Cation-Conduction Dominated Hydrogels for Durable Zinc-Iodine Batteries

Coupling Zn2+ Ferrying Effect With Anion-π Interaction to Mitigate Space Charge Layer Enables Ultra-High Utilization Rate Zn Anode

A major dilemma faced by Zn anodes at a high zinc utilization rate (ZUR) is the insufficient supply of ionic carriers that initiate the space charge layer (SCL) subject to the rampant growth of Zn dendrites. Herein, an "anion-cation co-regulation" strategy, associated with a fundamental principle for screening potential electrolyte additives coupling the Zn2+ ferrying effect with anion-retention capability, is put forward to construct dendrite-free, high-ZUR Zn anode. Taking ninhydrin-modified ZnSO4 system as a proof-of-concept, the multiple zincophilic polar groups of ninhydrin facilitate the transport of Zn2+ ions, while its electron-deficient aromatic ring retains SO4 2- counterions via anion-π interaction, constructing an ion-rich interface that minimizes the SCL-driven Zn deterioration. Consequently, the Zn anode can endure ∼240 h continuous cycling at an ultrahigh ZUR of 87.3%. The superiority brought by ninhydrin is further reflected by the ultralong cycling durability of Zn-I2 batteries (over 100 000 cycles). Even at an ultralow N/P ratio of 1.1 (∼90.6% ZUR), the battery with a capacity of ∼5.27 mAh cm-2 can still sustain for 350 cycles, which has been hardly achieved in aqueous Zn batteries. Furthermore, the effectiveness of this strategy is fully validated by a series of additives sharing similar fundamentals.

Micro-Macro Hierarchical Hydrogel Architectures with Dual-Scale Polyanionic Networks for High Energy Density PBA||Zn Batteries

Hydrogel electrolytes are promising for aqueous zinc metal batteries but face challenges in suppressing Zn dendrites and cathode dissolution. This study develops a polyanionic hydrogel electrolyte, poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide) (P(AMPS-co-AM)), featuring a dense porous structure that enables a higher Zn2+ transference number (tZn 2+ = 0.81) and homogeneous zinc deposition. Additionally, the dense porous structure further reduces the proportion of free water molecules, thereby suppressing side reactions. Based on the benefits of the hydrogel, the Zn||Zn symmetric cell demonstrates over 3000 h of continuous cycling, and the Zn||Cu asymmetric cell exhibits an exceptional Coulombic efficiency of 99.29% in the first cycle. Benefiting from the fixation of transition metals by polyanionic groups and the reduced content of free water molecules within a densely packed porous architecture, the PBA||Zn full cell achieves a high energy density of 267 Wh kg-1. This hydrogel electrolyte design strategy provides significant insights for achieving long cycle life through both the microscale and macroscale structure design to achieve low cost and high energy density of aqueous zinc-metal batteries (AZMBs).

Cation-driven phase transition and anion-enhanced kinetics for high energy efficiency zinc-interhalide complex batteries

Aqueous Zn-halogen batteries, valued for high safety, large capacity, and low cost, suffer from the polyhalide shuttle effect and chaotic zinc electrodeposition, reducing energy efficiency and lifespan. Here we show a cation-driven positive electrode phase transition to suppress the shuttle effect and achieve uniform zinc electrodeposition, along with an anion kinetic enhancement strategy to improve energy efficiency and lifespan. Taking tetramethylammonium halide (TMAX, X = F, Cl, Br) as a subject, TMA+ promotes oriented zinc (101) deposition on the negative electrode through electrostatic shielding, significantly extending cycling life. Concurrently, it captures I3- on the positive electrode, forming a stable solid-phase interhalide complex that enhances coulombic efficiency. Compared to I3- and TMAI3, X- anions lower the Gibbs free energy differences of I- → I2X- and I2X- → TMAI2X, accelerating I-/I2X-/TMAI2X conversions and improving voltage efficiency. In TMAF-modified electrolytes, zinc interhalide complex batteries achieve a high energy efficiency of 95.2% at 0.2 A g-1 with good reversibility, showing only 0.1% capacity decay per cycle over 1000 cycles. At 1 A g-1, they show a low decay rate of 0.1‰ per cycle across 10,000 cycles. This study provides insights into enhancing energy efficiency and long-term stability for sustainable energy storage.

Hydrogel Electrolyte With Ultrahigh Water-Locking Capability for Quasi-Solid Zinc-Ion Batteries with Extreme Environmental Safety

Aqueous zinc-ion batteries (AZIBs) are considered promising energy storage devices because of the intrinsic safety, low cost, and environmental friendliness. However, the electrochemical performance of AZIBs is often hindered by side reactions occurring in electrolytes across different temperatures. Herein, this work investigates a quasi-solid hydrogel electrolyte, named GPE-EG with wide-temperature adaptability by simple copolymerization [2-(methacryloyloxy)ethyl] dimethyl(3-sulfopropyl) (SBMA) and acrylamide (AM) with H2O and ethylene glycol (EG) as co-solvents. The ion transport channels provided by SBMA and the regulation of electric field distribution on the zinc anode surface significantly enhance the cycling performance of AZIBs. Moreover, the ultrahigh water-locking capability of GPE-EG significantly improves the stability of electrolytes at both low and high temperatures. The symmetrical batteries exhibit stable cycling for over 1000 h (-20 °C), 1300 h (25 °C), and 300 h (65 °C), and the Zn||PANI full batteries with GPE-EG electrolyte exhibit remarkable electrochemical performance across a range of temperatures. Moreover, the full batteries maintain stable performance even under simulated extreme environmental conditions with gradient temperature changes. This work presents a novel gel chemistry that regulates zinc behavior and water reactivity across temperature extremes, showing strong potential for AZIBs in harsh environments.

