Alkaline Double-Network Hydrogels with High Conductivities, Superior Mechanical Performances, and Antifreezing Properties for Solid-State Zinc-Air Batteries

Gel polymer electrolytes for rechargeable batteries toward wide-temperature applications

Rechargeable batteries, typically represented by lithium-ion batteries, have taken a huge leap in energy density over the last two decades. However, they still face material/chemical challenges in ensuring safety and long service life at temperatures beyond the optimum range, primarily due to the chemical/electrochemical instabilities of conventional liquid electrolytes against aggressive electrode reactions and temperature variation. In this regard, a gel polymer electrolyte (GPE) with its liquid components immobilized and stabilized by a solid matrix, capable of retaining almost all the advantageous natures of the liquid electrolytes and circumventing the interfacial issues that exist in the all-solid-state electrolytes, is of great significance to realize rechargeable batteries with extended working temperature range. We begin this review with the main challenges faced in the development of GPEs, based on extensive literature research and our practical experience. Then, a significant section is dedicated to the requirements and design principles of GPEs for wide-temperature applications, with special attention paid to the feasibility, cost, and environmental impact. Next, the research progress of GPEs is thoroughly reviewed according to the strategies applied. In the end, we outline some prospects of GPEs related to innovations in material sciences, advanced characterizations, artificial intelligence, and environmental impact analysis, hoping to spark new research activities that ultimately bring us a step closer to realizing wide-temperature rechargeable batteries.

Using bacterial cellulose to bridge covalent and physical crosslinks in hydrogels for fabricating multimodal sensors

Network optimization is vital for the polysaccharide based hydrogels with multiple crosslinks. In this study, we developed a 'two-step' strategy to activate synergistic effect of chemical and physical crosslinks using a poly (vinyl alcohol) (PVA)/bacterial cellulose (BC) hydrogel as a template. The BC nanofibers, on the one hand, acted as nucleating agents, participating in the crystallization of PVA, and on the other hand, were also involved in the formation of boronic ester bond, anchored with the PVA chains via chemical bonding. Therefore, the existence of BC nanofibers, as 'bridge', linked the crystalline regions and amorphous parts of PVA together, associating the two characteristic crosslinks, which was conducive to load transfer. The mechanical properties of resultant hydrogels, including the tensile elongation and strength, as well as fracture toughness, were significantly improved. Moreover, the dually cross-linked hydrogels possessed ionic conductivity, which was sensitive to the tensile deformation and environmental temperature. This study clarifies a unique role of BC nanofibers in hydrogels, and proposes an effective approach to construct multiple networks in the nanocellulose reinforced PVA hydrogels.

Synergistic effect of surfactant and 1,10-decanedithiol as corrosion inhibitor for zinc anode in alkaline electrolyte of zinc-air batteries

The self-discharge by corrosion of zinc-air batteries (ZABs) will result in the reduced coulombic efficiency and lower energy efficiency. The additives in electrolyte should not only inhibit the occurrence of self-corrosion during battery dormancy, but also achieve a stable cycle of adsorption-desorption during battery operation, improving the durability of discharge cycles. But the former requires strong binding between additives and zinc to form a dense protective film, while the latter requires easy desorption of additives and zinc without affecting discharge power, which is contradictory to balance. In this study, a dynamic combination of additives and zinc, as well as a design of multi-channel strategy for the corresponding protective layer, have been proposed to solve the issues of self-corrosion and discharge cycle stability. Specifically, the surfactant (octylphenol polyoxyethylene ether phosphate (OP-10P)) and 1,10-decanedithiol (DD) have been selected as the combined anti-corrosion additives in ZABs with concentrated alkaline solution. The synergistic inhibition mechanism and the stabilization mechanism in zinc-air full cells have been studied systematically. The results indicated that the combined inhibitors inhibited the self-corrosion of Zn efficiently in the dormancy, and the inhibition efficiency reached 99.9 % at the optimized proportion. OP-10P achieve the preferential adsorption on the zinc surface, and then the chelates of DD with Zn2+ deposit on the outer layer to form the protective film with fine hydrophobic performance. The stability of ZABs in discharge and charging cycles has been improved owing to the multilayer adsorption film on zinc surface, which retains ion transport channels with the homogeneously pores to weaken the dendrites and side reactions during galvanostatic cycles. A probable model on zinc surface was established to discuss the actual working mechanism.

