential to suppress the bromine diffusion. This work demonstrates a zinc-bromine static (non-flow) battery without these auxiliary parts and utilizing glass fiber separator, which overcomes the high self-discharge rate and low energy efficiency while the advantages of the zinc-bromine chemistry are well preserved. It is achieved by a multifunctional additive, tetrapropylammonium bromide (TPABr), which not only mitigates the bromine cross-diffusion by regulating the fluidic bromine to a condensed solid phase but also provides a favorable interface for zinc electrodeposition toward non-dendritic growth. The proposed zinc-bromine static battery demonstrates a high specific energy of 142 Wh kg-1 with a high energy efficiency up to 94%. By optimizing the porous electrode architecture, the battery shows an ultra-stable cycling life for over 11,000 cycles with controlled self-discharge rate.