Roadmap for advanced aqueous batteries: From design of materials to applications

Prioritizing Hetero-Metallic Interfaces via Thermodynamics Inertia and Kinetics Zincophilia Metrics for Tough Zn-Based Aqueous Batteries

Poor thermodynamic stability and sluggish electrochemical kinetics of metallic Zn anode in aqueous solution greatly hamper its practical application. To solve such problems, to date, various zincophilic surface modification strategies are developed, which can facilitate reversible Zn plating/stripping behavior. However, there is still a lack of systematic and fundamental understanding regarding the metrics of thermodynamics inertia and kinetics zincophilia in selecting zincophilic sites. Herein, hetero-metallic interfaces are prioritized for the first time via optimizing different hetero metals (Fe, Co, Ni, Sn, Bi, Cu, Zn, etc.) and synthetic solvents (ethanol, ethylene glycol, n-propanol, etc.). Specifically, both theoretical simulations and experimental results suggest that this Bi@Zn interface can exhibit high efficiency owing to the thermodynamics inertia and kinetics zincophilia. A best practice for prioritizing zincophilic sites in a more practical metric is also proposed. As a proof of concept, the Bi@Zn anode delivers ultralow overpotential of ≈55 mV at a high rate of 10 mA cm^-2 and stable cycle life over 4700 cycles. The elaborated "thermodynamics inertia and kinetics metalphilia" metrics for hetero-metallic interfaces can benchmark the success of other metal-based batteries.

Guest Ion-Dependent Reaction Mechanisms of New Pseudocapacitive Mg3 V4 (PO4 )6 /Carbon Composite as Negative Electrode for Monovalent-Ion Batteries

Polyanion-type phosphate materials, such as M₃ V₂ (PO₄ )₃ (M = Li/Na/K), are promising as insertion-type negative electrodes for monovalent-ion batteries including Li/Na/K-ion batteries (lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs)) with fast charging/discharging and distinct redox peaks. However, it remains a great challenge to understand the reaction mechanism of materials upon monovalent-ion insertion. Here, triclinic Mg₃ V₄ (PO₄ )₆ /carbon composite (MgVP/C) with high thermal stability is synthesized via ball-milling and carbon-thermal reduction method and applied as a pseudocapacitive negative electrode in LIBs, SIBs, and PIBs. In operando and ex situ studies demonstrate the guest ion-dependent reaction mechanisms of MgVP/C upon monovalent-ion storage due to different sizes. MgVP/C undergoes an indirect conversion reaction to form Mg⁰ , V⁰ , and Li₃ PO₄ in LIBs, while in SIBs/PIBs the material only experiences a solid solution with the reduction of V³⁺ to V²⁺ . Moreover, in LIBs, MgVP/C delivers initial lithiation/delithiation capacities of 961/607 mAh g^-1 (30/19 Li⁺ ions) for the first cycle, despite its low initial Coulombic efficiency, fast capacity decay for the first 200 cycles, and limited reversible insertion/deinsertion of 2 Na⁺ /K⁺ ions in SIBs/PIBs. This work reveals a new pseudocapacitive material and provides an advanced understanding of polyanion phosphate negative material for monovalent-ion batteries with guest ion-dependent energy storage mechanisms.

Fluoride-Based Stable Quasi-Solid-State Zinc Metal Battery with Superior Rate Capability

Aqueous zinc metal batteries are limited in practical applications due to their short lifespans. Herein, a LaF₃-coated Zn anode (LF@Zn) is investigated to induce the uniform Zn deposition and successfully build a separator-free quasi-solid-state zinc metal battery. The LF@Zn enables smooth and dendrite-free Zn deposition, owing to the homogeneous Zn²⁺ flux regulated by the LaF₃-based quasi-solid-state electrolyte. It can also suppress the corrosion side reactions by modulating the [Zn(H₂O)₆]²⁺ solvation sheath. The polarization of plating and stripping is relatively modest due to the reduced diffuse energy of desolvated Zn²⁺ in the quasi-solid-state electrolyte. In a separator-free symmetric cell, the LF@Zn anode shows a significantly prolonged lifespan of over 1300 h at 2 mA cm^-2 and a superior rate performance with only 156 mV at an ultrahigh current density of 50 mA cm^-2. A LF@Zn//VO₂ quasi-solid-state full cell exhibits outperforming rate capability and a long cyclic performance for up to 3000 cycles at 6.0 A g^-1. A stable Zn anode is established in this work with a fluoride-based quasi-solid-state electrolyte, opening up a new avenue for protecting metal anodes.

Metal-Organic Framework-Based Materials in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are promising for large-scale energy storage systems due to their high safety, large capacity, cost-effectiveness, and environmental friendliness. However, their commercialization is currently hindered by several challenging issues, including cathode degradation and zinc dendrite growth. Recently, metal-organic frameworks (MOFs) and their derivatives have gained significant attention and are widely used in AZIBs due to their highly porous structures, large specific surface area, and ability to design frameworks for Zn²⁺ shuttle. Based on preceding contributions, this review aims to generalize two design principles for MOF-based materials in AZIBs: cathode preparation and anode protection. For cathode preparation, we mainly introduce novel MOF-based electrode materials such as pure MOFs, porous carbon materials, metal oxides, and their compounds, focusing on the analysis of the specific capacity of AZIBs. For anode protection, we systematically analyze MOF-based materials used as 3D Zn architecture, solid electrolyte interfaces, novel separators, and solid-state electrolytes, highlighting the improvement in the cyclic stability of Zn anodes. Finally, we propose the future development of MOF-based materials in AZIBs. Our work can give some clues for raising the practical application level of aqueous ZIBs.