Coordination Chemistry toward Advanced Zn-I2 Batteries with Four-Electron I-/I0/I+ Conversion

Aqueous zinc-iodine (Zn-I2) batteries with four-electron (4e) I-/I0/I+ conversion (4eZIBs) offer high energy density but face significant challenges for application, including the polyiodide shuttle effect and I+ hydrolysis for the I2 cathodes and poor reversibility for the Zn anodes. Here, we report a coordination chemistry strategy to address these issues simultaneously by introducing hexamethylenetetramine (HMTA) as an electrolyte additive. In aqueous electrolytes, HMTA undergoes protonation to form positively charged nitrogen moieties that effectively precipitate the polyiodides and I+ species (ICl2-) to mitigate the polyiodides shuttle and I+ hydrolysis. This strategy enables 4eZIBs to achieve a near-theoretical specific capacity of 425 mA h g-1 (based on the mass of iodine) and a Coulombic efficiency (CE) exceeding 99%. On the Zn anode, HMTA preferentially adsorbs onto its surface, inhibiting competitive water adsorption to suppress both Zn dendrite formation and hydrogen evolution. As a result, for the first time, we achieve durable 4eZIB performance in pouch-cell configurations with limited Zn supply. A 0.5 A h pouch cell with 15% Zn utilization exhibits a high energy density of 113.0 W h kg-1 (based on the mass of cathodes and anodes) and excellent cycling stability for over 1400 cycles, highlighting the potential of 4eZIBs for next-generation energy storage systems.

An Mn2+ Cross-Linked Gel Electrolyte Enables Reversible Quasi-Solid-State Manganese Metal Batteries

Aqueous manganese metal batteries (AMMBs) have emerged as promising alternatives for stationary energy storage applications owing to their higher energy density and higher cost efficiency compared to Zn metal batteries. However, the higher reactivity of Mn metal results in severe parasitic reactions, hampering the development of AMMBs. Here, we design an ionic cross-linking gel electrolyte (SA@Mn) via the cross-linking reaction between sodium alginate (SA) and manganese cations (Mn2+). The hydrophilic polymer chains reduce the free water content, inhibiting water-related parasitic reactions. Moreover, the unique ionic transport channels facilitate orderly Mn2+ migration to suppress dendrite growth. With optimized concentration, the 3M SA@Mn displays a high ionic conductivity (172.5 mS cm-1) and transference number (0.89). Therefore, the Mn||Mn symmetric cell achieves a high plating/stripping reversibility over 450 h, and the Mn||AgVO full cell could operate over 400 cycles. More importantly, a quasi-solid-state Mn metal pouch cell marks progress toward secure AMMBs for future smart-grid energy storage.

Functionally Segregated Ion Regulation Enables Dual Confinement Effect for Highly Stable Zinc-Iodine Batteries

Conventional electrolytes in aqueous zinc-iodine batteries struggle to suppress the shuttle effect and enhance interfacial stability, resulting in high self-discharge rate, low areal capacity, and short cycle life. To address these issues, a dual-confinement hydrogel electrolyte (DCHE) is designed to simultaneously stabilize the iodine cathode and zinc anode at high areal capacities via a functionally segregated ion regulation strategy. As for the cathode, anion-functional groups in the DCHE repel polyiodides, while cation-functional groups adsorb those that escape repulsion, thereby reinforcing the suppression of polyiodide migration toward the zinc anode. This dual confinement effect, validated by theoretical simulations and in situ characterization, effectively mitigates the shuttle effect. Additionally, hydrophilic and zincophilic functional groups regulate the hydrogen-bond network and Zn2+ flux, strengthening the electrochemical stability of the zinc anode. As a result, a Zn//ZnI2 cell assembled with DCHE delivers a practical areal capacity of 4.5 mAh cm-2 and achieves a record-long lifespan exceeding 6000 h with 88.9% capacity retention at 100 mA g-1. Furthermore, the single-layer pouch cell exhibits good mechanical stability, retaining 80% of its capacity after 100 cycles of 90° bending. This work highlights the importance of functionally segregated ion regulation in advancing high-performance aqueous batteries.

Enabling I3-/I2 Redox Couple toward High-Voltage Zn-Polyiodide Batteries by the Iodide-π Conjugation Effect

Distinct from the conventional I3-/I- redox couple (1.299 V), the I3-/I2 redox couple (1.552 V) can enhance the output voltage and achieve higher energy density, which exhibits great development potential. However, the sluggish solid-liquid reaction rate, high conversion energy barrier, and high polyiodide solubility in aqueous electrolytes together hinder its development, especially at a low N/P ratio. Herein, we introduce an approach to achieve fast liquid-liquid reaction kinetics and a lower conversion barrier for high valence iodine electrochemistry of I3-/I2, by coupling chemical liquefaction (MPII ionic liquid) and chelating catalyst (triazine-based poly(ionic liquid), PIL-tri). The MPII can spontaneously react with solid I2 to generate liquid MPII3, increasing reaction contact sites and accelerating reaction kinetics. Besides, PIL-tri significantly lowers the conversion barrier from I3- to I2 and restricts the triiodide shuttling by distinctive iodide-π (I-π) conjugation with an I3- electron cloud. Such a synergistic effect kinetically and thermodynamically ensures a high valence I3-/I2 redox couple. Consequently, PIL-tri@GP Zn-polyiodide batteries demonstrate a high output voltage (1.47 V), long cycling (800 cycles), and high-areal-capacity twice that of graphite paper (1.2 V) at a harsh N/P ratio (1). Meanwhile, they exhibited a polarity-switchable characteristic that maintained stable cyclability of 300 cycles when the anode and cathode were reversed every 50 cycles.