A Review of Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Zinc-air batteries (ZABs) are gaining attention as an ideal option for various applications requiring high-capacity batteries, such as portable electronics, electric vehicles, and renewable energy storage. ZABs offer advantages such as low environmental impact, enhanced safety compared to Li-ion batteries, and cost-effectiveness due to the abundance of zinc. However, early research faced challenges due to parasitic reactions at the zinc anode and slow oxygen redox kinetics. Recent advancements in restructuring the anode, utilizing alternative electrolytes, and developing bifunctional oxygen catalysts have significantly improved ZABs. Scientists have achieved battery reversibility over thousands of cycles, introduced new electrolytes, and achieved energy efficiency records surpassing 70%. Despite these achievements, there are challenges related to lower power density, shorter lifespan, and air electrode corrosion leading to performance degradation. This review paper discusses different battery configurations, and reaction mechanisms for electrically and mechanically rechargeable ZABs, and proposes remedies to enhance overall battery performance. The paper also explores recent advancements, applications, and the future prospects of electrically/mechanically rechargeable ZABs.

Extending the Cycle Lifetime of Solid-State Zinc-Air Batteries by Arranging Stable Zinc Species Channels

The solid-state zinc-air batteries have attracted extensive attention due to their high theoretical energy density, high safety, and the compact structure. In this work, a novel hydrogel solid-state electrolyte was developed that was equipped with an interpenetrating network of zinc polyacrylate (PAZn) and polyacrylamide (PAM). At the same time, a cyclodextrin derivative with sulfonate groups was introduced as an additive. From the design of anionic groups in the network, effective and stable channels for zinc species have been established. The unique structure of the additives regulates the uniform deposition of zinc. After using this solid-state electrolyte, the cycle lifetime of solid-state zinc-air batteries assembled have been significantly extended. The byproducts were greatly suppressed and generated the smooth zinc electrode surface after the charge-discharge cycling.

Self-healable, Tolerant Superaerophobic Coating for Improving Electrochemical Hydrogen Production

Gas-evolving electrodes often suffer from the blocking of catalytic active sites-due to unwanted and unavoidable adhesion of generated gas bubbles, which elevates the overpotential for the electrochemical hydrogen evolution reaction (HER)- by raising the resistance of the electrode. Here, a catalyst-free and self-healable superaerophobic coating having ultra-low bubble adhesion is introduced for achieving significantly depleted overpotentials of 209 and 506 mV at both low (50 mA cm-2 ) and high (500 mA cm-2 ) current densities, respectively, compared to a bare nickel-foam electrode. The optimized coating ensured an early detachment of the generated tiny (0.8 ± 0.1 mm) gas bubble-and thus, prevented the undesired rise in resistance of the coated electrode. The systematic association of physical (i.e., ionic interactions, H-bonding, etc.) cross-linkage, β-amino ester type covalent cross-linkage and reinforced halloysite nano clay enables the design of such functional material embedded with essential characteristics-including improved mechanical (toughness of 63.7 kJ m-3 , and tensile modulus of 26 kPa) property and chemical (extremes of pH (1 and 14), salinity, etc.) stability, rapid (<10 min) self-healing ability (even at alkaline condition) and desired bubble-wettability (bubble contact angle of 158.2 ± 0.2° ) with ultralow force (4.2 ± 0.4 µN) of bubble adhesion.

Electrolyte Modulation Strategies for Low-Temperature Zn Batteries

Aqueous rechargeable Zn metal batteries (ARZBs) are extensively studied recently because of their low-cost, high-safety, long lifespan, and other unique merits. However, the terrible ion conductivity and insufficient interfacial redox dynamics at low temperatures restrict their extended applications under harsh environments such as polar inspections, deep sea exploration, and daily use in cold regions. Electrolyte modulation is considered to be an effective way to achieve low-temperature operation for ARZBs. In this review, first, the fundamentals of the liquid-solid transition of water at low temperatures are revealed, and an in-depth understanding of the critical factors for inferior performance at low temperatures is given. Furthermore, the electrolyte modulation strategies are categorized into anion/concentration regulation, organic co-solvent/additive introduction, anti-freezing hydrogels construction, and eutectic mixture design strategies, and emphasize the recent progress of these strategies in low-temperature Zn batteries. Finally, promising design principles for better electrolytes are recommended and future research directions about high-performance ARZBs at low temperatures are provided.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

Failure Mechanism, Electrolyte Design, and Electrolyte/Electrode Interface Regulation for Low-Temperature Zinc-Based Batteries

