Dendrites in Zn-Based Batteries

Weakly solvating effect optimized hydrated eutectic electrolyte towards reliable zinc anode interfacial chemistry

The inherent issues of aqueous Zn-ion batteries, including side reactions and dendrite growth, can be effectively addressed through designing solvation structures enriched with anions to facilitate the formation of an anion-derived solid electrolyte interphase (SEI) layer. Here, the weakly solvating effect is utilized to modulate Zn2+ solvation structure for constructing an anion-derived SEI layer. Trifluoroacetamide (TFACE), with a specific weak solvating ability, serves as an ideal ligand for preparing hydrated eutectic electrolytes (HEEs) combining the anion-containing solvation structures and high ionic conductivity. The results demonstrate that coordinated anions preferentially decompose and generate an inorganic/organic hybrid SEI layer on the Zn anode, which efficiently suppresses both side reactions and dendritic growth. Such an electrolyte enables assembled Zn//polyaniline (PANI) full cells to process an impressive capacity retention, maintaining 80 % after 3000 cycles at 0.5 A g-1. This work provides a fundamental insight into building the anion-derived SEI by the weakly solvating effect and gives a viable route for designing advanced aqueous electrolytes.

Zinc-Ion Conductive Metal-Organic Framework Interfaces for Comprehensive Anode Protection in High-Performance Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries have attracted intensive attention because of their safety, low cost, and high theoretical capacity; however, their practical application is hindered by challenges, such as Zn dendrite formation, the hydrogen evolution reaction, and a limited cycle life. Herein, a zinc anode interface is prepared by combining sodium alginate (SA) with hydroxyl and carboxyl groups as a binder and zeolite imidazole framework (ZIF-7) as the ion transport channel. The carboxyl groups in SA exhibit strong Zn2+-ion affinity, forming a cross-linked structure with ZIF-7 and creating a self-reinforcing coating that promotes uniform Zn2+ ion flux while the ZIF-7 provides suitable ionic channels to enable oriented deposition. A ZIF-7/SA coated Zn anode (ZIF-7/SA@Zn) exhibited a high Coulombic efficiency of 99.7% after 1500 cycles at 10 mA cm-2 and 1 mA h cm-2. Even under high-current and high-capacity conditions (20 mA cm-2, 20 mA h cm-2), ZIF-7/SA@Zn maintained stable cycling for 500 h. When ZIF-7/SA@Zn was paired with a Zn0.25V2O5 cathode, the resultant full cell retained more than 77.2% of its capacity after 10,000 cycles at 3000 mA g-1. This work proposes a strategy to stabilize Zn anodes under high currents, advancing high-performance Zn-based energy storage systems.

Breaking Performance Limits of Zn Anodes in Aqueous Batteries by Tailoring Anion and Cation Additives

Crystallographic engineering of Zn anodes to favor the exposure of (002) planes is an effective approach for improving stability in aqueous electrolytes. However, achieving non-epitaxial electrodeposition with a pronounced (002) texture and maintaining this orientation during extended cycling remains challenging. This study questions the prevailing notion that a single (002)-textured Zn anode inherently ensures superior stability, showing that such anodes cannot sustain their texture in ZnSO4 electrolytes. We then introduced a novel electrolyte additive, benzyltriethylammonium chloride (TEBAC), which preserves the (002) texture over prolonged cycling. Furthermore, we successfully converted commercial Zn foils into highly crystalline (002)-textured Zn without any pretreatment. Experiments and theoretical calculations revealed that the cationic TEBA+ selectively adsorbs onto the anode surface, promoting the exposure of the Zn(002) plane and suppressing dendrite formation. A critical discovery was the pitting corrosion caused by chloride ions from TEBAC, which we mitigated by anion substitution. This modification leads to a remarkable lifespan of 375 days for the Zn||Zn symmetric cells at 1 mA cm-2 and 1 mAh cm-2. Furthermore, a TEBA+-modified Zn||VO2 full cell demonstrates high specific capacity and robust cycle stability at 10.0 A g-1. These results provide valuable insights and strategies for developing long-life Zn 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.

Fast single metal cation conduction in ion-water aggregated aqueous battery electrolytes

Metal ion transport in solution is closely linked to its interactions with counter anions and solvent molecules. This interplay creates a longstanding trade-off between transference number (tMn⁺), ionic conductivity (δ), and solvation process. Advanced aqueous batteries with metal negative electrode require electrolytes with unity tMn⁺, high δ and low solvation energy. Here we introduce guanidinium sulfate (Gdm2SO4) into metal sulfate aqueous solutions to construct the ion-water aggregated electrolytes. These electrolytes exhibit fast single ion conduction, approaching unity tMn⁺ and high δ over 50 mS cm-1 for various metal cations (M= Zn, Cu, Fe, Sn and Li). The ion-water aggregates, dynamically formed by strong hydrogen bonding between sulfate anions, guanidinium cations and water, featuring an unfrustrated topological structure to suppress both anion mobility and water activity. This general configuration decouples the metal charge carrier from its coordination sheath, resulting in decreased solvation energy. These merits lead to homogeneous metal plating/stripping behavior with high coulombic efficiency of 99.9%. Moreover, the ion-water aggregates with reinforced kosmotropic characteristics significantly decrease the freezing point of the sulfate-based electrolytes to -28 oC, making them widely applicable in aqueous metal batteries for both intercalation and conversion positive electrodes.

Aspartame Endowed ZnO-Based Self-Healing Solid Electrolyte Interface Film for Long-Cycling and Wide-Temperature Aqueous Zn-Ion Batteries

Metallic Zn anodes suffer from hydrogen evolution and dendritic deposition in aqueous electrolytes, resulting in low Coulombic efficiency and poor cyclic stability for aqueous Zn-ion batteries (AZIBs). Constructing stable solid electrolyte interphase (SEI) with strong affinity for Zn and exclusion of water corrosion of Zn metal anodes is a promising strategy to tackle these challenges. In this study, we develop a self-healing ZnO-based SEI film on the Zn electrode surface by employing an aspartame (APM) as a versatile electrolyte additive. The hydrophobic nature and strong Zn affinity of APM can facilitate the dynamic self-healing of ZnO-based SEI film during cyclic Zn plating/stripping process. Benefiting from the superior protection effect of self-healing ZnO-based SEI, the Zn║Cu cells possess an average coulombic efficiency more than 99.59% over 1,000 cycles even at a low current density of 1 mA cm-2 - 1 mAh cm-2. Furthermore, the Zn║NH4+-V2O5 full cells display a large specific capacity of 150 mAh g-1 and high cyclic stability with a capacity retention of 77.8% after 1,750 cycles. In addition, the Zn║Zn cell delivers high temperature adaptability at a wide-temperature range from - 5 to 40 °C even under a high DOD of 85.2%. The enhanced capability and durability originate from the self-healing SEI formation enabled by multifunctional APM additives mediating both corrosion suppression and interfacial stabilization. This work presents an inspired and straightforward approach to promote a dendrite-free and wide-temperature rechargeable AZIBs energy storage system.

Electrochemical Responsive Alginate Chains Rendered Sol-to-Gel Gradient Electrolyte towards Practical Ah-level Zinc Metal Pouch Cell

Zinc metal batteries have been considered as an appealing candidate for grid-scale energy storage devices, but are hindered by the instable interface. Herein, we design a sol-to-gel gradient electrolyte through the simultaneous electrochemical deposition of Zn2+ and alginate. The electrochemical gelation of alginate creates a gradient sol-to-gel interface and enables the high ionic conductivity, where vehicular mechanism dominated transport is maintained in the bulk electrolyte, while a lean-water hydrogel like state is created at the Zn/electrolyte interface to reduce water activity. The electrochemical active alginate undergoes a gelation process to form an egg-shell to confine the Zn2+, rendering a 2D growth mode and inhibiting dendrite growth. By taking the advantages of both fast ion transport and stable interface, the full cell based on Zn/VO2 achieved a stable cycling of 400 cycles at an industrial-level areal capacity of over 4 mAh cm-2 with a capacity retention of 89.25 %. Additionally, we demonstrate the Ah-level pouch cell, which stably operates for over 200 cycles with an almost unity average coulombic efficiency (over 99.90 %). By demonstrating the remarkable performance, our work represents an advancement in zinc metal batteries toward a practical scale and is expected to set a stepping stone for transformative advancements in energy storage technologies.

