Does Water-in-Salt Electrolyte Subdue Issues of Zn Batteries?

Gains and losses in zinc-ion batteries by proton- and water-assisted reactions

Research on aqueous zinc-ion batteries (AZIBs) has expanded significantly over the last decade due to their promising performance, cost, and safety as well as environmentally friendly features. The use of aqueous electrolytes enables promising AZIB properties while simultaneously introducing undesired reactions and processes. This review focuses on fundamental and critical considerations of water-related equilibria and reactions in zinc-ion batteries. First, we examine Zn2+/water ionic equilibria and their consequences for the chemistry of electrodes. Then, we focus on the mechanisms and kinetics of proton and Zn2+ insertion in host frameworks. Next, special attention is given to the water-related dissolution, deposition, and amorphization phenomena of transition-metal-based cathode materials. Finally, we highlight the role of water- and proton-assisted reactions through a systematic comparison of aqueous and nonaqueous zinc-ion batteries.

Biomass-Derived Carbon Dots: Sustainable Solutions for Advanced Energy Storage Applications

The growing energy demand has underscored the importance of sustainable energy storage devices. Biomass-derived carbon dots (B-Cdots) have gained significant attention for their potential to address this challenge. Utilizing greener routes for the large-scale synthesis of B-Cdots is not only eco-friendly and cost-effective but also promotes sustainability. This review highlights various synthesis methods for B-Cdots, including microwave-assisted, hydrothermal, and pyrolysis-based carbonization processes. It also explores their electrochemical applications in supercapacitors, lithium-ion batteries, sodium-ion batteries, and other energy storage devices, along with recent advancements in the field. The fabrication of electrodes using B-Cdots offers several advantages, such as tunable chemical and physical properties, porous structures, efficient heteroatom doping, and excellent electrical conductivity. These attributes make B-Cdots highly attractive for energy storage applications. Overall, this review emphasizes the critical role of sustainable materials in shaping the future of energy storage technologies.

Suspension Electrolytes with Catalytically Self-Expediating Desolvation Kinetics for Low-Temperature Zinc Metal Batteries

The conventional electrolyte for rechargeable aqueous zinc metal batteries (AZMBs) breeds many problems such as Zn dendrite growth and side reaction of hydrogen evolution reaction, which are fundamentally attributed to the uneven ion flux owing to the high barriers of desolvation and diffusion of Zn[(H2O)6]2+ clusters. Herein, to modulate the [Zn(H2O)6]2+ solvation structure, the suspension electrolyte engineering employed with electron-delocalized catalytic nanoparticles is initially proposed to expedite desolvation kinetics. As a proof, the electron-density-adjustable CeO2- x is introduced into the commercial electrolyte and preferentially adsorbed on the Zn surface, regulating the Zn[(H2O)6]2+ structure. Meanwhile, the defect-rich CeO2- x redistributes the localized space electric field to uniformize ion flux kinetics and inhibits dendrite growth, as confirmed by a series of theoretical simulations, spectroscopical and experimental measurements. Encouragingly, the CeO2- x decorated suspension electrolyte enables a long stability over 1200 cycles at 5 mA cm-2 and an extended lifespan exceeding 6500 h with lower overpotentials of 34 mV under 0 °C. Matched with polyaniline cathodes, the full cells with suspension electrolyte exhibit a capacity-retention of 96.75% at 1 A g-1 under -20 °C as well as a long lifespan of up to 400 cycles in a large-areal pouch cell, showcasing promising potentials of suspension electrolyte for practical AZMBs.

Engineering electro-crystallization orientation and surface activation in wide-temperature zinc ion supercapacitors

Matching the capacity of the anode and cathode is essential for maximizing electrochemical cell performance. This study presents two strategies to balance the electrode utilization in zinc ion supercapacitors, by decreasing dendritic loss in the zinc anode while increasing the capacity of the activated carbon cathode. The anode current collector was modified with copper nanoparticles to direct zinc plating orientation and minimize dendrite formation, improving the Coulombic efficiency and cycle life. The cathode was activated by an electrolyte reaction to increase its porosity and gravimetric capacity. The full cell delivered a specific energy of 192 ± 0.56 Wh kg-1 at a specific power of 1.4 kW kg-1, maintaining 84% capacity after 50,000 full charge-discharge cycles up to 2 V. With a cumulative capacity of 19.8 Ah cm-2 surpassing zinc ion batteries, this device design is particularly promising for high-endurance applications, including un-interruptible power supplies and energy-harvesting systems that demand frequent cycling.

