Anode optimization strategies for aqueous zinc-ion batteries

Overcoming the Challenges in Aqueous Zinc Metal Batteries: Underlying Issues and Mitigation Strategies

The increasing demand for green and clean energy harvesting and their judicious storage call for pursuing new energy storage technologies. Building better batteries has drawn significant attention to fulfilling the energy demand by delivering the stored electrical energy at the anticipated time and minimal cost. Li-ion batteries play a crucial role in transitioning to a sustainable energy landscape. However, their safety and environmental issues are of concern. Zn-based batteries provide more sustainable solutions due to their low cost, enhanced safety, and environmental benignity. Still, poor thermodynamic reversibility and stability of Zn anode in the aqueous electrolytes prevent its practical application. Significant efforts such as Zn anode surface engineering and electrolyte and/or interface modification alleviate these issues. However, in-depth studies of the root causes associated with the reversibility and stability issues of Zn electrodes are still deficient. Hence, this review focuses on the underlying causes of the major issues (dendrite, hydrogen evolution, corrosion, and passivation) associated with Zn anodes. Furthermore, we have summarized the technological advances that have been made to address these issues. Finally, some promising future directions and perspectives are provided for a further in-depth understanding of thermodynamic irreversibility and to improve the overall performance of the Zn anode.

Hydrazide chemistry for durable, dendrite-free zinc anodes: Insights into solvation structures and electrolyte interfaces

The practical application of aqueous Zn-ion batteries (AZIBs) is impeded by intrinsic limitations of Zn metal anode. Herein, benzoyl hydrazine (BH), incorporating multiple functional groups, serves as an electrolyte additive to mitigate interfacial side reactions and thereby enhancing its cycling durability. Experimental characterizations incorporating with theoretical simulations reveal that the BH molecule can not only reconstruct the solvation configuration by replacing the coordinated water of hydrated Zn2+, but strengthen hydrogen-bond network by forming strong hydrogen bonds, thus suppressing proton transport and active water decomposition. Additionally, BH molecules adsorb onto the Zn anode surface, establishing a versatile protective interfacial layer: hydrophobic benzene ring ligand can impede the direct desolvation of [Zn(H2O)6]2+ on the anode surface; nucleophilic sites coordinate desolvated Zn2+, modulating its flux to promote uniform Zn deposition and stripping. Consequently, Zn//Zn cells with low dosage of BH display a extended cycle life of 1200 h, and Zn//Cu cells run stably for 600 h with maintaining a high coulombic efficiency of 99.55 %. Moreover, Zn//MnO2 full cells with 10 mM BH exhibited superior capacity retention after 1400 cycles. This work proposes a practical strategy by harnessing hydrazide chemistry to facilitate the development of AZIBs.

Multiple Regulation of Electrolyte with Trace Amounts of Sodium Dehydroacetate Additives Enables High-Performance Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) draw much attention for low cost and high safety. However, hydrogen evolution reaction (HER) and uneven Zn2+ deposition shorten lifespan, hampering commercial use. In this study, sodium dehydroacetate (SD) containing carbonyl and keto-carbonyl is introduced as multifunctional electrolyte additives, which effectively modifies the solvent shell structure, achieving a Zn2+ transference number of up to 0.72. Acting as a hydrogen bond acceptor, SD disrupts the water network structure, thereby increasing the HER overpotential by 22 mV and the corrosion potential by 9 mV. The polar functional groups in SD can reversibly capture H⁺ ions and dynamically neutralize OH⁻ ions, maintaining interfacial pH balance on the zinc anode and suppressing HER. Notably, SD not only alters the electrolyte's kinetic but also induces uniform Zn2+ deposition along the (002) plane, inhibiting dendrite growth and minimizing side reactions. This phenomenon is demonstrated in both symmetric and full-cell configurations. The Zn//Zn symmetric cell achieves an ultra-long cycling lifespan of 2800 hours at 5 mA cm⁻2, and the Zn//VO2 full battery maintains a capacity retention rate of 73.09% after 2000 cycles with a high average coulombic efficiency of 99.98%, underscoring the effectiveness of this electrolyte additive in enhancing battery performance.

