Ultra-High-Capacity and Dendrite-Free Zinc-Sulfur Conversion Batteries Based on a Low-Cost Deep Eutectic Solvent

Advances in Aqueous Zinc Ion Batteries based on Conversion Mechanism: Challenges, Strategies, and Prospects

Recently, aqueous zinc-ion batteries with conversion mechanisms have received wide attention in energy storage systems on account of excellent specific capacity, high power density, and energy density. Unfortunately, some characteristics of cathode material, zinc anode, and electrolyte still limit the development of aqueous zinc-ion batteries possessing conversion mechanism. Consequently, this paper provides a detailed summary of the development for numerous aqueous zinc-based batteries: zinc-sulfur (Zn-S) batteries, zinc-selenium (Zn-Se) batteries, zinc-tellurium (Zn-Te) batteries, zinc-iodine (Zn-I2) batteries, and zinc-bromine (Zn-Br2) batteries. Meanwhile, the reaction conversion mechanism of zinc-based batteries with conversion mechanism and the research progress in the investigation of composite cathode, zinc anode materials, and selection of electrolytes are systematically introduced. Finally, this review comprehensively describes the prospects and outlook of aqueous zinc-ion batteries with conversion mechanism, aiming to promote the rapid development of aqueous zinc-based batteries.

Building a High-Potential Silver-Sulfur Redox Reaction Based on the Hard-Soft Acid-Base Theory

Sulfur holds immense promise for battery applications owing to its abundant availability, low cost, and high capacity. Currently, sulfur is commonly combined with alkali or alkaline earth metals in metal-sulfur batteries. However, these batteries universally face challenges in cycling stability due to the inevitable issue of polysulfide dissolution and shuttling. Additionally, the inferior stability of metal sulfide discharge compounds results in low S0/S2- redox potentials (<-0.41 V vs SHE). Herein, we leverage the principle of the hard-soft acid-base theory to introduce a novel silver-sulfur (Ag-S) battery system, which operates on the reaction between the soft acid of Ag+ and the soft base of S2-. Due to their high reaction affinity, the discharge compound of silver sulfide (Ag2S) is intrinsically insoluble and fundamentally stable. This not only resolves the polysulfide dissolution issue but also leads to a predominantly high S0/S2- redox potential (+1.0 V vs. SHE). We thus exploit the Ag-S reaction for a primary zinc battery application, which exhibits a high capacity of ∼620 mAh g-1 and a high voltage of ∼1.45 V. This work offers valuable insights into the application of classic chemistry theories in the development of innovative energy storage devices.

An Ionic Liquid Supramolecular Gel Electrolyte with Unique Wide Operating Temperature Range Properties for Zinc-Ion Batteries

Zinc-ion batteries are promising candidates for large-scale energy storage. The side reactions of the hydrogen evolution reaction (HER) and zinc dendrite growth are major challenges for developing high-performance zinc-ion batteries. In this paper, a supramolecular gel electrolyte (BLO-ILZE) was self-assembled in an ionic liquid (EMIMBF4) with zinc tetrafluoroborate (Zn(BF4)2) on the separator in situ to obtain a gel electrolyte used in zinc-ion batteries. BLO-ILZE is demonstrated to significantly enhance conductivity over a broad temperature range between -70 and 100 °C. Interestingly, through testing and fitting, it is found that the supramolecular gel electrolyte satisfies the liquid state law over a wide temperature range, and even achieves high conductivity (2.12 mS cm-1) at -40 °C. It is equivalent to the conductivity of aqueous zinc-ion batteries (ZnSO4/H2O) at -10 °C, which is 2.33 mS cm-1. Moreover, the supramolecular gel electrolyte can effectively inhibit the HER, thus exhibiting a longer lifetime in Zn/Zn cells for 3470 h at 1 mA cm-2 compared to the aqueous zinc-ion batteries with the Zn(BF4)2 aqueous electrolyte (400 h at 1 mA cm-2). The assembled V2O5/BLO-ILZE/Zn full cells also showed cycling performance, with 5000 cycles at 0.5 mA g-1 at room temperature, a capacity of 98%, and a coulombic efficiency of about 100%.