With more renewable energy developed to satisfy the human need in the energy crisis, electricity storage is critical in power utilization and storage. Due to its high safety, high nature reserve, and high energy density, the zinc-based battery is drawing increasing attention. Together with the expansion of human activities, the low-temperature battery is developed to satisfy the power demand in extreme environments, and as a critical component, electrolytes shall have a low freezing point and satisfying electrochemical properties in cold conditions. In this review, the development of low-temperature electrolytes for zinc-based batteries will be comprehensively introduced. First, the failure mechanism of zinc-based battery at low temperature will be illustrated. Second, five main types of low-temperature electrolytes will be introduced in detail. Finally, the regulation of electrolyte/electrode surface at low temperature will be discussed. This review aims to provide a guideline for low-temperature electrolyte design from the perspective of molecular behavior regulation.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Preparation of lignin-based hydrogels, their properties and applications

Polymers obtained from biomass are a concerning alternative to petro-based polymers because of their low cost of manufacturing, biocompatibility, ecofriendly and biodegradability. Lignin as the second richest and the only polyaromatics bio-polymer in plant which has been most studied for the numerous applications in different fields. But, in the past decade, the exploitation of lignin for the preparation of new smart materials with improved properties has been broadly sought, because lignin valorization plays one of the primary challenging issues of the pulp and paper industry and lignocellulosic biorefinery. Although, well suited chemical structure of lignin comprises of many functional hydrophilic and active groups, such as phenolic hydroxyls, carboxyls and methoxyls, which provides a great potential to be applied in the preparation of biodegradable hydrogels. In this review, lignin hydrogel is covered with preparation strategies, properties and applications. This review reports some important properties, such as mechanical, adhesive, self-healing, conductive, antibacterial and antifreezing properties were then discussed. Furthermore, herein also reviewed the current applications of lignin hydrogel, including dye adsorption, smart materials for stimuli sensitive, wearable electronics for biomedical applications and flexible supercapacitors. Overall, this review covers recent progresses regarding lignin-based hydrogel and constitutes a timely review of this promising material.

Synthesis of Polymer Nanoweb via a Lipid Template

We report a generalized platform for synthesizing a polymer nanoweb with a high specific surface area via a bicellar template, composed of 1,2-dipalmitoyl phosphocholine (DPPC), 1,2-dihexanoyl phosphocholine (DHPC), and 1,2-dipalmitoyl phosphoglycerol (DPPG). The pristine bicelle (in the absence of monomer or polymer) yields a variety of well-defined structures, including disc, vesicle, and perforated lamella. The addition of styrene monomers in the mixture causes bicelles to transform into lamellae. Monomers are miscible with DPPC and DPPG initially, while polymerization drives polymers to the DHPC-rich domain, resulting in a polymer nanoweb supported by the outcomes of small angle neutron scattering, differential scanning calorimetry, and transmission electron microscopy.

Tough Hydrogel Electrolytes for Anti-Freezing Zinc-Ion Batteries

As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze-vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co-nonsolvency and "salting-out" that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open-cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6 MPa), freeze-tolerance (< -77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N-isopropylacrylamide) and poly(N-tertbutylacrylamide-co-acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

Hydrogen-bonding topological remodeling modulated ultra-fine bacterial cellulose nanofibril-reinforced hydrogels for sustainable bioelectronics

Bacterial cellulose (BC) with its inherent nanofibrils framework is an attractive building block for the fabrication of sustainable bioelectronics, but there still lacks an effective and green strategy to regulate the hydrogen-bonding topological structure of BC to improve its optical transparency and mechanical stretchability. Herein, we report an ultra-fine nanofibril-reinforced composite hydrogel by utilizing gelatin and glycerol as hydrogen-bonding donor/acceptor to mediate the rearrangement of the hydrogen-bonding topological structure of BC. Attributing to the hydrogen-bonding structural transition, the ultra-fine nanofibrils were extracted from the original BC nanofibrils, which reduced the light scattering and endowed the hydrogel with high transparency. Meanwhile, the extracted nanofibrils were connected with gelatin and glycerol to establish an effective energy dissipation network, leading to an increase in stretchability and toughness of hydrogels. The hydrogel also displayed tissue-adhesiveness and long-lasting water-retaining capacity, which acted as bio-electronic skin to stably acquire the electrophysiological signals and external stimuli even after the hydrogel was exposing to air condition for 30 days. Moreover, the transparent hydrogel could also serve as a smart skin dressing for optical identification of bacterial infection and on-demand antibacterial therapy after combined with phenol red and indocyanine green. This work offers a strategy to regulate the hierarchical structure of natural materials for designing skin-like bioelectronics toward green, low cost, and sustainability.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

Fabrication of an ion-enhanced low-temperature tolerant graphene/PAA/KCl hydrogel and its application for skin sensors