Spontaneous Anionic Double Capture/Displacement to Trigger Single-Ion Conducting Interpenetrating Polymer Networks with Desired Ionic Conductivity for Durable Zn Metal Batteries

The engineering of single-ion conductors (SICs) is a promising strategy to stabilize the anode/electrolyte interface in zinc-ion batteries. However, the commonly employed single-ion conductive solid or quasi-solid electrolytes often lead to a significant reduction in overall ionic conductivity, thereby impeding ion diffusion kinetics. Here, we propose a compromise strategy that effectively balances ionic conductivity and ion transference number. Specifically, a single-ion conductive interpenetrating polymer networks (IPNs) with phase-functional decoupling is developed solely on the anode side, while a liquid electrolyte is retained on the cathode side. This design facilitates a high ion transference number (0.84) while maintaining the ionic conductivity at an optimal level (12.1 mS cm-1). The single-ion-conductive mechanism is unveiled as a spontaneous anionic dual capture/displacement process, which synergizes zincophilic-hydrophobic functionality to ensure efficient reversible Zn2+ stripping and plating. The modified electrode demonstrates outstanding cycling stability of 1300 h at 5 mA cm-2. The assembled NH4V4O10//IPNs@Zn full cell exhibits an exceptional lifespan, enduring 9000 cycles with a remarkable retention of 80.8% at 10 A g-1. This work introduces an effective approach for balancing ionic conductivity and ion transference numbers in SICs, offering a promising pathway for the development of high-performance SICs.

α-Methyl Group Reinforced Amphiphilic Poly(Ionic Liquid) Additive for High-Performance Zinc-Iodine Batteries

Aqueous zinc-iodine (Zn-I2) batteries are prospective on energy storage, yet the practical application is severely hindered by side reactions of Zn metal and the shuttle effects for polyiodide. Herein, a polymer additive was copolymerized by 1-vinyl-3-ethylimidazolium trifluoromethanesulfonate (VEImOTf) and methacrylamide (MAAm) (PVEMA) to alleviate the above issues. The polymer chain of PVEMA endows amphipathic properties for Zn2+ diffusion and solvation structure regulation, and the α-methyl of MAAm enhances the hydrophobic properties to avoid side reactions on Zn metal. In addition, the imidazole groups adsorb onto Zn metal with electrostatic shielding effect for further side reaction alleviation and mitigate shuttle effects by electrostatic interactions with polyiodides. Consequently, the PVEMA confers the symmetrical Zn battery with great cycling stability for over 400 h at 20 mA cm-2 and high depth of discharge (DOD) of 77.7%. The Zn-I2 batteries with PVEMA also demonstrate stable cycling performance under various current densities and temperatures.

Interfacial Adsorption Layers Based on Amino Acid Analogues to Enable Dual Stabilization toward Long-Life Aqueous Zinc Iodine Batteries

Aqueous zinc-iodine (Zn-I2) batteries are promising candidates for large-scale energy storage due to the merits of low cost and high safety. However, their commercial application is hindered by Zn corrosion and polyiodide shuttle at I2 cathode. Herein, N,N-bis(2-hydroxyethyl)glycine (BHEG) based interfacial adsorption layers are constructed to stabilize Zn anodes and mitigate polyiodide shuttle according to ion-dipole interactions, by using a strategy of electrolyte additive. The tertiary amine (N(CH2)3) and carboxyl (─COO-) groups in the deprotonated BHEG can reversibly capture H+ and dynamically neutralize OH- ions, efficiently buffering the interfacial pH of Zn metal anodes and suppressing hydrogen evolution reactions. Additionally, the BHEG adsorption layers can repel 39.3% of H2O molecules at the Zn interface, creating a "water-deficient" inner Helmholtz plane and preventing Zn corrosion. Significantly, the N(CH2)3 groups in BHEG also inhibit polyiodide shuttle at the I2 cathode, which exhibits high adsorption energies of -0.88, -0.41, and -0.39 eV for I-, I2, and I3 -, respectively. Attributing to these benefits, the Zn-I2 battery can achieve a high areal capacity of 2.99 mAh cm-2 and an extended cycling life of 2,000 cycles, even at a high mass loading of I2 cathode (≈21.5 mg cm-2).

Solvent Chemistry Manipulated Iodine Redox Thermodynamics For Durable Iodine Batteries

The diverse valences of iodine enable it with multi-electron transfer capability for energy dense batteries. However, previous studies indicate that the primary I-/I2 redox couple exhibits distinct behaviors depending on electrolyte choice, with the mechanistic basis of aqueous versus nonaqueous systems remaining unclear. Here, we elucidated the solvent effect on iodine redox, particularly focusing on polyiodide formation and their molecular interaction correlations. We validate that a thermodynamically one-step conversion reaction (I2 ↔ I-) occurs in the protic solvents, while it is a two-step transformation (I2 ↔ I3 - ↔ I-) in aprotic solvents. This distinction arises from strong electron-donating properties in aprotic solvents that facilitate charge transfer with iodine, promoting complexation with iodide as solvent⋅I3 - species. Conversely, protic solvents form additional hydrogen bonds with iodine, alleviating polarization and reducing interaction with iodide. Furthermore, to address the limitations of single protic electrolytes - characterized by sluggish dissolution-precipitation and slow ion migration rates - we propose a hybrid electrolyte combining water and ethylene glycol. These hybrids enhance iodine redox kinetics, inhibits I3 - generation, and modifies the Zn2+ solvation structure to mitigate zinc anode corrosion and dendrite. The Zn-I2 batteries demonstrates exceptional long-term cycling stability in a wide temperature range, highlighting its potential for practical applications.

