Single-Ion Conducting Double-Network Hydrogel Electrolytes for Long Cycling Zinc-Ion Batteries

In-Built Hybrid SEI with Weak Hydrophilicity from a Hydrogel Electrolyte for Stable Zinc Anodes

Zinc metal anodes have great prospects but encounter challenges, such as dendrites and adverse reactions. A stable solid electrolyte interface (SEI) that not only enhances zinc plating/stripping but also suppresses side reactions is crucial for a highly reversible zinc anode. In this study, a double-network hydrogel electrolyte containing polysaccharide carrageenan with a low level of the lowest unoccupied molecular orbital energy (LUMO) is designed to develop an in-built organic-inorganic hybrid SEI. This hybrid SEI features weak hydrophilicity but low interfacial impedance, which endows zinc anodes with durable anticorrosion properties in the aqueous environment and facilitates uniform Zn2+ deposition, thereby boosting the overall cycling stability in both symmetric and asymmetric cells. Consequently, the Zn||MnVO@CNT full cell achieves 81.8% capacity retention for 1000 cycles with a high MnVO@CNT loading of 13.6 mg cm-2 at 1 A g-1. Additionally, a pouch cell with a capacity of 22 mAh retained 76.7% over 700 cycles at 1 A g-1. This work highlights the advantages of hydrogel electrolytes in the construction of in-built SEIs and provides significant insights for the future development of zinc metal batteries.

Progress in Developing Polymer Electrolytes for Advanced Zn Batteries

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

New understanding on iota-carrageenan oxidation by hydrogen peroxide with or without copper sulphate and the properties of oxidized iota-carrageenan

To explore the oxidation mechanism of iota-carrageenan (IC) and the potential applications of oxidized IC (OIC), a series of OIC samples were firstly synthesized under hydrogen peroxide or hydrogen peroxide/copper sulphate, their purification process, chemical composition, chemical structure, molecular weight, physical property, biosafety and antibacterial activity were systematically evaluated. Our results firstly found that only the combination of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) pre-treatment and dialysis could realize successful purification of OIC due to the strong electrostatic attractions between positive copper ions and negative sulphate ions. The introduction of copper ions could effectively speed up the oxidation of OIC, enhancing the content of aldehyde and carboxyl groups, resulting in the hydrolysis of the ether bonds in IC, inhibiting the detachment of sulphate, decreasing the molecular weight, thermal stability, characteristic temperature (i.e. gelling and melting temperature). It was worth mentioning that K+ and Zn2+ could also inhibit the de-sulfation during the oxidation via the chelating interactions. In addition, purified OIC samples possessed excellent cytocompatibility and blood compatibility, which could also promote the proliferation and migration of fibroblast cells. Meanwhile, all OIC samples possessed antibacterial activity. In summary, our investigations shed new lights on OIC synthesis, purification, properties and potential applications.

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

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

Non-consumable gamma-cyclodextrin additive constructing anti-solvation interphase realizing dendrites free and high-performance lithium metal batteries

Relying on surpassing high theoretical capacity (3,865 mAh/g) and the lowest relative electrode potential (0 V vs. metallic Li), lithium metal batteries (LMBs) have been regarded as the "holy grail" of next-generation energy storage technology. Whereases, the instability of pristine solid electrolyte interphase (SEI) layers and the disorderly growth of lithium dendrites are still significant challenges to the commercialisation of LMBs. In this study, a novel approach is introduced to homogenise Li deposition by incorporating an environmentally friendly electrolyte additive, gamma-cyclodextrin (γ-CD), in ether-based electrolytes. Through host-guest interactions, γ-CD additives not only form inclusion complexes to improve Li+ transference number to 0.86 but also encapsulate TFSI- anions and other solvent molecules within the "cavity effect" to relieve unfavourable solvent effect. Electrochemical characterisations demonstrate that introducing 1 wt% γ-CD elevates the oxidation decomposition voltage of ether electrolytes to 4.15 V, thereby inhibiting the decomposition of ether electrolytes and reducing the fracture of SEI layers. According to reduce the nucleate potential, the Li//Cu half battery exhibits improved stability for 100 cycles, with an improved average Coulombic efficiency (CE) maintained above 98.4 %. Even if applied at high current densities of 5.0 mA cm-2 for a capacity of 1.0 mAh cm-2, the Li//Li symmetric battery can cycle for over 800 h, and the Li//Li4Ti5O12 (LTO) full battery retains 98.8 % of the initial capacity after 1,400 cycles.