Flexible sensors based on conductive hydrogels show great potential in wearable displays and smart devices. However, a water-based hydrogel inevitably freezes or loses its conductivity under extremely cold temperatures, leading to inadequate fulfillment of sensor performance. Herein, a well-designed strategy is proposed for fabricating a low-temperature-tolerant water-based hydrogel for sensor applications. By immersing a multi-crosslinking graphene(GO)/polyacrylic acid (PAA)-Fe³⁺ hydrogel into a KCl solution, an ion-enhanced conductive (GO/PAA/KCl) hydrogel is obtained with excellent conductivity (24.4 S m^-1 at 20 °C; 16.2 S m^-1 at -20 °C; 0.8 S m^-1 at -80 °C) and outstanding antifreezing properties. The conductive hydrogel also possesses good mechanical properties with a fracture stress of 2.65 MPa and an elongation at break of 1511% and maintains its flexibility even at -35 °C. Then, a strain sensor is assembled to monitor the human motion at 20 °C and the movement of a wooden mannequin at -20 °C. Under both conditions, the sensor presents high sensitivity (GF = 8.66 at 20 °C, 7.93 at -20 °C) and good durability (300 cycles under 100% strain). Consequently, the anti-freezing ion-enhanced hydrogel will meet the needs of flexible sensors designed for intelligent robots, health monitoring, etc., which have to work in cold regions or extreme climates.

Mechanically Strong, Freeze-Resistant, and Ionically Conductive Organohydrogels for Flexible Strain Sensors and Batteries

Conductive hydrogels as promising material candidates for soft electronics have been rapidly developed in recent years. However, the low ionic conductivity, limited mechanical properties, and insufficient freeze-resistance greatly limit their applications for flexible and wearable electronics. Herein, aramid nanofiber (ANF)-reinforced poly(vinyl alcohol) (PVA) organohydrogels containing dimethyl sulfoxide (DMSO)/H₂ O mixed solvents with outstanding freeze-resistance are fabricated through solution casting and 3D printing methods. The organohydrogels show both high tensile strength and toughness due to the synergistic effect of ANFs and DMSO in the system, which promotes PVA crystallization and intermolecular hydrogen bonding interactions between PVA molecules as well as ANFs and PVA, confirmed by a suite of characterization and molecular dynamics simulations. The organohydrogels also exhibit ultrahigh ionic conductivity, ranging from 1.1 to 34.3 S m^-1 at -50 to 60 °C. Building on these excellent material properties, the organohydrogel-based strain sensors and solid-state zinc-air batteries (ZABs) are fabricated, which have a broad working temperature range. Particularly, the ZABs not only exhibit high specific capacity (262 mAh g^-1 ) with ultra-long cycling life (355 cycles, 118 h) even at -30 °C, but also can work properly under various deformation states, manifesting their great potential applications in soft robotics and wearable electronics.

Highly conductive and anti-freezing cellulose hydrogel for flexible sensors

Ionic conducting hydrogels (ICHs) are emerging materials for multi-functional sensors in the fields of healthcare monitoring and flexible electronics. However, there is a long-standing dilemma between ionic conductivity and mechanical properties of the ICHs. In this work, ionic conductive, flexible, transparent, and anti-freezing hydrogels are fabricated by dissolving cotton linter pulp in ZnCl2/CaCl2 solution and cross-linking with epichlorohydrin (ECH). The presence of inorganic salt imparts the hydrogel with high ionic conductivity and low-temperature tolerance. While the introduction of ECH as the second network gives the hydrogel with desirable mechanical performance. By tailoring the ECH addition, the tensile strength, compressive strength, elongation at break, and conductivity of the hydrogel could reach 0.82 MPa, 2.80 MPa, 260 %, and 5.48 S m-1, respectively. The prepared ICHs are fabricated into sensors for detecting full-range human body motions, and they demonstrate fast response and durable sensitivity to both tensile strain and compressive deformation. Moreover, flexible sensors can work at subzero temperatures. This work provides a new idea for the preparation of cellulose-based hydrogels with good ionic conductivity and mechanical properties under extreme conditions.

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.