Solid-Electrolyte Interphase Chemistries Towards High-Performance Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries (AZMBs) are deemed a promising technology for electrochemical energy storage due to their high safety, low cost, and high energy density. However, AZMBs still suffer from severe side reactions, including Zn dendrite formation and intrinsic hydrogen evolution reaction. In contrast to the solid-electrolyte interphase (SEI) layer that stabilizes Li/Na/K metal anodes in organic electrolytes, it is difficult to form an SEI layer on the Zn surface because of the difficulty in decomposing the salt anions within the narrow electrochemical potential window of water. A team from the University of Adelaide reports a novel pure or hybrid electrolyte with H₂ O by using dimethyl methylphosphonate (DMMP) as solvent or co-solvent to construct a uniform and stable phosphate-based SEI layer (ZnP₂ O₆ and Zn₃ (PO₄ )₂ ). As a result, high Coulombic efficiencies and improved capacity retentions are obtained.

A Novel Aqueous Asymmetric Supercapacitor based on Pyrene-4,5,9,10-Tetraone Functionalized Graphene as the Cathode and Annealed Ti3 C2 Tx MXene as the Anode

Asymmetric supercapacitors (ASCs), employing two dissimilar electrode materials with a large redox peak position difference as cathode and anode, have been designed to further broaden the voltage window and improve the energy density of supercapacitors. Organic molecule based electrodes can be constructed by combining redox-active organic molecules with conductive carbon-based materials such as graphene. Herein, pyrene-4,5,9,10-tetraone (PYT), a redox-active molecule with four carbonyl groups, exhibits a four-electron transfer process and can potentially deliver a high capacity. PYT is noncovalently combined with two different kinds of graphene (Graphenea [GN] and LayerOne [LO]) at different mass ratios. The PYT-functionalized GN electrode (PYT/GN 4-5) possesses a high capacity of 711 F g^-1 at 1 A g^-1 in 1 M H₂ SO₄ . To match with the PYT/GN 4-5 cathode, an annealed-Ti₃ C₂ T_x (A-Ti₃ C₂ T_x ) MXene anode with a pseudocapacitive character is prepared by pyrolysis of pure Ti₃ C₂ T_x . The assembled PYT/GN 4-5//A-Ti₃ C₂ T_x ASC delivers an outstanding energy density of 18.4 Wh kg^-1 at a power density of 700 W kg^-1 . The PYT-functionalized graphene holds great potential for high-performance energy storage devices.

Integrated Micro Space Electrostatic Field in Aqueous Zn-Ion Battery: Scalable Electrospray Fabrication of Porous Crystalline Anode Coating

The inhomogeneous consumption of anions and direct contact between electrolyte and anode during the Zn-deposition process generate Zn-dendrites and side reactions that can aggravate the space-charge effect to hinder the practical implementation of zinc-metal batteries (ZMBs). Herein, electrospray has been applied for the scalable fabrication (>10 000 cm² in a batch-experiment) of hetero-metallic cluster covalent-organic-frameworks (MCOF-Ti₆ Cu₃ ) nanosheet-coating (MNC) with integrated micro space electrostatic field for ZMBs anode protection. The MNC@Zn symmetric cell presents ultralow overpotential (≈72.8 mV) over 10 000 cycles at 1 mAh cm^-2 with 20 mA cm^-2 , which is superior to bare Zn and state-of-the-art porous crystalline materials. Theoretical calculations reveal that MNC with integrated micro space electrostatic field can facilitate the deposition-kinetic and homogenize the electric field of anode to significantly promote the lifespan of ZMBs.

Constructing Carbon Nanotube Hybrid Fiber Electrodes with Unique Hierarchical Microcrack Structure for High-Voltage, Ultrahigh-Rate, and Ultralong-Life Flexible Aqueous Zinc Batteries

Flexible aqueous zinc batteries are promising candidates as safe power sources for fast-growing portable and wearable electronics. However, the low working voltage, poor rate capability, and cycling stability have greatly restricted their development and applications. Here, a new family of flexible bimetallic phosphide/carbon nanotube hybrid fiber electrodes with unique macroscopic microcrack structure and microscopic porous nanoflower structure is reported. The hierarchical microcrack structure not only facilitates the penetration of electrolyte for effective exposure of active sites, but also can serve as buffers to relieve the stress concentrations of the fiber electrode under deformations, enabling impressive electrochemical performance and mechanical flexibility. Particularly, the fabricated flexible aqueous zinc batteries demonstrate high working voltage plateau and specific capacity (≈1.7 V, 258.9 mAh g^-1 at 2 A g^-1 ), ultrahigh rate capability (135.8 mAh g^-1 at 50 A g^-1 , fully charged in only 9.8 s) and impressive power density of 79 000 W kg^-1 . Moreover, the flexible batteries show ultralong cycling life with 74.6% capacity retention after 20 000 cycles. The fiber batteries are also highly flexible and can be easily knitted into soft electronic textiles to power a smartphone, which are particularly promising for the next-generation of flexible and wearable electronics.

