Stable Hydrogel Electrolytes for Flexible and Submarine-Use Zn-Ion Batteries

Advanced electrolyte strategies for dendrite-free aqueous Zn-metal batteries

Aqueous Zn-metal batteries have attracted wide attention due to the abundant Zn reserves and the safety and non-toxicity of aqueous electrolytes, and have become a research hotspot in the field of electrochemical energy. However, the dendrite defects of metal Zn anodes and the resulting interfacial instability limit the large-scale development. Despite comprehensive studies and in-depth understanding, this issue still affects the electrochemical performance of Zn-metal batteries dramatically. In light of the great progress and recent focus, this review analyzes the origin of critical Zn dendrite related problems, and recapitulates the development of electrolyte strategies. In the end, various outlooks are summarized from different angles to improve the performance of Zn metal anodes. This study is expected to provide new inspiration for innovation in aqueous Zn battery electrolytes in the future.

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.

Tough and highly conductive deep eutectic solvent-based gel electrolyte strengthened by high aspect ratio of hemp lignocellulosic nanofiber

Flexible electronic sensing and energy storage technology impose heightened demands on the mechanical and stable properties of gel electrolyte materials. Lignocellulosic nanofiber (LCNF) present a promising avenue for improving the properties of electrolyte networks and mechanical strength. In this study, LCNF derived from hemp fibers was prepared using lactic acid/choline chloride deep eutectic solvent (DES) through a combination of cooking and colloid mill mechanical treatment to achieve nanocellulose with a high aspect ratio and uniform dimensions. The outcomes demonstrated that LCNF, a width of below 20 nm and a length of over 5 μm, can be effectively produced through the DES cooking pretreatment in conjunction with colloid mill mechanical treatment. Meanwhile, DES lignin possessed a purity of ∼90 % and was obtained as a by-product. Subsequently, the as-prepared LCNF was integrated as a nanofiller into gel electrolyte. Ag-L NPs/LCNF/DES/PAA exhibited dense porous structures and showcased exceptional properties, including a high conductivity exceeding 10 mS/cm and remarkable adhesion strength surpassing 100 KPa. The presence of LCNF allowed Ag-L NPs/LCNF/DES/PAA to achieve strains above 1000 % and compression properties over 1000 KPa. The supercapacitor based on this assembly had a high specific capacitance of 271 F g-1 at 0.5 A g-1), along with an impressive capacity retention rate reaching ∼100 % after 3000 cycles. This investigation offers valuable insights into the utilization of lignocellulosic multi-component approaches in the development of flexible electronic devices.

Hygroscopic Nature of Lithium Ions: A Simple Key to Super Tough Atmosphere-Stable Hydrogel Electrolytes

Gel electrolytes have emerged as a versatile solution to address numerous limitations associated with liquid electrolytes in electrical energy storage (EES) devices, in terms of safety, flexibility, and affordability. Aqueous gel electrolytes, in particular, exhibit exceptional features by offering one of the highest ion solvation capacities and ionic conductivities. The two main challenges with hydrogel electrolytes are their easy freezing at subzero temperatures and rapid dehydration under open conditions, leading to the failure of the EES device. In response, we present an uncomplicated and quick-to-make hydrogel electrolyte system offering impressive mechanical properties (205.5 kPa tensile strength, 2880 kJ/m3 toughness, and 3030% strain at the break), along with antifreezing and antiflammability attributes. Notably, the hydrogel electrolyte demonstrates high ionic conductivity and superior performance in supercapacitor cells over a wide range of temperatures (-40 to 80 °C) and under various deformations. The hydrogel electrolyte maintains its capabilities under open conditions over an extended period of time, even at 50 °C, showcased by powering a wristwatch. The atmospheric stability of the hydrogel electrolyte demonstrated in this study introduces promising prospects for the future of EES devices spanning from production to end-user consumption.

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.