Formulation principles and synergistic effects of high-voltage electrolytes

The energy density of lithium-ion batteries (LIBs) is primarily determined by the working potential of devices and the specific capacity of cathode compounds. Carbonate-based electrolytes have received considerable attention due to their significance for advancing current cell-assembly process. However, the commercially available liquid LiPF6 based electrolytes cannot withstand the harsh high-voltage environment and the effects of cathode, due to issues such as the undesired oxidative decomposition of ethylene carbonate (EC), the catalytic influence of dissolved transition metal ions (TMs), and the poor performance of interphases with unstable morphologies and components. Furthermore, the complex working mechanisms of high-voltage electrolytes (HVEs) are not fully understood. This review presents a comprehensive summary of the HVEs, including their physical properties, solvation structures, and interface chemistry. Specifically, chemical environment of high-voltage cathode compounds and failure mechanisms of commercial electrolytes are investigated, followed by a discussion of expected functions of HVEs. Then, screening criteria for single-component electrolytes, considering their oxidation resistance and decomposition mechanism, and screening mechanism of interphase species are explored based on their energy level positions. Next, a cross-scale evolution framework is proposed, from the solvation structure to interphase characteristics, aimed at uncovering the formulation principles and synergistic effects of HVEs. Operational mechanisms are systematically scrutinized, starting from the conventional tuning of solvation structure to the incorporation of multiple components and further to the role of entropy-driven effects, all of which will favor the understanding of formulation principles and synergistic effects. Finally, integration of advanced computational methods and mature experimental techniques is expected to foster the development of novel perspectives and promising electrolyte candidates.

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.

Fatty acid methyl ester ethoxylate additive for enhancing high-temperature aqueous zinc-ion battery performance

Aqueous zinc-ion batteries (AZIBs) have attracted significant attention due to their high theoretical capacity, low cost, and excellent safety. Nevertheless, their practical applications are hindered by challenges such as electrolyte decomposition, Zn corrosion/passivation, and dendrite growth, which become more severe under high-temperature conditions. To address these issues, innovative electrolyte design has become a key strategy. In this study, we propose a simple and effective electrolyte modification strategy by introducing fatty acid methyl ester ethoxylate (FMEE) as an additive. FMEE functions as both a solvation structure regulator and a water cluster stabilizer, effectively suppressing side reactions and promoting the formation of a robust solid electrolyte interphase enriched with ZnS and ZnF2. This significantly improves the interfacial chemical stability of the Zn anode. As a result, the Zn anode achieves an extended cycling lifespan of up to 3000 h at 1 mA cm-2 and 1 mAh cm-2. Furthermore, the Zn-V2O5 full cell using the FMEE-modified electrolyte exhibits an excellent rate performance and long-term cycling stability. Notably, the cell maintains a superior electrochemical performance even at 60 °C, demonstrating remarkable thermal stability. This study offers a new strategy for developing high-performance, temperature-tolerant AZIBs.

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

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

Benchmarking Corrosion with Anionic Polarity Index for Stable and Fast Aqueous Batteries Even in Low-Concentration Electrolyte

Despite aqueous electrolyte endowing batteries with the merits of safe operation, low-cost fabrication, and high ionic conductivity, water-induced corrosion, including spontaneous chemical and electrochemical hydrogen evolution corrosion, adversely affects lifespan and rate capability. There is still a lack of selection criteria for benchmarking corrosion behavior qualitatively. Through theoretical simulation, an anionic polarity index (API) tactic is proposed to resist corrosion by manipulating interfacial and solvated water concomitantly, thus realizing stable and fast Zn aqueous batteries (ZABs). As proof of concept, a low-cost zinc salt of 0.5 m zinc bis(4-hydroxybenzenesulphonate) (Zn(HBS)2) with low-API anion is prioritized. Combined in situ spectroscopic and electrochemical analyses reveal that, even in a low-concentration electrolyte, the low-API anion reduces interfacial water in the inner Helmholtz plane, shielding the chemical water dissociation. Meanwhile, their entering into the solvation sheath of Zn2+ lowers the solvent-separated ion pair, suppressing the electrochemical corrosion. The elaborated API-screened zinc salt endows fast plating kinetics of 50 mA cm-2 (119.1 mV polarization), high coulombic efficiency of 99.8%, dendrite-free cycling over 1600 h, and prolonged lifespan over 5000 cycles for the Zn-V cell. The results provide new metrics that can benchmark the success of ZABs for large-scale energy storage.

Disrupting the Crystal Growth of Corrosion Byproducts to Stabilize Aqueous Zn Metal Batteries

There is a key missing point in the research field of zinc metal batteries (ZMBs); that is, no one pays attention to the crystal growth of byproducts. In this paper, from the perspective of crystallography, a method of disturbing the crystal growth of corrosion byproducts is proposed to improve the serious water-related corrosion in ZMBs. In consideration of the thermodynamics and dynamics of crystallization, the corrosion inhibitor (CI) molecules are introduced to envelop the active growth sites of byproduct crystals and increase the surface free energy change (ΔGs), thus leading to a positive free energy change (ΔG) unfavorable for crystal formation and growth. Therefore, the expansion of the crystal lattice (that is, crystal growth) is impeded, resulting in the distortion and inhibition of the byproduct crystal structure, which have been comprehensively validated through density functional theory (DFT) calculations and in situ X-ray diffraction (XRD) measurements. Extensive research is also conducted on various CI compounds with an adjusted number of phosphate groups and carbon chain length. This work innovatively focuses on inhibiting the crystallization process of byproducts, which is of great significance for the study of water-related corrosion, showcasing the significant potential in enhancing the longevity of aqueous batteries.