Supramolecular Crystals based Fast Single Ion Conductor for Long-Cycling Solid Zinc Batteries

The solid polymer electrolytes (SPEs) used in Zn-ion batteries (ZIBs) have low ionic conductivity due to the sluggish dynamics of polymer segments. Thus, only short-range movement of cations is supported, leading to low ionic conductivity and Zn2+ transference (tZn 2+). Zn-based supramolecular crystals (ZMCs) have considerable potential for supporting long-distance Zn2+ transport; however, their efficiency in ZIBs has not been explored. The present study developed a ZMC consisting of succinonitrile (SN) and zinc bis (trifluoromethylsulfonyl) imide (Zn(TFSI)2), with a structural formula identified as Zn(TFSI)2SN3. The ZMC has ordered three-dimensional tunnels in the crystalline lattices for ion conduction, providing high ionic conductivities (6.02×10-4 S cm-1 at 25 °C and 3.26×10-5 S cm-1 at -35 °C) and a high tZn 2+ (0.97). We demonstrated that a Zn‖Zn symmetrical battery with ZMCs has long-term cycling stability (1200 h) and a dendrite-free Zn plating/stripping process, even at a high plating areal density of 3 mAh cm-2. The as-fabricated solid-state Zn battery exhibited excellent performance, including high discharge capacity (1.52 mAh cm-2), long-term cycling stability (83.6 % capacity retention after 70000 cycles (7 months)), wide temperature adaptability (-35 to 50 °C) and fast charging ability. The ZMC differs from SPEs in its structure for transporting Zn2+ ions, significantly improving solid-state ZIBs while maintaining safety, durability, and sustainability.

Polyacrylamide-based hydrogel electrolyte for modulating water activity in aqueous hybrid batteries

While zinc-ion and hybrid aqueous battery systems have emerged as potential substitutes for expensive lithium-ion batteries, issues like side reactions, limited electrochemical stability, and electrolyte leakage hinder their commercialization. Due to their low cost, high stability, minimal leakage risks, and a wide variety of modification opportunities, hydrogel electrolytes are considered the most promising solution compared to liquid or solid electrolytes. Here, we synthesized a dual-function hydrogel electrolyte based on polyacrylamide and poly(ethylene dioxythiophene):polystyrene (PPP). This electrolyte reduces water content and enhances stability by minimizing side reactions while swelling in a binary ethylene glycol and water solution (EG 10%) further stabilizes the battery system. The developed hydrogel exhibits relatively good ionic conductivity (1.6 × 10-3 S cm-1) and excellent electrochemical stability, surpassing 2.5 V on linear sweep voltammetry tests. The PPP-based system reached a value of 119.2 mA g-1, while the aqueous electrolyte reached only 80.4 mA g-1 specific capacity. The rechargeable PPP hydrogel electrolyte-based hybrid aqueous battery with zinc anode achieved more than 600 cycles. Coulombic efficiency (CE) remained at 99%, indicating good electrochemical reaction stability and reversibility. This study highlights the potential of polyacrylamide-based hydrogel electrolytes with dual functionality as the electrolyte and separator, inspiring further development in hydrogel electrolytes for aqueous battery systems. This study highlights the potential of polyacrylamide-based hydrogel electrolytes with dual functionality as the electrolyte and separator, inspiring further development in hydrogel electrolytes for aqueous battery systems.

A Metalgel with Liquid Metal Continuum Immobilized in Polymer Network

Gels are formed by fluids that expand throughout the whole volume of 3D polymer networks. To unlock unprecedented properties, exploring new fluids immobilized in polymer networks is crucial. Here, a new liquid metal-polymer gel material termed "metalgel" is introduced via fluid replacement strategy, featuring 92.40% vol liquid metal fluid as a continuum immobilized by interconnected nanoscale polymer network. The unique structure endows metalgel with high electrical conductivity (up to 3.18 × 106 S·m‒1), tissue-like softness (Young's modulus as low as 70 kPa), and low gas permeability (4.50 × 10‒22 m2·s‒1·Pa‒1). Besides, metalgel demonstrates electrical stability under extreme deformations, such as being run over by a 4.5-metric-tonne truck, and maintains its integrity in various environments for up to 180 days. The immobilization of high-volume-fraction liquid metal fluid is realized by electrostatic interactions is further revealed. Potential applications for metalgel are diverse and include soft electromagnetic shielding, hermetic sealing, and stimulating/sensing electrodes in implantable bioelectronics, underscoring its broad applicability.

Zinc Chemistries of Hybrid Electrolytes in Zinc Metal Batteries: From Solvent Structure to Interfaces

Along with the booming research on zinc metal batteries (ZMBs) in recent years, operational issues originated from inferior interfacial reversibility have become inevitable. Presently, single-component electrolytes represented by aqueous solution, "water-in-salt," solid, eutectic, ionic liquids, hydrogel, or organic solvent system are hard to undertake independently the task of guiding the practical application of ZMBs due to their specific limitations. The hybrid electrolytes modulate microscopic interaction mode between Zn2+ and other ions/molecules, integrating vantage of respective electrolyte systems. They even demonstrate original Zn2+ mobility pattern or interfacial chemistries mechanism distinct from single-component electrolytes, providing considerable opportunities for solving electromigration and interfacial problems in ZMBs. Therefore, it is urgent to comprehensively summarize the zinc chemistries principles, characteristics, and applications of various hybrid electrolytes employed in ZMBs. This review begins with elucidating the chemical bonding mode of Zn2+ and interfacial physicochemical theory, and then systematically elaborates the microscopic solvent structure, Zn2+ migration forms, physicochemical properties, and the zinc chemistries mechanisms at the anode/cathode interfaces in each type of hybrid electrolytes. Among of which, the scotoma and amelioration strategies for the current hybrid electrolytes are actively exposited, expecting to provide referenceable insights for further progress of future high-quality ZMBs.