Wood Cell Wall Nanoengineering toward Anisotropic, Strong, and Flexible Cellulosic Hydrogel Sensors

Achieving highly ionic conductive hydrogels from natural wood remains challenging owing to their insufficient surface area and low number of active sites on the cell wall. This study proposes a viable strategy to design a strong and anisotropic wood-based hydrogel through cell wall nanoengineering. By manipulating the microstructure of the wood cell wall, a flexible cellulosic hydrogel is achieved through Schiff base bonding via the polyacrylamide and cellulose molecular chains. This results in excellent flexibility and mechanical properties of the wood hydrogel with tensile strengths of 22.3 and 6.1 MPa in the longitudinal and transverse directions, respectively. Moreover, confining aqueous salt electrolytes within the porous structure gives anisotropic ionic conductivities (19.5 and 6.02 S/m in the longitudinal and transverse directions, respectively). The wood-based hydrogel sensor has a favorable sensitivity and a stable working performance at a low temperature of -25 °C in monitoring human motions, thereby demonstrating great potential applications in wearable sensor devices.

Cathode|Electrolyte Interface Engineering by a Hydrogel Polymer Electrolyte for a 3D Porous High-Voltage Cathode Material in a Quasi-Solid-State Zinc Metal Battery by In Situ Polymerization

This work highlights the development of a superior cathode|electrolyte interface for the quasi solid-state rechargeable zinc metal battery (QSS-RZMB) by a novel hydrogel polymer electrolyte using an ultraviolet (UV) light-assisted in situ polymerization strategy. By integrating the cathode with a thin layer of the hydrogel polymer electrolyte, this technique produces an integrated interface that ensures quick Zn2+ ion conduction. The coexistence of nanowires for direct electron routes and the enhanced electrolyte ion infiltration and diffusion by the 3D porous flower structure with a wide open surface of the Zn-MnO electrode complements the interface formation during the in situ polymerization process. The QSS-RZMB configured with an integrated cathode (i-Zn-MnO) and the hydrogel polymer electrolyte (PHPZ-30) as the separator yields a comparable specific energy density of 214.14 Wh kg-1 with that of its liquid counterpart (240.38 Wh kg-1, 0.5 M Zn(CF3SO3)2 aqueous electrolyte). Other noteworthy features of the presented QSS-RZMB system include its superior cycle life of over 1000 charge-discharge cycles and 85% capacity retention with 99% coulombic efficiency at the current density of 1.0 A g-1, compared to only 60% capacity retention over 500 charge-discharge cycles displayed by the liquid-state system under the same operating conditions.

Aqueous AlCl3/ZnCl2 solution room-induced the self-growing strategy of expanded topological network for cellulose/polyacrylamide-based solid-state electrolytes

The green synthesis strategy for cellulose-containing hydrogel electrolytes is significant for effectively managing resources, energy, and environmental concerns in the contemporary world. Herein, we propose an all-green strategy using AlCl3/ZnCl2/H2O solvent to create cellulose/polyacrylamide-based hydrogel (AZ-Cel/PAM) with expanded hierarchical topologies. The aqueous AlCl3/ZnCl2 facilitates the efficient dissolution of cellulose at room temperature, and the dispersed Al3+-Zn2+ ions autocatalytic system catalyzes in-situ polymerization of acrylamide (AM) monomer. This expands the AM network within the cellulose framework, forming multiple bonding interactions and stable ion channels. The resulting hybrid hydrogel exhibits improved mechanical properties (tensile strength of 56.54 kPa and compressive strength of 359.43 kPa) and enhanced ionic conductivity (1.99 S/m). Furthermore, it also demonstrates excellent adhesion, freeze resistance (-45 °C), and water retention capabilities. Quantum simulations further clarify the mechanical composition and ion transport mechanism of AZ-Cel/PAM hydrogels. The assembled supercapacitor with the hydrogel electrolyte, demonstrates an ideal area-specific capacitance of 203.80 mF/cm2. This all-green strategy presents a novel approach to developing sustainable energy storage devices.

Gels/Hydrogels in Different Devices/Instruments-A Review

Owing to their physical and chemical properties and stimuli-responsive nature, gels and hydrogels play vital roles in diverse application fields. The three-dimensional polymeric network structure of hydrogels is considered an alternative to many materials, such as conductors, ordinary films, constituent components of machines and robots, etc. The most recent applications of gels are in different devices like sensors, actuators, flexible screens, touch panels, flexible storage, solar cells, batteries, and electronic skin. This review article addresses the devices where gels are used, the progress of research, the working mechanisms of hydrogels in those devices, and future prospects. Preparation methods are also important for obtaining a suitable hydrogel. This review discusses different methods of hydrogel preparation from the respective raw materials. Moreover, the mechanism by which gels act as a part of electronic devices is described.