Surface Laser Texturing and Alloying: Front-End Design Optimization of Zinc Metal Anode for Dendrite-Free Deposition

A critical barrier to commercializing aqueous Zn-metal batteries lies in the dual challenges of dendritic Zn growth and parasitic side reactions at the anode/electrolyte interface. Here, this study presents a front-end design optimization strategy for Zn metal anodes (ZMAs), combining surface laser texturing with alloying treatment to stabilize the interfacial chemistry. Specifically, laser texturing creates a geometrically ordered microstructure on the Zn surface, while subsequent chemical permeation induces the in situ transformation of this microstructured layer into a CuZn5 alloy, forming the LT-Zn@CuZn5 anode. The geometrically ordered alloy coating homogenizes the electronic filed distribution across the zinc surface and enhances corrosion resistance. Thereby, the LT-Zn@CuZn5 anode demonstrated optimized electrochemical reversibility, sustaining over 3000 cycles at 3 mA cm-2/1 mAh cm-2. This performance translates into a high improvement in the cycling behavior of the assembled Zn||I2 soft pack battery, which acquired an initial capacity of 225.8 mAh g-1 and retained 79.1% after 4000 cycles. In contrast, the counterpart employing untreated Zn foil started with a lower initial capacity of 180.7 mAh g-1 and failed after less than 478 cycles. The demonstrated effective approach improves the front-end design strategy of ZMAs and contributes to the development of dendrite-free ZMAs.

Dextran stabilised hematite: a sustainable anode in aqueous electrolytes

During the last decades, the use of innovative hybrid materials in energy storage devices has led to notable advances in the field. However, further enhancement of their electrochemical performance faces significant challenges nowadays, imposed by the materials used in the electrodes and the electrolyte. Such problems include the high solubility of both the organic and the inorganic anode components in the electrolyte as well as the limited intrinsic electronic conductivity and substantial volume variation of the materials during cycling. The present work focuses on the fabrication of novel and sustainable anode electrodes for use in energy storage devices, utilizing cross-linked oxidized dextran (Ox-Dex) as the binder and hematite (α-Fe2O3) cubes as the active component. The ion diffusion mechanism within the anode electrode materials, as well as their cycling stability, were studied via cyclic voltammetry measurements, using Li+, Zn2+ and Al3+ aqueous electrolytes. The hybrid iron oxide electrodes exhibited the highest electrochemical performance in the Al2(SO4)3 electrolyte (3000 mA g-1), followed by ZnSO4 (2000 mA g-1) and Li2SO4 (800 mA g-1). The differences in the performance of the anodes for the three investigated electrolytes were attributed to the ionic radii of Li+, Zn2+ and Al3+, which affect the rate of ion diffusion within the material lattice exhibiting the highest diffusion coefficient of 4.64 × 10-9 cm2 s-1 in Al3+. Notably, the hybrid anodes demonstrated superior cycling performance (with the lowest variance percentage of 1.3% for hybrid compared to 38.1% for the bare in the presence of Zn2+), underlining the pivotal role of the natural binder. This was attributed to hydrogen bonding interactions, which increase the contact points between the inorganic and polymeric components, resulting in a more uniform network structure. Additionally, the cross-linking of Ox-Dex promotes stability and tolerance to the volume expansion of the electrodes. These results underscore the immense potential of the proposed hybrid electrodes in the field of energy storage.

Emerging two dimensional MXene for corrosion protection in new energy systems: Design and mechanisms

With the development of new and clean energy (offshore wind power, fuel cells, aqueous zinc ion batteries, lithium-ion batteries, etc.), the corrosion and security problems in special environments of the new energy system have attracted much attention. Corrosion protection on the metals applied in new energy system can reduce the economic loss, security risk, and energy consumption, as well as guarantee the efficiency of energy system. Traditional coatings face challenges in agglomeration of nano fillers, structural control, environmental issues, and poor conductivity, which limits the applications. With features in controllable surface chemistry and composition, rich surface terminations, better conductivity than graphene oxide, high aspect-ratio, strong impermeability, and low friction coefficient, the two-dimensional (2D) MXene presents potential for applications in corrosion protection in new energy systems. Despite progress has been made in the MXene for corrosion protection, there is still a lack of comprehensive review regarding the design and mechanisms of anti-corrosive MXene-based materials for corrosion protection in new energy system. In this review, a brief induction of MXene and the specially four corrosive environments (offshore wind power at deep sea, bipolar plates in PEMFC environments, zinc anode in AZIBs, and current collectors in Li-ion battery) are presented. Importantly, the design strategies and mechanisms of the MXene-based anti-corrosive coatings on metals used in the special environments are discussed in detail. Finally, the challenges and research trends in the MXene-based coatings for new energy systems are prospected. This review provides further understanding of corrosion in new energy and would expand the application prospects of MXene.