Hydrogel Electrolytes-Based Rechargeable Zinc-Ion Batteries under Harsh Conditions

Rechargeable zinc (Zn)-ion batteries (RZIBs) with hydrogel electrolytes (HEs) have gained significant attention in the last decade owing to their high safety, low cost, sufficient material abundance, and superb environmental friendliness, which is extremely important for wearable energy storage applications. Given that HEs play a critical role in building flexible RZIBs, it is urgent to summarize the recent advances in this field and elucidate the design principles of HEs for practical applications. This review systematically presents the development history, recent advances in the material fundamentals, functional designs, challenges, and prospects of the HEs-based RZIBs. Firstly, the fundamentals, species, and flexible mechanisms of HEs are discussed, along with their compatibility with Zn anodes and various cathodes. Then, the functional designs of hydrogel electrolytes in harsh conditions are comprehensively discussed, including high/low/wide-temperature windows, mechanical deformations (e.g., bending, twisting, and straining), and damages (e.g., cutting, burning, and soaking). Finally, the remaining challenges and future perspectives for advancing HEs-based RZIBs are outlined.

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.

Remolding the Interface Stability for Practical Aqueous Zn/I2 Batteries via Sulfonic Acid-Rich Electrolyte and Separator Design

The electrolyte-electrode interface plays a crucial role in aqueous Zn/I2 battery and is largely determined by the properties of electrolyte and separator. Here, the synergistic effect of sulfonic acid-rich electrolyte additive and separator impacts the interface stability of Zn/I2 batteries is comprehensively investigated using operando synchrotron-based Fourier-transform infrared spectroscopy, cryo-electron microscopy, and in situ spectroscopy. As a case study, a cost-effective additive known as lignosulfonic acid sodium (LAS) and a flexible sulfonated polyether sulfone membrane are employed to facilitate the formation of a stable solid electrolyte interface (SEI) on the Zn anode and effectively suppress the shuttle effect. The chemisorption of LAS on Zn, its interaction with Zn2+, and the impact on the Zn desolvation process are systematically investigated through both theoretical simulations and operando measurements. Furthermore, the formation of an in situ SEI consisting of ZnS and ZnF2 is identified, which facilitates the uniform nucleation and planar plating of Zn(002), while effectively suppressing detrimental side reactions. Additionally, visualization experiments and in situ spectroscopy confirm that R-SO3- groups effectively impede the shuttle process of I3-/I5- anions through electrostatic repulsion. This work provides valuable insights for designing robust electrolyte interfaces for high-performance aqueous Zn/I2 batteries.

Ionic Liquid Induced Static and Dynamic Interface Double Shields for Long-Lifespan All-Temperature Zn-Ion Batteries

Aqueous Zn-ion batteries (ZIBs) have experienced substantial advancements recently, while the aqueous electrolytes exhibit limited thermal adaptability. The low-cost Zn(BF4)2 salt possesses potential low-temperature application, while brings unsatisfied stability of Zn anodes. To address this challenge, an ionic liquid based eutectic electrolyte (ILEE) utilizing the Zn(BF4)2 presenting remarkable stability across a temperature range of ≈-100-150 °C is developed, enabling ZIBs to operate in diverse thermal conditions. The inner Zn2+ solvation structure can be modulated to a BF4 --rich state within the ILEE system, forming a static ZnF₂ layer at the electrolyte-Zn anode interface, as evidenced by ab initial molecular dynamic simulations. Moreover, the positively charged EMIM+ can accumulate on the Zn anodes to form the secondary electrostatic dynamic shield that mitigates the uncontrollable Zn dendrites growth, enhancing the overall cycling life of Zn anodes to over 10 times compared with the pure Zn(BF4)2 system. When utilizing the ILEE as the electrolyte, PANI||Zn full cells demonstrate acceptable performances under the all-temperature environments, especially presenting a long life of over 9500 cycles at a low temperature of -40 °C and 500 cycles at a high temperature of 60 °C. This special ILEE holds significant promise for future aqueous batteries in extreme environment.

Elimination of Concentration Polarization Under Ultra-High Current Density Zinc Deposition by Nanofluid Self-Driven Ion Enrichment

The commercialization of zinc metal batteries aims at high-rate capability and lightweight, which requires zinc anodes working at high current density, high areal capacity, and high depth of discharge. However, frequent zinc anode fades drastically under extreme conditions. Herein, it is revealed that the primary reason for the anode instability is the severe concentration polarization caused by the imbalanced consumption rate and transfer rate of Zn2+ under extreme conditions. Based on this finding, a nanofluid layer is constructed to rapidly absorb Zn2+ and mitigate the polarization induced by the nonlinear transport of interfacial ions. The modified zinc anode sustains at extreme conditions for over 1573 h (40 mA cm-2, 40 mAh cm-2, DOD = 75.97%) and 490 h (100 mA cm-2, 100 mAh cm-2, DOD = 90.91%), and achieving an unprecedented cumulative capacity of 62.92 Ah cm-2. This work offers both fundamental and practical insights for the interface design in energy storage devices.