Dual-functional cellulose separator regulating Zn deposition for long lifespan zinc-based energy storage

Aqueous zinc-ion batteries (ZIBs) hold significant promise for next-generation energy storage due to their low cost and inherent safety, but their lifespan is limited by zinc dendrites and hydrogen evolution during the cycling. Herein, long lifespan ZIBs are achieved using a dual-functional separator with ultrathin (18 μm) and high mechanical strength (91 MPa). The abundant polar hydroxyl groups in the separator's structure repel free SO42- anions, accelerate the Zn2+ ion transport, and modify the solvation structure of Zn(H2O)62+, ensuring uniform Zn2+ ion flux and inhibiting zinc dendrite formation. Consequently, the Zn//Zn symmetrical cells with dual-functional separators exhibit superior electrochemical performance, with a cycle life exceeding 1100 h at 1 mA cm-2 and 440 h at 10 mA cm-2, 5-9 times longer than cells using glass-fiber separators. The zinc-ion hybrid supercapacitor with the functional separator achieves a high capacity retention ratio (93.1 %) after 1000 cycles at 1.0 A g-1. This work offers new insights into the development of multifunctional separator for high-performance zinc-based energy storage.

Selective Acid Etching Construction of High (101) Texture Zinc Metal Anodes for High-Performance Zinc Ion Batteries

Aqueous zinc-ion batteries (AZIB) are significantly constrained by the poor stability of Zn anodes in aqueous electrolytes, which is caused by uncontrollable deposition behavior and parasitic reactions. The construction of specific crystalline surfaces represents an effective method for stabilizing Zn anodes. Therefore, a stable Malic acid@Zn (MA@Zn) anode with a highly (101) texture configuration is developed through acid etching. The mechanism of MA selective etching is investigated through theoretical calculations, where Zn atoms detach from the (002) crystal surface due to the strong interaction of MA with the (002) surface, leading to the preferential corrosion of the (002) surface and the formation of a unique (101) texture configuration morphology. This texture is conducive to the MA@Zn anode, as it enhances the affinity of MA@Zn for Zn2+ and optimizes the electric field distribution on the surface, thereby facilitating a more stable Zn deposition. Consequently, the MA@Zn symmetric battery is subjected to stable cycling for a period exceeding 2400 h at a current density of 5 mA cm-2. In comparison, the cycle life of the Zn//V2O5 full battery is significantly improved by >6000 cycles, pouch battery also shows better performance.

Reaction Kinetics and Mass Transfer Synergistically Enhanced Electrodes for High-Performance Zinc-Bromine Flow Batteries

Zinc-bromine flow batteries (ZBFBs) hold great promise for grid-scale energy storage owing to their high theoretical energy density and cost-effectiveness. However, conventional ZBFBs suffer from inhomogeneous zinc deposition and sluggish Br2/Br- redox kinetics, resulting in a short cycle life and low power density. Herein, a multiscale porous electrode with abundant nitrogen-containing functional groups is developed by growing zeolitic imidazolate framework-8 in situ on graphite felts, followed by a facile carbonization process to simultaneously tackle both the challenges. Theoretical and experimental results reveal that nitrogen-containing functional groups exhibit a high adsorption energy toward zinc atoms, while the microstructures promote pore-level mass transport, thereby resulting in compact and uniform zinc deposition. In the meantime, the electrode boosts the Br2/Br- reaction kinetics due to its high catalytic activity and large surface area. As a result, the ZBFBs equipped with optimized electrodes at both negative and positive sides can operate at an ultrahigh current density of 250 mA cm-2 while maintaining an energy efficiency of 68.0%, far surpassing that with pristine graphite felts (50.7%). Remarkably, the battery exhibits excellent cycling stability over 2000 cycles without obvious decay. This study provides a simple yet effective method for developing high-performance electrodes to tackle the critical challenges in ZBFBs, thereby promoting the commercialization of this promising energy storage technology.

Anisotropic Ion-Guiding Hydrogel Electrolyte with High-Water Affinity for Zn Ion Battery

Aqueous zinc-ion batteries (AZIBs) are a promising alternative to lithium-ion batteries, boasting superior safety, eco-friendliness, and cost-effectiveness. Despite these advantages, performance issues such as irregular Zn deposition and cathode material dissolution remain challenging. This study introduces an intrinsically anisotropic ion-guiding hydrogel electrolyte (APHE) fabricated via a double-stabilization anisotropic freezing strategy. The synergistic effect of anisotropic structure and high water affinity of APHE effectively suppress water-induced parasitic reactions. In brief, the anisotropic structure promotes rapid Zn2+ ion diffusion, leading to the uniform Zn2+ ion flux. Additionally, abundant hydroxyl groups in APHE facilitate Zn2+ ion dissociation and adjust the solvation structure, setting it apart from an isotropic matrix. Furthermore, the improvement of ion diffusion tortuosity enhances the electrode/electrolyte kinetics, thereby improving the rate-capability and reversibility of Zn2+ ion (de)-intercalation. Thus, APHE demonstrates a thin and dense Zn deposition layer of 31.7 µm, which is less than half the thickness of IPHE (67.5 µm) after 500 cycles. This research addresses fundamental challenges in the performance of AZIBs and provides valuable insights into the design of advanced electrolytes for future energy storage systems.

Insights into Dendrite Regulation by Polymer Hydrogels for Aqueous Batteries

Aqueous batteries, renowned for their high capacity, safety, and low cost, have emerged as promising candidates for next-generation, sustainable energy storage. However, their large-scale application is hindered by challenges, such as dendrite formation and side reactions at the anode. Hydrogel electrolytes, which integrate the advantages of liquid and solid phases, exhibit superior ionic conductivity and interfacial compatibility, giving them potential to suppress dendrite evolution. This Perspective first briefly introduces the fundamentals underlying dendrite formation and the unique features of hydrogels. It then identifies the key role of water and polymer networks in inhibiting dendrite formation, highlighting their regulation of water activity, ion transport, and electrode kinetics. By elucidating the principles of hydrogels in dendrite suppression, this work aims to provide valuable insights to advance the implementation of aqueous batteries incorporating polymer hydrogels.

Progress in Developing Polymer Electrolytes for Advanced Zn Batteries

Aqueous Zn batteries (ZBs) are promising candidates for large-scale energy storage, considering their intrinsically safe features, competitive cost, and environmental friendliness. However, the fascinating metallic Zn anode is subjected to severe issues, such as dendrite growth, hydrogen evolution, and corrosion. Additionally, traditional aqueous electrolytes' narrow electrochemical windows and temperature ranges further hinder the practical application of ZBs. Solid-state electrolytes, including solid polymer electrolytes and hydrogel electrolytes, offer distinct paths to mitigate these issues and simultaneously endow the ZBs with customizable functions such as flexibility, self-healing, anti-freezing, and regulated Zn deposition, etc, due to their tuneable structures. This review summarizes the latest progress in developing polymer electrolytes for ZBs, focusing on modifying the ionic conductivity, interfacial compatibility, Zn anode stability, electrochemical stability windows, and improving the environmental adaptability under harsh conditions. Although some achievements are obtained, many critical challenges still exist, and it is hoped to offer guidance for future research, accelerating the development and application of polymer electrolytes.