Current status and advances in zinc anodes for rechargeable aqueous zinc-air batteries

To promote sustainable development and reduce fossil fuel consumption, there is a growing demand for high-performance, cost-effective, safe and environmentally friendly batteries for large-scale energy storage systems. Among the emerging technologies, zinc-air batteries (ZABs) have attracted significant interest. By integrating the principles of traditional zinc-ion batteries and fuel cells, ZABs offer remarkably high theoretical energy density at lower production cost compared to the current state-of-the-art lithium-ion batteries (LIBs). However, the critical challenge remains in developing high-performance zinc anode. Herein, this review provides a comprehensive analysis of the current status and advancements in zinc anodes for rechargeable aqueous ZABs. We begin by highlighting the major challenges and underlying mechanisms associated with zinc anodes including issues such as uneven zinc deposition, dendrite growth and hydrogen evolution reaction. Then, this review discusses the recent advancements in zinc anode modifications, focusing on strategies such as alloying, surface porosity and zincophilicity. By reviewing the latest research, we also identify existing gaps and pose critical questions that need further exploration to push the field forward. The goal of this review is to inspire new research directions and promote the development of more efficient zinc anodes.

A Feasible Strategy for High-Performance Aqueous Zinc-Ion Batteries: Introducing Conducting Polymer

Aqueous zinc ion batteries (AZIBs) offer great potential for large-scale energy storage because of their high safety, low cost and acceptable energy density. However, the cycle life of AZIBs is inevitably affected by parasitic reactions and dendritic growth caused by multiple factors such as electrode, electrolyte and separator, which pose significant obstacles to the practical application of AZIBs. To address these challenges, conducting polymer (CP) based materials have gained widespread attention in the realm of rechargeable batteries due to the adjustable band gap, controllable morphology, and excellent flexibility of CPs. In particular, CPs exhibit remarkable conductivity, low dimensionality, and doping characteristics, making them highly promising for integration into the AZIB system. In this review, the problems associated with the cathode, anode, electrolyte, and separator of AZIBs are discussed, and the application of CPs for their modification is summarized. The review provides a comprehensive analysis of the action mechanisms involved in the CP modification process and offers valuable insights for the design and development of CPs that can be effectively utilized in AZIBs. Additionally, the review presents a promising outlook of this research field, aiming to further advance the application of low-cost and high-performance CPs and their composites in AZIBs.

Advancing Anode Performance in Aqueous Zinc-Ion Batteries: A Review of Metal-Organic Framework-Based Strategies

Aqueous zinc-ion batteries (AZIBs) are garnering substantial research interest in electric vehicles, energy storage systems, and portable electronics, primarily for the reason that the inexpensive cost, high theoretical specific capacity, and environmental sustainability of zinc metal anodes, which are an essential component to their design. Nonetheless, the progress of AZIBs is hindered by significant obstacles, such as the occurrence of anodic side reactions (SR) and the formation of zinc dendrites. Metal-organic framework (MOF)-based materials are being explored as promising alternatives owing to homogeneous porous structure and large specific surface areas. There has been a rare overview and discussion on strategies for protecting anodes using MOF-based materials. This review specifically aims to investigate cutting-edge strategies for the design of highly stable MOF-based anodes in AZIBs. Firstly, the mechanisms of dendrites and SR are summarized. Secondly, the recent advances in MOF-based anodic protection including those of pristine MOFs, MOF composites, and MOF derivatives are reviewed. Furthermore, the strategies involving MOF-based materials for zinc anode stabilization are presented, including the engineering of surface coatings, three-dimensional zinc structures, artificial solid electrolyte interfaces, separators, and electrolytes. Finally, the ongoing challenges and prospective directions for further enhancement of MOF-based anodic protection technologies in AZIBs are highlighted.