Enabling Low-Temperature Zinc-Bromine Microbatteries with an Additive-Free Electrolyte Design

Aqueous zinc-bromine microbatteries (Zn-Br2 MBs) are promising energy storage devices for miniaturized electronic applications. However, their performance in low-temperature environments remains a challenge due to poor compatibility between antifreeze agents and complexing agents. In this work, we propose an additive-free electrolyte design to address this incompatibility from the source. An electrochemically active 7.5 m zinc bromide solution was found to have a low freezing point of -105 °C, while also inhibiting polybromide dissolution. Zn-Br2 microbatteries using this electrochemically active electrolyte demonstrated excellent cycling stability, with over 10,000 cycles (99% capacity retention) at 25 °C and more than 2000 cycles (98% capacity retention) at -60 °C. Both experimental data and theoretical calculations demonstrate that low-temperature environments inhibit polybromide dissolution. This work addresses the issue of incompatibility between antifreeze and complexing agents, challenging the traditional reliance on organic complexing agents to prevent polybromide dissolution in Zn-Br2 systems.

Zinc Single-Atom Catalysts Encapsulated in Hierarchical Porous Bio-Carbon Synergistically Enhances Fast Iodine Conversion and Efficient Polyiodide Confinement for Zn-I2 Batteries

Aqueous zinc iodine (Zn-I2) batteries have attracted attention due to their low cost, environmental compatibility, and high specific capacity. However, their development is hindered by the severe shuttle effect of polyiodides and the slow redox conversion kinetics of the iodine (I2) cathode. Herein, a long-life Zn-I2 battery is developed by anchoring iodine within an edible fungus slag-derived carbon matrix encapsulated with Zn single-atom catalysts (SAZn@CFS). The high N content and microporous structure of SAZn@CFS provide a strong iodine confinement, while the Zn-N4-C sites chemical interact with polyiodides effectively mitigating the iodine dissolution and the polyiodide shuttle effect. Additionally, the uniformly distributed SAZn sites significantly enhance the redox conversion efficiency of I-/I3 -/I5 -/I2, leading to improved capacity. At a high current density of 10 A g-1, the designed Zn-I2 battery delivers an excellent capacity of 147.2 mAh g-1 and a long lifespan of over 80 000 cycles with 93.6% capacity retention. Furthermore, the battery exhibits stable operation for 3500 times even at 50 °C, demonstrating significant advances in iodine reversible storage. This synergistic strategy optimizes composite structure, offering a practical approach to meet the requirements of high-performance Zn-I2 batteries.

Synchronous Modulation of H-bond Interaction and Steric Hindrance via Bio-molecular Additive Screening in Zn Batteries

In addressing challenges such as side reaction and dendrite formation, electrolyte modification with bio-molecule sugar species has emerged as a promising avenue for Zn anode stabilization. Nevertheless, considering the structural variability of sugar, a comprehensive screening strategy is meaningful yet remains elusive. Herein, we report the usage of sugar additives as a representative of bio-molecules to develop a screening descriptor based on the modulation of the hydrogen bond component and electron transfer kinetics. It is found that xylo-oligosaccharide (Xos) with the highest H-bond acceptor ratio enables efficient water binding, affording stable Zn/electrolyte interphase to alleviate hydrogen evolution. Meanwhile, sluggish reduction originated from the steric hindrance of Xos contributes to optimized Zn deposition. With such a selected additive in hand, the Zn||ZnVO full cells demonstrate durable operation. This study is anticipated to provide a rational guidance in sugar additive selection for aqueous Zn batteries.

Pyrrolic-Nitrogen Chemistry in 1-(2-hydroxyethyl)imidazole Electrolyte Additives toward a 50,000-Cycle-Life Aqueous Zinc-Iodine Battery

Rechargeable aqueous zinc iodine (Zn-I2) batteries offer benefits such as low cost and high safety. Nevertheless, their commercial application is hindered by hydrogen evolution reaction (HER) and polyiodide shuttle, which result in a short lifespan. In this study, 1-(2-hydroxyethyl)imidazole (HEI) organic molecules featuring pyrrole-N groups are introduced as dually-functional electrolyte additives to simultaneously stabilize Zn anode and confine polyiodide through ion-dipole interactions. The pyrrole-N groups in HEI can preserve the interfacial pH equilibrium at Zn anode by reversibly capturing H+ ions and dynamically neutralizing OH- ions, thereby suppressing the HER. Notably, the H2 evolution rate at the Zn anode is reduced to a mere 2.20 μmol h-1 cm-2. Furthermore, the pyrrole-N moieties in HEI effectively curtail the polyiodide shuttle at I2 cathode, which show adsorption energies of -0.174 eV for I2, -0.521 eV for I3 -, and -0.768 eV for I-, as indicated by density functional theory calculations. Electrochemical testing demonstrates that the Zn//Zn symmetric cell maintains stable cycling for up to 4,200 hours at 1 mA cm-2. Most strikingly, at a high I2 mass loading of 9.7 mg cm-2, the Zn-I2 battery achieves an extraordinary cycle life of 50,000 cycles.