Immobile polyanionic backbone enables a 900-μm-thick electrode for compact energy storage with unprecedented areal capacitance

Thickening of electrodes is crucial for maximizing the proportion of active components and thus improving the energy density of practical energy storage cells. Nevertheless, trade-offs between electrode thickness and electrochemical performance persist because of the considerably increased ion transport resistance of thick electrodes. Herein, we propose accelerating ion transport through thick and dense electrodes by establishing an immobile polyanionic backbone within the electrode pores; and as a proof of concept, gel polyacrylic electrolytes as such a backbone are in situ synthesized for supercapacitors. During charge and discharge, protons rapidly hop among RCOO- sites for oriented transport, fundamentally reducing the effects of electrode tortuosity and polarization resulting from concentration gradients. Consequently, nearly constant ion transport resistance per unit thickness is achieved, even in the case of a 900-μm-thick dense electrode, leading to unprecedented areal capacitances of 14.85 F cm-2 at 1 mA cm-2 and 4.26 F cm-2 at 100 mA cm-2. This study provides an efficient method for accelerating ion transport through thick and dense electrodes, indicating a significant solution for achieving high energy density in energy storage devices, including but not limited to supercapacitors.

Phytic Acid Customized Hydrogel Polymer Electrolyte and Prussian Blue Analogue Cathode Material for Rechargeable Zinc Metal Hydrogel Batteries

Zinc anode deterioration in aqueous electrolytes, and Zn dendrite growth is a major concern in the operation of aqueous rechargeable Zn metal batteries (AZMBs). To tackle this, the replacement of aqueous electrolytes with a zinc hydrogel polymer electrolyte (ZHPE) is presented in this study. This method involves structural modifications of the ZHPE by phytic acid through an ultraviolet (UV) light-induced photopolymerization process. The high membrane flexibility, high ionic conductivity (0.085 S cm-1), improved zinc corrosion overpotential, and enhanced electrochemical stability value of ≈2.3 V versus Zn|Zn2+ show the great potential of ZHPE as an ideal gel electrolyte for rechargeable zinc metal hydrogel batteries (ZMHBs). This is the first time that the dominating effect of chelation of phytic acid with M2+ center over H-bonding with water is described to tune the gel electrolyte properties for battery applications. The ZHPE shows ultra-high stability over 360 h with a capacity of 0.50 mAh cm-2 with dendrite-free plating/stripping in Zn||Zn symmetric cell. The fabrication of the ZMHB with a high-voltage zinc hexacyanoferrate (ZHF) cathode shows a high-average voltage of ≈1.6 V and a comparable capacity output of 63 mAh g-1 at 0.10 A g-1 of the current rate validating the potential application of ZHPE.

Green and Low-Cost Alkali-Polyphenol Synergetic Self-Catalysis System Access to Fast Gelation of Self-Healable and Self-Adhesive Conductive Hydrogels for Self-Powered Triboelectric Nanogenerators

Biomass-based hydrogels have attracted great attention in flexible and sustainable self-powered power sources but struggled to fabricate in a green, high-efficiency, and low-cost manner. Herein, a novel and facile alkali-polyphenol synergetic self-catalysis system is originally employed for the fast gelation of self-healable and self-adhesive lignin-based conductive hydrogels, which can be regarded as hydrogel electrodes of flexible triboelectric nanogenerators (TENGs). This synergy self-catalytic system comprises aqueous alkali and polyphenol-containing lignin, in which alkali-activated ammonium persulfate (APS) significantly accelerates the generation of radicals and initiates the polymerization of monomers, while polyphenol acts as a stabilizer to avoid bursting polymerization from inherent radical scavenging ability. Furthermore, multiple hydrogen bonds between lignin biopolymers and polyacrylamide (PAM) chains impart lignin-based hydrogels with exceptional adhesiveness and self-healing properties. Intriguingly, the alkaline conditions not only contribute to the solubility of lignin but also impart superior ionic conductivity of lignin-based hydrogel that is applicable to flexible TENG in self-powered energy-saving stair light strips, which holds great promise for industrial applications of soft electronics.