Tailoring Water-in-DMSO Electrolyte for Ultra-stable Rechargeable Zinc Batteries

Rechargeable zinc batteries (RZBs) are hindered by two primary challenges: instability of Zn anode and deterioration of the cathode structure in traditional aqueous electrolytes, largely attributable to the decomposition of active H2O. Here, we design and synthesize a non-flammable water-in-dimethyl sulfoxide electrolyte to address these issues. X-ray absorption spectroscopy, in situ techniques and computational simulations demonstrate that the activity of H2O in this electrolyte is extremely compressed, which not only suppresses the side reactions and increases the reversibility of Zn anode, but also diminishes the cathode dissolution and proton intercalation. The hybrid solid-electrolyte interface (SEI), formed in situ, helps Zn-Zn symmetric cell a prolonged lifespan exceeding 10000 h at 0.5 mA cm-2 and 600 h at a 60 % discharge depth. The versatility of this electrolyte endows the Zn-VO2 full batteries ultra-stable cycling performance. This work provides insights into electrolyte structure-property relationships, and facilitates the design of high-performance RZBs.

Voltammetry Prediction and Electrochemical Analysis of Carbon Material from "Salt-In-Water" to "Water-In-Salt"

Cyclic voltammetry (CV) is a standard method for assessing electrochemical properties in the electrochemical cells, typically in conventional aqueous contexts like 1 m solutions ("salt-in-water"). However, recent advancements have extended electrochemistry into superconcentrated regimes, such as "water-in-salt" solutions with concentrations above 10 to 20 m, which require large amounts of salt for experiments. To address this, machine learning (ML) has been applied, coupled with in-house data collection using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolytes. This work demonstrates the electrochemistry of YEC-8B in LiTFSI, given their broad potential window of up to 3.0 V across concentrations from 1 to 20 m. The CV profiles were divided into two models: the upper curve for charging and the lower curve for discharging. Data were normalized and segmented by percentiles, and a decision tree model was developed to predict outputs based on input parameters like LiTFSI concentration, scan rates, and potential window. The model predicted nine target variables with a mean absolute percentage error of approximately 2% for both the upper and the lower CV profile curves. Trapezoidal rule was then used to calculate the system's capacitance. Additionally, tests showed a 75% accuracy in predicting the potential window and a suitable scan rate. Overall, the model effectively demonstrated the relationship between "water-in-salt" electrolytes and CV profiles in an electrochemical context using a simple machine learning (ML) algorithm, which continues to expand the integration of data science and electrochemistry.

From Salt in Water, Water in Salt to Beyond

Traditional aqueous electrolytes have a limited electrochemical stability window due to the decomposition voltage of water (≈1.23 V). "Water-in-Salt" (WIS) electrolytes are developed, which expand the stability window of aqueous electrolytes from 1.23 to 3 V and sparked a global surge of research in aqueous batteries. This breakthrough revealed novel aspects of solvation structure, ion transport mechanisms, and interfacial properties in WIS electrolytes, marking the start of a new era in solution chemistry that extends beyond traditional dilute electrolytes and has implications across electrolyte research. In this review, the current mechanistic understanding of WIS electrolytes and their derivative designs, focusing on the construction of solvation structures is presented. The insights gained and limitations encountered in bulk solvation structure engineering is further discussed. Finally, future directions beyond WIS for advancing aqueous electrolyte design is proposed.

Interfacial Confinement Effect of Self-Adsorbed Monolayer Enables Highly Reversible Zn Metal Anodes

The practical applications of aqueous Zn metal batteries are promising, yet still impeded by the corrosion reactions and dendrite growth on the Zn metal anode. Here, a self-adsorbed monolayer (SAM) is designed to stabilize the Zn metal anode. Theory and experiment results show that the interfacial confinement effect of the SAM, for one thing, greatly suppresses the corrosion reactions through the H2O-poor inner Helmholtz plane because of the steric-hindrance effect, and for another, alleviates the Zn2+ concentration gradient on the anode surface through the Zn2+ enrichment behavior and eventually inhibits the dendrite growth. Consequently, the Zn||Cu cell maintains a Coulombic efficiency of 99.3% at 10 mA cm-2/1 mAh cm-2 for 2000 cycles, and the Zn||Zn cell can stably cycle for 1400 h at 1 mA cm-2/1 mAh cm-2. Additionally, the NVO||Zn pouch cell shows impressive cycling stability (over 200 cycles) and low gassing behavior at 3 A g-1. This work provides a novel perspective for the interface engineering of Zn metal anodes.

Enhancing Zn Deposition Reversibility on MXene Current Collectors by Forming ZnF2-Containing Solid-Electrolyte Interphase for Anode-Free Zinc Metal Batteries

Anode-free aqueous zinc metal batteries (AZMBs) offer significant potential for energy storage due to their low cost and environmental benefits. Ti3C2Tx MXene provides several advantages over traditional metallic current collectors like Cu and Ti, including better Zn plating affinity, lightweight, and flexibility. However, self-freestanding MXene current collectors in AZMBs remain underexplored, likely due to challenges with Zn deposition reversibility. This study investigates the combination of a Ti3C2Tx self-freestanding film with advanced electrolyte engineering, specifically examining the effects of Li-salt and propylene carbonate (PC) as additives on Zn plating reversibility. While using Li+ ions as an additive alone facilitates uniform Zn deposition on bulk metals through the electrostatic shielding effect, the addition of Li-salt negatively impacts Zn plating uniformity on Ti3C2Tx. Meanwhile, using PC additive alone forms an organic SEI layer on Ti3C2Tx and causes Zn agglomeration. The use of both additives together results in a ZnF2-containing hybrid SEI layer with improved interfacial kinetics, promoting more uniform Zn deposition. This approach achieves an average Coulombic efficiency (CE) of 96.8% over 150 cycles (a maximum CE of 97.8%). The study highlights the strategic difference in electrolyte design, emphasizing the need for tailored approaches to optimize Zn deposition on MXenes, contrasting with traditional metallic current collectors.

