Anti-Fatigue Hydrogel Electrolyte for All-Flexible Zn-Ion Batteries

An integrated all-flexible zinc-ion battery based on leather gel electrolyte

An integrated all-flexible zinc-ion battery was fabricated. It tightly integrates the cathode/anode with a quasi-solid leather gel electrolyte, which effectively eliminates mechanical deformation-induced displacement. The battery retains over 97% capacity under 100 bending cycles, perforation, and 6 kg of pressure. It can also power devices under extreme conditions, like waterlogging and fire.

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.

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.

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.

High-Energy-Density Aqueous Zinc-Ion Batteries: Recent Progress, Design Strategies, Challenges, and Perspectives

Aqueous zinc-ion batteries (AZIBs) are emerging as a promising energy storage technique supplementary to Li-ion batteries, attracting much research attention owing to their intrinsic safety, cost economy, and environmental friendliness. However, energy densities for AZIBs still do not fulfill practical requirements because of the low specific and areal capacity, limited working potential, and excessive negative-to-positive electrode capacity (N/P) ratio. In this review, a comprehensive overview of basic requirements and major challenges for achieving high-energy-density AZIBs is provided. Following that, recent progress in the optimization of each component and the overall configuration is summarized, and crucial design principles are discussed. Apart from conventional emphasis on each part, especially cathode materials, separately, the comprehensive discussion about the synergistic interactions among all components is conducted. Finally, the outlook and research direction are given to provide valuable guidance for the further holistic development of high-energy-density aqueous zinc-ion batteries.

Mass production of robust hydrogel electrolytes for high-performance zinc-ion batteries

Hydrogel electrolytes are crucial for solving the problems of random zinc dendrite growth, hydrogen evolution reactions, and uncontrollable passivation. However, their complex fabrication processes pose challenges to achieving large-scale production with excellent mechanical properties required to withstand multiple cycles of mechanical loads while maintaining high electrochemical performance needed for the new-generation flexible zinc-ion batteries. Herein, we present a superspreading-based strategy to produce robust hydrogel electrolytes consisting of polyvinyl alcohol, sodium alginate and sodium acetate. The hydrogel electrolytes have a tensile strength of 54.1 ± 2.5 MPa, a fracture strain of up to 1113 ± 37%, and a fracture toughness of 374.1 ± 6.1 MJ m-3, showcasing endurance of 2500 cycles at 80% strain without damage. Besides, the hydrogel electrolytes feature a high ionic conductivity of 14 mS cm-1 and a Zn2+ transference number of 0.62, as interfacial regulation enables the symmetric cell to achieve 1300 hours of highly stable and reversible zinc plating/stripping. As a preliminary attempt toward mass production, soft-pack batteries assembled using modified hydrogel electrolytes demonstrate robust machinability, with minimal voltage change after being bent and deformed 100 times. This work is expected to pave the way for developing a convenient hydrogel electrolyte for effective and stable zinc-ion batteries.

Electrolyte Regulation toward Cathodes with Enhanced-Performance in Aqueous Zinc Ion Batteries

Enhancing cathodic performance is crucial for aqueous zinc-ion batteries, with the primary focus of research efforts being the regulation of the intrinsic material structure. Electrolyte regulation is also widely used to improve full-cell performance, whose main optimization mechanisms have been extensively discussed only in regard to the metallic anode. Considering that ionic transport begins in the electrolyte, the modulation of the electrolyte must influence the cathodic performance or even the reaction mechanism. Despite its importance, the discussion of the optimization effects of electrolyte regulation on the cathode has not garnered the attention it deserves. To fill this gap and raise awareness of the importance of electrolyte regulation on cathodic reaction mechanisms, this review comprehensively combs the underlying mechanisms of the electrolyte regulation strategies and classifies the regulation mechanisms into three main categories according to their commonalities for the first time, which are ion effect, solvating effect, and interfacial modulation effect, revealing the missing puzzle piece of the mechanisms of electrolyte regulation in optimizing the cathode.