Selective Interface Engineering with Large π-Conjugated Molecules Enables Durable Zn Anodes

Undesirable dendrite growth and side reactions at the electrical double layer (EDL) of Zn/electrolyte interface are critical challenges limiting the performance of aqueous zinc ion batteries. Through density functional theory calculations, we demonstrate that grafting large π-conjugated molecules (e.g. bilirubin, biliverdin, lumirubin, and hemoglobin) onto Zn surface induces preferential adsorption on non-(002) facets, leading to interfacial charge redistribution, upshifted Zn d-band center, and enhanced H+ fixation capability. Among these, bilirubin (BR) is identified as the most effective, preferentially adsorbing onto non-Zn(002) facets to inhibit hydrogen evolution reaction and promote Zn(002) planar growth during plating. This approach results in average Coulombic efficiency of 99.86 % over 4000 cycles in Zn||BR-1@Cu cells and prolonged lifespan exceeding 1600 h in BR-1@Zn||BR-1@Zn cells at 10 mA cm-2 and 1 mAh cm-2. Even under harsh conditions of 25 mA cm-2 and 10 mAh cm-2, BR-1@Zn||BR-1@Zn cell maintains a lifespan of over 400 h. Furthermore, BR-1@Zn||MnO2 and BR-1@Zn||NVO full cells achieve 76.4 % and 86.1 % capacity retention after 800 and 1400 cycles at 1.0 A g-1, respectively. This study underscores the importance of grafting large π-conjugated molecules to allow selective Zn(002) exposure, Zn d-band center upshift, and EDL structure regulation, paving the way towards durable Zn anodes.

Dissecting ionic favorable hydrogen bond chemistry in hybrid separators for aqueous zinc-ion batteries

Separators, regulating the ion transport channels between electrodes, are crucial for maintaining the properties of electrochemical batteries. However, sluggish ion transport and desolvation kinetics in aqueous zinc-ion batteries (AZIBs) cause uneven ion flux at the separator-electrode interface, accelerating Zn dendrite growth. Herein, we systematically dissect ionic favorable hydrogen bond chemistry in a hybrid separator engineered through rational boron nitride (BN) doping into polyacrylonitrile (PAN) separators. Notably, in situ Fourier transform infrared spectroscopy (FTIR) analyses reveal that the hydrogen bond network in a BN-PAN separator improved the desolvation of Zn2+ by immobilizing water molecules through hydrogen bond interactions, thus effectively increasing the transference number of zinc ions. Capitalizing on the ionic favorable properties, uniform electric field distribution and zinc plating/stripping behavior are achieved at the separator-electrode interface, efficiently suppressing the formation of zinc dendrites and by-products. As a result, the BN-PAN separator demonstrates extended cycling stability, exceeding 1100 h at a current density of 1.0 mA cm-2 and 700 h at a current density of 5.0 mA cm-2, while exhibiting enhanced rate capability and stability in full cells. This work offers valuable insights into leveraging hydrogen bond chemistry for the design of fast ion-transport separators in aqueous batteries.

Electrochemical engineering in aqueous metal-ion batteries

Aqueous metal ion batteries (AMIBs), with merits of safety, ambient assembly, and eco-friendliness, demonstrate great potential in various energy storage scenarios. Despite the laboratory-scale progress in battery components and mechanisms featured by large specific capacities and long lifespans, AMIBs' practical use meets challenges with electrodes and electrolytes. It is crucial to prepare a review discussing the problems and solutions for the battery performance degradation during the electrode/battery scaleup from the perspectives of ion mass transfer and electrode reaction, which is proposed as the electrochemical engineering in AMIBs. We first introduce the anodic reactions and their effective reinforcement by molecule chemistry and electrodeposition. Then, we discuss the ion diffusion in electrolytes by learning from the Nernst-Planck theory, followed by the interphase ion diffusion at the electrolyte-cathode interface. After that, we highlight the lattice-void and particle-gap ion diffusion in cathodes and the cathodic reactions reinforced by catalysis and micro-reactor construction. Finally, we present the challenge and perspective of this blooming field toward the lab-to-market transition of AMIBs.

Ion flux regulating with Au-modified separator to realize a homogenize Zn metal deposition

Aqueous Zn-ion batteries (AZIBs) have attracted widespread attention owing to the feature of low cost, inherent safety and eco-friendliness. However, the poor reversibility of Zn anode severely hinders the practical applicability of AZIBs. Separator modification is an effective way to functionalize the electrode/electrolyte interface and improve the cycling performance. Here, we propose a modified glass fiber separator with Au coating (Au@GF), which could realize uniform Zn2+ distribution at the electrode/electrolyte interface and regulate the plating/stripping behaviors, achieving a dense and homogenous deposition. Zn||Zn symmetric cells assembled with Au@GF separator demonstrate evidently prolonged cycle life over 1600 h at the current density of 5 mA cm-2 and the capacity of 1 mAh cm-2, while symmetric cells with GF fail in less than 40 h. Even at the condition of 15 mA cm-2/3 mAh cm-2, lifespan of Zn||Zn cells with Au@GF is extended to 750 h, which is more than 3 times compared with that of GF. The modified separator with highly conductive coating is capable of a longtime stable Zn plating/stripping. Moreover, an enhanced cycling performance is also detected in a series of full cells with different cathode materials. This work provides an easy and efficient approach to homogenize Zn2+ deposition.

The synergistic effect induced by "Z-bond" between cations and anions achieving a highly reversible zinc anode

Due to their high energy density, low cost, and environmental friendliness, aqueous zinc-ion batteries are considered a potential alternative to Li-ion batteries. However, dendrite growth and parasitic reactions of water molecules limit their practical applications. Herein, an ionic liquid additive, 1-butyl-3-methylimidazolium Bis(fluorosulfonyl)imide (BMImFSI), is introduced to regulate the electrical double layer (EDL). Both BMIm+ and FSI- can preferentially adsorb on the Zn anode, constructing a water-poor EDL and thus effectively suppressing side reactions. Additionally, under the synergistic effect of the mineralized solid-electrolyte interphase (SEI) formed by the decomposition of FSI- and the ion dispersion layer constructed by BMIm+ on the mineralized SEI, the deposition of zinc ions is effectively dispersed, preventing excessive aggregation of zinc ions and thus dendrite formation. The Zn‖Zn symmetric cells using the BMImFSI/ZnSO4 electrolyte operate stably for 1060 h and 560 h at 10 mA cm-2-10 mAh cm-2 and 20 mA cm-2-20 mAh cm-2, respectively. The Zn‖Cu asymmetric cell maintains an average Coulombic efficiency of 99.4 % after 1000 cycles. The capacity retention of a full cell using α-MnO2 as the cathode is significantly improved at 1 A g-1.

Promoting Migration Kinetic of Desolvated Zn2+ by Functional Interlayer Toward Superior Zn Metal Anode

The development of Zn metal anodes is challenged by non-uniformity of ion flux causing inhomogeneous deposition and strong solvation of Zn(H2O)6 2+ resulting in adverse side reactions. Applying intermediate protecting layers with high affinity to Zn2+ is a popular and effective solution, but it also limits the ion migration. A functional MXene-based interlayer is designed in this work to modify the glass fiber separator achieving balanced adsorption energy and ion migration. By coating porous silica on the MXene surface, the instinct advanatges of MXene are mostly reserved while the adsorption energy to Zn2+ is optimized. Such an interlayer enables high flux and uniformity of desolvated Zn2+, contributing to rapid deposition kinetic for excellent rate performance and inhibited side reactions for long-term cycling stability. As a result, the functionalized Zn metal anode delivers steady plating/stripping cycles for more than 5000 h at 0.1 mA cm-2 and 700 h at 5.0 mA cm-2. The Zn||MnO2 full cells with this separator also exhibit superior rate capabilities (173 mAh g-1 at 2.0 A g-1) and excellent cycle performance (254.7 mAh g-1 after 1000 cycles at 0.5 A g-1). This work provides a feasible strategy for preparing functional interlayers toward superior Zn or other metal anodes.