Zn(TFSI)2-Mediated Ring-Opening Polymerization for Electrolyte Engineering Toward Stable Aqueous Zinc Metal Batteries

Practical Zn metal batteries have been hindered by several challenges, including Zn dendrite growth, undesirable side reactions, and unstable electrode/electrolyte interface. These issues are particularly more serious in low-concentration electrolytes. Herein, we design a Zn salt-mediated electrolyte with in situ ring-opening polymerization of the small molecule organic solvent. The Zn(TFSI)2 salt catalyzes the ring-opening polymerization of (1,3-dioxolane (DOL)), generating oxidation-resistant and non-combustible long-chain polymer (poly(1,3-dioxolane) (pDOL)). The pDOL reduces the active H2O molecules in electrolyte and assists in forming stable organic-inorganic gradient solid electrolyte interphase with rich organic constituents, ZnO and ZnF2. The introduction of pDOL endows the electrolyte with several advantages: excellent Zn dendrite inhibition, improved corrosion resistance, widened electrochemical window (2.6 V), and enhanced low-temperature performance (freezing point =  - 34.9 °C). Zn plating/stripping in pDOL-enhanced electrolyte lasts for 4200 cycles at 99.02% Coulomb efficiency and maintains a lifetime of 8200 h. Moreover, Zn metal anodes deliver stable cycling for 2500 h with a high Zn utilization of 60%. A Zn//VO2 pouch cell assembled with lean electrolyte (electrolyte/capacity (E/C = 41 mL (Ah)-1) also demonstrates a capacity retention ratio of 92% after 600 cycles. These results highlight the promising application prospects of practical Zn metal batteries enabled by the Zn(TFSI)2-mediated electrolyte engineering.

Synchronous Modulation of H-bond Interaction and Steric Hindrance via Bio-molecular Additive Screening in Zn Batteries

In addressing challenges such as side reaction and dendrite formation, electrolyte modification with bio-molecule sugar species has emerged as a promising avenue for Zn anode stabilization. Nevertheless, considering the structural variability of sugar, a comprehensive screening strategy is meaningful yet remains elusive. Herein, we report the usage of sugar additives as a representative of bio-molecules to develop a screening descriptor based on the modulation of the hydrogen bond component and electron transfer kinetics. It is found that xylo-oligosaccharide (Xos) with the highest H-bond acceptor ratio enables efficient water binding, affording stable Zn/electrolyte interphase to alleviate hydrogen evolution. Meanwhile, sluggish reduction originated from the steric hindrance of Xos contributes to optimized Zn deposition. With such a selected additive in hand, the Zn||ZnVO full cells demonstrate durable operation. This study is anticipated to provide a rational guidance in sugar additive selection for aqueous Zn batteries.

Simultaneous Inhibition of Vanadium Dissolution and Zinc Dendrites by Mineral-Derived Solid-State Electrolyte for High-Performance Zinc Metal Batteries

Designing solid electrolyte is deemed as an effective approach to suppress the side reaction of zinc anode and active material dissolution of cathodes in liquid electrolytes for zinc metal batteries (ZMBs). Herein, kaolin is comprehensively investigated as raw material to prepare solid electrolyte (KL-Zn) for ZMBs. As demonstrated, KL-Zn electrolyte is an excellent electronic insulator and zinc ionic conductor, which presents wide voltage window of 2.73 V, high ionic conductivity of 5.08 mS cm-1, and high Zn2+ transference number of 0.79. For the Zn//Zn cells, superior cyclic stability lasting for 2200 h can be achieved at 0.2 mA cm-2. For the Zn//NH4V4O10 batteries, stable capacity of 245.8 mAh g-1 can be maintained at 0.2 A g-1 after 200 cycles along with high retention ratio of 81 %, manifesting KL-Zn electrolyte contributes to stabilize the crystal structure of NH4V4O10 cathode. These satisfying performances can be attributed to the enlarged interlayer spacing, zinc (de)solvation-free mechanism and fast diffusion kinetics of KL-Zn electrolyte, availably guaranteeing uniform zinc deposition for zinc anode and reversible zinc (de)intercalation for NH4V4O10 cathode. Additionally, this work also verifies the application possibility of KL-Zn electrolyte for Zn//MnO2 batteries and Zn//I2 batteries, suggesting the universality of mineral-based solid electrolyte.