Fully Solar-Powered Uninterrupted Highway Tunnel-Lighting System Enabled by Cement-Based Aqueous Ni-Zn Structural Batteries

Highway tunnel lighting working 24 h a day, 365 days a year largely enables traffic safety but consumes a large amount of electric energy. Moreover, these tunnel lighting installations are powered by lithium-based batteries, which rely on Li sources and flammable organic electrolytes, leading to safety and space issues, or by electric power grids facing geographic limitations and high operating costs. Thus, taking advantage of cement-based materials to create low-cost and high-safety aqueous structural batteries and further develop a self-driven tunnel-lighting system is greatly desirable. Herein, the cement-based aqueous Ni-Zn structural batteries (CNZSBs), solar panels, and LEDs are successfully assembled together to realize a fully solar-powered uninterrupted lighting system, in which the CNZSBs can deliver a maximum energy density of 2.56 kWh m-3, as well as enough compressive strength to act as part of the tunnel structure. Specifically, the solar panels featuring a sustainable energy input can enable the charging of CNZSBs for energy storage and provide stable energy for LEDs during the day, while the fully-charged CNZSBs offer a steady output voltage for lighting at night. Such an uninterrupted lighting system provides exciting opportunities for developing energy storage in building materials and exploiting renewable energy sources.

Water-Restrained Hydrogel Electrolytes with Repulsion-Driven Cationic Express Pathways for Durable Zinc-Ion Batteries

The development of flexible zinc-ion batteries (ZIBs) faces a three-way trade-off among the ionic conductivity, Zn2+ mobility, and the electrochemical stability of hydrogel electrolytes. To address this challenge, we designed a cationic hydrogel named PAPTMA to holistically improve the reversibility of ZIBs. The long cationic branch chains in the polymeric matrix construct express pathways for rapid Zn2+ transport through an ionic repulsion mechanism, achieving simultaneously high Zn2+ transference number (0.79) and high ionic conductivity (28.7 mS cm-1). Additionally, the reactivity of water in the PAPTMA hydrogels is significantly inhibited, thus possessing a strong resistance to parasitic reactions. Mechanical characterization further reveals the superior tensile and adhesion strength of PAPTMA. Leveraging these properties, symmetric batteries employing PAPTMA hydrogel deliver exceeding 6000 h of reversible cycling at 1 mA cm-2 and maintain stable operation for 1000 h with a discharge of depth of 71%. When applied in 4 × 4 cm2 pouch cells with MnO2 as the cathode material, the device demonstrates remarkable operational stability and mechanical robustness through 150 cycles. This work presents an eclectic strategy for designing advanced hydrogels that combine high ionic conductivity, enhanced Zn2+ mobility, and strong resistance to parasitic reactions, paving the way for long-lasting flexible ZIBs.

Active Water Optimization in Different Electrolyte Systems for Stable Zinc Anodes

Zinc (Zn) metal, with abundant resources, intrinsic safety, and environmental benignity, presents an attractive prospect as a novel electrode material. However, many substantial challenges remain in realizing the widespread application of aqueous Zn-ion batteries (AZIBs) technologies. These encompass significant material corrosion challenges (This can lead to battery failure in an unloaded state.), hydrogen evolution reactions, pronounced dendrite growth at the anode interface, and a constrained electrochemical stability window. Consequently, these factors contribute to diminished battery lifespan and energy efficiency while restricting high-voltage performance. Although numerous reviews have addressed the potential of electrode and separator design to mitigate these issues to some extent, the inherent reactivity of water remains the fundamental source of these challenges, underscoring the necessity for precise regulation of active water molecules within the electrolyte. In this review, the failure mechanism of AZIBs (unloaded and in charge and discharge state) is analyzed, and the optimization strategy and working principle of water in the electrolyte are reviewed, aiming to provide insights for effectively controlling the corrosion process and hydrogen evolution reaction, further controlling dendrite formation, and expanding the range of electrochemical stability. Furthermore, it outlines the challenges to promote its practical application and future development pathways.