Recent Progress in Aqueous Zinc-ion Batteries at High Zinc Utilization

Aqueous zinc ion batteries (AZIBs) are promising candidates for next-generation energy storage systems due to their low cost, high safety, and environmental friendliness. As the critical component, Zn metal with high theoretical capacity (5855 mAh cm-3), low redox potential (-0.76 V vs. standard hydrogen electrode), and low cost has been widely applied in AZIBs. However, the low Zn utilization rate (ZUR) of Zn metal anode caused by the dendrite growth, hydrogen evolution, corrosion, and passivation require excess Zn installation in current AZIBs, thus leading to increased unnecessary battery weight and decreased energy density. Herein, approaches to the historical progress toward high ZUR AZIBs through the perspective of electrolyte optimization, anode protection, and substrate construction are comprehensively summarized, and an in-depth understanding of ZUR is highlighted. Specifically, the main challenges and failure mechanisms of Zn anode are analyzed. Then, the persisting issues and promising solutions in the reaction interface, aqueous electrolyte, and Zn anode are emphasized. Finally, the design of 100 % ZUR AZIBs free of Zn metal is presented in detail. This review aims to provide a better understanding and fundamental guidelines on the high ZUR AZIBs design, which can shed light on research directions for realizing high energy density AZIBs.

Enabling Gradient-Structured Solid Electrolyte Interphase by a Hydrated Eutectic Electrolyte for High-Performance Zn Metal Batteries

Aqueous Zn metal batteries are attracting tremendous interest as promising energy storage systems due to their intrinsic safety and cost-effectiveness. Nevertheless, the reversibility of Zn metal anodes (ZMAs) is hindered by water-induced parasitic reactions and dendrite growth. Herein, a novel hydrated eutectic electrolyte (HEE) consisting of Zn(BF4)2·xH2O and sulfolane (SL) is developed to prevent the side reactions and achieve the outstanding cyclability of ZMAs. The strong coordination between Zn2+ and SL triggers the eutectic feature, enabling the low-temperature availability of HEEs. The restriction of BF4 - hydrolysis in the eutectic system can realize favorable compatibility between Zn(BF4)2-based electrolyte and ZMAs. Besides, the newly-established solvation structure with the participation of SL, H2O, and BF4 -, can induce in situ formation of desirable SEI with gradient structure consisting of B,O-rich species, ZnS, and ZnF2, to offer satisfactory protection toward ZMAs. Consequently, the HEE allows the Zn||Zn symmetric cell to cycle over 1650 h at 2 mA cm-2 and 1 mA h cm-2. Moreover, the Zn||NH4V4O10 full batteries can deliver a prolonged lifespan for 1000 cycles with a high capacity retention of 83.4%. This work represents a feasible approach toward the elaborate design of advanced electrolyte systems for next-generation batteries.

Dendrite-free Zn anode enabled by combining carbon nanoparticles hydrophobic layer with crystal face reconstruction toward high-performance Zn-ion battery

Aqueous zinc ion batteries (ZIBs) have been considered promising energy storage systems due to their excellent electrochemical performance, environmental toxicity, high safety and low cost. However, uncontrolled dendrite growth and side reactions at the zinc anode have seriously hindered the development of ZIBs. Herein, we prepared the carbon nanoparticles layer coated zinc anode with (103) crystal plane preferential oriented crystal structure (denoted as C@RZn) by a facile one-step vapor deposition method. The preferential crystallographic orientation of (103) crystal plane promotes zinc deposition at a slight angle, effectively preventing the formation of Zn dendrites on the surface. In addition, the hydrophobic layer of carbon layer used as an inert physical barrier to prevent corrosion reaction and a buffer during volume changes, thus improving the reversibility of the zinc anode. As a result. the C@RZn anode achieves a stable cycle performance of more than 3000 h at 1 mA cm-2 with CE of 99.77 % at 5 mA cm-2. The full battery with C@RZn anode and Mn-doped V6O13 (MVO) cathode show stability for 5000 cycles at the current density of 5 A g-1. This work provides a new approach for the design of multifunctional interfaces for Zn anode.