Achieving Ultra-Long Cycle Life of Zn Anode Using ZnSn(OH)6 Coating Layer via Desolvation Effect and Uniform Zn2+ Flux

Aqueous zinc-ion batteries (AZIBs) are considered as a promising energy storage system because of good safety, low cost, abundant resources, and environmental friendliness. However, the bottlenecks including dendrite growth, hydrogen evolution, and corrosion seriously limit their practical application. Herein, a novel ZnSn(OH)6 coating layer with rich hydroxyl groups is employed to achieve highly stable Zn anode. The hydroxyl groups can feasibly interact with H2O molecules, contributing to the desolvation of hydrated Zn2+ and the inhibition of side reactions on Zn anode surface. Furthermore, according to the DFT calculation, the adsorption energy of Zn2+ among various sites on the surface of ZnSn(OH)6 coating layer is relatively large, which helps the uniform distribution of Zn2+ flux and the prevention of dendrite growth. Consequently, the ZnSn(OH)6@Zn anode delivers ultra-long cycle life (6770 h), low polarization voltage (27 mV), and high Coulombic efficiency (99.2% over 800 cycles) at 1 mA cm-2, 1 mAh cm-2. Besides, the assembled NaV3O8·xH2O//ZnSn(OH)6@Zn full cell can operate stably for 1500 cycles at 2 A g-1 with a high specific capacity of 144.9 mAh g-1, demonstrating an excellent application potential. This simple and effective coating layer with high electrochemical performance provides an appealing strategy for the development of rechargeable AZIBs.

Potential-Dependent ATR-SEIRAS and EQCM-D Analysis of Interphase Formation in Zinc Battery Electrolytes

With the heightening interest in bivalent battery technology, there arises a necessity for a thorough investigation into zinc-ion battery (ZIB) electrolytes, accommodating their chemical attributes and potential-dependent structural dynamics. While the phenomenon of in situ solid electrolyte interphase formation is extensively documented in lithium-ion batteries, its analogous occurrences in ZIBs remain limited. Herein is a comparative study of three zinc electrolytes of interest: ZnSO4, ZnOTF, and Zn(TFSI)2/LiTFSI hybrid water-in-salt electrolyte. Additionally, the impact of an acetonitrile additive is scrutinized, with a comparative assessment of the interfacial behavior in aqueous solutions. Utilizing ATR-SEIRAS, potential-dependent alterations in the composition of the electrolyte/electrode interface were monitored, while EQCM-D facilitated a comprehensive understanding of variations in the mass and structural properties of the adsorbed layer. Aqueous ZnSO4 demonstrated the accumulation of porous Zn4SO4(OH)6·xH2O at negative potentials, leading to a mass of 1.47 μg cm-2 after five cycles. Bisulfate formation was observed at positive potentials. SEIRAS measurements for ZnOTF demonstrated reorientation and surface adsorption of CF3SO3- to favor CF3 at the surface for positive potentials, and acetonitrile showed increased stability for the electrode at negative potentials. The additive was also reported to lead to the accumulation of a substantial passivation layer with viscoelastic properties. The zinc water-in-salt showed exceptional surface stability at negative potentials and a widened potential window. A thin rigid zinc SEI layer is reported with a mass of 0.7 μg cm-2. The compositional intricacies of these surface structures are discussed in relation to their solvent conditions. This investigation not only sheds light on the initial charge/discharge cycles in ZIBs but also underscores their pivotal role in instigating enduring transformations that can significantly influence their long-term cycling performance.

Inorganic Electrolyte Additive Promoting the Interfacial Stability for Durable Zn-Ion Batteries

The development of Zn-ion batteries (ZIBs) is always hindered by the ruleless interface reactions between the solid electrode and liquid electrolyte, and seeking appropriate electrolyte additives is considered as a valid approach to stabilize the electrode/electrolyte interphases for high-performance ZIBs. Benefiting from the unique solubility of TiOSO4 in acidic solution, the composite electrolyte of 2 m ZnSO4+30 mm TiOSO4 (ZSO/TSO) is configured and its positive contribution to Zn//Zn cells, Zn//Cu cells, and Zn//NH4V4O10 batteries are comprehensively investigated by electrochemical tests and theoretical calculations. Based on the theoretical calculations, the introduction of TiOSO4 contributes to facilitating the desolvation kinetics of Zn2+ ions and guarantees the stable interface reactions of both zinc anode and NH4V4O10 cathode. As expected, Zn//Zn cells keep long-term cycling behavior for 3750 h under the test condition of 1 mA cm-2-1 mAh cm-2, Zn//Cu cells deliver high Coulombic efficiency of 99.9% for 1000 cycles under the test condition of 5 mA cm-2-1 mAh cm-2, and Zn//NH4V4O10 batteries maintain reversible specific capacity of 193.8 mAh g-1 after 1700 cycles at 5 A g-1 in ZSO/TSO electrolyte. These satisfactory results manifest that TiOSO4 additive holds great potential to improve the performances of ZIBs.

A Crystalline-Water Electrolyte Enabled High Depth-of-Discharge Anodes in Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries are regarded as a promising energy storage solution for a green and sustainable society in the future. However, the practical application of metallic zinc anode is plagued by the thermodynamic instability issue of water molecules in conventional electrolytes, which leads to severe dendrite growth and side reactions. In this work, an ultra-thin and high areal capacity metallic zinc anode is achieved by utilizing crystalline water with a stable stoichiometric ratio. Unlike conventional electrolytes, the designed electrolyte can effectively suppress the reactivity of water molecules and diminish the detrimental corrosion on the metallic zinc anode, while preserving the inherent advantages of water molecules, including great kinetic performance in electrolytes and H+ capacity contribution in cathodes. Based on the comprehensive performance of the designed electrolyte, the 10 µm Zn||10 µm Zn symmetric cell stably ran for 1000 h at the current density of 1 mA cm-2, and the areal capacity of 1 mAh cm-2, whose depth-of-discharge is over 17.1%. The electrochemical performance of the 10 µm Zn||9.3 mg cm-2 polyaniline (PANI) full-cell demonstrates the feasibility of the designed electrolyte. This work provides a crucial understanding of balancing activity of water molecules in aqueous zinc metal batteries.