Realizing Textured Zinc Metal Anodes through Regulating Electrodeposition Current for Aqueous Zinc Batteries

Crystallography modulation of zinc (Zn) metal anode is promising to promote Zn reversibility in aqueous electrolytes, but efficiently constructing Zn with specific crystallographic texture remains challenging. Herein, we report a current-controlled electrodeposition strategy to texture the Zn electrodeposits in conventional aqueous electrolytes. Using the electrolytic cell with low-cost Zn(CH₃ COO)₂ electrolyte and Cu substrate as a model system, the texture of as-deposited Zn gradually transforms from (101) to (002) crystal plane as increasing the current density from 20 to 80 mA cm^-2 . Moreover, the high current accelerates the Zn nucleation rate with abundant nuclei, enabling uniform deposition. The (002) texture permits stronger resistance to dendrite growth and interfacial side reactions than the (101) texture. The resultant (002)-textured Zn electrode achieves deep cycling stability and supports the stable operation of full batteries with conventional V/Mn-based oxide cathodes.

A Liquid Crystal Ionomer-Type Electrolyte toward Ordering-Induced Regulation for Highly Reversible Zinc Ion Battery

Novel electrolyte is being pursued toward exploring Zn chemistry in zinc ion batteries. Here, a fluorine-free liquid crystal (LC) ionomer-type zinc electrolyte is presented, achieving simultaneous regulated water activity and long-range ordering of conduction channels and SEI. Distinct from water network or local ordering in current advances, long-range ordering of layered water channels is realized. Via manipulating water activity, conductivities range from ≈0.34 to 15 mS cm^-1 , and electrochemical window can be tuned from ≈2.3-4.3 V. The Zn|Zn symmetric cell with LC gel exhibits highly reversible Zn stripping/plating at 5 mA cm^-2 and 5 mAh cm^-2 for 800 h, with retained ordering of water channels. The capability of gel for inducing in situ formation of long-range ordered layer SEI associated with alkylbenzene sulfonate anion is uncovered. V₂ O₅ /Zn cell with the gel shows much improved cycling stability comparing to conventional zinc electrolytes, where the preserved structure of V₂ O₅ is associated with the efficiently stabilized Zn anode by the gel. Via long-range ordering-induced regulation on ion transport, electrochemical stability, and interfacial reaction, the development of LC electrolyte provides a pathway toward advancing aqueous rechargeable batteries.

Supramolecular Engineering of Cathode Materials for Aqueous Zinc-ion Energy Storage Devices: Novel Benzothiadiazole Functionalized Two-Dimensional Olefin-Linked COFs

Two-dimensional covalent organic frameworks (COFs) have emerged as promising materials for energy storage applications exhibiting enhanced electrochemical performance. While most of the reported organic cathode materials for zinc-ion batteries use carbonyl groups as electrochemically-active sites, their high hydrophilicity in aqueous electrolytes represents a critical drawback. Herein, we report a novel and structurally robust olefin-linked COF-TMT-BT synthesized via the aldol condensation between 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4'-(benzothiadiazole-4,7-diyl)dibenzaldehyde (BT), where benzothiadiazole units are explored as novel electrochemically-active groups. Our COF-TMT-BT exhibits an outstanding Zn²⁺ storage capability, delivering a state-of-the-art capacity of 283.5 mAh g^-1 at 0.1 A g^-1 . Computational and experimental analyses reveal that the charge-storage mechanism in COF-TMT-BT electrodes is based on the supramolecularly engineered and reversible Zn²⁺ coordination by the benzothiadiazole units.

pH-Triggered Molecular Switch Toward Texture-Regulated Zn Anode

Zn electrodes in aqueous media exhibit an unstable Zn/electrolyte interface due to severe parasitic reactions and dendrite formation. Here, a dynamic Zn interface modulation based on the molecular switch strategy is reported by hiring γ-butyrolactone (GBL) in ZnCl₂ /H₂ O electrolyte. During Zn plating, the increased interfacial alkalinity triggers molecular switch from GBL to γ-hydroxybutyrate (GHB). GHB strongly anchors on Zn surface via triple Zn-O bonding, leading to suppressive hydrogen evolution and texture-regulated Zn morphology. Upon Zn stripping, the fluctuant pH turns the molecular switch reaction off through the cyclization of GHB to GBL. This dynamic molecular switch strategy enables high Zn reversibility with Coulombic efficiency of 99.8 % and Zn||iodine batteries with high-cyclability under high Zn depth of discharge (50 %). This study demonstrates the importance of dynamic modulation for Zn electrode and realizes the reversible molecular switch strategy to enhance its reversibility.

Solvothermal Engineering of NaTi2(PO4)3 Nanomorphology for Applications in Aqueous Na-Ion Batteries

Aqueous Na-ion batteries are among the most discussed alternatives to the currently dominating Li-ion battery technology, in the area of stationary storage systems because of their sustainability, safety, stability, and environmental friendliness. The electrochemical properties such as ion insertion kinetics, practical capacity, cycling stability, or Coulombic efficiency are strongly dependent on the structure, morphology, and purity of an electrode material. The selection and optimization of materials synthesis route in many cases allows researchers to engineer materials with desired properties. In this work, we present a comprehensive study on size- and shape-controlled hydro(solvo)thermal synthesis of NaTi₂(PO₄)₃ nanoparticles. The effects of different alcohol/water synthesis media on nanoparticle phase purity, morphology, and size distribution are analyzed. Water activity in the synthesis media of different alcohol solutions is identified as the key parameter governing the nanoparticle phase purity, size, and shape. The careful engineering of NaTi₂(PO₄)₃ nanoparticle morphology allows control of the electrochemical performance and degradation of these materials as aqueous Na-ion battery electrodes.