Molecular Bridging Induced Anti-Salting-Out Effect Enabling High Ionic Conductive ZnSO4-Based Hydrogel for Quasi-Solid-State Zinc Ion Batteries

Hydrogel electrolytes (HEs) hold great promise in tackling severe issues emerging in aqueous zinc-ion batteries, but the prevalent salting-out effect of kosmotropic salt causes low ionic conductivity and electrochemical instability. Herein, a subtle molecular bridging strategy is proposed to enhance the compatibility between PVA and ZnSO4 from the perspective of hydrogen-bonding microenvironment re-construction. By introducing urea containing both an H-bond acceptor and donor, the broken H-bonds between PVA and H2O, initiated by the SO4 2--driven H2O polarization, could be re-united via intense intermolecular hydrogen bonds, thus leading to greatly increased carrying capacity of ZnSO4. The urea-modified PVA-ZnSO4 HEs featuring a high ionic conductivity up to 31.2 mS cm-1 successfully solves the sluggish ionic transport dilemma at the solid-solid interface. Moreover, an organic solid-electrolyte-interphase can be derived from the in situ electro-polymerization of urea to prohibit H2O-involved side reactions, thereby prominently improving the reversibility of Zn chemistry. Consequently, Zn anodes witness an impressive lifespan extension from 50 h to 2200 h at 0.1 mA cm-2 while the Zn-I2 full battery maintains a remarkable Coulombic efficiency (>99.7 %) even after 8000 cycles. The anti-salting-out strategy proposed in this work provides an insightful concept for addressing the phase separation issue of functional HEs.

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.

Double Network Gel Electrolyte with High Ionic Conductivity and Mechanical Strength for Zinc-Ion Batteries

Gel electrolytes hold promise for stabilizing zinc-ion batteries (ZIBs), but achieving both high ionic conductivity and strong mechanical properties remains challenging. This work presents a double network gel electrolyte based on poly(N-hydroxymethyl acrylamide) (PNMA) and sodium alginate (SA), overcoming this trade-off. The PNMA network provides mechanical strength and water retention, while the SA network facilitates rapid zinc-ion (Zn2+) diffusion through tailored solvation. This double network gel exhibits a tensile strength of up to 838 kPa, significantly higher than previous reports. The SA network provides ion channels for rapid transport of hydrated Zn2+, enhancing the ionic conductivity to a ground-breaking 33.1 mS cm-1. This value is even higher than the liquid electrolytes. The growth of Zn dendrites is also suppressed due to the mechanical constraint and rapid ion conduction. In symmetrical cells, the PNMA/SA gel demonstrates exceptional cycling stability (>2000 h). Characterizations show this is because of reduced free water amount, hindering cathode material dissolution. The full cells with sodium vanadate cathode manifest a high capacity (364.8 mA h g-1 at 0.5 A g-1) and excellent capacity retention (83% after 2500 cycles at 10 A g-1). This double network design offers a way to achieve high-performance and stable ZIBs.

Recent Advancements in Gel Polymer Electrolytes for Flexible Energy Storage Applications

Since the last decade, the need for deformable electronics exponentially increased, requiring adaptive energy storage systems, especially batteries and supercapacitors. Thus, the conception and elaboration of new deformable electrolytes becomes more crucial than ever. Among diverse materials, gel polymer electrolytes (hydrogels, organogels, and ionogels) remain the most studied thanks to the ability to tune the physicochemical and mechanical properties by changing the nature of the precursors, the type of interactions, and the formulation. Nevertheless, the exploitation of this category of electrolyte as a possible commercial product is still restrained, due to different issues related to the nature of the gels (ionic conductivity, evaporation of filling solvent, toxicity, etc.). Therefore, this review aims to resume different strategies to tailor the properties of the gel polymer electrolytes as well as to provide recent advancements in the field toward the elaboration of deformable batteries and supercapacitors.

An ionically cross-linked composite hydrogel electrolyte based on natural biomacromolecules for sustainable zinc-ion batteries

Zinc-ion batteries (ZIBs) are regarded as promising power sources for flexible and biocompatible devices due to their good sustainability and high intrinsic safety. However, their applications have been hindered by the issues of uncontrolled Zn dendrite growth and severe water-induced side reactions in conventional liquid electrolytes. Herein, an ionically cross-linked composite hydrogel electrolyte based on natural biomacromolecules, including iota-carrageenan and sodium alginate, is designed to promote highly efficient and reversible Zn plating/stripping. The abundant functional groups of macromolecules effectively suppress the reactivity of water molecules and facilitate uniform Zn deposition. Moreover, the composite hydrogel electrolyte exhibits a high ionic conductivity of 5.89 × 10-2 S cm-1 and a Zn2+ transference number of 0.58. Consequently, the Zn‖Zn symmetric cell with the composite hydrogel electrolyte shows a stable cycle life of more than 500 h. Meanwhile, the Zn‖NH4V4O10 coin cell with the composite hydrogel electrolyte retains a high specific capacity of approximately 200 mA h g-1 after 600 cycles at 2 A g-1. The Zn‖NVO pouch cell based on the composite hydrogel electrolyte also shows a high specific capacity of 246.1 mA h g-1 at 0.5 A g-1 and retains 70.7% of its initial capacity after 150 cycles. The pouch cell performs well at different bending angles and exhibits a capacity retention rate of 98% after returning to its initial state from 180° folding. This work aims to construct high-performance hydrogel electrolytes using low-cost natural materials, which may provide a solution for the application of ZIBs in flexible biocompatible devices.