Bridging Electrolyte Bulk and Interfacial Chemistry: Dynamic Protective Strategy Enable Ultra-Long Lifespan Aqueous Zinc Batteries

The main bottleneck of rechargeable aqueous zinc batteries (AZBs) is their limited cycle lifespans stemming from the unhealthy electrolyte bulk and fragile interface, especially in the absence of dynamic protection mechanism between them. To overcome this limitation, benefitting from their synergistic physical and chemical properties, chitin nanocrystals (ChNCs) are employed as superior colloid electrolyte to bridge electrolyte bulk and interfacial chemistry for ultra-long lifespan AZBs. This unique strategy not only enables continuous optimization of the electrolyte bulk and interfacial chemistry within the battery but also facilitates self-repairing of mechanical damage both internally and externally, thereby achieving comprehensive, persistent, and dynamic protection. As a result, the modified zinc (Zn) cells present high Zn plating/stripping coulombic efficiencies of 97.71 % ~99.81 % from 5 to 100 mA cm-2, and remarkably service lifespan up to 8,200 h (more than 11 months). Additionally, the Zn//MnO2 full cell exhibits a high capacity retention of 70.1 % after 3,000 cycles at 5 A g-1. This dynamic protective strategy to challenge aqueous Zn chemistry may open up a new avenue for building better AZBs and beyond.

Sn(II)-Pyrophosphate Complex with Low Plating/Stripping Potential for Sn-I Flow Battery Applications

Exploring electrolyte formulations that can effectively reduce the plating/stripping potentials of metallic electrodes holds great significance in advancing the development of high-voltage redox flow batteries. In this study, we introduce a novel Sn-based chelated electrolyte, namely, Sn(P2O7)26-, by directly reacting the Sn2+ solution with an excess of P2O74- solution. Electrochemical tests prove that the incorporation of high-concentration P2O74- ligands could shift the plating/stripping potential to -0.67 V. Thus, the demonstrated Sn-I flow battery reveals an average cell voltage of nearly 1.2 V and maintains stable cycling over 250 cycles at a high current density of 80 mA cm-2, with an average energy efficiency of about 70%. Moreover, no dendrite formation formed during the Sn deposition on the carbon felt. This study offers broad prospects for the future development of high-voltage Sn-based flow batteries.

Polyelectrolyte Membrane Enables Highly Reversible Zinc Battery Chemistry via Immobilizing Anion and Stabilizing Water

The integration of water-based electrolytes into zinc-ion batteries encounters challenges due to the limited voltage window of water, interfacial side reactions of mobile counterions, and the growth of zinc metal (Zn0) dendrites during charge. In this study, we introduce a nonfluorinated, cation-conducting polyelectrolyte membrane (PEM) designed to alleviate these challenges by suppressing the reactivities of both water and counterions. This PEM forms hydrogen bonds with water molecules through its proton-accepting side chains, thus shifting the lowest unoccupied molecular orbital (LUMO) energy of water from -0.37 to -0.14 eV and inducing a negative shift in the onset potential for hydrogen evolution by 110 mV. Additionally, it immobilizes the counteranions onto the polymer backbones via covalent bonding, hence making the Zn2+ transference number nearly unity (0.96). Meanwhile, the high modulus PEM establishes a solid-state diffusion barrier to homogenize the interfacial Zn2+ flux, leading to 3D in-plane interfacial Zn2+ diffusion and compact Zn0 plating within the (002) plane. Atomic resolution scanning transmission electron microscopy (STEM) reveals corrosion-free Zn0 deposition without electrolyte degradation, while operando transition X-ray microscopy (TXM) further illustrates the real-time dendrite-free Zn0 plating process at 5 mA/cm2. Consequently, the unique properties of this water-binding and anion-tethering PEM enable enhanced electrochemical performance without employing highly fluorinated and expensive anions. This PEM demonstrates a durability of 3800 h in Zn0-Zn0 symmetric cells and a lifetime of 6000 cycles in Zn0-LiV3O8 full cells.

Manipulating Interphase Chemistry for Aqueous Zn Stabilization: The Role of Supersaturation

The limited cycling durability of Zn anode, attributed to the absence of a robust electrolyte-derived solid electrolyte interphase (SEI), remains the bottleneck for the practical deployment of aqueous zinc batteries. Herein, we highlight the role of local supersaturation in governing the fundamental crystallization chemistry of Zn4SO4(OH)6⋅xH2O (ZSH) and propose a subtle supersaturation-controlled morphology strategy to tailor the interphase chemistry of Zn anode. By judiciously creating local high-supersaturation environment with organic caprolactam to manipulate the precipitation manner of zinc sulfate hydroxide (ZSH), lattice-lattice matched heterogeneous nucleation of ZSH (001) and Zn (002) is realized in aqueous ZnSO4, producing a dense, pseudo-coincidence interface capable of functioning as decent SEI. The plating/stripping efficacy of Zn is significantly boosted, as reflected by the great improvement of the initial (from 59.9 % to 84.1 %) and average (from 94.0 % to 99.2 %) coulombic efficiency. Accordingly, the cyclic endurance of Zn anode is prolonged from 50 h to over 7000 h at 0.5 mA cm-2/1 mAh cm-2, witnessing 140-fold lifespan extension. Profiting from supersaturation-mediated Zn stabilization, the performance deterioration of zinc-vanadium batteries is also alleviated, even with a low N/P ratio of 4.4. This work is anticipated to deepen current understanding on the control of the anodic chemistry.

Interface regulation and electrolyte design strategies for zinc anodes in high-performance zinc metal batteries

Rechargeable zinc metal batteries (ZMBs) represent a promising solution for large-scale energy storage due to their safety, cost-effectiveness, and high theoretical capacity. However, the development of zinc metal anodes is hindered by challenges such as dendrite formation, hydrogen evolution reaction (HER), and low Coulombic efficiency stemming from undesirable interfacial processes in aqueous electrolytes. This review explores various strategies to enhance zinc anode performance, focusing on artificial SEI, morphology adjustments, electrolyte regulation, and flowing electrolyte. These approaches aim to suppress dendrite growth, mitigate side reactions, and optimize the electric double layer (EDL) and Zn2+ solvation structures. By addressing these interfacial challenges, the insights presented here pave the way for designing high-performance ZMBs, offering directions for future research into scalable and sustainable battery technologies.

Strategies and Prospects for Engineering a Stable Zn Metal Battery: Cathode, Anode, and Electrolyte Perspectives