A bio-inspired, ultra-tough, high-sensitivity, and anti-swelling conductive hydrogel strain sensor for motion detection and information transmission

Conductive hydrogels are excellent candidates for the next-generation wearable materials and are being extensively investigated for their potential use in health monitoring devices, human-machine interfaces, and other fields. However, their relatively low mechanical strength and performance degradation due to swelling have presented challenges in their practical application. Inspired by the multiscale heterogeneous architecture of biological tissue, a dynamic cross-linked, ultra-tough, and high-sensitivity hydrogel with a swelling resistance characteristic was fabricated by the principle of multiple non-covalent interaction matching and a step-by-step construction strategy. A heterogeneous structure was constructed by the combination of a 'soft' hydrophobic-conjugated micro-region structural domain with inter/intra-molecular hydrogen bonding and π-π stacking along with 'rigid' cross-linking via strong ionic coordination interactions. Reversible cross-linking synergies and variations in the content of rigid and flexible components guaranteed the hydrogel to undergo flexible and efficient modulation of the structures and gain excellent mechanics, including elongation at break (>2000%), toughness (∼60 MJ m^-3), and recovery (>88%). Notably, hydrogels displayed good anti-swelling properties even in solutions with different pH (pH 2-11) and solvents. Moreover, the hydrogel further exhibited fast response (47.4 ms) and high sensitivity due to the presence of dynamic ions (Fe³⁺, Na⁺, and Cl^-); therefore, it was assembled into a sensor to detect various human motions and used as a signal transmitter for the encryption and decryption of information according to Morse code. This study provides basis for the development of a variety of robust and flexible conductive hydrogels with multifunctional sensing applications in next-generation wearable devices.

Enhanced Copolymer Gel Modified by Dual Surfactants for Flexible Zinc-Air Batteries

Zinc-air batteries using gels as carriers for electrolyte absorption have attracted extensive attention due to their flexibility, deformability, and high specific capacity. However, traditional mono-polymer gel electrolytes display poor mechanical properties and low ionic conductivity at wide-window temperatures. Here, the enhanced gel polymer (PAM-F/G) modified by dual surfactants is present by way of pluronic F127 and layered graphene oxide introduced into the polyacrylamide (PAM) matrix. The gel electrolyte procured by absorbing 6 M KOH exhibits improved mechanical characteristics, temperature adaptability, and a satisfactory ionic conductivity (276 mS cm^-1). The results demonstrate that a flexible zinc-air battery assembled by PAM-F/G electrolyte outputs a high power density (155 mW cm^-2) and can even operate reliably (>40 h) at -20 °C. These findings are available for promoting the research and popularization of flexible zinc-air batteries with high performance.

Design of co-continuous structure of cellulose/PAA-based alkaline solid polyelectrolyte for flexible zinc-air battery

In order to prepare high ionic conductivity and robust mechanical properties of alkaline solid polyelectrolyte (ASPE) for applications in flexible wearable devices, a co-continuous structure membrane was designed using in-situ polymerization to introduce cross-linked polyacrylic acid (N-PAA) into the cellulose network constructed by regenerated degreasing cotton (RDC). The resultant ASPE membrane showed high ionic conductivity (430 mS·cm^-1 at 25 °C), strong mechanical properties, and excellent alkaline stabilities, proving the viability of cellulose for use in energy storage systems. Surprisingly, the sandwich-shaped zinc-air battery assembled using RDC/N-PAA/KOH membranes as electrolytes exhibits superior values of cycling stability, discharge time, specific capacity (731.5 mAh·g^-1), peak power density (40.25 mW·cm^-2), and mechanical flexibility. Even under bending conditions, the zinc-air batteries still possess stable energy supply performance, suggesting this novel solid polyelectrolyte has promising application for wearable technology.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Supramolecular Gel-Derived Highly Efficient Bifunctional Catalysts for Omnidirectionally Stretchable Zn-Air Batteries with Extreme Environmental Adaptability

Most existing stretchable batteries can generally only be stretched uniaxially and suffer from poor mechanical and electrochemical robustness to withstand extreme mechanical and environmental challenges. A highly efficient bifunctional electrocatalyst is herein developed via the unique self-templated conversion of a guanosine-based supramolecular hydrogel and presents a fully integrated design strategy to successfully fabricate an omnidirectionally stretchable and extremely environment-adaptable Zn-air battery (ZAB) through the synergistic engineering of active materials and device architecture. The electrocatalyst demonstrates a very low reversible overpotential of only 0.68 V for oxygen reduction/evolution reactions (ORR/OER). This ZAB exhibits superior omnidirectional stretchability with a full-cell areal strain of >1000% and excellent durability, withstanding more than 10 000 stretching cycles. Promisingly, without any additional pre-treatment, the ZAB exhibits outstanding ultra-low temperature tolerance (down to -60 °C) and superior waterproofness, withstanding continuous water rinsing (>5 h) and immersion (>3 h). The present work offers a promising strategy for the design of omnidirectionally stretchable and high-performance energy storage devices for future on-skin wearable applications.