Stable Zinc Anode Facilitated by Regenerated Silk Fibroin-modified Hydrogel Protective Layer

Inherent dendrite growth and side reactions of zinc anode caused by its unstable interface in aqueous electrolytes severely limit the practical applications of zinc-ion batteries (ZIBs). To overcome these challenges, a protective layer for Zn anode inspired by cytomembrane structure is developed with PVA as framework and silk fibroin gel suspension (SFs) as modifier. This PVA/SFs gel-like layer exerts similar to the solid electrolyte interphase, optimizing the anode-electrolyte interface and Zn2+ solvation structure. Through interface improvement, controlled Zn2+ migration/diffusion, and desolvation, this buffer layer effectively inhibits dendrite growth and side reactions. The additional SFs provide functional improvement and better interaction with PVA by abundant functional groups, achieving a robust and durable Zn anode with high reversibility. Thus, the PVA/SFs@Zn symmetric cell exhibits an ultra-long lifespan of 3150 h compared to bare Zn (182 h) at 1.0 mAh cm-2-1.0 mAh cm-2, and excellent reversibility with an average Coulombic efficiency of 99.04% under a large plating capacity for 800 cycles. Moreover, the PVA/SFs@Zn||PANI/CC full cells maintain over 20 000 cycles with over 80% capacity retention under harsh conditions at 5 and 10 A g-1. This SF-modified protective layer for Zn anode suggests a promising strategy for reliable and high-performance ZIBs.

An Electrochemically Initiated Self-Limiting Hydrogel Electrolyte for Dendrite-Free Zinc Anode

The zinc dendrite growth generally relies upon a "positive-feedback" mode, where the fast-grown tips receive higher current densities and ion fluxes. In this study, a self-limiting polyacrylamide (PAM) hydrogel that presents negative feedback to dendrite growth is developed. The monomers are purposefully polymerized at the dendrite tips, then the hydrogel reduces the local current density and ion flux by limiting zinc ion diffusion with abundant functional groups. As a consequence, the accumulation at the dendrite tips is restricted, and the (002) facets-oriented deposition is achieved. Moreover, the refined porous structure of the gel enhances Coulombic Efficiency by reducing water activity. Due to the synergistic effects, the zinc anodes perform an ultralong lifetime of 5100 h at 0.5 mA cm-2 and 1500 h at 5 mA cm-2, which are among the best records for PAM-based gel electrolytes. Further, the hydrogel significantly prolongs the lifespan of zinc-ion batteries and capacitors by dozens of times. The developed in situ hydrogel presents a feasible and cost-effective way to commercialize zinc anodes and provides inspiration for future research on dendrite suppression using the negative-feedback mechanism.

Thermally Healable Electrolyte-Electrode Interface for Sustainable Quasi-Solid Zinc-ion Batteries

Quasi-solid zinc-ion batteries using hydrogel electrolytes show great potential in energy storage devices owing to their intrinsic safety, fewer side reactions and wide electrochemical windows. However, the dendrite issues on the zinc anodes cannot be fundamentally eliminated and the intrinsic anode-electrolyte interfacial interspace is rarely investigated. Here, we design a dynamically healable gelatin-based hydrogel electrolyte with a highly reversible sol-gel transition, which can construct a conformal electrode-electrolyte interface and further evolve into a stable solid-solid interface by in situ solidification. The unique helical gelatin chain structure provides a uniform channel for zinc ion transport by the bridging effect of sulfate groups. As a consequence, the dynamically healable interface enables dendrite-free zinc anodes and repeatedly repairs the anode-electrolyte interfacial interspaces by the reversible sol-gel transition of gelatin electrolyte to retain long-lasting protection for sustainable zinc-ion batteries.

Nanofiber-Reinforced Composite Gel Enabling High Ionic Conductivity and Ultralong Cycle Life for Zn Ion Batteries

Despite the impressive merits of gel electrolytes for aqueous Zn-ion batteries, it remains a significant challenge to design and develop the gel electrolyte with high ionic conductivity, excellent dimensional stability, and long cycle life. Herein, a composite electrolyte (PTP) with thermolastic polyurethane -poly(m-phenylene isophthalamide)  nanofiber-reinforced polyvinyl alcohol gel strategy is proposed for highly reversible Zn plating/stripping. Mechanically robust and ultrathin PTP contains functional groups for building ion migration channels and immobilizing water molecules, which accelerates Zn2+ migration and mitigates water-related side reactions. Thus, the Zn anodes exhibit excellent electrochemical performance involving high cycling stability (6500 h at 5 mA cm-2 , 5 mA h cm-2 ) and achieving an exceptional cumulative capacity of more than 16 000 mA h cm-2 . This enhancement is well maintained when combined with MnO2 cathode. This work provides a reasonable solution for stabilizing Zn anodes and also provides new ideas for the modification of nanofiber-reinforced gel electrolytes.