Charge Carriers for Aqueous Dual-Ion Batteries

Environmental and safety concerns of energy storage systems call for application of aqueous battery systems which have advantages of low cost, environmental benignity, safety, and easy assembling. Among the aqueous battery systems, aqueous dual-ion batteries (ADIBs) provide high possibility for achieving excellent battery performance. Compared with the "rocking chair" batteries with only one type of carrier involved in the charging and discharging, ADIBs with both cations and anions as charge carriers possess diverse selections of electrodes and electrolytes. Charge carriers are the basis of the configuration of ADIBs. In this Review, cations and anions that could be applied in ADIBs are demonstrated with corresponding electrode materials and favorable electrolytes. Some insertion mechanisms are emphasized to provide insights for the possibilities to enhance the practical performances of ADIBs.

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

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

Influence of Water on Gel Electrolytes for Zinc-Ion Batteries

Gel electrolytes are being intensively explored for aqueous rechargeable zinc-ion batteries, especially towards high performance and multi-functionalities. Water plays a central role on the fundamental properties, interface reaction/interaction, and performance of the gel-type zinc electrolyte. In this review, the influence of water on the physiochemical properties of gel electrolytes is focused on. The correlation between water activity and the fundamental properties of zinc electrolytes is presented. Current approaches and challenges in manipulating water activity and the consequent influence on the electrochemical stability, transport, and interface kinetics of gel electrolytes are summarized. An outlook on approaches to tuning and investigating water activity is provided to shed light on the design of advanced gel electrolytes.

Step by Step Induced Growth of Zinc-Metal Interface on Graphdiyne for Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc ion batteries (AZIBs) promise high energy density, low redox potential, low cost and safety; however, their cycle performances are seriously insufficient to restrict the progress in this field. We propose a new concept of atomic electrode formed on the graphdiyne (GDY). This new idea electrode was synthesized by selectively, uniformly, and stably anchoring Zn atoms on GDY at the beginning of plating. The Zn atoms are induced to grow into larger size Zn clusters, which continue to grow into nanoflat. Finally, a new heterojunction interface is formed on GDY without any Zn dendrites and side reactions, even at high current densities. Such stepwise induction of growth greatly suppresses the formation of Zn dendrites, resulting in high electroplating/stripping reversibility and lifespan of AZIBs.

3D Co-Doping α-Ni(OH)2 Nanosheets for Ultrastable, High-Rate Ni-Zn Battery

The α-Ni(OH)₂ is regarded as one promising cathode for aqueous nickel-zinc batteries due to its high theoretical capacity of ≈480 mAh g^-1 , its practical deployment however suffers from the poor stability in strong alkaline solution, intrinsic low electrical conductivity as well as the retarded ionic diffusion. Herein, a 3D (three dimensional) macroporous α-Ni(OH)₂ nanosheets with Co doping is designed through a facile and easily scalable electroless plating combined with electrodeposition strategy. The unique micrometer-sized 3D pores come from Ni substrate and rich voids between Co-doping α-Ni(OH)₂ nanosheets can synergistically afford facile, interconnected ionic diffusion channels, sufficient free space for accommodating its volume changes during cycling; meanwhile, the Co-doping can stabilize the structural robustness of the α-Ni(OH)₂ in the alkaline electrolyte during cycling. Thus, the 3D α-Ni(OH)₂ shows a high capacity of 284 mAh g^-1 at 0.5 mA cm^-2 with an excellent retention of 78% even at 15 mA cm^-2 , and more than 2000 stable cycles at 6 mA cm^-2 , as well as the robust cycling upon various flexible batteries. This work provides a simple and efficient pathway to enhance the electrochemical performance of Ni-Zn batteries through improving ionic transport kinetics and stabilizing crystal structure of cathodes.

Intrinsic Interfacial Dynamic Engineering of Zincophilic Microbrushes via Regulating Zn Deposition for Highly Reversible Aqueous Zinc Ion Battery

Aqueous rechargeable zinc ion batteries are promising efficient energy storage systems due to remarkable safety and satisfactory capacity. However, zinc metal anode instability including dendrite growth and side reactions severely hinders widespread applications. Herein, zincophilic microbrushes have been in situ anchored on zinc plates through simple freeze-drying and mild reduction of graphene oxide, successfully overcoming these thorny issues. By introducing suitable oxygen-containing groups, the microbrushes exhibit a good affinity for zinc ions, thereby providing sufficient depositing sites, promoting zinc plating and stripping during cycling, and suppressing side reactions. The delicate zincophilic microbrushes can not only function as protective layer to guide the deposition of zinc ions, but also act as high-speed pathways to redistribute the zinc ion flux for rapid kinetics. Consequently, the microbrushes-covered zinc anode displays long lifespan and good durability, whenever in symmetric cell or full battery tests. This work paves a feasible bridge to design advanced aqueous anodes by architecting both structures and compositions of metal coverings.

Promoting Effect of Common Marine Cations on Hydrate Dissociation and Structural Evolution under a Static Electric Field

Natural gas hydrate, a potential energy resource, is attracting worldwide attention. In this study, we propose a new method of hydrate dissociation which uses seawater and electrostatic fields (SE method) cooperatively. The hydrate molecular dissociation mechanism of gas hydrate is a key issue in studying the kinetic properties of gas hydrate using the SE method. Therefore, molecular dynamics simulations were used to investigate the thermodynamic properties and structural changes of methane hydrate (MH) in multiple kinds of salt solutions under an electrostatic field. The results show that the electric field can drive cations into the MH phase to form a series of random semiopen cages, which are essentially temporary and metastable. The variation in free energy indicates that it is more difficult for divalent cations to enter the hydrate phase than monovalent cations, meaning that the hydrate structures formed with divalent cations are more unstable. Then, the ion current occurred in the hydrate phase (called ion migration in this study), which greatly accelerated hydrate dissociation. In contrast, the promotion effect of cations with the same charge on MH dissociation is as follows: Sr²⁺ > K⁺ ≈ Na⁺ > Ca²⁺ ≈ Mg²⁺. In general, the presence of common marine cations enhanced the promotion effect of the electric field on gas hydrate dissociation.