Strong, tough, ionic conductive, and freezing-tolerant all-natural hydrogel enabled by cellulose-bentonite coordination interactions

Ionic conductive hydrogels prepared from naturally abundant cellulose are ideal candidates for constructing flexible electronics from the perspective of commercialization and environmental sustainability. However, cellulosic hydrogels featuring both high mechanical strength and ionic conductivity remain extremely challenging to achieve because the ionic charge carriers tend to destroy the hydrogen-bonding network among cellulose. Here we propose a supramolecular engineering strategy to boost the mechanical performance and ionic conductivity of cellulosic hydrogels by incorporating bentonite (BT) via the strong cellulose-BT coordination interaction and the ion regulation capability of the nanoconfined cellulose-BT intercalated nanostructure. A strong (compressive strength up to 3.2 MPa), tough (fracture energy up to 0.45 MJ m⁻³), yet highly ionic conductive and freezing tolerant (high ionic conductivities of 89.9 and 25.8 mS cm⁻¹ at 25 and −20 °C, respectively) all-natural cellulose-BT hydrogel is successfully realized. These findings open up new perspectives for the design of cellulosic hydrogels and beyond.

Cellulose based ion conductive hydrogels are emerging materials for application in flexible electronics but achieving simultaneously high conductivity and good mechanical properties remains challenging. Here, the authors propose a supramolecular engineering strategy to strengthen cellulosic hydrogel and to improve simultaneously its ionic conductivity and freezing tolerance.

Lignin-containing hydrogels with anti-freezing, excellent water retention and super-flexibility for sensor and supercapacitor applications

We developed a highly conductive and flexible, anti-freezing sulfonated lignin (SL)-containing polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogel, with a high concentration of NiCl₂. Ni²⁺ contributes multi-functions to the preparation of the hydrogel and its final properties, such as fast polymerization reaction as a result of the presence of redox pairs of Ni³⁺/Ni²⁺ and hydroquinone/quinone, and anti-freezing properties of the hydrogel due to the salt effects of NiCl₂ so that at -20 °C the hydrogel shows similar properties to those at the room temperature. Thanks to the effective coordinations of Ni²⁺ with catecholic groups and carboxylic groups, as well as the rich hydrogen bonding capacity, the resultant hydrogel possesses excellent mechanical properties. High ionic conductivity (6.85 S·m^-1) of the hydrogel is obtained due to the supply of high concentration of Ni²⁺. Moreover, the ionic solvation effect of NiCl₂ in the hydrogel imparts excellent water retention ability, with water retention of ~93 % after 21-day storage. The SL-g-PAA-Ni hydrogel can accurately detect various human motions at -20 °C. The supercapacitor assembled from SL-g-PAA-Al hydrogel at -20 °C manifests a high specific capacitance of 252 F·g^-1, with maximum energy density of 26.97 Wh·kg^-1, power density of 2667 W·kg^-1, and capacitance retention of 96.7 % after 3000 consecutive charge-discharge cycles.

From room temperature to harsh temperature applications: Fundamentals and perspectives on electrolytes in zinc metal batteries

As one of the most competitive candidates for the next-generation energy storage systems, the emerging rechargeable zinc metal battery (ZMB) is inevitably influenced by beyond-room-temperature conditions, resulting in inferior performances. Although much attention has been paid to evaluating the performance of ZMBs under extreme temperatures in recent years, most academic electrolyte research has not provided adequate information about physical properties or practical testing protocols of their electrolytes, making it difficult to assess their true performance. The growing interest in ZMBs is calling for in-depth research on electrolyte behavior under harsh practical conditions, which has not been systematically reviewed yet. Hence, in this review, we first showcase the fundamentals behind the failure of ZMBs in terms of temperature influence and then present a comprehensive understanding of the current electrolyte strategies to improve battery performance at harsh temperatures. Last, we offer perspectives on the advance of ZMB electrolytes toward industrial application.

Engineering Dual Single-Atom Sites on 2D Ultrathin N-doped Carbon Nanosheets Attaining Ultra-Low-Temperature Zinc-Air Battery

Herein, a novel dual single-atom catalyst comprising adjacent Fe-N₄ and Mn-N₄ sites on 2D ultrathin N-doped carbon nanosheets with porous structure (FeMn-DSAC) was constructed as the cathode for a flexible low-temperature Zn-air battery (ZAB). FeMn-DSAC exhibits remarkable bifunctional activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Control experiments and density functional theory calculations reveal that the catalytic activity arises from the cooperative effect of the Fe/Mn dual-sites aiding *OOH dissociation as well as the porous 2D nanosheet structure promoting active sits exposure and mass transfer during the reaction process. The excellent bifunctional activity of FeMn-DSAC enables the ZAB to operate efficiently at ultra-low temperature of -40 °C, delivering 30 mW cm^-2 peak power density and retaining up to 86 % specific capacity from the room temperature counterpart.