Solvent Polarity-Induced Regulation of Cation Solvation Sheaths for High-Voltage Zinc-Based Batteries with a 1.94 V Discharge Platform

To address the challenge of low discharge platforms (<1.5 V) in aqueous zinc-based batteries, highly concentrated salts have been explored due to their wide electrochemical window (~3 V). However, these electrolytes mainly prevent hydrogen evolution and dendrite growth at the anode without significantly enhancing voltage performance. Herein we introduce an approach by adjusting solvent polarity to regulate cation solvation sheaths in hybrid electrolytes, reducing Zn/Zn2+ oxidation potential and water activity. Through strong cation-water coordination and hydrogen bonding between dimethylsulfoxide and water, the designed electrolyte, at a low concentration, achieves a broader electrochemical window (4 V) than conventional concentrated electrolytes. Using this electrolyte, a Zn/Zn battery showed an impressive cycle life of 4340 cycles, while a Zn/lithium manganate battery delivered a high discharge platform of over 1.9 V with exceptional cycling stability. A Zn-based micro-battery with a polyvinyl alcohol-based hybrid electrolyte also achieved a record-high discharge platform of 1.94 V. This work presents a promising strategy for developing low-concentration electrolytes for high-performance sustainable energy storage.

Breaking the Ice: Hofmeister Effect-Inspired Hydrogen Bond Network Reconstruction in Hydrogel Electrolytes for High-Performance Zinc-Ion Batteries

Gel electrolytes have emerged as a promising solution for enhancing the performance of zinc-ion batteries (ZIBs), particularly in flexible devices. However, they face challenges such as low-temperature inefficiency, constrained ionic conductivity, and poor mechanical strength. To address these issues, this study presents a novel PAMCD gel electrolyte with tunable freezing point and mechanical properties for ZIBs, blending the high ionic conductivity of polyacrylamide with the anion interaction capability of β-cyclodextrin. Leveraging the Hofmeister effect, the chaotropic anions of ClO4 - are integrated to weaken hydrogen bonds, enhancing the mechanical and anti-freezing properties. The chaotropic salt disrupts the hydrogen bond network within water molecules, increasing weaker bonds and forming contact ion pairs, while polyacrylamide chains bind water molecules, further destabilizing hydrogen bonds. These changes improve Zn2+ ion mobility, mechanical resilience, and reduce the freezing point, significantly boosting ZIB performance. Consequently, the Zn-Zn symmetric cells achieve remarkable lifespans over 5290 hours at 0.5 mA cm-2 and 1960 hours at 5 mA cm-2, and the Zn-polyaniline full batteries maintain a high capacity of 100.8 mAh g-1 at 2 A g-1, even at -40 °C, over 7600 cycles, showcasing superior cyclability and rate performance.

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.

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.

An Ultrahigh-Modulus Hydrogel Electrolyte for Dendrite-Free Zinc Ion Batteries

Quasi-solid-state aqueous zinc ion batteries suffer from anodic zinc dendrite growth during plating/stripping processes, impeding their commercial application. The inhibition of zinc dendrites by high-modulus electrolytes has been proven to be effective. However, hydrogel electrolytes are difficult to achieve high modulus owing to their inherent high water contents. This work reports a hydrogel electrolyte with ultrahigh modulus that can overcome the growth stress of zinc dendrites through mechanical suppression effect. By combining wet-annealing, solvent-exchange, and salting-out processes and tuning the hydrophobic and crystalline domains, a hydrogel electrolyte is obtained with substantial water content (≈70%), high modulus (198.5 MPa), high toughness (274.3 MJ m-3), and high zinc-ion conductivity (28.9 mS cm-1), which significantly outperforms the previously reported poly(vinyl alcohol)-based hydrogels. As a result, the hydrogel electrolyte exhibits excellent dendrite-suppression effect and achieves stable performance in Zn||Zn symmetric batteries (1800 h of cycle life at 1 mA cm-2). Moreover, the Zn||V2O5 pouch batteries display excellent cycling life and operate stably even under extreme conditions, such as large bending angle (180°) and automotive crushing. This work provides a promising approach for designing mechanically reliable hydrogel electrolytes for advanced aqueous zinc ion batteries.