Nanoscale Ultrafine Zinc Metal Anodes for High Stability Aqueous Zinc Ion Batteries

Aqueous Zn batteries (AZBs) are a promising energy storage technology, due to their high theoretical capacity, low redox potential, and safety. However, dendrite growth and parasitic reactions occurring at the surface of metallic Zn result in severe instability. Here we report a new method to achieve ultrafine Zn nanograin anodes by using ethylene glycol monomethyl ether (EGME) molecules to manipulate zinc nucleation and growth processes. It is demonstrated that EGME complexes with Zn²⁺ to moderately increase the driving force for nucleation, as well as adsorbs on the Zn surface to prevent H-corrosion and dendritic protuberances by refining the grains. As a result, the nanoscale anode delivers high Coulombic efficiency (ca. 99.5%), long-term cycle life (over 366 days and 8800 cycles), and outstanding compatibility with state-of-the-art cathodes (ZnVO and AC) in full cells. This work offers a new route for interfacial engineering in aqueous metal-ion batteries, with significant implications for the commercial future of AZBs.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Nitrogen-Doped and Sulfonated Carbon Dots as a Multifunctional Additive to Realize Highly Reversible Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) using the Zn metal anode have been considered as one of the next-generation commercial batteries with high security, robust capacity, and low price. However, parasitic reactions, notorious dendrites and limited lifespan still hamper their practical applications. Herein, an eco-friendly nitrogen-doped and sulfonated carbon dots (NSCDs) is designed as a multifunctional additive for the cheap aqueous ZnSO₄ electrolyte, which can overcome the above difficulties effectively. The abundant polar groups (-COOH, -OH, -NH₂ , and -SO₃ H) on the CDs surfaces can regulate the solvation structure of Zn²⁺ through decreasing the coordinated active H₂ O molecules, and thus redistribute Zn²⁺ deposition to avoid side reactions. Some of the negatively charged NSCDs are adsorbed on Zn anode surface to isolate the H₂ O/SO₄ ^2- corrosion through the electrostatic shielding effect. The synergistic effect of the doped nitrogen species and the surface sulfonic groups can induce a uniform electrolyte flux and a homogeneous Zn plating with a (002) texture. As a result, the excellent cycle life (4000 h) and Coulombic efficiency (99.5%) of the optimized ZIBs are realized in typical ZnSO₄ electrolytes with only 0.1 mg mL^-1 of NSCDs additive.

High-Capacity Zinc Anode with 96 % Utilization Rate Enabled by Solvation Structure Design

Aqueous zinc-ion batteries (AZBs) show promises for large-scale energy storage. However, the zinc utilization rate (ZUR) is generally low due to side reactions in the aqueous electrolyte caused by the active water molecules. Here, we design a novel solvation structure in the electrolyte by introduction of sulfolane (SL). Theoretical calculations, molecular dynamics simulations and experimental tests show that SL remodels the primary solvation shell of Zn²⁺ , which significantly reduces the side reactions of Zn anode and achieves high ZUR under large capacities. Specifically, the symmetric and asymmetric cells could achieve a maximum of ∼96 % ZUR at an areal capacity of 24 mAh cm^-2 . In a ZUR of ∼67 %, the developed Zn-V₂ O₅ full cell can be stably cycled for 500 cycles with an energy density of 180 Wh kg^-1 and Zn-AC capacitor is stable for 5000 cycles. This electrolyte structural engineering strategy provides new insight into achieving high ZUR of Zn anodes for high performance AZBs.

Redox Flow Batteries: Electrolyte Chemistries Unlock the Thermodynamic Limits

Redox flow batteries (RFBs) represent a promising approach to enabling the widespread integration of intermittent renewable energy. Rapid developments in RFB materials and electrolyte chemistries are needed to meet the cost and performance targets. In this review, special emphasis is given to the recent advances how electrolyte design could circumvent the main thermodynamic restrictions of aqueous electrolytes. The recent success of aqueous electrolyte chemistries has been demonstrated by extending the electrochemical stability window of water beyond the thermodynamic limit, the operating temperature window beyond the thermodynamic freezing temperature of water and crystallization of redox-active materials, and the aqueous solubility beyond the thermodynamic solubility limit. They would open new avenues towards enhanced energy storage and all-climate adaptability. Depending on the constituent, concentration and condition of electrolytes, the performance gain has been correlated to the specific solvation environment, interactions among species and ion association at a molecular level.

Surface-redox sodium-ion storage in anatase titanium oxide

Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO₂(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO₂(A) becomes amorphous but still undergoes Ti⁴⁺/Ti³⁺ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO₂(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g^-1, at high charge/discharge rates. Kinetic studies of TiO₂(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO₂(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO₂(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO₂(A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.

High-performance aqueous rechargeable nickel//bismuth batteries with Bi2MoO6@rGO and Co0.5Ni0.5MoO4@rGO as electrode materials

Aqueous rechargeable nickel–bismuth batteries have surfaced as a prospective energy storage and conversion system because of their merits of good safety, high power density, and low cost.