A wide-temperature-range sensor based on wide-strain-range self-healing and adhesive organogels

Adaptable physicochemically double cross-linked network organogels for use at different temperatures are demonstrated.

Flexible Zinc-Air Battery with High Energy Efficiency and Freezing Tolerance Enabled by DMSO-Based Organohydrogel Electrolyte

With the emergence of various flexible electronics, the flexible zinc-air battery (ZAB) is considered a promising energy source with low cost, high energy density, and safety. However, gel electrolytes that improve the freezing tolerance and energy efficiency of ZABs are rarely explored. Herein, an organohydrogel electrolyte (OHE) is fabricated by soaking poly(2-acrylamido-2-methylpropanesulfonic acid)/polyacrylamide (PAMPS/PAAm) double-network hydrogel in aqueous KOH electrolyte with dimethyl sulfoxide (DMSO) additive. The prepared OHE exhibits high mechanical strength and excellent ionic conductivity. In addition, the introduction of DMSO effectively improves freezing tolerance and electrochemical performance especially in energy efficiency of ZABs due to that DMSO can break the hydrogen bonds between water molecules and alter the path of the conventional oxygen evolution reaction in ZAB simultaneously. Compared with the control hydrogel electrolyte, the optimized OHE enables flexible ZABs to not only exhibit an exceptionally low charge voltage of 1.63 V, high energy efficiency of 74.2%, and long cycling life of 177 cycles, but also to operate with an excellent specific capacity of 562 mAh g^-1 and energy density of 523.4 Wh kg^-1 at -40 °C. Moreover, the obtained flexible ZABs keep a stable output under deformations and extreme low temperature, manifesting a great potential for functional wearable devices.

Printed Zinc Paper Batteries

Paper electronics offer an environmentally sustainable option for flexible and wearable systems and perfectly fit the available printing technologies for high manufacturing efficiency. As the heart of energy-consuming devices, paper-based batteries are required to be compatible with printing processes with high fidelity. Herein, hydrogel reinforced cellulose paper (HCP) is designed to serve as the separator and solid electrolyte for paper batteries. The HCP can sustain higher strain than pristine papers and are biodegradable in natural environment within four weeks. Zinc-metal (Ni and Mn) batteries printed on the HCP present remarkable volumetric energy density of ≈26 mWh cm-3 , and also demonstrate the feature of cuttability and compatibility with flexible circuits and devices. As a result, self-powered electronic system could be constructed by integrating printed paper batteries with solar cells and light-emitting diodes. The result highlights the feasibility of hydrogel reinforced paper for ubiquitous flexible and eco-friendly electronics.

Lignin reinforced hydrogels with multi-functional sensing and moist-electric generating applications

Hydrogels, including PVA hydrogels, have numerous applications in many fields; however, their poor mechanical strength limits their utilization potential. Lignin, the most abundant aromatic biopolymer in nature from lignocellulosic biomass, is presently under-utilized. Herein, we used lignin to improve strength and impart pH-responsive properties of PVA hydrogel. The lignin reinforced PVA (LRP) hydrogel has a maximum storage modulus of 83.1 kPa, which is much higher than the PVA hydrogel. The LRP hydrogel exhibits great ionic conductivity, mechanical properties, and strain-sensitivity even at -30 °C. The LRP hydrogel is subsequently applied for a moisture-induced electric generator, which delivers a voltage output of 226.6 mV from moisture flow. The eco-friendly, pH responsive, high antifreezing, ionic conductive, strain sensitive, and moist-electric generating hydrogels have potential applications in many fields, including biomedicine, flexible electrodes, pH-responsive switch, strain sensor, and next-generation self-powered device systems.

Dynamic Polymer Networks: A New Avenue towards Sustainable and Advanced Soft Machines

While the fascinating field of soft machines has grown rapidly over the last two decades, the materials they are constructed from have remained largely unchanged during this time. Parallel activities have led to significant advances in the field of dynamic polymer networks, leading to the design of three-dimensionally cross-linked polymeric materials that are able to adapt and transform through stimuli-induced bond exchange. Recent work has begun to merge these two fields of research by incorporating the stimuli-responsive properties of dynamic polymer networks into soft machine components. These include dielectric elastomers, stretchable electrodes, nanogenerators, and energy storage devices. In this Minireview, we outline recent progress made in this emerging research area and discuss future directions for the field.