Advanced electrolytes for high-performance aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have garnered significant attention in the realm of large-scale and sustainable energy storage, primarily owing to their high safety, low cost, and eco-friendliness. Aqueous electrolytes, serving as an indispensable constituent, exert a direct influence on the electrochemical performance and longevity of AZIBs. Nonetheless, conventional aqueous electrolytes often encounter formidable challenges in AZIB applications, such as the limited electrochemical stability window and the zinc dendrite growth. In response to these hurdles, a series of advanced aqueous electrolytes have been proposed, such as "water-in-salt" electrolytes, aqueous eutectic electrolytes, molecular crowding electrolytes, and hydrogel electrolytes. This comprehensive review commences by presenting an in-depth overview of the fundamental compositions, principles, and distinctive characteristics of various advanced aqueous electrolytes for AZIBs. Subsequently, we systematically scrutinizes the recent research progress achieved with these advanced aqueous electrolytes. Furthermore, we summarizes the challenges and bottlenecks associated with these advanced aqueous electrolytes, along with offering recommendations. Based on the optimization of advanced aqueous electrolytes, this review outlines future directions and potential strategies for the development of high-performance AZIBs. This review is anticipated to provide valuable insights into the development of advanced electrolyte systems for the next generation of stable and sustainable multi-valent secondary batteries.

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.

Rationally Engineering a Quasi-Solid-State Flexible Self-Healing Secondary Battery Using Hydrogel-Based Water-in-Salt Electrolyte and Electrochemically Deposited Electrodes

High safety and low cost are essential for energy-storage systems. Here, an aqueous zinc ion battery composed of a hydrogel-based water-in-salt electrolyte prepared by photoinitiated polymerization of acrylamide in ZnCl2 solution (named as PZC) and flexible electrodes is developed. The stable performance in Zn||Zn symmetric cells and high Coulombic efficiency of PZC in Zn||Cu asymmetric cells verify dendrite suppression. VO2 nanobelts coated with polyaniline (PANI) are grown on a carbon cloth (CC). The battery shows a capacity of 221.5 mAh g-1 after 200 cycles. The batteries present high recovery performance after bending/cutting. After bending of 60°, 90°, and 180°, capacities remain at 240.0, 205.4, and 175.2 mAh g-1, respectively; while the battery healed from 1, 2, 3, and 4 times of cutting shows 197.5, 174.3, 124.7, and 101.2 mAh g-1, respectively. Our findings enable the engineering of a quasi-solid-state battery to have good capability for flexible and portable electronics.

Selection of Negative Charged Acidic Polar Additives to Regulate Electric Double Layer for Stable Zinc Ion Battery

Zinc-ion batteries are promising for large-scale electrochemical energy storage systems, which still suffer from interfacial issues, e.g., hydrogen evolution side reaction (HER), self-corrosion, and uncontrollable dendritic Zn electrodeposition. Although the regulation of electric double layer (EDL) has been verified for interfacial issues, the principle to select the additive as the regulator is still misted. Here, several typical amino acids with different characteristics were examined to reveal the interfacial behaviors in regulated EDL on the Zn anode. Negative charged acidic polarity (NCAP) has been unveiled as the guideline for selecting additive to reconstruct EDL with an inner zincophilic H2O-poor layer and to replace H2O molecules of hydrated Zn2+ with NCAP glutamate. Taking the synergistic effects of EDL regulation, the uncontrollable interface is significantly stabilized from the suppressed HER and anti-self-corrosion with uniform electrodeposition. Consequently, by adding NCAP glutamate, a high average Coulombic efficiency of 99.83% of Zn metal is achieved in Zn|Cu asymmetrical cell for over 2000 cycles, and NH4V4O10|Zn full cell exhibits a high-capacity retention of 82.1% after 3000 cycles at 2 A g-1. Recapitulating, the NCAP principle posted here can quicken the design of trailblazing electrolyte additives for aqueous Zn-based electrochemical energy storage systems.

Reconstruction of zinc-metal battery solvation structures operating from -50 ~ +100 °C

Serious solvation effect of zinc ions has been considered as the cause of the severe side reactions (hydrogen evolution, passivation, dendrites, and etc.) of aqueous zinc metal batteries. Even though the regulation of cationic solvation structure has been widely studied, effects of the anionic solvation structures on the zinc metal were rarely examined. Herein, co-reconstruction of anionic and cationic solvation structures was realized through constructing a new multi-component electrolyte (Zn(BF4)2-glycerol-boric acid-chitosan-polyacrylamide, simplified as ZGBCP), which incorporates double crosslinking network via the esterification, protonation and polymerization reactions, thereby combining multiple advantages of 'liquid-like' high conductivity, 'gel-like' robust interface, and 'solid-like' high Zn2+ transfer number. Based on the ZGBCP electrolyte, the Zn anodes achieve record-low polarization and stable cycling. Furthermore, the ZGBCP electrolyte renders the AZMBs ultrawide working temperature (-50 °C ~ +100 °C) and ultralong cycle life (30000 cycles), which further validates the feasibility of the dual solvation structure strategy and provides a innovative perspective for the development of high-performance AZMBs.