Flexible electrochemical energy storage devices and related applications: recent progress and challenges

Given the escalating demand for wearable electronics, there is an urgent need to explore cost-effective and environmentally friendly flexible energy storage devices with exceptional electrochemical properties. However, the existing types of flexible energy storage devices encounter challenges in effectively integrating mechanical and electrochemical performances. This review is intended to provide strategies for the design of components in flexible energy storage devices (electrode materials, gel electrolytes, and separators) with the aim of developing energy storage systems with excellent performance and deformability. Firstly, a concise overview is provided on the structural characteristics and properties of carbon-based materials and conductive polymer materials utilized in flexible energy storage devices. Secondly, the fabrication process and strategies for optimizing their structures are summarized. Subsequently, a comprehensive review is presented regarding the applications of carbon-based materials and conductive polymer materials in various fields of flexible energy storage, such as supercapacitors, lithium-ion batteries, and zinc-ion batteries. Finally, the challenges and future directions for next-generation flexible energy storage systems are proposed.

Advances of Nanomaterials for High-Efficiency Zn Metal Anodes in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) have emerged as one of the most promising candidates for next-generation energy storage devices due to their outstanding safety, cost-effectiveness, and environmental friendliness. However, the practical application of zinc metal anodes (ZMAs) faces significant challenges, such as dendrite growth, hydrogen evolution reaction, corrosion, and passivation. Fortunately, the rapid rise of nanomaterials has inspired solutions for addressing these issues associated with ZMAs. Nanomaterials with unique structural features and multifunctionality can be employed to modify ZMAs, effectively enhancing their interfacial stability and cycling reversibility. Herein, an overview of the failure mechanisms of ZMAs is presented, and the latest research progress of nanomaterials in protecting ZMAs is comprehensively summarized, including electrode structures, interfacial layers, electrolytes, and separators. Finally, a brief summary and optimistic perspective are given on the development of nanomaterials for ZMAs. This review provides a valuable reference for the rational design of efficient ZMAs and the promotion of large-scale application of AZIBs.

Achieving Long Cycle Life of Zn-Ion Batteries through Three-Dimensional Copper Foam

Metallic zinc anodes in aqueous zinc-ion batteries (ZIBs) suffer from dendritic growth, low Coulombic efficiency, and high polarization during cycling. To mitigate these challenges, current collectors based on three-dimensional (3D) commercial copper foam (CCuF) are generally preferred. However, their utilization is constrained by their thickness, low electroactive surface area, and increased manufacturing expenses. In this study, the synthesis of cost-effective current collectors with exceptionally large surface areas designed for ZIBs that can be cycled hundreds of times is reported. A zinc-coated CuF anode (Zn/CuF) was prepared with a 3D porous CuF current collector produced by the dynamic hydrogen bubble template (DHBT) method. Electrochemically generated copper foam could be obtained within seconds while offering a thickness as low as 30-40 μm (CuF5 achieved a thickness of ∼38 μm in 5 s) via the DHBT method. The excellent electrical conductivity and open pore structure of the 3D porous copper scaffold ensured the uniform deposition/stripping of Zn during cycling. During the 500 h Zn deposition/stripping process, the as-synthesized CuF5 current collector offered fast electrochemical kinetics and low polarization as well as a relatively high average Coulombic efficiency of 99% (at a current density of 5 mA cm-2 and a capacity of 1 mAh cm-2). Furthermore, the symmetric cell exhibited low voltage polarization and a stable voltage profile for 1000 h at a current density of 0.1 mA cm-2. In addition, full cells containing the Zn/CuF anode coupled with an as-synthesized α-MnO2 nanoneedle cathode in aqueous electrolyte were also prepared. Capacities of 266 mAh g-1 at 0.1 A g-1 and 94 mAh g-1 at 2 A g-1 were achieved after 200 charge/discharge cycles with a stable Coulombic efficiency value close to 99.9%.