Composite Polymer Anion Exchange Membranes with Sandwich Structure and Improved Performance for Zn-Air Battery

In this study, we fabricated a composite polymer anion exchange membrane (AEM) with a sandwich structure. This prepared AEM demonstrated high ionic conductivity (0.25 Scm⁻¹), excellent alkali resistance (8 M KOH), and good mechanical properties (tensile strength of 0.455 MPa and elongation at break of 82.13%). Here, degrease cotton (DC) treated with LiOH/urea aqueous solution was used and immersed into a coagulation bath to form a film. This film was immersed in acrylic acid (AA) monomers, and in-suit polymerization was carried out in the presence of KOH and an initiator. Finally, a composite polymer membrane with sandwich structure was achieved, in which the upper and bottom layers were mainly composed of polymerized AA (PAA) while the central layer was mainly composed of DC derived film. The central layer acted as a skeleton to improve the mechanical properties and alkali resistance. The top and bottom layers (PAA-rich layers) acted as OH- ion transport carriers, making basic cations migrate along the main chain of PAA. This newly developed composite membrane showed increased tensile strength and an elongation at break of 2.7 and 1.5 times, respectively, when compared to a control PAA/KOH AEM film. Furthermore, an electrochemical stability window of 2.0 V was measured via the cyclic voltammetry curve test, showing a wide electrochemical window and promising application in Zn–Air batteries.

Preparation of Alkaline Polyelectrolyte Membrane Based on Quaternary Ammonium Salt-Modified Cellulose and Its Application in Zn-Air Flexible Battery

In this study, a type of alkaline solid polyelectrolyte (ASPE) membrane was developed via the introduction of microcrystalline cellulose (MCC) and its modified product (QMCC) into the polyvinyl alcohol (PVA) matrix. In this process, green NaOH/urea-based solvent was used to achieve a good dispersion of MCC in the PVA matrix; meanwhile, the OH^- groups in the NaOH/urea-based solvent provided an alkaline environment for good ion conductivity. Compared to the MCC-incorporated ASPE, further improved conductivity was achieved when the MCC was modified with quantitative quaternary ammonium salt. TGA showed that the addition of QMCC improved the water retention of the matrix, which was beneficial to the OH^- conduction in the system. Compared to the control (50 mS cm^-1), a maximum conductivity of 238 mS cm^-1 was obtained after the incorporation of QMCC in the PVA matrix. Moreover, the tensile strength of the polymer electrolyte were also significantly increased with the addition of QMCC. Finally, this developed ASPE membrane was used in assembling a flexible Zn-air battery and showed a promising potential in the development of flexible electronic devices.

An Ultrastable Na-Zn Solid-State Hybrid Battery Enabled by a Robust Dual-Cross-linked Polymer Electrolyte

This work proposes a dual-cross-linked gel solid electrolyte (SE), here defined as Zn-re-inforced sodium alginate-polyacrylamide SE (Zn-reinforced SA-PAM SE), in which Na⁺ and Zn²⁺ coexist. The SE shows a high conductivity of 19.74 mS cm^-1. Compared to the pure PAM gel, the tensile strength and compressive strength of Zn-reinforced SA-PAM SE are significantly enhanced to be 674.28 kPa and 16.29 MPa, respectively, because of the strengthening mechanism of Zn²⁺ cross-linked SA. Based on such a robust electrolyte, a novel hybrid cell is developed by involving Na_0.5FeFe(CN)₆-carbon nanotube composites (PB@CNT) as the Na⁺ intercalation-type cathode and metallic Zn as the plating anode. The hybrid cell shows an extremely high stability for 10,000 cycles with a record little capacity loss of 0.0027% per cycle, as Zn-reinforced SA-PAM SE successfully inhibits free water molecules from occupying low-spinning metallic sites (Fe-C) in Na_0.5FeFe(CN)₆. Ex situ X-ray photoelectron spectroscopy reveals that the dissolution of Na_0.5FeFe(CN)₆ is highly reduced by 79.5%. It is further noted that the corrosion and dendrites at the Zn²⁺/Zn plating anode are greatly hindered for the robust electrolyte. This work gives a pathway for the development of new aqueous ion batteries.

Engineering hydrogels by soaking: from mechanical strengthening to environmental adaptation

The soaking strategy could not only strengthen hydrogels with superior mechanical properties but also provide the hydrogels with environmentally adapting properties.