ConspectusZinc metal batteries (ZMBs) appear to be promising candidates to replace lithium-ion batteries owing to their higher safety and lower cost. Moreover, natural reserves of Zn are abundant, being approximately 300 times greater than those of Li. However, there are some typical issues impeding the wide application of ZMBs. Traditional inorganic cathodes exhibit an unsatisfactory cycling lifetime because of structure collapse and active materials dissolution. Apart from inorganic cathodes, organic materials are now gaining extensive attention as ZMBs cathodes because of their sustainability, high environmental friendliness, and tunable molecule structure which make them usually exhibit superior cycling life. Nevertheless, due to the inferior conductivity of organic materials, their mass loading and volumetric energy density still cannot meet our demands. In addition, the specific working mechanism of inorganic/organic cathodes also needs further investigation, necessitating the use of advanced in situ characterization technologies. Reversibility of metallic Zn anodes is also crucial in determining the overall cell performances. Like Li and Na anodes, uncontrolled dendrite growth is also an annoying problem for Zn anodes, which may penetrate the separator and cause inner short circuit. In aqueous electrolyte, highly reactive H2O molecules easily attack metallic Zn anode, leading to undesired Zn corrosion. Furthermore, during cell operation, hydrogen evolution reaction (HER) occurs, which leads to continuous consumption of electrolytes and formation of insulating byproducts on Zn anodes. Although strategies like novel Zn anode design and artificial SEI layer construction are proposed to inhibit dendrites growth and protect Zn anodes from active H2O attack, the corresponding manufacturing process remains complex. Modifying electrolyte components is relatively simple to implement and effectively stabilizes Zn anodes. However, HER cannot be completely eliminated when H2O exists in the modified electrolytes. Under such conditions, nonaqueous electrolytes appear to be a promising solution for ZMBs in the future due to their aprotic nature and high stability with the Zn anodes. However, the ionic conductivity of nonaqueous electrolytes is relatively low compared to that of aqueous electrolytes. Most of the previous reviews focus only on the individual components of ZMBs. A review of ZMBs from a higher perspective, focusing on advanced ZMBs system design, is currently lacking.In this Account, we begin with a brief overview of ZMBs, highlighting their advantages and current challenges. Subsequently, we give a summary of the development of inorganic cathodes (such as MnO2) for ZMBs. Specifically, development history and representative modification strategy of inorganic cathodes are illustrated. Following this, representative organic cathodes are discussed, along with introduction of novel modification strategies for organic cathodes. Afterward, Zn anode form design, additive selection and artificial solid electrolyte interface (SEI) layer are briefed for development of Zn anodes. Thereafter, formulation of electrolyte components is systematically discussed, highlighting potential future of nonaqueous electrolyte in ZMBs. Unlike other reviews giving very detailed information in one aspect, this Account offers an overview of current opportunities and challenges faced by ZMBs. We hope this Account can provide researchers with deeper insights into the evolution of ZMBs, encouraging them to devise effective and innovative strategies that will accelerate widespread application of ZMB technology.

Hydrogen/Electron Amphiphilic Bi-Functional Water Molecular Inactivator-Assisted Interface Stabilization in Highly Reversible Zn Metal Batteries

Continuous hydrogen-bond-network in aqueous electrolytes can lead to uncontrollable hydrogen transfer, and combining the interfacial parasitic electron consumption cause the side reaction in aqueous zinc metal batteries (AZMBs). Herein, hydrogen/electron amphiphilic bi-functional 1,5-Pentanediol (PD) molecule was introduced to stabilize the electrode/electrolyte interface. Stronger proton affinity of -OH in PD can break bulk-H2O hydrogen-bond-network to inhibit the activity of water, and electron affinity can enhance electron acceptation capability, which ensures that PD is preferentially bound to electrode material over H2O. Besides, the participation of PD in the Zn2+ solvation structure reduces water content at the solid-liquid interface and promotes uniform deposition process by optimizing Zn2+ de-solvation energy. Accordingly, dense and vertical zinc texture based on intrinsic steric hindrance effect of PD and formed SEI protective layer to induce stable Zinc anode-electrolyte interface. Moreover, an organic-inorganic shielding water layer was formed at the cathode side to suppress vanadium dissolution in vanadium Oxide. Consequently, Zn//Zn symmetric cell could cycle for more than 5600 hours at 1 mAh cm-2@1 mA cm-2 (more than 250 hours at 50 °C). Besides, the VO2 and I2 cathode all achieved stable cycling performance and former pouch cell could reach average capacity of 0.13 Ah.

Popularizing Holistic High-Index Crystal Plane via Nonepitaxial Electrodeposition Toward Hydrogen-Embrittlement-Relieved Zn Anode

Electrodeposition is promising to fabricate Zn electrodes affording nonepitaxial single-crystal textures. Previous research endeavors focus on achieving Zn(002) faceted deposition, nevertheless, the popularization of a high-index Zn plane with favorable electrochemical activity remains poorly explored. There also exists a deficiency in the assessment of the electrodeposited quality of Zn. Here, a straightforward strategy to address such concerns by cultivating predominant Zn(112) texture via a potentiostatic electrodeposition mode is reported. By precisely identifying the "limiting" conditions for electrodeposition, a striking balance between improved deposition quality, tailored deposition kinetics, and suppressed hydrogen evolution is found. (002) Faceted Zn electrode is shown that be indeed produced, yet the rampant hydrodynamic convection and hydrogen embrittlement issue under such "over-limiting" preparation conditions pose challenges in the electrode lifespan. In contrast, an optimized deposition minimizes hydrodynamic disturbances and mitigates the hydrogen embrittlement effect, where the thus-generated high-index (112)-textured Zn electrode manifests impressive deposition quality and demonstrates holistic cycling stability. The pouch cell by pairing a ZnxV2O5 (ZnVO) cathode manages a reversible capacity of ≈130 mAh and a capacity retention of 98.42%. This study offers guidance for the development of dendrite-free and hydrogen-embrittlement-relieved Zn anodes, unleashing the potential of high-index plane textures for advanced Zn batteries.

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.

Epitaxy Orientation and Kinetics Diagnosis for Zinc Electrodeposition

An accurate assessment of the electrodeposition mechanism is essential for evaluating the electrochemical stability and reversibility of the metal anodes. Multiple strategies aimed at uniform Zn deposition have been extensively reported, yet it is challenging to clarify the Zn crystal growth regularity and activity due to the obscured physicochemical properties of as-deposited Zn. Herein, we present a protocol for elucidating the controlled epitaxial growth process of Zn crystals and quantifying their surface electrochemical activity using scanning electrochemical microscopy. We find that the early-stage epitaxy tends to form a stacked-multilayer structure accompanied by intermittent rotation. The site-dependent kinetics and morphology correlation reveal a distinct evolution path at early and final stages. Our exploration advances the understanding of the Zn growth mechanism and facilitates the realization of the interface kinetics of metal batteries in situ.

Optimization of the Zinc Deposition Interface by Sn Nanoparticles for Fast-Charging Zinc Metal Anodes

The electrodeposition behavior of zinc metal anodes critically correlates with the electrode surface properties. The tendency for inhomogeneous deposition of zinc is more severe, especially under high current density. Herein, the surface structure of zinc and zinc deposition substrates is reconstructed with a uniform metal tin (Sn) coating via a simple evaporation method. Sn nanoparticles can serve on metal nuclei to reduce the Zn nucleation barrier and enable more nucleation sites for even deposition. Moreover, the mechanical stability of the electrode surface is improved via Zn-Sn alloying. Consequently, the uniform Zn deposition/dissolution behavior on Sn-modified two- and three-dimensional copper substrates is reflected in the stable Coulombic efficiency and reduced polarization. The Sn@Zn electrode is endowed with superior stability at a high current density (800 h at 20 mA cm-2). More encouragingly, the full cell installed with a carbon nanotube/MnO2 cathode maintains enduring stability (700 cycles) at 1 A g-1. This work enlightens metal alloy as an effective and instructive modification strategy toward stabilized zinc anodes.

Revitalizing Dead Zinc with Ferrocene/Ferrocenium Redox Chemistry for Deep-Cycle Zinc Metal Batteries

Aqueous zinc (Zn) batteries are highly desirable for sustainable and large-scale electrochemical energy storage technologies. However, the ceaseless dendrite growth and the derived dead Zn are principally responsible for the capacity decay and insufficient lifespan. Here, we propose a dissolved oxygen-initiated revitalization strategy to reactivate dead Zn via ferrocene redox chemistry, which can be realized by incorporating a trace amount of poly(ethylene glycol) as a solubilizer to improve the solubility of water-insoluble ferrocene derivatives. Ferrocene scaffold can be spontaneously oxidized to ferricenium cations by dissolved oxygen, which eradicates the dissolved oxygen-involved Zn corrosion and insulating by-product generation. Subsequently, the generated ferricenium cations as the scavenger can rejuvenate electrically isolated dead Zn into electroactive Zn2+ ions to compensate the zinc loss. Through this design, the symmetric cell exhibited improved cycle life of 3700 h at 10 mA cm-2, and 220 h under a high depth of discharge of 80 %. Importantly, the Zn||NaV3O8 ⋅ 1.5H2O full cells demonstrated the impressive cycling stability over 1500 cycles at a low N/P ratio of 3.0. This work presents an innovative solution for the revitalization of dead Zn to extend the lifespan of deep-cycling metal batteries.