Functional Hydrogels for Aqueous Zinc-Based Batteries: Progress and Perspectives

Aqueous zinc batteries (AZBs) hold great potential for green grid-scale energy storage due to their affordability, resource abundance, safety, and environmental friendliness. However, their practical deployment is hindered by challenges related to the electrode, electrolyte, and interface. Functional hydrogels offer a promising solution to address such challenges owing to their broad electrochemical window, tunable structures, and pressure-responsive mechanical properties. In this review, the key properties that functional hydrogels must possess for advancing AZBs, including mechanical strength, ionic conductivity, swelling behavior, and degradability, from a perspective of the full life cycle of hydrogels in AZBs are summarized. Current modification strategies aimed at enhancing these properties and improving AZB performance are also explored. The challenges and design considerations for integrating functional hydrogels with electrodes and interface are discussed. In the end, the limitations and future directions for hydrogels to bridge the gap between academia and industries for the successful deployment of AZBs are discussed.

Zinc Affinity and Hydrogen Evolution Trade-Off for Homogenous Zn Deposition in Reversible Zn Ion Batteries

Zn ion batteries (ZIBs) are promising for large-scale energy storage but their practical application is plagued by inhomogeneous Zn deposition. Despite much effort, the harm of simultaneous hydrogen evolution reaction (HER) during plating to Zn deposition, has not received sufficient studies. Herein, Sn-modified Cu nanowires (Sn@CuNWs) with Sn-Cu core-shell nanostructure to achieve uniform Zn deposition by zinc affinity-HER tendency trade-off are fabricated. Confirmed by both theoretical calculation and practical characterization, the nanowires with high zinc affinity and large deposition sites facilitate Zn deposition, while the enlarged HER tendency harmful to Zn plating is inhibited by Sn nanoshell. Therefore, the Zn deposited Sn@CuNWs anode delivers a long lifespan of 800 h at 5 mA cm-2, and the full cell exhibits a high capacity of 294.4 mAh g-1 at 5 A g-1 and a high capacity retention of 97.8% after 2500 cycles. This work reveals the importance of HER regulation for reversible Zn deposition, which should be noticed in further research.

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.

Sieving-type Electric Double Layer with Hydrogen Bond Interlocking to Stable Zinc Metal Anode

The stability of aqueous zinc metal batteries is significantly affected by side reactions and dendrite growth on the anode interface, which primarily originate from water and anions. Herein, we introduce a multi H-bond site additive, 2, 2'-Sulfonyldiethanol (SDE), into an aqueous electrolyte to construct a sieving-type electric double layer (EDL) by hydrogen bond interlock in order to address these issues. On the one hand, SDE replaces H2O and SO4 2- anions that are adsorbed on the zinc anode surface, expelling H2O/SO4 2- from the EDL and thereby reducing the content of H2O/SO4 2- at the interface. On the other hand, when Zn2+ are de-solvated at the interface during the plating, the strong hydrogen bond interaction between SDE and H2O/SO4 2- can trap H2O/SO4 2- from the EDL, further decreasing their content at the interface. This effectively sieves them out of the zinc anode interface and inhibits the side reactions. Moreover, the unique characteristics of trapped SO4 2- anions can restrict their diffusion, thereby enhancing the transference number of Zn2+ and promoting dendrite-free deposition and growth of Zn. Consequently, utilizing an SDE/ZnSO4 electrolyte enables excellent cycling stability in Zn//Zn symmetrical cells and Zn//MnO2 full cells with lifespans exceeding 3500 h and 2500 cycles respectively.

Molecular Synergistic Effects Mediate Efficient Interfacial Chemistry: Enabling Dendrite-Free Zinc Anode for Aqueous Zinc-Ion Batteries

The primary cause of the accelerated battery failure in aqueous zinc-ion batteries (AZIBs) is the uncontrollable evolution of the zinc metal-electrolyte interface. In the present research on the development of multiadditives to ameliorate interfaces, it is challenging to elucidate the mechanisms of the various components. Additionally, the synergy among additive molecules is frequently disregarded, resulting in the combined efficacy of multiadditives that is unlikely to surpass the sum of each component. In this study, the "molecular synergistic effect" is employed, which is generated by two nonhomologous acid ester (NAE) additives in the double electrical layer microspace. Specifically, ethyl methyl carbonate (EMC) is more inclined to induce the oriented deposition of zinc metal by means of targeted adsorption with the zinc (002) crystal plane. Methyl acetate (MA) is more likely to enter the solvated shell of Zn2+ and will be profoundly reduced to produce SEI that is dominated by organic components under the "molecular synergistic effect" of EMC. Furthermore, MA persists in a spontaneous hydrolysis reaction, which serves to mitigate the pH increase caused by the hydrogen evolution reaction (HER) and further prevents the formation of byproducts. Consequently, the 1E1M electrolyte not only extends the cycle life of the zinc anode to 3140 cycles (1 mA h cm-2 and 1 mA cm-2) but also extends the life of the Zn//MnO2 full battery, with the capacity retention rate still at 89.9% after 700 cycles.

Asymmetric Kosmotropism-Stabilized Double-Layer Hydrogel for Low-Cost Neutral Zinc-Air Battery

Zinc air battery (ZAB) provides a low-cost and high-energy density power source, particularly in wearable and portable devices. Despite the extensive research on air cathode catalysts, their practical application is hindered by low zinc utilization rate and severe corrosion and passivation in liquid-based alkaline electrolytes. Herein, a double-layer gel (DLKgel) is developed by leveraging the distinct kosmotropic properties of ZnCl2 and ZnSO4. Through phase separation induced by the kosmotropic differentiation (instead of membrane in decoupled systems), this DLKgel electrolyte serves a dual purpose of shielding cathode from irreversible reaction products and protecting Zn anode from passivation. Neutral ZABs with DLKgel demonstrate high zinc utilization rate of 89.3% and stable cycling over 800 h under a current density of 0.1 mA cm-2. The integration of DLKgel-based ZABs into a flexible GPS tracking device is demonstrated, highlighting the potential for broad adoption of flexible ZABs in wearable and logistics applications.