Water-in-Salt Gel Biopolymer Electrolytes for Flexible and Wearable Zn/Alkali Metal Dual-Ion Batteries

Zn/alkali metal dual-ion batteries (ZM DIBs) with highly concentrated water-in-salt (WiS) electrolytes are promising next-generation energy storage systems. This enhanced design of Zn-ion rechargeable batteries offers intrinsic safety, high operating voltage, satisfactory capacity, and outstanding cyclic stability. Herein, taking the concept of highly concentrated electrolytes one step further, we introduce water-in-salt gel biopolymer electrolytes (WiS-GBEs) by encapsulating Zn/Li or Zn/Na bisalt compositions in a cellulose membrane. WiS-GBEs inherit the electrochemical merits of highly concentrated electrolytes (i.e., wide voltage window, high ionic conductivity, etc.) and excellent durability of gel biopolymer structures. Both types of WiS-GBEs apply to coin- and pouch-cell compartments of ZM DIBs, offering a high plateau voltage (>1.8 V vs. Zn2+/Zn), good and reversible capacity (118 and 57 mAh g-1 for Zn/Li and Zn/Na cells, respectively), and outstanding cycling stability (more than 90% after 1,000 cycles). Essentially, the pouch cells with WiS-GBEs present superior durability, flexibility, and capacity endurance under various bending stress conditions (90% capacity retention under 0-180° bending modes), indicating their potential capability to power wearable electronics. The practical powering ability of Li- and Na-based pouch systems is demonstrated by the example of a wearable digital timer.

Interface Engineering for Aqueous Aluminum Metal Batteries: Current Progresses and Future Prospects

Aqueous aluminum metal batteries (AMBs) have attracted numerous attention because of the abundant reserves, low cost, high theoretical capacity, and high safety. Nevertheless, the poor thermodynamics stability of metallic Al anode in aqueous solution, which is caused by the self-corrosion, surface passivation, or hydrogen evolution reaction, dramatically limits the electrochemical performance and hampers the further development of AMBs. In this comprehensive review, the key scientific challenges of Al anode/electrolyte interface (AEI) are highlighted. A systematic overview is also provided about the recent progress on the rational interface engineering principles toward a relatively stable AEI. Finally, suggestions and perspectives for future research are offered on the optimization of Al anode and aqueous electrolytes to enable a stable and durable AEI, which may pave the way for developing high-performance AMBs.

Rescuing zinc anode-electrolyte interface: mechanisms, theoretical simulations and in situ characterizations

The research interest in aqueous zinc-ion batteries (AZIBs) has been surging due to the advantages of safety, abundance, and high electrochemical performance. However, some technique issues, such as dendrites, hydrogen evolution reaction, and corrosion, severely prohibit the development of AZIBs in practical utilizations. The underlying mechanisms regarding electrochemical performance deterioration and structure degradation are too complex to understand, especially when it comes to zinc metal anode-electrolyte interface. Recently, theoretical simulations and in situ characterizations have played a crucial role in AZIBs and are exploited to guide the research on electrolyte engineering and solid electrolyte interphase. Herein, we present a comprehensive review of the current state of the fundamental mechanisms involved in the zinc plating/stripping process and underscore the importance of theoretical simulations and in situ characterizations in mechanism research. Finally, we summarize the challenges and opportunities for AZIBs in practical applications, especially as a stationary energy storage and conversion device in a smart grid.

The Construction of Anion-Induced Solvation Structures in Low-concentration Electrolyte for Stable Zinc Anodes

Aqueous zinc-ion batteries (ZIBs) are promising large-scale energy storage devices because of their low cost and high safety. However, owing to the high activity of H2O molecules in electrolytes, hydrogen evolution reaction and side reactions usually take place on Zn anodes. Herein, additive-free PCA-Zn electrolyte with capacity of suppressing the activity of free and solvated H2O molecules was designed by selecting the cationophilic and solventophilic anions. In such electrolyte, contact ion-pairs and solvent-shared ion-pairs were achieved even at low concentration, where PCA- anions coordinate with Zn2+ and bond with solvated H2O molecules. Simultaneously, PCA- anions also induce the construction of H-bonds between free H2O molecules and them. Therefore, the activity of free and solvated H2O molecules is effectively restrained. Furthermore, since PCA- anions possess a strong affinity with metal Zn, they can also adsorb on Zn anode surface to protect Zn anode from the direct contact of H2O molecules, inhibiting the occurrence of water-triggered side reactions. As a result, plating/stripping behavior of Zn anodes is highly reversible and the coulombic efficiency can reach to 99.43 % in PCA-Zn electrolyte. To illustrate the feasibility of PCA-Zn electrolyte, the Zn||PANI full batteries were assembled based on PCA-Zn electrolyte and exhibited enhanced cycling performance.