A Bio-Inspired Multifunctional Hydrogel Network with Toughly Interfacial Chemistry for Dendrite-Free Flexible Zinc Ion Battery

Flexible and high-performance aqueous zinc-ion batteries (ZIBs), coupled with low cost and safe, are considered as one of the most promising energy storage candidates for wearable electronics. Hydrogel electrolytes present a compelling alternative to liquid electrolytes due to their remarkable flexibility and clear advantages in mitigating parasitic side reactions. However, hydrogel electrolytes suffer from poor mechanical properties and interfacial chemistry, which limits them to suppressed performance levels in flexible ZIBs, especially under harsh mechanical strains. Herein, a bio-inspired multifunctional hydrogel electrolyte network (polyacrylamide (PAM)/trehalose) with improved mechanical and adhesive properties was developed via a simple trehalose network-repairing strategy to stabilize the interfacial chemistry for dendrite-free and long-life flexible ZIBs. As a result, the trehalose-modified PAM hydrogel exhibits a superior strength and stretchability up to 100 kPa and 5338 %, respectively, as well as strong adhesive properties to various substrates. Also, the PAM/trehalose hydrogel electrolyte provides superior anti-corrosion capability for Zn anode and regulates Zn nucleation/growth, resulting in achieving a high Coulombic efficiency of 98.8 %, and long-term stability over 2400 h. Importantly, the flexible Zn//MnO2 pouch cell exhibits excellent cycling performance under different bending conditions, which offers a great potential in flexible energy-related applications and beyond.

Ether-Based Gel Polymer Electrolyte for High-Voltage Potassium Ion Batteries

Low-concentration ether electrolytes cannot efficiently achieve oxidation resistance and excellent interface behavior, resulting in severe electrolyte decomposition at a high voltage and ineffective electrode-electrolyte interphase. Herein, we utilize sandwich structure-like gel polymer electrolyte (GPE) to enhance the high voltage stability of potassium-ion batteries (PIBs). The GPE contact layer facilitates stable electrode-electrolyte interphase formation, and the GPE transport layer maintains good ionic transport, which enabled GPE to exhibit a wide electrochemical window and excellent electrochemical performance. In addition, Al corrosion under a high voltage is suppressed through the restriction of solvent molecules. Consequently, when using the designed GPE (based on 1 m), the K||graphite cell exhibits excellent cycling stability of 450 cycles with a capacity retention of 91%, and the K||FeFe-Prussian blue cell (2-4.2 V) delivers a high average Coulombic efficiency of 99.9% over 2200 cycles at 100 mA g-1. This study provides a promising path in the application of ether-based electrolytes in high-voltage and long-lasting PIBs.

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.

Asymmetric Hydrogel Electrolyte Featuring a Customized Anode and Cathode Interfacial Chemistry for Advanced Zn-I2 Batteries

An integrated asymmetric hydrogel electrolyte with a tailored composition and chemical structure on the cathode/anode-electrolyte interface is designed to boost the cost-effective, high-energy Zn-I2 battery. Such a configuration concurrently addresses the parasitic reactions on the Zn anode side and the polyiodide shuttle issue afflicting the cathode. Specifically, the Zn2+-cross-linked sodium alginate and carrageenan dual network (Carra-Zn-Alg) is adopted to guide the Zn2+ transport, achieving a dendrite-free morphology on the Zn surface and ensuring long-term stability. For the cathode side, the poly(vinyl alcohol)-strengthened poly(3,4-ethylenedioxythiophene)polystyrenesulfonate hydrogel (PVA-PEDOT) with high conductivity is employed to trap polyiodide and accelerate electron transfer for mitigating the shuttle effect and facilitating I2/I- redox kinetics. Attributing to the asymmetrical architecture with a customized interfacial chemistry, the optimized Zn-I2 cell exhibits a superior Coulombic efficiency of 99.84% with a negligible capacity degradation at 0.1 A g-1 and an enhanced stability of 10 000 cycles at 5 A g-1. The proposed asymmetric hydrogel provides a promising route to simultaneously resolve the distinct challenges encountered by the cathode and anode interfaces in rechargeable batteries.

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

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

Biomass Solid-State Electrolyte with Abundant Ion and Water Channels for Flexible Zinc-Air Batteries

Flexible zinc-air batteries are the leading candidates as the next-generation power source for flexible/wearable electronics. However, constructing safe and high-performance solid-state electrolytes (SSEs) with intrinsic hydroxide ion (OH-) conduction remains a fundamental challenge. Herein, by adopting the natural and robust cellulose nanofibers (CNFs) as building blocks, the biomass SSEs with penetrating ion and water channels are constructed by knitting the OH--conductive CNFs and water-retentive CNFs together via an energy-efficient tape casting. Benefiting from the abundant ion and water channels with interconnected hydrated OH- wires for fast OH- conduction under a nanoconfined environment, the biomass SSEs reveal the high water-uptake, impressive OH- conductivity of 175 mS cm-1 and mechanical robustness simultaneously, which overcomes the commonly existed dilemma between ion conductivity and mechanical property. Remarkably, the flexible zinc-air batteries assemble with biomass SSEs deliver an exceptional cycle lifespan of 310 h and power density of 126 mW cm-2. The design methodology for water and ion channels opens a new avenue to design high-performance SSEs for batteries.