High-Conductivity and Ultrastretchable Self-Healing Hydrogels for Flexible Zinc-Ion Batteries

Aqueous zinc-ion batteries are promising candidates for flexible energy storage devices due to their safety, economic efficiency, and environmental friendliness. However, the uncontrollable dendrite growth and side reactions at the zinc anode hinder their commercial application. Herein, we designed and synthesized a dual network self-healing hydrogel electrolyte with zwitterionic groups (PAM-PAAS-QCS), which can be used for the large deformations of flexible devices due to its excellent stretchability (ε = 5100%). The incorporation of zwitterionic groups into the PAM-PAAS-QCS hydrogel electrolyte endows it with high ionic conductivity (33.61 mS/cm), a wide electrochemical stability window, and the ability to suppress zinc dendrite formation and side reactions. Besides, the Zn//Zn symmetric cell with PAM-PAAS-QCS can stably plate and strip zinc for 1500 h at 0.5 mA/cm2, and the Zn//Polyaniline full cell retains 82.4% of its capacity after 1500 cycles at 1 A/g. Additionally, flexible batteries based on both the original and self-healed PAM-PAAS-QCS hydrogel electrolytes demonstrate good cycling stability and stable charge-discharge performance under various bending conditions. This self-healing hydrogel electrolyte with excellent stretchability and high ionic conductivity is expected to pave the way for the development of high-performance flexible energy storage and wearable devices.

Toward High-Energy-Density Aqueous Zinc-Iodine Batteries: Multielectron Pathways

Aqueous zinc-iodine batteries (ZIBs) based on the reversible conversion between various iodine species have garnered global attention due to their advantages of fast redox kinetics, good reversibility, and multielectron conversion feasibility. Although significant progress has been achieved in ZIBs with the two-electron I-/I2 pathway (2eZIBs), their relatively low energy density has hindered practical application. Recently, ZIBs with four-electron I-/I2/I+ electrochemistry (4eZIBs) have shown a significant improvement in energy density. Nonetheless, the practical use of 4eZIBs is challenged by poor redox reversibility due to polyiodide shuttling during I-/I2 conversion and I+ hydrolysis during I2/I+ conversion. In this Review, we thoroughly summarize the fundamental understanding of two ZIBs, including reaction mechanisms, limitations, and improvement strategies. Importantly, we provide an intuitive evaluation on the energy density of ZIBs to assess their practical potential and highlight the critical impacts of the Zn utilization rate. Finally, we emphasize the cost issues associated with iodine electrodes and propose potential closed-loop recycling routes for sustainable energy storage with ZIBs. These findings aim to motivate the practical application of advanced ZIBs and promote sustainable global energy storage.

Tailoring the Whole Deposition Process from Hydrated Zn2+ to Zn0 for Stable and Reversible Zn Anode

The practical application of aqueous zinc-ion batteries (ZIBs) indeed faces challenges primarily attributed to the inherent side reactions and dendrite growth associated with the Zn anode. In the present work, N-Methylmethanesulfonamide (NMS) is introduced to optimize the transfer, desolvation, and reduction of Zn2+, achieving highly stable and reversible Zn plating/stripping. The NMS molecule can substitute one H2O molecule in the solvation structure of hydrated Zn2+ and be preferentially chemisorbed on the Zn surface to protect Zn anode against corrosion and hydrogen evolution reaction (HER), thereby suppressing byproducts formation. Additionally, a robust N-rich organic and inorganic (ZnS and ZnCO3) hybrid solid electrolyte interphase is in situ generated on Zn anode due to the decomposition of NMS, resulting in enhanced Zn2+ transport kinetics and uniform Zn2+ deposition. Consequently, aqueous cells with the NMS achieve a long lifespan of 2300 h at 1 mA cm-2 and 1 mAh cm-2, high cumulative plated capacity of 3.25 Ah cm-2, and excellent reversibility with an average coulombic efficiency (CE) of 99.7 % over 800 cycles.

Mullite Mineral-Derived Robust Solid Electrolyte Enables Polyiodide Shuttle-Free Zinc-Iodine Batteries

Zinc dendrite, active iodine dissolution, and polyiodide shuttle caused by the strong interaction between liquid electrolyte and solid electrode are the chief culprits for the capacity attenuation of aqueous zinc-iodine batteries (ZIBs). Herein, mullite is adopted as raw material to prepare Zn-based solid-state electrolyte (Zn-ML) for ZIBs through zinc ion exchange strategy. Owing to the merits of low electronic conductivity, low zinc diffusion energy barrier, and strong polyiodide adsorption capability, Zn-ML electrolyte can effectively isolate the redox reactions of zinc anode and AC@I2 cathode, guide the reversible zinc deposition behavior, and inhibit the active iodine dissolution as well as polyiodide shuttle during cycling process. As expected, wide operating voltage window of 2.7 V (vs Zn2+/Zn), high Zn2+ transference number of 0.51, and low activation energy barrier of 29.7 kJ mol-1 can be achieved for the solid-state Zn//Zn cells. Meanwhile, high reversible capacity of 127.4 and 107.6 mAh g-1 can be maintained at 0.5 and 1 A g-1 after 3 000 and 2 100 cycles for the solid-state Zn//AC@I2 batteries, corresponding to high-capacity retention ratio of 85.2% and 80.7%, respectively. This study will inspire the development of mineral-derived solid electrolyte, and facilitate its application in Zn-based secondary batteries.