Breaking the trade-off between capacity and stability in vanadium-based zinc-ion batteries

Water in electrolytes is a double-edged sword in zinc-ion batteries (ZIBs). While it allows for proton insertion in the cathode, resulting in a significant increase in capacity compared to that of organic ZIBs, it also causes damage to electrodes, leading to performance degradation. To overcome the capacity-stability trade-off, organic solvents containing a small amount of water are proposed to mitigate the harmful effects of water while ensuring sufficient proton insertion. Remarkably, in a Zn(OTf)2 electrolyte using 8% H2O in acetonitrile as the solvent, Zn‖(NH4)0.5V2O5·0.5H2O exhibited a capacity as high as 490 mA h g-1 at a low current (0.3 A g-1), with a capacity retention of 80% even after 9000 cycles at high current (6 A g-1), simultaneously achieving the high capacity as in pure aqueous electrolytes and excellent stability as in organic electrolytes. We also found that the water content strongly impacts the kinetics and reversibility of ion insertion/extraction and zinc stripping/plating. Furthermore, compared to electrolytes with pure acetonitrile or H2O solvents, electrolytes with only 8% H2O in acetonitrile provide higher capacities at temperatures ranging from 0 to -50 °C. These discoveries enhance our understanding of the mechanisms involved in ZIBs and present a promising path toward enhancing electrolyte solutions for the creation of high-performance ZIBs.

A hydrophobic alloy-coated Zn anode for durable electrochromic devices

To mitigate Zn corrosion, dendrite growth and hydrogen evolution reactions (HER) in Zn-anode based electrochromic devices, hydrophobic CuZn5 alloy was coated on Zn@CuZn with lower nucleation potential, high coulombic efficiency, inhibited HER, and prolonged reversibility, enabling improved switching kinetics and cycling stability in an electrochromic Zn@CuZn||Prussian Blue (PB) device.

Polymer Coating with Balanced Coordination Strength and Ion Conductivity for Dendrite-Free Zinc Anode

Decorating Zn anodes with functionalized polymers is considered as an effective strategy to inhibit dendrite growth. However, this normally brings extra interfacial resistance rendering slow reaction kinetics of Zn2+ . Herein, a poly(2-vinylpyridine) (P2VP) coating with modulated coordination strength and ion conductivity for dendrite-free Zn anode is reported. The P2VP coating favors a high electrolyte wettability and rapid Zn2+ migration speed (Zn2+ transfer number, tZn 2+ = 0.58). Electrostatic potential calculation shows that P2VP mildly coordinates with Zn2+ (adsorption energy = -0.94 eV), which promotes a preferential deposition of Zn along the (002) crystal plane. Notably, the use of partially (26%) quaternized P2VP (q-P2VP) further reduces the interfacial resistance to 126 Ω, leading to a high ion migration speed (tZn 2+ = 0.78) and a considerably low nucleation overpotential (18 mV). As a result of the synergistic effect of mild coordination and partial electrolysis, the overpotential of the q-P2VP-decorated Zn anode retains at a considerably low level (≈46 mV) over 1000 h at a high current density of 10 mA cm-2 . The assembled (NH4 )2 V6 O16 ·1.5H2 O || glass fiber || q-P2VP-Zn full cell reveals a lower average capacity decay rate of only 0.018% per cycle within 500 cycles at 1 A g-1 .

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.

Durable modulation of Zn(002) plane deposition via reproducible zincophilic carbon quantum dots towards low N/P ratio zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are promising candidates for next-generation energy storage systems due to their intrinsic safety, environmental friendliness, and low cost. However, the uncontrollable Zn dendrite growth during cycling is still a critical challenge for the long-term operation of ZIBs, especially under harsh lean-Zn conditions. Herein, we report nitrogen and sulfur-codoped carbon quantum dots (N,S-CDs) as zincophilic electrolyte additives to regulate the Zn deposition behaviors. The N,S-CDs with abundant electronegative groups can attract Zn2+ ions and co-deposit with Zn2+ ions on the anode surface, inducing a parallel orientation of the (002) crystal plane. The deposition of Zn preferentially along the (002) crystal direction fundamentally avoids the formation of Zn dendrites. Moreover, the co-depositing/stripping feature of N,S-CDs under an electric field force ensures the reproducible and long-lasting modulation of the Zn anode stability. Benefiting from these two unique modulation mechanisms, stable cyclability of the thin Zn anodes (10 and 20 μm) at a high depth of discharge (DOD) of 67% and high Zn||Na2V6O16·3H2O (NVO, 11.52 mg cm-2) full-cell energy density (144.98 W h Kg-1) at a record-low negative/positive (N/P) capacity ratio of 1.05 are achieved using the N,S-CDs as an additive in ZnSO4 electrolyte. Our findings not only offer a feasible solution for developing actual high-energy density ZIBs but also provide in-depth insights into the working mechanism of CDs in regulating Zn deposition behaviors.