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.

Anti-dendrite separator interlayer enabling staged zinc deposition for enhanced cycling stability of aqueous zinc batteries

Aqueous zinc ion batteries exhibit great prospects due to their low cost and high safety, while their lifespan is limited by severe dendritic growth problems. Herein, we develop an anti-dendrite hot-pressing separator interlayer through a mass-producible hot-pressing strategy, by spreading metal-organic framework (MOF) precursor on nonwoven matrix followed by a simple hot-pressing process. The in situ modification of MOF crystals on fiber surface processes abundant nitrogenous functional groups and high specific surface area (190.8 m2 g-1) with a strong attraction to Zn2+. These features contribute to a staged deposition behavior to promote uniform nucleation at high concentrations and two-dimensional grain growth at low concentrations. Consequently, Zn | |Zn symmetrical cells with hot-pressing separator interlayer demonstrate cycle lives of 3000 hours at 2 mA cm-2, 2 mAh cm-2. Moreover, Zn | |I2 pouch batteries with hot-pressing separator interlayer realizes 840 cycles lifespan with a capacity retention of 90.9% and a final discharge capacity of 110.6 mAh at 25 °C.

A carrageenan-induced highly stable Zn anode by regulating interface chemistry

Zinc-ion batteries (ZIBs) are promising on account of the inherent safety, minimal toxicity, cost-effectiveness, and high theoretical capacity. However, the critical issues including the Zn dendrites and side reactions impede their commercial application. Here, we propose green, non-toxic and biological carrageenan (Carr) serving as an electrolyte additive to address the aforementioned issues. Owing to the multifunctional groups, Carr has the capacity to interact with Zn2+, thereby modulating the solvation configuration of Zn2+ and changing the ion distribution at the electrode-electrolyte interface. Moreover, it can adsorb on the Zn electrode and induce the formation of a solid electrolyte interphase (SEI) consisting of ZnO, ZnS and R-SO2 species. It contributes to uniform Zn2+ ion diffusion and even Zn deposition with the preferable (002) plane. Consequently, the Zn||Zn cells exhibit a stable cycle performance for 800 h at 5 mA cm-2 and 5 mA h cm-2. An elevated coulombic efficiency of 99.2% over 1800 cycles is obtained in the Zn||Cu cells using the electrolyte with Carr. Benefitting from the highly stable and reversible Zn anode, the Zn||VO2 full cell also delivers a high performance in comparison with the bare ZnSO4 electrolyte, favoring the practical implementation of ZIBs.

Novel in situ SEI fabrication on Zn anodes for ultra-high current density tolerance enabled by electrical excitation–conjugation of iminoacetonitriles

Electroplating enriched IDAN on zinc surface, forming IDA to complex with Zn2+, thus creating an in situ SEI. This SEI optimized Zn2+ transport, shielded water molecules, and stabilized the anode interface, enhancing long-term performance.

"Anchoring Capture" Effect Mimicking Proline in Hardy Deep-Sea Fish to Stabilize the Zinc Anode with Lower Operating Temperature

The low plating/stripping efficiency of zinc anodes, dendrite growth, and high freezing points of aqueous solutions hinder the practical application of aqueous zinc-ion batteries. This paper proposes a zwitterionic permeable network solid-state electrolyte based on the "anchor-capture" effect to address these problems by incorporating proline (Pro, a biological antifreeze agent) into the electrolyte. Extensive validation tests, Quantum Chemistry (QC) calculations, Molecular Dynamics (MD) Simulations, and ab initio molecular dynamics simulations consistently indicate that the amino groups in proline adsorb onto the Zn metal surface, stabilizing the zinc anode-electrolyte interface, suppressing side reactions from water decomposition, and homogenizing zinc-ion flux. This electrolyte demonstrates excellent reversibility in Zn-Mn2O3 cells and Zn-Zn half-cells, achieving a high coulombic efficiency of over 99.4% across 2000 cycles in Zn-Mn2O3 full cells, and delivering a discharge-specific capacity of 175.2 mAh g-1 at -35 °C and 1 A g-1. Additionally, an appropriate concentration of proline lowers the electrolyte's freezing point to -45 °C through the network's solid-state effect, ensuring the stable operation of the solid-state battery at -35 °C. This innovative concept of network solid-state electrolytes injects new vitality into the development of multifunctional solid-state electrolytes.

Breaking Mass Transport Limit for Hydrogen Evolution-Inhibited and Dendrite-Free Aqueous Zn Batteries

It is commonly accepted that batteries perform better at low current densities below the mass-transport limit, which restricts their current rate and capacity. Here, it is demonstrated that the performance of Zn metal electrodes can be dramatically enhanced at current densities and cut-off capacities exceeding the mass-transport limit by using pulsed-current protocols. These protocols achieve cumulative plating/stripping capacities of 11.0 Ah cm-2 and 3.8 Ah cm-2 at record-high current densities of 80 and 160 mA cm-2, respectively. The study identifies and understands the promoted (002)-textured Zn growth and suppressed hydrogen evolution based on the thermodynamics and kinetics of competing reactions. Furthermore, the over-limiting pulsed-current protocol enables long-life Zn batteries with high mass loading (29 mgcathode cm-2) and high areal capacity (7.9 mAh cm-2), outperforming cells using constant-current protocols at equivalent energy and time costs. The work provides a comprehensive understanding of the current-capacity-performance relationship in Zn plating/stripping and offers an effective strategy for dendrite-free metal batteries that meet practical requirements for high capacity and high current rates.

Functionalized Quasi-Solid-State Electrolytes in Aqueous Zn-Ion Batteries for Flexible Devices: Challenges and Strategies

The rapid development of wearable and intelligent flexible devices has posed strict requirements for power sources, including excellent mechanical strength, inherent safety, high energy density, and eco-friendliness. Zn-ion batteries with aqueous quasi-solid-state electrolytes (AQSSEs) with various functional groups that contain electronegative atoms (O/N/F) with tunable electron accumulation states are considered as a promising candidate to power the flexible devices and tremendous progress has been achieved in this prospering area. Herein, this review proposes a comprehensive summary of the recent achievements using the AQSSE in flexible devices by focusing on the significance of different functional groups. The fundamentals and challenges of the ZIBs are introduced from a chemical view in the first place. Then, the mechanism behind the stabilization of the flexible ZIBs with the functionalized AQSSE is summarized and explained in detail. Then the recent progress regarding the enhanced electrochemical stability of the ZIBs with the AQSSE is summarized and classified based on the functional groups on the polymer chain. The advanced characterization methods for the AQSSE are briefly introduced in the following sections. Last but not least, current challenges and future perspectives for this promising area are provided from the authors' point of view.