Recent Advances on Stretchable Aqueous Zinc-Ion Batteries for Wearable Electronics

The rapid development of wearable electronics has stimulated the pursuit of advanced stretchable power sources. As a promising candidate, stretchable aqueous zinc-ion batteries (AZIBs), have attracted unprecedented attention owing to their intrinsic safety, low cost, environmental benignity, and high performance, and can be endowed with additional functionalities to broaden the applications of wearable electronics. Here, a comprehensive review on the latest advances of stretchable AZIBs is presented. The materials and methods for stretchable components in AZIBs are first summarized, covering current collectors, electrodes, electrolytes/separators, and encapsulating layers. Subsequently, the benefits of the coplanar, fiber-shaped, and sandwiched configurations for stretchable AZIBs are analyzed. Moreover, the additional features integrated into stretchable AZIBs are highlighted. Finally, the challenges and prospects of stretchable AZIBs for wearable applications in the future are proposed.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Biodegradable Pea Protein Fibril Hydrogel-Based Quasi-Solid-State Zn-Ion Battery

Zinc-ion batteries show great potential as the next-generation power source due to their nontoxic, low-cost, and safe properties. However, issues with zinc anodes, such as dendrite growth and parasitic hydrogen evolution reactions (HERs), must be addressed to commercialize them. Solutions, such as quasi-solid-state electrolytes made from synthetic polymer hydrogels, have been proposed to improve battery flexibility and energy density. However, most polymers used are nonbiodegradable, posing a challenge to sustainability. In this study, hydrogels made from biodegradable poly(vinyl alcohol) and protein nanofibrils from pea protein, a renewable plant-based source, are used as an electrolyte in aqueous zinc-ion batteries. Results show that the flexible and biodegradable hydrogel can enhance the zinc anode stability and effectively restrict HER. This phenomenon is because of the hydrogen-bond network between nanofibril functional groups and water molecules. In addition, the interaction between functional groups on nanofibrils and Zn2+ constructs ion channels for the even migration of Zn2+, avoiding dendrite growth. The Zn||Zn symmetric cell using the hydrogel electrolyte exhibits a long lifespan of over 3000 h and improved capacity retention in the Zn||AC-I2 hybrid ion batteries by suppressing cathode material dissolution. This study suggests the potential of biodegradable hydrogels as a sustainable and effective solution for biodegradable soft powering sources.

Recent advances in cellulosic materials for aqueous zinc-ion batteries: An overview

Aqueous zinc-ion batteries (AZIBs), with the merits of high safety, environmental friendliness, abundant resources, and competitive energy density are recognized as a promising secondary battery technology and are anticipated to be a great alternative to organic lithium-ion batteries (LIBs). However, the commercial application of AZIBs is severely hindered by intractable issues, including high desolvation barrier, sluggish ion transport kinetics, growth of zinc dendrite, and side reactions. Nowadays, cellulosic materials are frequently employed in the fabrication of advanced AZIBs, because of the intrinsically excellent hydrophilicity, strong mechanical strength, sufficient active groups, and unexhaustible production. In this paper, we start from reviewing the success and dilemma of organic LIBs, followed by introducing the next-generation power source of AZIBs. After summarizing the features of cellulose with great potential in advanced AZIBs, we comprehensively and logically analyze the applications and superiorities of cellulosic materials in AZIBs electrodes, separators, electrolytes, and binders with an in-depth perspective. Finally, a clear outlook is delivered for future development of cellulose in AZIBs. Hopefully, this review can offer a smooth avenue for future direction of AZIBs by means of cellulosic material design and structure optimization.

Single-Ion-Conducting Hydrogel Electrolytes Based on Slide-Ring Pseudo-Polyrotaxane for Ultralong-Cycling Flexible Zinc-Ion Batteries