Peptide Gel Electrolytes for Stabilized Zn Metal Anodes

The rechargeable aqueous Zn ion battery (AZIB) is considered a promising candidate for future energy storage applications due to its intrinsic safety features and low cost. However, Zn dendrites and side reactions (e.g., corrosion, hydrogen evolution reaction, and inactive side product (Zn hydroxide sulfate) formation) at the Zn metal anode have been serious obstacles to realizing a satisfactory AZIB performance. The application of gel electrolytes is a common strategy for suppressing these problems, but the normally used highly cross-linked polymer matrix (e.g., polyacrylamide (PAM)) brings additional difficulties for battery assembly and recycling. Herein, we have developed a gel electrolyte for Zn metal anode stabilization, where a peptide matrix, a highly biocompatible material, is used for gel construction. Various experiments and simulations elucidate the sulfate anion-assisted self-assembly gel formation and its effect in stabilizing Zn metal anodes. Unlike polymer gel electrolytes, the peptide gel electrolyte can reversibly transform between gel and liquid states, thus facilitating the gel-involved battery assembly and recycling. Furthermore, the peptide gel electrolyte provides fast Zn ion diffusion (comparable to conventional liquid electrolyte) while suppressing side reactions and dendrite growth, thus achieving highly stable Zn metal anodes as validated in various cell configurations. We believe that our concept of gel electrolyte design will inspire more future directions for Zn metal anode protection based on gel electrolyte design.

Hydrogel Electrolyte Enabled High-Performance Flexible Aqueous Zinc Ion Energy Storage Systems toward Wearable Electronics

To cater to the swift advance of flexible wearable electronics, there is growing demand for flexible energy storage system (ESS). Aqueous zinc ion energy storage systems (AZIESSs), characterizing safety and low cost, are competitive candidates for flexible energy storage. Hydrogels, as quasi-solid substances, are the appropriate and burgeoning electrolytes that enable high-performance flexible AZIESSs. However, challenges still remain in designing suitable and comprehensive hydrogel electrolyte, which provides flexible AZIESSs with high reversibility and versatility. Hence, the application of hydrogel electrolyte-based AZIESSs in wearable electronics is restricted. A thorough review is required for hydrogel electrolyte design to pave the way for high-performance flexible AZIESSs. This review delves into the engineering of desirable hydrogel electrolytes for flexible AZIESSs from the perspective of electrolyte designers. Detailed descriptions of hydrogel electrolytes in basic characteristics, Zn anode, and cathode stabilization effects as well as their functional properties are provided. Moreover, the application of hydrogel electrolyte-based flexible AZIESSs in wearable electronics is discussed, expecting to accelerate their strides toward lives. Finally, the corresponding challenges and future development trends are also presented, with the hope of inspiring readers.

Dynamic Regulation of the Interfacial pH for Highly Reversible Aqueous Zinc Ion Batteries

An electrolyte additive, with convenient operation and remarkable functions, has been regarded as an effective strategy for prolonging the cycle life of aqueous zinc ion batteries. However, it is still difficult to dynamically regulate the unstable Zn interface during long-term cycling. Herein, tricine was introduced as an efficient regulator to achieve a pH-stable and byproduct-free interface. The functional zwitterion of tricine not only inhibits interfacial pH perturbation and parasitic reactions by the trapping effect of an anionic group (-COO-) but also simultaneously creates a uniform electric field by the electrostatic shielding effect of a cationic group (-NH2+). Such synergy accordingly eliminates dendrite formation and creates a chemical equilibrium in the electrolyte, endowing the Zn||Zn cell with long-term Zn plating/stripping for 2060 h at 5 mA cm-2 and 720 h at 10 mA cm-2. As a result, the Zn||VS2 full cell under a high cathodic loading mass (8.6 mg cm-2) exhibits exceptional capacity retention of 93% after 1000 cycles.

GO-enhanced Gel Polymer Electrolyte for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) assembled with gel polymer electrolyte (GPE) have gained great popularity due to their low cost and safety. Nevertheless, the extensive utilization of GPE based AZIBs is hindered by various challenges, such as inadequate conductivity, limited mechanical strength, and unstable electrochemical properties. Herein, through the multiple cross-linking reaction of sodium alginate (SA), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and graphene oxide (GO), a hydrated GPE with high conductivity and excellent mechanical property was prepared. GO formed strong hydrogen-bonding interaction with polymers to build a three-dimensional network structure for ion migration and improved the mechanical property of GPE. The prepared GPE showed high ionic conductivity of 2.89×10-3  S cm-1 and excellent tensile strength of 900 kPa. In addition, the assembled Zn-Li hybrid battery provided a discharge specific capacity retention rate of 67.6 % and a Coulombic efficiency (CE) of approximate 100 % after 1000 cycles at 1 C.

Solid State Zinc and Aluminum ion batteries: Challenges and Opportunities

Solid-state zinc ion batteries (ZIBs) and aluminum-ion batteries (AIBs) are deemed as promising candidates for supplying power in wearable devices due to merits of low cost, high safety, and tunable flexibility. However, their wide-scale practical application is limited by various challenges, down to the material level. This Review begins with elaboration of the root causes and their detrimental effect for four main limitations: electrode-electrolyte interface contact, electrolyte ionic conductivity, mechanical strength, and electrochemical stability window of the electrolyte. Thereafter, various strategies to mitigate each of the described limitation are discussed along with future research direction perspectives. Finally, to estimate the viability of these technologies for wearable applications, economic-performance metrics are compared against Li-ion batteries.