High-Energy and Long-Lived Zn-MnO2 Battery Enabled by a Hydrophobic-Ion-Conducting Membrane

Alkaline Zn-MnO₂ batteries feature high security, low cost, and environmental friendliness while suffering from severe electrochemical irreversibility for both the Zn anode and MnO₂ cathode. Although neutral electrolytes are supposed to improve the reversibility of the Zn anode, the MnO₂ cathode indeed experiences a capacity degradation caused by the Jahn-Teller effect of the Mn³⁺ ion, thus shortening the lifespan of the neutral Zn-MnO₂ batteries. Theoretically, the MnO₂ cathode undergoes a highly reversible two-electron redox reaction of the MnO₂/Mn²⁺ couple in strongly acidic electrolytes. However, acidic electrolytes would inevitably accelerate the corrosion of the Zn anode, making long-lived acidic Zn-MnO₂ batteries impossible. Herein, to overcome the challenges faced by Zn-MnO₂ batteries, we propose a hybrid Zn-MnO₂ battery (HZMB) by coupling the neutral Zn anode with the acidic MnO₂ cathode, wherein the neutral anode and acidic cathode are separated by a proton-shuttle-shielding and hydrophobic-ion-conducting membrane. Benefiting from the optimized reaction conditions for both the MnO₂ cathode and Zn anode as well as the well-designed membrane, the HZMB exhibits a high working voltage of 2.05 V and a long lifespan of 2275 h (2000 cycles), breaking through the limitations of Zn-MnO₂ batteries in terms of voltage and cycle life.

In Situ Formation of Nitrogen-Rich Solid Electrolyte Interphase and Simultaneous Regulating Solvation Structures for Advanced Zn Metal Batteries

Zn metal as one of promising anode materials for aqueous batteries suffers from notorious dendrite growth, serious Zn corrosion and hydrogen evolution. Here, a bifunctional electrolyte additive, N-methyl pyrrolidone (NMP), is developed to improve the electrochemical performance of Zn anode. NMP not only alters the solvation structure of Zn²⁺ , but also in situ produces a dense N-rich solid-electrolyte-interphase layer on Zn foils. This layer protects Zn foils from corrosive electrolytes and benefits the uniform plating/stripping of Zn. Hence, the asymmetrical cells with NMP in the electrolyte retain a high coulombic efficiency of 99.8 % over 1000 cycles. The symmetric cells survive ≈200 h for 10 mAh cm^-2 at a high Zn utilization of 85.6 %. The full cells of Zn||MnO₂ show an impressive cumulative capacity even with lean electrolyte (E/C ratio=10 μL mAh^-1 ), limited Zn supply (N/P ratio=2.3) and high areal capacity (5.0 mAh cm^-2 ).

A Sustainable NH4 + Ion Battery by Electrolyte Engineering

Aqueous ammonium ion battery is a promising sustainable energy storage system. However, the side reactions originating from electrolytes (the water decomposition and host material dissolution) preclude its practical applications. Unlike the metal-based aqueous batteries, the idea of "ultrahigh concentrated electrolyte" is not feasible due to the strong hydrolysis of ammonium ions. Therefore, we propose an effective and sustainable strategy for the water hydrogen bond network modulation by adding sucrose into the electrolytes. The sucrose can form sucrose-water hydrogen bond networks to break the continuous water hydrogen bond network, thereby inhibiting water decomposition significantly. Moreover, the weak hydrogen bond interaction between ammonium and sucrose facilitates rapid ion migration, leading to an improved ionic conductivity. This work presents a new electrolyte modulating strategy for the practical application of aqueous ammonium ion batteries.

Coupling dual metal active sites and low-solvation architecture toward high-performance aqueous ammonium-ion batteries

Aqueous rechargeable ammonium-ion batteries (AIBs) possess the characteristics of safety, low cost, environmental friendliness, and fast diffusion kinetics. However, their energy density is often limited due to the low specific capacity of cathode materials and narrow electrochemical stability windows of electrolytes. Herein, high-performance aqueous AIBs were designed by coupling Fe-substituted manganese-based Prussian blue analog (FeMnHCF) cathodes and highly concentrated NH₄CF₃SO₃ electrolytes. In FeMnHCF, Mn³⁺/Mn²⁺-N redox reaction at high potential was introduced, and two metal active redox species of Mn and Fe were achieved. To match such FeMnHCF cathodes, highly concentrated NH₄CF₃SO₃ electrolyte was further developed, where NH₄⁺ ion displays low-solvation structure because of the increased coordination number of CF₃SO₃^- anions. Furthermore, the water molecules are confined by NH₄⁺ and CF₃SO₃^- ions in their solvation sheath, leading to weak interaction between water molecules and thus effectively extending the voltage window of electrolyte. Consequently, the FeMnHCF electrodes present high reversibility during the charge/discharge process. Moreover, owing to a small amount of free water in concentrated electrolyte, the dissolution of FeMnHCF is also inhibited. As a result, the assembled aqueous AIBs exhibit enhanced energy density, excellent rate capability, and stable cycling behavior. This work provides a creative route to construct high-performance aqueous AIBs.