A Tellurium-Boosted High-Areal-Capacity Zinc-Sulfur Battery

Aqueous rechargeable zinc-sulfur (Zn-S) batteries are a promising, cost-effective, and high-capacity energy storage technology. Still, they are challenged by the poor reversibility of S cathodes, sluggish redox kinetics, low S utilization, and unsatisfactory areal capacity. This work develops a facile strategy to achieve an appealing high-areal-capacity (above 5 mAh cm-2) Zn-S battery by molecular-level regulation between S and high-electrical-conductivity tellurium (Te). The incorporation of Te as a dopant allows for manipulation of the Zn-S electrochemistry, resulting in accelerated redox conversion, and enhanced S utilization. Meanwhile, accompanied by the S-ZnS conversion, Te is converted to zinc telluride during the discharge process, as revealed by ex-situ characterizations. This additional redox reaction contributes to the S cathode's total excellent discharge capacity. With this unique cathode structure design, the carbon-confined TeS cathode (denoted as Te1S7/C) delivers a high reversible capacity of 1335.0 mAh g-1 at 0.1 A g-1 with a mass loading of 4.22 mg cm-2, corresponding to a remarkable areal capacity of 5.64 mAh cm-2. Notably, a hybrid electrolyte design uplifts discharge plateau, reduces overpotential, suppresses Zn dendrites growth, and extends the calendar life of Zn-Te1S7 batteries. This study provides a rational S cathode structure to realize high-capacity Zn-S batteries for practical applications.

Trace Selenium Doping for Improving the Reaction Kinetics of ZnS Cathode for Aqueous Zn-S Batteries

Aqueous Zinc-sulfur (Zn-S) batteries are promising for the field of energy storage due to their low cost, high theoretical capacity, and safety. However, the large volume expansion and the inherently poor conductivity of sulfur would result in electrode cracking and sluggish reaction kinetics, limiting the practical application of Zn-S batteries. Herein, commercial zinc sulfide (ZnS) is employed instead of S as cathode and proposed a doping modification strategy to solve the above problems. The designed ZnS0.93Se0.07 cathode shows good cycle stability and much-improved reaction kinetics, which is due to the smaller bandgap of ZnS0.93Se0.07 (1.40 eV) compared to ZnS (1.86 eV). As a result, the obtained ZnS0.93Se0.07 cathode exhibits a high specific capacity of 552 mAh g-1 (1672.6 mAh g-1 based on S) at 0.1 A g-1 and 330 mAh g-1 (1000 mAh g-1 based on S) at 2 A g-1. Moreover, the ZnS0.93Se0.07 cathode can provide a high areal capacity of 3.8 mAh cm-2 at a high mass loading of 10 mg cm-2 and limited electrolyte (4 µL mg-1). This work provides a simple and effective cathode modification strategy, which is conducive to promoting the practical application of Zn-S batteries.

Facilitating the Electrochemical Oxidation of ZnS through Iodide Catalysis for Aqueous Zinc-Sulfur Batteries

Aqueous zinc-sulfur (Zn-S) batteries show great potential for unlocking high energy and safety aqueous batteries. Yet, the sluggish kinetic and poor redox reversibility of the sulfur conversion reaction in aqueous solution challenge the development of Zn-S batteries. Here, we fabricate a high-performance Zn-S battery using highly water-soluble ZnI2 as an effective catalyst. In situ experimental characterizations and theoretical calculations reveal that the strong interaction between I- and the ZnS nanoparticles (discharge product) leads to the atomic rearrangement of ZnS, weakening the Zn-S bonding, and thus facilitating the electrochemical oxidation reaction of ZnS to S. The aqueous Zn-S battery exhibited a high energy density of 742 Wh kg(sulfur) -1 at the power density of 210.8 W kg(sulfur) -1 and good cycling stability over 550 cycles. Our findings provide new insights about the iodide catalytic effect for cathode conversion reaction in Zn-S batteries, which is conducive to promoting the future development of high-performance aqueous batteries.

Highly stable and reversible Zn anodes enabled by an electrolyte additive of sucrose

Aqueous zinc-ion batteries (ZIBs) are one of the most promising candidates for electric energy storage devices due to their merits of low cost and high safety. However, the notorious side reactions and dendrite formation on zinc anodes impede the commercialization of ZIBs. In this work, a cheap and edible electrolyte additive sucrose is applied to address the above issues. Sucrose with hydroxyl groups can react as zincophilic sites to adsorb Zn2+. As verified by Raman and FT-IR spectroscopy, the solvation structure of Zn2+ and the hydrogen bonds can be regulated by the sucrose molecule. The weakened solvated structure of Zn2+ and lowered coupling degree between Zn2+ and SO42- can inhibit the hydrogen evolution reaction (HER) and the generation of the sulfate by-product. Furthermore, a solid electrolyte interphase (SEI)-like ion buffer layer is formed because of the preferentially adsorbed sucrose, which can increase the nucleation overpotential and equalize the ion distribution. The enriched Zn nucleation sites and inhibited 2D diffusion of Zn2+ resulting from the sucrose additive enable uniform Zn deposition. Thus, improved performances of symmetric Zn||Zn, asymmetric Zn||Cu and Zn||VO2 cells are realized. The Zn||Zn cell exhibits a highly reversible cycling performance for 1200 h and 400 h at 5 mA cm-2/1 mA h cm-2 and 10 mA cm-2/5 mA h cm-2, respectively. This work provides a readily available and edible additive to improve the performance of ZIBs.