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.

Recent progress in structural modification of polymer gel electrolytes for use in solid-state zinc-ion batteries

Zinc-ion batteries are one of the promising energy storage devices, which have the advantages of environmental friendliness, high safety and low price and are expected to be used in large-scale battery application fields. However, four prominent water-induced adverse reactions, including zinc dendrite formation, zinc corrosion, passivation and the hydrogen evolution reaction in aqueous systems, seriously shorten the cycling life of zinc-ion batteries and greatly hinder their development. Based on this, polymer gel electrolytes have been developed to alleviate these issues due to their unique network structure, which can reduce water activity and suppress water-induced side reactions. Based on the challenges of polymer gel electrolytes, this review systematically summarizes the latest research progress in the use of additives in them and explores new perspectives in response to the existing problems with polymer electrolytes. In order to expand the performance of polymer gel electrolytes in zinc-ion batteries, a range of different types of additives are added via physical/chemical crosslinking, such as organic or inorganic substances, natural plants, etc. In addition, different types of additives and polymerization crosslinking from different angles essentially improve the ionic conductivity of the gel electrolyte, inhibit the growth of zinc dendrites, and reduce hydrogen evolution and oxygen-absorbed corrosion. After these modifications of polymer gel electrolytes, a more stable and superior electrochemical performance of zinc-ion batteries can be obtained, which provides some strategies for solid-state zinc-ion batteries.

Production of gas-releasing electrolyte-replenishing Ah-scale zinc metal pouch cells with aqueous gel electrolyte

Aqueous zinc batteries are ideal candidates for grid-scale energy storage because of their safety and low-cost aspects. However, the production of large-format aqueous Zn batteries is hindered by electrolyte consumption, hydrogen gas evolution and accumulation, and Zn dendrites growth. To circumvent these issues, here we propose an "open" pouch cell design for large-format production of aqueous Zn batteries, which can release hydrogen gas and allow the refilling of the electrolyte components consumed during cell cycling. The cell uses a gel electrolyte containing crosslinked kappa (k)-carrageenan and chitosan. It bonds water molecules and hinders their side reaction with Zn, preventing electrolyte leakage and fast evaporation. As a proof-of-concept, we report the assembly and testing of a Zn | |Zn_xV₂O₅·nH₂O multi-layer "open" pouch cell using the carrageenan/chitosan gel electrolyte, which delivers an initial discharge capacity of 0.9 Ah and 84% capacity retention after 200 cycles at 200 mA g^‒1, 370 kPa and 25 °C.

Trace Amounts of Triple-Functional Additives Enable Reversible Aqueous Zinc-Ion Batteries from a Comprehensive Perspective

Although their cost-effectiveness and intrinsic safety, aqueous zinc-ion batteries suffer from notorious side reactions including hydrogen evolution reaction, Zn corrosion and passivation, and Zn dendrite formation on the anode. Despite numerous strategies to alleviate these side reactions have been demonstrated, they can only provide limited performance improvement from a single aspect. Herein, a triple-functional additive with trace amounts, ammonium hydroxide, was demonstrated to comprehensively protect zinc anodes. The results show that the shift of electrolyte pH from 4.1 to 5.2 lowers the HER potential and encourages the in situ formation of a uniform ZHS-based solid electrolyte interphase on Zn anodes. Moreover, cationic NH4+ can preferentially adsorb on the Zn anode surface to shield the "tip effect" and homogenize the electric field. Benefitting from this comprehensive protection, dendrite-free Zn deposition and highly reversible Zn plating/stripping behaviors were realized. Besides, improved electrochemical performances can also be achieved in Zn//MnO2 full cells by taking the advantages of this triple-functional additive. This work provides a new strategy for stabilizing Zn anodes from a comprehensive perspective.

Application of Biomass Materials in Zinc-Ion Batteries

Currently, aqueous zinc-ion batteries, with large reserves of zinc metal and maturity of production, are a promising alternative to sustainable energy storage. Nevertheless, aqueous solution has poor frost resistance and is prone to side reactions. In addition, zinc dendrites also limit the performance of zinc-ion batteries. Biomass, with complex molecular structure and abundant functional groups, makes it have great application prospects. In this review, the research progress of biomass and its derived materials used in zinc-ion batteries are reviewed. The different regulation strategies and characteristics of biomass used in zinc-ion battery electrodes, electrolyte separators and binders are demonstrated. The regulation mechanism is analyzed. At the end, the development prospect and challenges of biomass in energy materials application are proposed.