Improvements and Challenges of Hydrogel Polymer Electrolytes for Advanced Zinc Anodes in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are widely regarded as desirable energy storage devices due to their inherent safety and low cost. Hydrogel polymer electrolytes (HPEs) are cross-linked polymers filled with water and zinc salts. They are not only widely used in flexible batteries but also represent an ideal electrolyte candidate for addressing the issues associated with the Zn anode, including dendrite formation and side reactions. In HPEs, an abundance of hydrophilic groups can form strong hydrogen bonds with water molecules, reducing water activity and inhibiting water decomposition. At the same time, special Zn2+ transport channels can be constructed in HPEs to homogenize the Zn2+ flux and promote uniform Zn deposition. However, HPEs still face issues in practical applications, including poor ionic conductivity, low mechanical strength, poor interface stability, and narrow electrochemical stability windows. This Review discusses the issues associated with HPEs for advanced AZIBs, and the recent progresses are summarized. Finally, the Review outlines the opportunities and challenges for achieving high performance HPEs, facilitating the utilization of HPEs in AZIBs.

Intrinsically Decoupled Coordination Chemistries Enable Quasi-Eutectic Electrolytes with Fast Kinetics toward Enhanced Zinc-Ion Capacitors

Eutectic electrolytes show potential beyond conventional low-concentration electrolytes (LCEs) in zinc (Zn)-ion capacitors (ZICs) yet suffer from high viscosity and sluggish kinetics. Herein, we originally propose a universal theory of intrinsically decoupling to address these issues, producing a novel electrolyte termed "quasi-eutectic" electrolyte (quasi-EE). Joint experimental and theoretical analyses confirm its unique solution coordination structure doped with near-LCE domains. This enables the quasi-EE well inherit the advanced properties at deep-eutectic states while provide facilitated kinetics as well as lower energy barriers via a vehicle/hopping-hybridized charge transfer mechanism. Consequently, a homogeneous electroplating pattern with much enhanced Sand's time is achieved on the Zn surface, followed by a twofold prolonged service-life with drastically reduced concentration polarization. More encouragingly, the quasi-EE also delivers increased capacitance output in ZICs, which is elevated by 12.4 %-144.6 % compared to that before decoupling. Furthermore, the pouch cell with a cathodic mass loading of 36.6 mg cm-2 maintains competitive cycling performances over 600 cycles, far exceeding other Zn-based counterparts. This work offers fresh insights into eutectic decoupling and beyond.

Organic solid-electrolyte interface layers for Zn metal anodes

Zinc ion batteries (ZIBs) have emerged as promising candidates for renewable energy storage owing to their affordability, safety, and sustainability. However, issues with Zn metal anodes, such as dendrite growth, hydrogen evolution reaction (HER), and corrosion, significantly hinder the practical application of ZIBs. To address these issues, organic solid electrolyte interface (SEI) layers have gained traction in the ZIB community as they can, for instance, help achieve uniform Zn plating/stripping and suppress side reactions. This article summarizes recent advances in organic artificial SEI layers for ZIB anodes, including their fabrication methods, electrochemical performance, and degradation suppression mechanisms.

Ultrastable Electrolytic Zn-I2 Batteries Based on Nanocarbon Wrapped by Highly Efficient Single-Atom Fe-NC Iodine Catalysts

Aqueous Zn-iodine (Zn-I2) conversion batteries with iodine redox chemistry suffers the severe polyiodide shuttling and sluggish redox kinetics, which impede the battery lifespan and rate capability. Herein, an ultrastable Zn-I2 battery is introduced based on single-atom Fe-N-C encapsulated high-surface-area carbon (HC@FeNC) as the core-shell cathode materials, which accelerate the I-/I3 -/I° conversion significantly. The robust chemical-physical interaction between polyiodides and Fe-N4 sites tightly binds the polyiodide ions and suppresses the polyiodide shuttling, thereby significantly enhancing the coulombic efficiency. As a result, the core-shell HC@FeNC cathode endows the electrolytic Zn-I2 battery with an excellent capacity, remarkable rate capability, and an ultralong lifespan over 60 000 cycles. More importantly, a practical 253 Wh kg-1 pouch cell shows good capacity retention of 84% after 100 cycles, underscoring its considerable potential for commercial Zn-I2 batteries.

In-Situ Spontaneous Electropolymerization Enables Robust Hydrogel Electrolyte Interfaces in Aqueous Batteries

Hydrogels hold great promise as electrolytes for emerging aqueous batteries, for which establishing a robust electrode-hydrogel interface is crucial for mitigating side reactions. Conventional hydrogel electrolytes fabricated by ex situ polymerization through either thermal stimulation or photo exposure cannot ensure complete interfacial contact with electrodes. Herein, we introduce an in situ electropolymerization approach for constructing hydrogel electrolytes. The hydrogel is spontaneously generated during the initial cycling of the battery, eliminating the need of additional initiators for polymerization. The involvement of electrodes during the hydrogel synthesis yields well-bonded and deep infiltrated electrode-electrolyte interfaces. As a case study, we attest that, the in situ-formed polyanionic hydrogel in Zn-MnO2 battery substantially improves the stability and kinetics of both Zn anode and porous MnO2 cathode owing to the robust interfaces. This research provides insight to the function of hydrogel electrolyte interfaces and constitutes a critical advancement in designing highly durable aqueous batteries.