Flexible zinc-ion batteries (ZIBs) with high capacity and long cycle stability are essential for wearable electronic devices. Hydrogel electrolytes have been developed to provide ion-transfer channels while maintaining the integrity of ZIBs under mechanical strain. However, hydrogel matrices are typically swollen with aqueous salt solutions to increase ionic conductivity, which can hinder intimate contact with electrodes and reduce mechanical properties. To address this, a single-Zn-ion-conducting hydrogel electrolyte (SIHE) is developed by integrating polyacrylamide network and pseudo-polyrotaxane structure. The SIHE exhibits a high Zn2+ transference number of 0.923 and a high ionic conductivity of 22.4 mS cm-1 at room temperature. Symmetric batteries with SIHE demonstrate stable Zn plating/stripping performance for over 160 h, with a homogenous and smooth Zn deposition layer. Full cells with La-V2 O5 cathodes exhibit a high capacity of 439 mA h g-1 at 0.1 A g-1 and excellent capacity retention of 90.2% after 3500 cycles at 5 A g-1 . Moreover, the flexible ZIBs display stable electrochemical performance under harsh conditions, such as bending, cutting, puncturing, and soaking. This work provides a simple design strategy for single-ion-conducting hydrogel electrolytes, which could pave the way for long-life aqueous batteries.

Dual-network polyvinyl alcohol/polyacrylamide/xanthan gum ionic conductive hydrogels for flexible electronic devices

Ionic conductive hydrogels (ICHs) have received widespread attention as an ideal candidate for flexible electronic devices. However, conventional ICHs failed in widespread applications due to their inability to simultaneously possess high toughness, high ionic conductivity, and anti-freezing properties. Here, polyvinyl alcohol (PVA) and polyacrylamide (PAAm) were first dissolved in the zinc chloride solution, in which zinc ions (Zn²⁺) act as ionic cross-linkers and conducting ions, followed by the introduction of xanthan gum (XG) with a unique structure of trisaccharide side chains into the PVA/PAAm semi-interpenetrating network to prepare a dual-network ICHs (refers as PPXZ). Enabled by the synergistic effect of intermolecular chemical covalent cross-linking and physical cross-linking, PPXZ hydrogels exhibit significantly improved mechanical properties without sacrificing electrical conductivity. Furthermore, PPXZ hydrogels are successfully applied to flexible electronic devices, such as strain sensors and zinc ion hybrid supercapacitors, exhibiting satisfactory sensing sensitivity and cycling stability at a wide temperature range, respectively. Even at a high current density (10 A g^-1), the capacity of the supercapacitor retains 88.24 % after 10,000 cycles. This strategy provides new insight for ICHs in wide temperature-applied flexible electronic devices.

Synthesis of V2O5·nH2O nanobelts@polyaniline core-shell structures with highly efficient Zn2+ storage

Aqueous zinc-ion batteries (AZIBs) are regarded as attractive candidates for next-generation energy storage devices. Among various cathode materials, V₂O₅·nH₂O (VOH) possesses a high theoretical capacity but poor cycle stability due to the susceptibility of its open structure to damage by the quick shuttling of Zn²⁺. Herein, the structural stability of VOH is directly improved by wrapping polyaniline (PANI) on the VOH nanobelts (VOH@PANI). As a cathode material for AZIBs, the VOH nanobelts@PANI core-shell structures exhibit an outstanding cycle stability of 98% after 2000 cycles at 2 A g^-1. The improved conductivity and additional energy storage contribution of the PANI endow VOH@PANI with a specific capacity as high as 440 mAh g^-1 at 0.1 A g^-1, substantially higher than pure VOH (291 mAh g^-1). At the same time, high energy and power densities of 349 Wh kg^-1 and 3347 W kg^-1 are achieved. This work not only demonstrates that p-type doped PANI coatings on VOH can boost the Zn²⁺ storage of VOH, but also provides a novel method to enhance cathode materials for high electrochemical performance.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Manipulating Deposition Behavior by Polymer Hydrogel Electrolyte Enables Dendrite-Free Zinc Anode for Zinc-Ion Hybrid Capacitors