Polysaccharide hydrogel electrolytes with robust interfacial contact to electrodes for quasi-solid state flexible aqueous zinc ion batteries with efficient suppressing of dendrite growth

Flexible aqueous zinc-ion batteries (AZIBs) require high conductive and adhesive hydrogel electrolytes. However, high adhesion tends to hinder ion conduction rate. Herein, we designed a water/glycerol binary solvent coordinating the hydrophilic polymers to reconstruct the water molecules' environment in the hydrogel. As a consequence, the interface adhesion strength between Zn and the hydrogel reached 3.0 kPa and the ionic conductivity was up to 16.8 mS cm^-1. In addition, inspired by the slurry electrode preparation method, we developed a simple blade coating technique using a non-Newtonian polysaccharide liquid solution to construct an ultra-thin hydrogel electrolyte in situ on the cathode. The thickness of the obtained hydrogel reached 70 μm, and the ultrathin flexible AZIBs were easily constructed by pasting a Zn anode directly on the adhesive hydrogel, showing the potential of flexible AZIBs scalable assembly. In addition, the Zn//Zn symmetrical cells with the hydrogel electrolyte provided stable cycling performance for over 400 h at 0.1 mA cm^-2 with suppressed dendrite growth. The assembled Zn//Polyaniline battery and Zn//V₂O₅ battery also exhibited excellent capacity retention after cycles. This work has realized the hydrogel electrolyte with high adhesion and conductivity, which has good adaptability to metal electrodes and opened up a new practical way for large-scale assembly of flexible energy storage devices.

A blade-like CoZn metal organic framework-based flexible quasi-solid Zn-ion battery

Wearable flexible electronics has become more and more significant and popular in daily life. Here, a flexible quasi-solid Zn-ion battery consisting of CoZn-metal organic frameworks (MOFs) grown on carbon cloth as an all-in-one cathode working with a hydrogel electrolyte is developed. CoZn MOFs display a blade-like morphology, which is significant for rapid transfer of ions and electrons. The battery bending at angles from 0° to 180° displays high capacities and good capacity retention, and the capacity remains stable as the flexible battery twists to 90°. In addition, the capacity exceeds 101.4 mA h g^-1 as the battery is folded to 180° for 30 times, which indicates that the developed Zn-ion batteries would be applicable for a large variety of wearable devices such as foldable cellphones and pads.

Recent advances in exploiting carrageenans as a versatile functional material for promising biomedical applications

Carrageenans are a group of biopolymers widely found in red seaweeds. Commercial carrageenans have been traditionally used as emulsifiers, stabilizers, and thickening and gelling agents in food products. Carrageenans are regarded as bioactive polysaccharides with disease-modifying and microbiota-modulating activities. Novel biomedical applications of carrageenans as biocompatible functional materials for fabricating hydrogels and nanostructures, including carbon dots, nanoparticles, and nanofibers, have been increasingly exploited. In this review, we describe the unique structural characteristics of carrageenans and their functional relevance. We summarize salient physicochemical features, including thixotropic and shear-thinning properties, of carrageenans. Recent results from clinical trials in which carrageenans were applied as both antiviral and antitumor agents and functional materials are discussed. We also highlight the most recent advances in the development of carrageenan-based targeted drug delivery systems with various pharmaceutical formulations. Promising applications of carrageenans as a bioink material for 3D printing in tissue engineering and regenerative medicine are systematically evaluated. We envisage some key hurdles and challenges in the commercialization of carrageenans as a versatile material for clinical practice. This comprehensive review of the intimate relationships among the structural features, unique rheological properties, and biofunctionality of carrageenans will provide novel insights into their biomedicine application potential.

A Foldable Aqueous Zn-Ion Battery with Gear-Structured Composite as Freestanding Cathode

A foldable battery with high flexibility provides great potential in various wearable electronic devices for health and fitness tracking, chronic disease management, performance monitoring, navigation tracking, and portable gears for soldiers. We report a highly flexible, self-healing Zn-ion battery with a free-standing cathode that is composed of a 3D gear-like NH₄ V₄ O₁₀ @C composite on carbon paper. The battery retained a capacity of up to 102.4 mAh g^-1 even after being folded 60 times with a high angle of 180°. An aqueous hydrogel consisting polyvinyl alcohol, glycerin and Zn(CF₃ SO₃ )₂ was used as electrolyte, which showed as high as 580 % tensile strain under a loading weight of 78 N. The battery exhibited a better capacity retention of over 100 mAh g^-1 and Coulombic efficiency of over 99.8 % after cutting and twisting to 90°, thereby indicating a great self-healing performance. The gear-like geometry greatly improved the volume accommodation due to the increased interval space between the blades and the outward configuration. Meanwhile the Zn²⁺ ionic conductivity was improved by rapid re-binding of many existing hydroxy groups from the electrolyte and the enhanced contact surface area and diffusion route from the cathode material. The highly flexible, safe aqueous Zn-ion battery opens a practical way to power various carry-on electronics under mechanical agitation.

Polymeric Single-Ion Conductors with Enhanced Side-Chain Motion for High-Performance Solid Zinc-Ion Batteries

Zn-based solid polymer electrolytes (SPEs) have enormous potential in realizing high-performance zinc-ion batteries. Polymeric single-ion conductor (PSIC)-based SPEs can largely eradicate anion migration and side reactions of electrodes with decreased polarization, but the ionic conductivity is still unsatisfactory due to the tight localized ion interactions and sluggish chain motion. Herein, by employing the heterocyclic tetrazole as the anionic center of the side chain, a novel PSIC is fabricated with optimized charge delocalization and enhanced side-chain motion. The as-prepared PSIC delivers an ionic conductivity up to 5.4 × 10^-4 S cm^-1 with an ultrahigh Zn²⁺ transference number of 0.94. Based on the PSIC, dendrite-free and hydrogen-free Zn plating/stripping cycling (2000 h) is achieved. A further assembled Zn‖V₂ O₅ battery exhibits superior performances to other solid ZIBs, including a high discharge capacity, excellent rate capability, and long cycling life. In addition, a remarkable shelf-life (90 d), low self-discharge rate, and good temperature adaptability of the solid battery can be achieved benefiting from the high stability of the SPE during operation. The PSIC-based SPEs with advanced ion-transport structure endow solid ZIBs with significant performance improvement, high safety, and durability.