Wood-inspired anisotropic hydrogel electrolyte with large modulus and low tortuosity realizing durable dendrite-free zinc-ion batteries

While aqueous zinc-ion batteries exhibit great potential, their performance is impeded by zinc dendrites. Existing literature has proposed the use of hydrogel electrolytes to ameliorate this issue. Nevertheless, the mechanical attributes of hydrogel electrolytes, particularly their modulus, are suboptimal, primarily ascribed to the substantial water content. This drawback would severely restrict the dendrite-inhibiting efficacy, especially under large mass loadings of active materials. Inspired by the structural characteristics of wood, this study endeavors to fabricate the anisotropic carboxymethyl cellulose hydrogel electrolyte through directional freezing, salting-out effect, and compression reinforcement, aiming to maximize the modulus along the direction perpendicular to the electrode surface. The heightened modulus concurrently serves to suppress the vertical deposition of the intermediate product at the cathode. Meanwhile, the oriented channels with low tortuosity enabled by the anisotropic structure are beneficial to the ionic transport between the anode and cathode. Comparative analysis with an isotropic hydrogel sample reveals a marked enhancement in both modulus and ionic conductivity in the anisotropic hydrogel. This enhancement contributes to significantly improved zinc stripping/plating reversibility and mitigated electrochemical polarization. Additionally, a durable quasi-solid-state Zn//MnO2 battery with noteworthy volumetric energy density is realized. This study offers unique perspectives for designing hydrogel electrolytes and augmenting battery performance.

Cation-Conduction Dominated Hydrogels for Durable Zinc-Iodine Batteries

Zinc-iodine batteries have the potential to offer high energy-density aqueous energy storage, but their lifetime is limited by the rampant dendrite growth and the concurrent parasite side reactions on the Zn anode, as well as the shuttling of polyiodides. Herein, a cation-conduction dominated hydrogel electrolyte is designed to holistically enhance the stability of both zinc anode and iodine cathode. In this hydrogel electrolyte, anions are covalently anchored on hydrogel chains, and the major mobile ions in the electrolyte are restricted to be Zn2+. Specifically, such a cation-conductive electrolyte results in a high zinc ion transference number (0.81) within the hydrogel and guides epitaxial Zn nucleation. Furthermore, the optimized Zn2+ solvation structure and the reconstructed hydrogen bond networks on hydrogel chains contribute to the reduced desolvation barrier and suppressed corrosion side reactions. On the iodine cathode side, the electrostatic repulsion between negative sulfonate groups and polyiodides hinders the loss of the iodine active material. This all-round electrolyte design renders zinc-iodine batteries with high reversibility, low self-discharge, and long lifespan.

Binder-free barium-implanted MnO2 nanosheets on carbon cloth for flexible zinc-ion batteries

The intrinsically low electrical conductivity and poor structural fragility of MnO2 have significantly hampered the zinc storage performance. In this work, Ba2+-implanted δ-MnO2 nanosheets have been hydrothermally grown on a carbon cloth (Ba-MnO2@CC) as an extremely stable and efficient cathode material of aqueous zinc-ion batteries. The three-dimensionally porous architecture composed of interwoven thin MnO2 nanosheets effectively shortens the electron/ion transport distances, enlarges the electrode/electrolyte contact area, and increases the active sites for the electrochemical reaction. Meanwhile, Ba2+ could function as an interlayer pillar to stabilize the crystal structure of MnO2. Consequently, the as-optimized Ba-MnO2@CC exhibits remarkable Zn2+ storage capabilities, such as a high capacity (305 mAh g-1 at 0.2 A g-1), prolonged lifespan (95% retention after a 200-cycling test), and superb rate capability. The binder-free cathode is also applicable for flexible energy storage devices with attractive properties. The present investigation provides important insights into designing advanced cathode materials toward wearable electronics.