Molecular-Level Zn-Ion Transfer Pump Specifically Functioning on (002) Facets Enables Durable Zn Anodes

The modification of metallic Zn anode contributes to solving the cycling issue of Zn-ion batteries (ZIBs) by restraining the dendrite growth and side reactions. In this regard, modulating (002) Zn is an effective way to prolong the lifespan of ZIBs with a parallel arrangement of Zn deposition. Herein, the authors propose to add trace amounts of Zn(BF₄ )₂ additive in 3 M ZnSO₄ to promote in-plane Zn deposition by forming a BF₄ ^- -[Zn(H₂ O)₆ ]²⁺ -[Zn(BF₄ )₃ ]^- transfer process and specifically functioning on (002) facets. In this way, the optimized electrolyte highly boosts the cycling stability of Zn anodes with a long lifespan at 34.2% Zn utilization (500 h/10 mA cm^-2 ) and 51.3% Zn utilization (360 h/10 mA cm^-2 ; 834 h/1 mA cm^-2 ). Moreover, the electroplated Zn on Cu substrate exhibits a competitive cumulative plating capacity (CPC) of 2.87 Ah cm^-2 under harsh conditions. The assembled Zn|(NH₄ )₂ V₆ O₁₆ ·3H₂ O full cells with a high cathode loading of 29.12 mg cm^-2 also realizes almost no capacity degradation even after 2000 cycles at 2 A g^-1 . With this cost-effective strategy, it is promising to push the development of aqueous ZIBs as well as provide inspiration for metal anode optimization in other energy storage systems.

Porous Metal Current Collectors for Alkali Metal Batteries

Alkali metals (i.e., Li, Na, and K) are promising anode materials for next-generation high-energy-density batteries due to their superior theoretical specific capacities and low electrochemical potentials. However, the uneven current and ion distribution on the anode surface probably induces undesirable dendrite growth, which leads to significant safety hazards and severely hinders the commercialization of alkali metal anodes. A smart and versatile strategy that can accommodate alkali metals into porous metal current collectors (PMCCs) has been well established to resolve the issues as well as to promote the practical applications of alkali metal anodes. Moreover, the proposal of PMCCs can meet the requirement of the dendrite-free battery fabrication industry, while the electrode material loading exactly needs the metal current collector component as well. Here, a systematic survey on advanced PMCCs for Li, Na, and K alkali metal anodes is presented, including their development timeline, categories, fabrication methods, and working mechanism. On this basis, some significant methodology advances to control pore structure, surface area, surface wettability, and mechanical properties are systematically summarized. Further, the existing issues and the development prospects of PMCCs to improve anode performance in alkali metal batteries are discussed.

Proton-Trapping Agent for Mitigating Hydrogen Evolution Corrosion of Zn for an Electrolytic MnO2/Zn Battery

A rechargeable aqueous electrolytic MnO₂/Zn battery (EMZB) based on a reversible Mn²⁺/MnO₂ two-electron redox reaction in an acidic electrolyte is very attractive for large-scale energy storage due to its high output voltage, large gravimetric capacity, and low cost. However, severe hydrogen evolution corrosion (HEC) of the Zn anode in an acidic electrolyte limits its application. Here, a proton-trapping agent (PTA) is introduced in the electrolyte to improve the electrochemical performance of the EMZB. Experimental results and theoretical calculations demonstrate that HEC of the Zn electrode can be effectively mitigated through high binding energy between the protons and PTA. The optimized EMZB regulated by a PTA of acetate (EMZB-20% Ac) delivers a high discharge voltage of 1.91 V and over 400 stable cycles at 1 C, which is more than 5 times the cycle life of the battery without PTA. EMZB-20% Ac also shows a Coulombic efficiency of 90.7% at a high areal capacity of 8 mAh cm^-2 and an energy retention of 83.6% after 1000 cycles at 5 C with an areal capacity of 1 mAh cm^-2. This work provides a promising electrolyte regulation strategy for the design and application of a high-performance EMZB and other energy storage systems.

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

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

In Situ Nitrogen Functionalization of 2D-Ti3C2Tx-MXenes for High-Performance Zn-Ion Supercapacitor

Zinc (Zn) ion supercapacitors (ZISCs) have attracted considerable attention as a viable energy storage technology because they are cost-effective, safe, and environmentally friendly. However, cathode materials with suitable properties are rare and need to be explored. In this regard, metal carbides (MXenes) are a good choice for capacitive energy storage, but they exhibit low capacitance. The energy storage performance of MXenes can be bossed using functionalization with heteroatom doping, e.g., nitrogen (N), to simultaneously modify ZISCs’ fundamental characteristics and electrochemical properties. Herein, we present an in-situ N-functionalization of Ti₃C₂T_x-MXene via a hydrothermal reaction with urea (denoted as N-Ti₃C₂T_x-MXene). N-functionalization into Ti₃C₂T_x-MXene raised Ti₃C₂T_x-MXene’s interlayer spacing and boosted the Zn-ion storage in 1 M ZnSO₄ electrolyte. The N-Ti₃C₂T_x-MXene electrode delivered an excellent specific capacitance of 582.96 F/g at 1 A/g and retained an outstanding cycle stability of 94.62% after 5000 cycles at 10 A/g, which is 1.8 times higher than pristine Ti₃C₂T_x-MXene at identical conditions. Moreover, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum capacitance of 153.55 F/g at 1 A/g, retained 92% of its initial value after 5000 cycles, and its Coulombic efficiency was ~100%. This strategy considerably reduced Ti₃C₂T_x-MXene nanosheet restacking and aggregation and enhanced electrochemical performance. Further, this research elucidated N-Ti₃C₂T_x-MXene’s charge–storage process and offered a fresh approach to the rational design of novel electrode materials for ZISCs.