In situ visualization of interfacial processes at nanoscale in non-alkaline Zn-air batteries

Zn-air batteries (ZABs) present high energy density and high safety but suffer from low oxygen reaction reversibility and dendrite growth at Zn electrode in alkaline electrolytes. Non-alkaline electrolytes have been considered recently for improving the interfacial processes in ZABs. However, the dynamic evolution and reaction mechanisms regulated by electrolytes at both the positive and Zn negative electrodes remain elusive. Herein, using in situ atomic force microscopy, we disclose that thin ZnO2 nanosheets deposit in non-alkaline electrolyte during discharge, followed by the formation of low-modulus products encircled around them. During recharge, the nanosheets are completely decomposed, revealing the favorable reversibility of the O2/ZnO2 chemistry. The circular outlines with low-modulus, composed of C = C and ZnCO3, are left which play a key role in promoting the oxygen reduction reaction (ORR) during the subsequent cycles. In addition, in situ optical microscopy shows that Zn can be uniformly dissolved and deposited in non-alkaline electrolyte, with the formation of homogeneous solid electrolyte interphase. Our work provides straightforward evidence and in-depth understanding of the interfacial reactions at both electrode interfaces in non-alkaline electrolyte, which can inspire strategies of interfacial engineering and material design of advanced ZABs.

Carbon Dioxide Evolution in Aqueous Zinc Metal Batteries

Gas evolution reactions in aqueous zinc metal batteries (AZMBs) cause gas accumulation and battery swelling that negatively affect their performance. However, previous work often reported hydrogen as the main, if not the only, gas species evolved in AZMBs; the complexity of gas evolution has been overlooked. For the first time, this work found the CO2 evolution reaction (CER) in AZMBs, pinpointed its sources, and identified electrolyte modulation strategies. Using differential electrochemical mass spectrometry, CER was detected in V2O5||Zn full cells, instead of in asymmetric Cu||Zn cells, and it became substantial when being charged to 2.0 V. By using a carbon isotope tracing method, the primary origin of CER was identified as the electrochemical corrosion of conductive carbon at the cathode. Among six representative electrolytes, the weakly solvating electrolyte (3 m Zn(OTf)2 in acetonitrile/water) presented a high CER resistance by reducing water solvating and disturbing hydrogen bonding. This work sheds light on interfacial parasitic reactions for practical aqueous metal (Zn and Al) batteries.

Highly Reversible Dendrite-Free Zinc Anode Enabled by a Bilayered Inorganic-Metal Interface Layer

The unavoidable dendrite growth and side reactions are two major issues that lead to unsatisfactory cycling stability of the Zn metal anode and premature battery failure, which constrains the wide practical application of aqueous Zn-ion batteries. Herein, a bilayered zinc fluoride-indium interface-modified zinc anode (ZnF2-In@Zn) is in situ-constructed to solve these two issues through a simple solution-dipping strategy. The outer ZnF2 layer assures sufficient desolvation of hydrated Zn2+ and even Zn2+ flux; meanwhile, the interior In layer further contributes to the uniform distribution of the electric field and lower energy barrier of Zn2+ nucleation, achieving dendrite-free and side reaction-free Zn deposition. With synergistic regulation from the bilayered composite interface, the ZnF2-In@Zn anode exhibits outstanding cycling stability (over 4200 h at 1 mA cm-2), achieving a cumulative capacity of over 5250 mAh cm-2 even under a high current density of 5 mA cm-2. This work proposes an advanced understanding of reasonable interface engineering for tackling multiple challenges faced by metal anodes.

Constructing Quasi-Single Ion Conductors by a β-Cyclodextrin Polymer to Stabilize Zn Anode

Aqueous Zn-ion batteries (AZIBs) are promising for the next-generation large-scale energy storage. However, the Zn anode remains facing challenges. Here, we report a cyclodextrin polymer (P-CD) to construct quasi-single ion conductor for coating and protecting Zn anodes. The P-CD coating layer inhibited the corrosion of Zn anode and prevented the side reaction of metal anodes. More important is that the cyclodextrin units enabled the trapping of anions through host-guest interactions and hydrogen bonds, forming a quasi-single ion conductor that elevated the Zn ion transference number (from 0.31 to 0.68), suppressed the formation of space charge regions and hence stabilized the plating/striping of Zn ions. As a result, the Zn//Zn symmetric cells coated with P-CD achieved a 70.6 times improvement in cycle life at high current densities of 10 mA cm-2 with 10 mAh cm-2. Importantly, the Zn//K1.1V3O8 (KVO) full-cells with high mass loading of cathode materials and low N/P ratio of 1.46 reached the capacity retention of 94.5 % after 1000 cycles at 10 A g-1; while the cell without coating failed only after 230 cycles. These results provide novel perspective into the control of solid-electrolyte interfaces for stabilizing Zn anode and offer a practical strategy to improve AZIBs.

Toward Quantitative Electrodeposition via In Situ Liquid Phase Transmission Electron Microscopy: Studying Electroplated Zinc Using Basic Image Processing and 4D STEM

High energy density electrochemical systems such as metal batteries suffer from uncontrollable dendrite growth on cycling, which can severely compromise battery safety and longevity. This originates from the thermodynamic preference of metal nucleation on electrode surfaces, where obtaining the crucial information on metal deposits in terms of crystal orientation, plated volume, and growth rate is very challenging. In situ liquid phase transmission electron microscopy (LPTEM) is a promising technique to visualize and understand electrodeposition processes, however a detailed quantification of which presents significant difficulties. Here by performing Zn electroplating and analyzing the data via basic image processing, this work not only sheds new light on the dendrite growth mechanism but also demonstrates a workflow showcasing how dendritic deposition can be visualized with volumetric and growth rate information. These results along with additionally corroborated 4D STEM analysis take steps to access information on the crystallographic orientation of the grown Zn nucleates and toward live quantification of in situ electrodeposition processes.

Electrical Double Layer and In Situ Polymerization SEI Enables High Reversible Zinc Metal Anode

Aqueous zinc-ion batteries (AZIBs) stand out among new energy storage devices due to their excellent safety and environmental friendliness. However, the formation of dendrites and side reactions on the zinc metal anode during cycling have become the major obstacles to their commercialization. This study innovatively selected Sodium 4-vinylbenzenesulfonate (VBS) as a multifunctional electrolyte additive to address the issues. The dissociated VBS- anions can not only significantly alter the hydrogen bond network structure of H2O in the electrolyte, but also preferentially adsorb on the surface of the zinc anode before H2O molecules, which will result in the development of organic anion-rich interface and alterations to the electrical double layer (EDL) structure. Furthermore, the ─C═C─ structure in VBS leads to the formation of an in situ polymerized organic anion solid electrolyte interface (SEI) layer that adheres to the surface of the zinc anode. The mechanisms work together to significantly improve the performance of Zn//Zn symmetric batteries, achieving a cycle life of over 1800 h at 1 mA cm-2 and 1 mAh cm-2. The introduction of VBS also enhances the cycling performance and capacity of Zn//δ-MnO2 full cells. This study provides a low-cost solution for the development of AZIBs.

Achieving Ultra-Long Cycle Life of Zn Anode Using ZnSn(OH)6 Coating Layer via Desolvation Effect and Uniform Zn2+ Flux

Aqueous zinc-ion batteries (AZIBs) are considered as a promising energy storage system because of good safety, low cost, abundant resources, and environmental friendliness. However, the bottlenecks including dendrite growth, hydrogen evolution, and corrosion seriously limit their practical application. Herein, a novel ZnSn(OH)6 coating layer with rich hydroxyl groups is employed to achieve highly stable Zn anode. The hydroxyl groups can feasibly interact with H2O molecules, contributing to the desolvation of hydrated Zn2+ and the inhibition of side reactions on Zn anode surface. Furthermore, according to the DFT calculation, the adsorption energy of Zn2+ among various sites on the surface of ZnSn(OH)6 coating layer is relatively large, which helps the uniform distribution of Zn2+ flux and the prevention of dendrite growth. Consequently, the ZnSn(OH)6@Zn anode delivers ultra-long cycle life (6770 h), low polarization voltage (27 mV), and high Coulombic efficiency (99.2% over 800 cycles) at 1 mA cm-2, 1 mAh cm-2. Besides, the assembled NaV3O8·xH2O//ZnSn(OH)6@Zn full cell can operate stably for 1500 cycles at 2 A g-1 with a high specific capacity of 144.9 mAh g-1, demonstrating an excellent application potential. This simple and effective coating layer with high electrochemical performance provides an appealing strategy for the development of rechargeable AZIBs.