Recent progress in structural modification of polymer gel electrolytes for use in solid-state zinc-ion batteries

Zinc-ion batteries are one of the promising energy storage devices, which have the advantages of environmental friendliness, high safety and low price and are expected to be used in large-scale battery application fields. However, four prominent water-induced adverse reactions, including zinc dendrite formation, zinc corrosion, passivation and the hydrogen evolution reaction in aqueous systems, seriously shorten the cycling life of zinc-ion batteries and greatly hinder their development. Based on this, polymer gel electrolytes have been developed to alleviate these issues due to their unique network structure, which can reduce water activity and suppress water-induced side reactions. Based on the challenges of polymer gel electrolytes, this review systematically summarizes the latest research progress in the use of additives in them and explores new perspectives in response to the existing problems with polymer electrolytes. In order to expand the performance of polymer gel electrolytes in zinc-ion batteries, a range of different types of additives are added via physical/chemical crosslinking, such as organic or inorganic substances, natural plants, etc. In addition, different types of additives and polymerization crosslinking from different angles essentially improve the ionic conductivity of the gel electrolyte, inhibit the growth of zinc dendrites, and reduce hydrogen evolution and oxygen-absorbed corrosion. After these modifications of polymer gel electrolytes, a more stable and superior electrochemical performance of zinc-ion batteries can be obtained, which provides some strategies for solid-state zinc-ion batteries.

Stabilizing a zinc anode via a tunable covalent organic framework-based solid electrolyte interphase

Zinc (Zn) is an excellent material for use as an anode for rechargeable batteries in water-based electrolytes. Nevertheless, the high activity of water leads to Zn corrosion and hydrogen evolution, along with the formation of dendrites on the Zn surface during repeated charge-discharge (CD) cycles. To protect the Zn anode and limit parasitic side reactions, an artificial solid electrolyte interphase (ASEI) protective layer is an effective strategy. Herein, an ASEI made of a covalent organic framework (COFs: HqTp and BpTp) was fabricated on the surface of a Zn anode via Schiff base reactions of aldehyde and amine linkers. It is seen that COFs can regulate the Zn-ion flux, resulting in dendritic-free Zn. COFs can also mitigate the formation of an irreversible passive layer and the hydrogen evolution reaction (HER). Zn plating/stripping tests using a symmetrical cell suggest that HqTpCOF@Zn shows superior stability and greater coulombic efficiency (CE) compared to bare Zn. The full cell having COFs@Zn also displays much improved cyclability. As a result, the COF proves to be a promising ASEI material to enhance the stability of the Zn anode in aqueous media.

Improved Strategies for Separators in Zinc-Ion Batteries

The demand for energy storage is growing, and the disadvantages of lithium-ion batteries are being explored to overcome them. Accordingly, aqueous zinc-ion batteries (ZIBs) are developing very rapidly, owing to their high safety, environmental friendliness, high abundance of resources, and high cost performance. Over the last decade, ZIBs have made remarkable progress through extensive efforts in the field of electrode materials and through fundamental understanding of non-electrode components, such as solid-electrolyte interphase, electrolytes, separators, binders, and current collectors. In particular, the breakthrough in using separators on non-electrode elements should not be overlooked as such separators have proven key to conferring ZIBs with high energy and power density. In this Review, recent progress in the development of separators in ZIBs is comprehensively summarized based on their functions and roles in ZIBs, including the modification of conventional separators and the development of novel separators. Finally, the prospects and future challenges of separators are also discussed to facilitate ZIBs development.