An Electric-Field-Reinforced Hydrophobic Cationic Sieve Lowers the Concentration Threshold of Water-In-Salt Electrolytes

High-concentration water-in-salt (WIS) electrolytes expand the stable electrochemical window of aqueous electrolytes, leading to the advent of high-voltage (above 2 V) aqueous Li-ion batteries (ALIBs). However, the high lithium salt concentration electrolytes of ALIBs result in their high cost and deteriorate kinetic performance. Therefore, it is challenging for ALIBs to explore aqueous electrolytes with appropriate concentration to balance the electrochemical window and kinetic performance as well as the cost. In contrast to maintaining high concentrations of aqueous electrolytes (>20 m), a small number of hydrophobic cations are introduced to a much lower electrolyte concentration (13.8 m), and it is found that, compared with WIS electrolytes, ALIBs with these concentration-lowered electrolytes possess a compatible stable electrochemical window (3.23 V) and achieve better kinetic performance. These findings originate from the added cations, which form an electric-field-reinforced hydrophobic cationic sieve (HCS) that blocks water away from the anode and suppresses the hydrogen evolution reaction. Meanwhile, the lower electrolyte concentration provides significant benefits to ALIBs, including lower cost, better rate capability (lower viscosity of 18 cP and higher ionic conductivity of 22 mS cm^-1 at 25 °C), and improved low-temperature performance (liquidus temperature of -10.18 °C).

Molecular and Morphological Engineering of Organic Electrode Materials for Electrochemical Energy Storage

Organic electrode materials (OEMs) can deliver remarkable battery performance for metal-ion batteries (MIBs) due to their unique molecular versatility, high flexibility, versatile structures, sustainable organic resources, and low environmental costs. Therefore, OEMs are promising, green alternatives to the traditional inorganic electrode materials used in state-of-the-art lithium-ion batteries. Before OEMs can be widely applied, some inherent issues, such as their low intrinsic electronic conductivity, significant solubility in electrolytes, and large volume change, must be addressed. In this review, the potential roles, energy storage mechanisms, existing challenges, and possible solutions to address these challenges by using molecular and morphological engineering are thoroughly summarized and discussed. Molecular engineering, such as grafting electron-withdrawing or electron-donating functional groups, increasing various redox-active sites, extending conductive networks, and increasing the degree of polymerization, can enhance the electrochemical performance, including its specific capacity (such as the voltage output and the charge transfer number), rate capability, and cycling stability. Morphological engineering facilitates the preparation of different dimensional OEMs (including 0D, 1D, 2D, and 3D OEMs) via bottom-up and top-down methods to enhance their electron/ion diffusion kinetics and stabilize their electrode structure. In summary, molecular and morphological engineering can offer practical paths for developing advanced OEMs that can be applied in next-generation rechargeable MIBs.

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Triple-Function Electrolyte Regulation toward Advanced Aqueous Zn-Ion Batteries

The poor Zn reversibility has been criticized for limiting applications of aqueous Zn-ion batteries (ZIBs); however, its behavior in aqueous media is not fully uncovered yet. Here, this knowledge gap is addressed, indicating that Zn electrodes face a O₂ -involving corrosion, besides H₂ evolution and dendrite growth. Differing from aqueous Li/Na batteries, removing O₂ cannot enhance ZIB performance because of the aggravated competing H₂ evolution. To address Zn issues, a one-off electrolyte strategy is reported by introducing the triple-function C₃ H₇ Na₂ O₆ P, which can take effects during the shelf time of battery. It regulates H⁺ concentration and reduces free-water activity, inhibiting H₂ evolution. A self-healing solid/electrolyte interphase (SEI) can be triggered before battery operation, which suppresses O₂ adsorption corrosion and dendritic deposition. Consequently, a high Zn reversibility of 99.6% is achieved under a high discharge depth of 85%. The pouch full-cell with a lean electrolyte displays a record lifespan with capacity retention of 95.5% after 500 cycles. This study not only looks deeply into Zn behavior in aqueous media but also underscores rules for the design of active metal anodes, including Zn and Li metals, during shelf time toward real applications.

Working Aqueous Zn Metal Batteries at 100 °C

Reliable power supplies at extremely high temperatures are urgently needed to broaden the application scenarios for electric devices. Aqueous zinc metal batteries (ZMBs) with intrinsic safety are a promising alterative for high-temperature energy storage. However, the reversibility and long-term cycling stability of aqueous ZMBs at extremely high temperatures (≥100 °C) have rarely been explored. Herein, we reveal that spontaneous Zn corrosion and severe electrochemical hydrogen evolution at high temperature are vital restrictions for traditional aqueous ZMBs. To address this, a crowding agent, 1,5-pentanediol, was introduced into an aqueous electrolyte to suppress water reactivity by strengthening O-H bonds of H₂O and decreasing H₂O content in the Zn²⁺ solvation sheath, while maintaining flame resistance of the electrolyte. Importantly, this electrolyte enabled reversible Zn deposition with a Coulombic efficiency of 98.1% and a long cycling life of Zn//Zn batteries for over 500 cycles (at 1 mA cm^-2 and 0.5 mAh cm^-2) at 100 °C. Moreover, stable cycling of Zn//Te full batteries at 100 °C was demonstrated.

Hydrogels Enable Future Smart Batteries

The growing trend of intelligent devices ranging from wearables and soft robots to artificial intelligence has set a high demand for smart batteries. Hydrogels provide opportunities for smart batteries to self-adjust their functions according to the operation conditions. Despite the progress in hydrogel-based smart batteries, a gap remains between the designable functions of diverse hydrogels and the expected performance of batteries. In this Perspective, we first briefly introduce the fundamentals of hydrogels, including formation, structure, and characteristics of the internal water and ions. Batteries that operate under unusual mechanical and temperature conditions enabled by hydrogels are highlighted. Challenges and opportunities for further development of hydrogels are outlined to propose future research in smart batteries toward all-climate power sources and intelligent wearables.