Rechargeable aqueous zinc-ion hybrid capacitors (ZHCs) have aroused unprecedented attention because of their high safety, cost effectiveness, and environmental compatibility. However, the intractable issues of dendrite growth and side reactions at the electrode-electrolyte interface deteriorate durability and reversibility of Zn anodes, deeply encumbering the large-scale application of ZHCs. Concerning these obstacles, a negatively charged carboxylated chitosan-intensified hydrogel electrolyte (CGPPHE) with cross-linked networks is reported to stabilize Zn anodes. Beyond possessing good mechanical characteristics, the CGPPHE with polar groups can reduce the desolvation energy barrier of hydrated Zn²⁺ , constrain the 2D Zn²⁺ diffusion, and uniformize electric field and Zn²⁺ flux distributions, assuring dendrite-free Zn deposition with high plating-stripping efficiency. Concurrently, the hydrophilic CGPPHE alleviates harmful hydrogen evolution and corrosion by abating water activity. Accordingly, Zn|CGPPHE|Zn and Zn|CGPPHE|Cu cells exhibit an extended life exceeding 350 h (1600 mAh cm^-2 cumulative capacity under 20 mA cm^-2 ) and a high average coulombic efficiency of 98.2%, respectively. The resultant flexible ZHCs with CGPPHE and template-regulated carbon cathode present perfect properties in capacity retention (97.7% over 10 000 cycles), energy density (91.8 Wh kg^-1 ), and good mechanical adaptability. This study provides insight into developing novel hydrogel electrolytes toward highly rechargeable and stable ZHCs.

Regulating Zn Ion Desolvation and Deposition Chemistry Toward Durable and Fast Rechargeable Zn Metal Batteries

The high Zn ion desolvation energy, sluggish Zn deposition kinetics, and top Zn plating pattern are the key challenges toward practical Zn anodes. Herein, these key issues are addressed by introducing zinc pyrovanadate (ZVO) as a solid zinc-ion conductor interface to induce smooth and fast Zn deposition underneath the layer. Electrochemical studies, computational analysis, and in situ observations reveal the boosted desolvation and deposition kinetics, and uniformity by ZVO interface. In addition, the anti-corrosion ability of Zn anodes is improved, resulting in high Zn stripping/plating reversibility. Consequently, the ZVO layer renders fast rechargeability and durable life in both Zn symmetric cells (1050 h at 10 mA cm^-2 , 1 mAh cm^-2 ) and Zn/V₂ O₅ batteries (79.1% capacity retention after 1000 cycles at 2 A g^-1 ) with low electrode polarization. This work provides insights into the design of solid zinc-ion conductor interface to enhance the interface stability and kinetics of Zn metal anodes.

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

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

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Comparative Review on the Aqueous Zinc-Ion Batteries (AZIBs) and Flexible Zinc-Ion Batteries (FZIBs)

Lithium-ion batteries (LIBs) have been considered an easily accessible battery technology because of their low weight, cheapness, etc. Unfortunately, they have significant drawbacks, such as flammability and scarcity of lithium. Since the components of zinc-ion batteries are nonflammable, nontoxic, and cheap, AZIBs could be a suitable replacement for LIBs. In this article, the advantages and drawbacks of AZIBs over other energy storage devices are briefly discussed. This review focused on the cathode materials and electrolytes for AZIBs. In addition, we discussed the approaches to improve the electrochemical performance of zinc batteries. Here, we also discussed the polymer gel electrolytes and the electrodes for flexible zinc-ion batteries (FZIBs). Moreover, we have outlined the importance of temperature and additives in a flexible zinc-ion battery. Finally, we have discussed anode materials for both AZIBs and FZIBs. This review has summarized the advantages and disadvantages of AZIBs and FZIBs for future applications in commercial battery technology.

Electrolyte Engineering Enables High Performance Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) feature high safety, low cost, environmental-friendliness, and promising electrochemical performance, and are therefore regarded as a potential technology to be applied in large-scale energy storage devices. However, ZIBs still face some critical challenges and bottlenecks. The electrolyte is an essential component of batteries and its properties affect the mass transport, energy storage mechanisms, reaction kinetics, and side reactions of ZIBs. The adjustment of electrolyte formulas usually has direct and obvious impacts on the overall output and performance. In this review, advanced electrolyte strategies are overviewed for optimizing the compatibility between cathode materials and electrolytes, inhibiting anode corrosion and dendrite growth, extending electrochemical stability windows, enabling wearable applications, and enhancing temperature tolerance. The underlying scientific mechanisms, electrolyte design principles, and recent progress are presented to provide a better understanding and inspiration to readers. In addition, a comprehensive perspective about electrolyte design and engineering for ZIBs is included.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Molecular Crowded ″Water-in-Salt″ Polymer Gel Electrolyte for an Ultra-stable Zn-Ion Battery