Nanosheet-structured ZnCo-LDH microsphere as active material for rechargeable zinc batteries

We report zinc cobalt-layered double hydroxides (ZnCo-LDH) as the active cathode materials for the development of high-performance Zn-ZnCo batteries. Electrochemical investigations show the battery's capacity increases linearly with increasing the ZnCo-LDH loading (up to 60 mg cm-2). The resulting Zn-ZnCo battery exhibits excellent rate performance and cycle stability, retaining 86% of its capacity even after 5000 cycles of testing. By incorporating ZnCo-LDH with a Pt/C-coated gas diffusion layer to form an integrated multifunctional air-cathode, we demonstrate a hybrid Zn battery, which combines the merits of Zn-ZnCo and Zn-air batteries to show a characteristic two-stage charge-discharge voltage profile. The current work demonstrates the linear relationship between the battery capacity and the active material loading. The results also highlight that a greater battery capacity requires further increasing of loading though very challenging.

Advancements in aqueous zinc-iodine batteries: a review

Aqueous zinc-iodine batteries stand out as highly promising energy storage systems owing to the abundance of resources and non-combustible nature of water coupled with their high theoretical capacity. Nevertheless, the development of aqueous zinc-iodine batteries has been impeded by persistent challenges associated with iodine cathodes and Zn anodes. Key obstacles include the shuttle effect of polyiodine and the sluggish kinetics of cathodes, dendrite formation, the hydrogen evolution reaction (HER), and the corrosion and passivation of anodes. Numerous strategies aimed at addressing these issues have been developed, including compositing with carbon materials, using additives, and surface modification. This review provides a recent update on various strategies and perspectives for the development of aqueous zinc-iodine batteries, with a particular emphasis on the regulation of I2 cathodes and Zn anodes, electrolyte formulation, and separator modification. Expanding upon current achievements, future initiatives for the development of aqueous zinc-iodine batteries are proposed, with the aim of advancing their commercial viability.

Electrochemical Hydrophobic Tri-layer Interface Rendered Mechanically Graded Solid Electrolyte Interface for Stable Zinc Metal Anode

The aqueous zinc-ion battery is promising as grid scale energy storage device, but hindered by the instable electrode/electrolyte interface. Herein, we report the lean-water ionic liquid electrolyte for aqueous zinc metal batteries. The lean-water ionic liquid electrolyte creates the hydrophobic tri-layer interface assembled by first two layers of hydrophobic OTF- and EMIM+ and third layer of loosely attached water, beyond the classical Gouy-Chapman-Stern theory based electrochemical double layer. By taking advantage of the hydrophobic tri-layer interface, the lean-water ionic liquid electrolyte enables a wide electrochemical working window (2.93 V) with relatively high zinc ion conductivity (17.3 mS/cm). Furthermore, the anion crowding interface facilitates the OTF- decomposition chemistry to create the mechanically graded solid electrolyte interface layer to simultaneously suppress the dendrite formation and maintain the mechanical stability. In this way, the lean-water based ionic liquid electrolyte realizes the ultralong cyclability of over 10000 cycles at 20 A/g and at practical condition of N/P ratio of 1.5, the cumulated areal capacity reach 1.8 Ah/cm2 , which outperforms the state-of-the-art zinc metal battery performance. Our work highlights the importance of the stable electrode/electrolyte interface stability, which would be practical for building high energy grid scale zinc-ion battery.

Bio-Inspired Trace Hydroxyl-Rich Electrolyte Additives for High-Rate and Stable Zn-Ion Batteries at Low Temperatures

High-rate and stable Zn-ion batteries working at low temperatures are highly desirable for practical applications, but are challenged by sluggish kinetics and severe corrosion. Herein, inspired by frost-resistant plants, we report trace hydroxyl-rich electrolyte additives that implement a dual remodeling effect for high-performance low-temperature Zn-ion batteries. The additive with high Zn absorbability not only remodels Zn2+ primary solvent shell by alternating H2 O molecules, but also forms a shielding layer thus remodeling the Zn surface, which effectively enhances fast Zn2+ de-solvation reaction kinetics and prohibits Zn anode corrosion. Taking trace α-D-glucose (αDG) as a demonstration, the electrolyte obtains a low freezing point of -55.3 °C, and the Zn//Zn cell can stably cycle for 2000 h at 5 mA cm-2 under -25 °C, with a high cumulative capacity of 5000 mAh cm-2 . A full battery that stably operates for 10000 cycles at -50 °C is also demonstrated.

Rechargeable Aqueous Zinc-Halogen Batteries: Fundamental Mechanisms, Research Issues, and Future Perspectives

Aqueous zinc-halogen batteries (AZHBs) have emerged as promising candidates for energy storage applications due to their high security features and low cost. However, several challenges including natural subliming, sluggish reaction kinetics, and shuttle effect of halogens, as well as dendrite growth of the zinc (Zn) anode, have hindered their large-scale commercialization. In this review, first the fundamental mechanisms and scientific issues associated with AZHBs are summarized. Then the research issues and progresses related to the cathode, separator, anode, and electrolyte are discussed. Additionally, emerging research opportunities in this field is explored. Finally, ideas and prospects for the future development of AZHBs are presented. The objective of this review is to stimulate further exploration, foster the advancement of AZHBs, and contribute to the diversified development of electrochemical energy storage.

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

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