High-Performance Aqueous Zinc-Organic Battery with a Photo-Responsive Covalent Organic Framework Cathode

Covalent organic framework (COF) materials, known for their robust porous character, sustainability, and abundance, have great potential as cathodes for aqueous Zn-ion batteries (ZIBs). However, their application is hindered by low reversible capacity and discharge voltage. Herein, a donor-acceptor configuration COF (NT-COF) is utilized as the cathode for ZIBs. The cell exhibits a high discharge voltage plateau of ≈1.4 V and a discharge capacity of 214 mAh g-1 at 0.2 A g-1 when utilizing the Mn2+ electrolyte additive in the ZnSO4 electrolyte. A synergistic combination mechanism is proposed, involving the deposition/dissolution reactions of Zn4SO4(OH)6·4H2O and the co-(de)insertion reactions of H+ and SO4 2- in NT-COF. Meanwhile, the NT-COF with a donor-acceptor configuration facilitates efficient generation and separation of electron-hole pairs upon light exposure, thereby enhancing electrochemical reactions within the battery. This leads to a reduction in charging voltage and internal overvoltage, ultimately minimizing electricity consumption. Under ambient weather conditions, the cell exhibits an average discharge capacity of 430 mAh g-1 on sunny days and maintains consistent cycling stability for a duration of 200 cycles (≈19 days) at 0.2 A g-1. This research inspires the advancement of Zn-organic batteries for high-energy-density aqueous electrochemical energy storage systems or photo-electrochemical batteries.

Versatile Biopolymers for Advanced Lithium and Zinc Metal Batteries

Lithium (Li) and zinc (Zn) metals are emerging as promising anode materials for next-generation rechargeable metal batteries due to their excellent electronic conductivity and high theoretical capacities. However, issues such as uneven metal ion deposition and uncontrolled dendrite growth result in poor electrochemical stability, limited cycle life, and rapid capacity decay. Biopolymers, recognized for their abundance, cost-effectiveness, biodegradability, tunable structures, and adjustable properties, offer a compelling solution to these challenges. This review systematically and comprehensively examines biopolymers and their protective mechanisms for Li and Zn metal anodes. It begins with an overview of biopolymers, detailing key types, their structures, and properties. The review then explores recent advancements in the application of biopolymers as artificial solid electrolyte interphases, electrolyte additives, separators, and solid-state electrolytes, emphasizing how their structural properties enhance protection mechanisms and improve electrochemical performance. Finally, perspectives on current challenges and future research directions in this evolving field are provided.

Electrolyte Design via Cation-Anion Association Regulation for High-Rate and Dendrite-Free Zinc Metal Batteries at Low Temperature

Low-temperature zinc metal batteries (ZMBs) are highly challenged by Zn dendrite growth, especially at high current density. Here, starting from the intermolecular insights, we report a cation-anion association modulation strategy by matching different dielectric constant solvents and unveil the relationship between cation-anion association strength and Zn plating/stripping performance at low temperatures. The combination of comprehensive characterizations and theoretical calculations indicates that moderate ion association electrolytes with high ionic conductivity (12.09 mS cm-1 at -40 °C) and a stable anion-derived solid electrolyte interphase (SEI) jointly account for highly reversible and dendrite-free Zn plating/stripping behavior, resulting in high-rate and ultrastable cycle performance at low temperature. As a result, at -40 °C, Zn//Zn cells can stably cycle over 400 h at 5 mA cm-2 and 10 mAh cm-2 and Zn//Cu cells exhibit an exceptional average Coulombic efficiency (CE) of 99.91% for 1800 cycles at 1 mA cm-2 and 1 mAh cm-2. Benefiting from enhanced low-temperature Zn plating/stripping performance, Zn//PANI full cells stably operate for 12,000 cycles at 0.5A g-1 and 35,000 cycles at 5 A g-1. Impressively, at -60 °C, Zn//Cu cells still display a high average CE of 99.68% for 2000 cycles. This work underscores the crucial effect of cation-anion association regulation for high-rate and dendrite-free Zn metal anodes, deepening the understanding of intermolecular interaction insights for low-temperature electrolyte design.

Mechanistic Understanding of Long Duration Fast Charge Aqueous Zinc Batteries Using Physically Adsorbed Oligomers as Interphases

Polymers have been used as additives in the liquid electrolytes typically used for secondary batteries that utilize metals as anode. Such additives are conventionally argued to improve long-term anode performance by suppressing morphological and hydrodynamic instabilities thought to be responsible for out-of-plane and dendritic metal deposition during battery charging. More recent studies have reported that the polymer additives provide even more fundamental mechanisms for stabilizing metal electrodeposition through their ability to regulate metal electrodeposit crystallography and, thereby, morphology. Few studies explore how polymers carried in a liquid electrolyte achieve these functions, and fewer still provide rules for choosing among the various polymer types, the additive polymer molecular weight (Mw), and concentration in the electrolyte. Here, we investigate how these generally easy-to-control variables influence electrochemical interphase formation inside battery cells and their impact on the morphology and reversibility of Zn electrodes in aqueous electrolytes. We focus on aqueous Zn-iodine electrochemical cells containing linear polyethylene glycol (PEG) chains as additives and find that in electrolytes where the polymer concentration is maintained in the dilute solution regime there is an optimum polymer molecular weight (Mw ≈ 1000 Da), above which beneficial effects of polymers on Zn electrode reversibility and Zn-I2 battery lifetime are progressively lost. By means of optical ellipsometry and theoretical calculations, we show that the optimal Mw is associated with saturation of the thickness of a physiosorbed PEG coating on the Zn metal electrode. Electron microscopy and X-ray photoelectron spectroscopy analysis of Zn electrodeposits formed in such electrolytes reveal that the physiosorbed polymer coating has two primary effects─it regulates the deposit morphology and suppresses parasitic reactions between the electrode and electrolyte components. The parasitic reactions produce species like ZnO, which are known to passivate the Zn electrode and promote nonuniform deposition. Galvanostatic cycling measurements in aqueous Zn-I2 cells containing the PEG additives at the optimal Mw show that the cells maintain very high Coulombic efficiencies (≥99%) at current densities as high as 50 mA/cm2─close to the maximum values permissible across the Celgard separator membranes used in our studies.

High-Entropy Multiple-Anion Aqueous Electrolytes for Long-Life Zn-Metal Anodes

Aqueous zinc-ion batteries (AZIBs) hold great promise for large-scale energy storage applications, however, their practical use is significantly hindered by issues such as zinc dendrite growth and hydrogen evolution. To address these challenges, we propose a high-entropy (HE) electrolyte design strategy that incorporates multiple zinc salts, aimed at enhancing ion kinetics and improving the electrochemical stability of the electrolyte. The interactions between multiple anions and Zn2+ increase the complexity of the solvation structure, resulting in smaller ion clusters while maintaining weakly anion-rich solvation structures. This leads to improved ion mobility and the formation of robust interphase layers on the electrode-electrolyte interface. Moreover, the HE electrolyte effectively suppresses hydrogen evolution and corrosion side reactions while facilitating uniform and reversible Zn plating/stripping processes. Impressively, the optimized electrolyte enables dendrite-free Zn plating/stripping for over 3000 h in symmetric cells and achieves a high Coulombic efficiency of 99.5% at 10 mA cm-2 in asymmetric cells. Inspiringly, full cells paired with Ca-VO2 cathodes demonstrate excellent performance, retaining 81.5% of the initial capacity over 1800 cycles at 5 A g-1. These significant findings highlight the potential of this electrolyte design strategy to improve the performance and lifespan of Zn-metal anodes in AZIBs.