Recently, the use of a gel polymer electrolyte for the development of robust, flexible, quasi-solid, ultra-stable, high-performance zinc-ion batteries (ZiBs) as an alternative to lithium-ion batteries has attracted widespread attention. However, the performance of ZiBs is limited due to the lack of suitable gel electrolytes. Herein, a ″water-in-salt″ (WiS)-based hydrophilic molecular crowded polymer gel electrolyte and binder free V₂O₅@MnO₂ cathode are introduced to augment the durability, flexibility, safety, and electrochemical performance of ZiBs. The ″free water trapping″ capability of the WiS-based cross-linked molecular crowded polymer electrolyte provides an extended electrochemical stability window (ESW) of the device. The quasi-solid-state ZiB delivers ∼422 mAh g^-1 discharge capacity and shows excellent cycling stability as high as ∼79.83% retention of the initial capacity after 5000 cycles. The durable, flexible, and ultra-stable ZiB with the polymer gel electrolyte performs well under various severe conditions where both the battery safety and energy density are of high priority. This work demonstrates a new approach and application for the development of durable, flexible, ultra-stable, quasi-solid-state ZiBs.

A flexible self-healing Zn3V2O7(OH)2·2H2O-based Zn-ion battery under continuous folding and twisting

Engineering flexible and self-healing batteries is significant for wearable electronics. Here, we develop a flexible self-healing Zn-ion battery with a three-dimensional Zn3V2O7(OH)2·2H2O cathode working with polyvinyl alcohol and Zn2+/Mn2+ ions-based...

Electrolyte Dynamics Engineering for Flexible Fiber-Shaped Aqueous Zinc-Ion Battery with Ultralong Stability

Flexible aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices for wearable electronics due to their cost-effectiveness, environmental friendliness, and high theoretical energy density. Herein, a flexible fiber-shaped aqueous ZIB is demonstrated by using a self-assembled Co₃O₄ nanosheet array on a carbon nanotube fiber as the cathode and Zn nanosheets deposited on a carbon nanotube fiber as the anode. The cycle life span of the fiber-shaped battery is largely enhanced by a simple electrolyte dynamics engineering strategy of preadding a trace amount of Co²⁺ cations in the mild aqueous electrolyte. The assembled fiber-shaped ZIB shows a high specific capacity (158.70 mAh g^-1 at 1 A g^-1), superior rate capacity, and excellent cycling life span (97.27% capacity retention after 10,000 cycles). Additionally, the fiber-shaped ZIB also shows superior flexibility, which can charge a smart watch under deformed states. This work provides new opportunities for the development of flexible, safe, and high-performance energy storage devices for wearable electronics.

Conductive hydrogel-based flexible strain sensors with superior chemical stability and stretchability for mechanical sensing in corrosive solvents

Wearable flexible sensors face many harsh environments in practical applications.

Fundamental Concepts of Hydrogels: Synthesis, Properties, and Their Applications

In the present review, we focused on the fundamental concepts of hydrogels-classification, the polymers involved, synthesis methods, types of hydrogels, properties, and applications of the hydrogel. Hydrogels can be synthesized from natural polymers, synthetic polymers, polymerizable synthetic monomers, and a combination of natural and synthetic polymers. Synthesis of hydrogels involves physical, chemical, and hybrid bonding. The bonding is formed via different routes, such as solution casting, solution mixing, bulk polymerization, free radical mechanism, radiation method, and interpenetrating network formation. The synthesized hydrogels have significant properties, such as mechanical strength, biocompatibility, biodegradability, swellability, and stimuli sensitivity. These properties are substantial for electrochemical and biomedical applications. Furthermore, this review emphasizes flexible and self-healable hydrogels as electrolytes for energy storage and energy conversion applications. Insufficient adhesiveness (less interfacial interaction) between electrodes and electrolytes and mechanical strength pose serious challenges, such as delamination of the supercapacitors, batteries, and solar cells. Owing to smart and aqueous hydrogels, robust mechanical strength, adhesiveness, stretchability, strain sensitivity, and self-healability are the critical factors that can identify the reliability and robustness of the energy storage and conversion devices. These devices are highly efficient and convenient for smart, light-weight, foldable electronics and modern pollution-free transportation in the current decade.