Recent Progress and Challenges of Flexible Zn-Based Batteries with Polymer Electrolyte

Zn-based batteries have been identified as promising candidates for flexible and wearable batteries because of their merits of intrinsic safety, eco-efficiency, high capacity and cost-effectiveness. Polymer electrolytes, which feature high solubility of zinc salts and softness, are especially advantageous for flexible Zn-based batteries. However, many technical issues still need to be addressed in Zn-based batteries with polymer electrolytes for their future application in wearable electronics. Recent progress in advanced flexible Zn-based batteries based on polymer electrolytes, including functional hydrogel electrolytes and solid polymer electrolytes, as well as the interfacial interactions between polymer electrolytes and electrodes in battery devices, is comprehensively reviewed and discussed with a focus on their fabrication, performance validation, and intriguing affiliated functions. Moreover, relevant challenges and some potential strategies are also summarized and analyzed to help inform the future direction of polymer-electrolyte-based flexible Zn-based batteries with high practicability.

Hexagonal WO3/3D Porous Graphene as a Novel Zinc Intercalation Anode for Aqueous Zinc-Ion Batteries

Aqueous Zn-ion batteries (ZIBs) have acquired great attention because of their high safety and environmentally friendly properties. However, the uncontrollable Zn dendrites and the irreversibility of electrodes seriously affect their practical application. Herein, hexagonal WO₃/three-dimensional porous graphene (h-WO₃/3DG) is investigated as an intercalation anode for ZIBs. As a result, the h-WO₃/3DG//Zn half-battery shows excellent electrochemical performance with a high capacity of 115.6 mAh g^-1 at 0.1 A g^-1 and 89% capacity retention at 2.0 A g^-1 after 10 000 cycles. The reason could be that the crystalline structure of WO₃, which has hexagonal channels, with a diameter of 5.36 Å, much higher than the diameter of Zn²⁺ (0.73 Å), accelerating the insertion/extraction of Zn ions. A zinc metal-free full battery using h-WO₃/3DG as the anode and ZnMn₂O₄/carbon black (ZnMn₂O₄/CB) as the cathode is constructed, exhibiting an initial capacity of 66.8 mAh g^-1 at 0.1 A g^-1 corresponding to an energy density of 73.5 W h kg^-1 (based on the total mass of anode and cathode-active materials) and a capacity retention of 76.6% after 1000 cycles at 0.5 A g^-1. This work demonstrates the high potential of hexagonal WO₃ as an advanced intercalation anode material for Zn metal-free batteries and may inspire new ideas for the development of other intercalation anode hosts for ZIBs.

Dual Porous 3D Zinc Anodes toward Dendrite-Free and Long Cycle Life Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) have been proven to be an alternative energy storage system because of their high safety, low cost, and eco-friendliness. However, the poor stability of metallic Zn anodes suffering from uncontrolled dendrite formation and electrochemical corrosion has brought troublesome hindrances for their practical application. In this work, we report a dual porous Zn-3D@600 anode prepared by coating a Zn@C protective layer on a 3D zinc skeleton. The Zn-3D@600 anode exhibits a highly stable and low polarization voltage during the Zn plating/stripping process and possesses a smooth and dendrite-free interface after long-term cycling. Moreover, the assembled Zn-3D@600 cell shows excellent cycle stability and superlative rate performance, delivering a discharge capacity of 198.8 mAh g^-1 after 1000 cycles at 1 A g^-1. Such excellent electrochemical performance can be credited to the Zn@C protective layer regulating uniform Zn nucleation and the 3D zinc skeleton accommodating Zn deposition at a high current density.

High-performance reversible aqueous Zinc-Ion battery based on Zn2+ pre-intercalation alpha-manganese dioxide nanowires/carbon nanotubes

Rechargeable aqueous zinc ion batteries (ZIBs) have attracted more and more attention due to the advantages of high safety, low cost, and environmental friendly in recent years. However, the lack of high-performance cathode materials and uncertain reaction mechanisms hinder the large-scale application of ZIBs. Herein, a 1D structure interlaced by Zn_xMnO₂ nanowires and carbon nanotubes is synthesized as cathode material for ZIB. The Zn_xMnO₂/CNTs cathode exhibits excellent specific capacity of 400 mAh g^-1 at 100 mA g^-1 and outstanding long-cycle stability (with a capacity retention of 93% after 100 cycles at 1000 mA g^-1), which indicates the Zn²⁺ pre-intercalation and composite carbon nanotubes can effectively change the storage space of the tunnel structure and increase the electron transmission rate. In addition, the energy storage mechanism of the highly reversible co-insertion of H⁺ and Zn²⁺ is further elaborated. This work has enlightenment and promotion for the future research of ZIBs cathode materials. Moreover, the simple preparation method, low cost and excellent performance of Zn_xMnO₂/CNTs cathode material provide a new way for the practical application of ZIBs.