Comprehensive Understandings of Hydrogen Bond Chemistry in Aqueous Batteries

Aqueous batteries are emerging as highly promising contenders for large-scale grid energy storage because of uncomplicated assembly, exceptional safety, and cost-effectiveness. The unique aqueous electrolyte with a rich hydrogen bond (HB) environment inevitably has a significant impact on the electrode materials and electrochemical processes. While numerous reviews have focused on the materials design and assembly of aqueous batteries, the utilization of HB chemistry is overlooked. Herein, instead of merely compiling recent advancements, this review presents a comprehensive summary and analysis of the profound implication exerted by HB on all components of the aqueous batteries. Intricate links between the novel HB chemistry and various aqueous batteries are ingeniously constructed within the critical aspects, such as self-discharge, structural stability of electrode materials, pulverization, solvation structures, charge carrier diffusion, corrosion reactions, pH sensitivity, water splitting, polysulfides shuttle, and H2 S evolution. By adopting a vantage point that encompasses material design, binder and separator functionalization, electrolyte regulation, and HB optimization, a critical examination of the key factors that impede electrochemical performance in diverse aqueous batteries is conducted. Finally, insights are rendered properly based on HB chemistry, with the aim of propelling the advancement of state-of-the-art aqueous batteries.

High-Reversibility Sulfur Anode for Advanced Aqueous Battery

Despite being extensively explored as cathodes in batteries, sulfur (S) can function as a low-potential anode by changing charge carriers in electrolytes. Here, a highly reversible S anode that fully converts from S8 0 to S2- in static aqueous S-I2 batteries by using Na+ as the charge carrier is reported. This S anode exhibits a low potential of -0.5 V (vs standard hydrogen electrode) and a near-to-theoretical capacity of 1404 mA h g-1 . Importantly, it shows significant advantages over the widely used Zn anode in aqueous media by obviating dendrite formation and H2 evolution. To suppress "shuttle effects" faced by both S and I2 electrodes, a scalable sulfonated polysulfone (SPSF) membrane is proposed, which is superior to commercial Nafion in cost (US$1.82 m-2  vs $3500 m-2 ) and environmental benignity. Because of its ultra-high selectivity in blocking polysulfides/iodides, the battery with SPSF displays excellent cycling stability. Even under 100% depth of discharge, the battery demonstrates high capacity retention of 87.6% over 500 cycles, outperforming Zn-I2 batteries with 3.1% capacity under the same conditions. These findings broaden anode options beyond metals for high-energy, low-cost, and fast-chargeable batteries.

Synergistic Cation Solvation Reorganization and Fluorinated Interphase for High Reversibility and Utilization of Zinc Metal Anode

Batteries based on zinc (Zn) chemistry offer a great opportunity for large-scale applications owing to their safety, cost-effectiveness, and environmental friendliness. However, the poor Zn reversibility and inhomogeneous electrodeposition have greatly impeded their practical implementation, stemming from water-related passivation/corrosion. Here, we present a multifunctional electrolyte comprising gamma-butyrolactone (GBL) and Zn(BF4)2·xH2O to resolve these intrinsic challenges. The systematic results confirm that water reactivity toward a Zn anode is minimized by forcing GBL solvents into the Zn2+ solvation shell and constructing a fluorinated interphase on the Zn anode surface via anion decomposition. Furthermore, NMR was selected as an auxiliary testing protocol to elevate and understand the role of electrolyte composition in building the interphase. The combined factors in synergy guarantee high Zn reversibility (average Coulombic efficiency is 99.74%), high areal capacity (55 mAh/cm2), and high Zn utilization (∼91%). Ultimately, these merits enable the Zn battery utilizing a VO2 cathode to operate smoothly over 5000 cycles with a low-capacity decay rate of ∼0.0083% per cycle and a 0.23 Ah VO2/Zn pouch cell to operate over 400 cycles with a capacity retention of 77.3%.

A High-Energy Four-Electron Zinc Battery Enabled by Evoking Full Electrochemical Activity in Copper Sulfide Electrode

The growing global demand for sustainable and cost-effective energy storage solutions has driven the rapid development of zinc batteries. Despite significant progress in recent years, enhancing the energy density of zinc batteries remains a crucial research focus. One prevalent strategy involves the development of high-capacity and/or high-voltage cathode materials. CuS, a commonly used electrode material, exhibits a two-electron transfer mechanism; however, the reduced sulfion lacks electrochemical activity and thereby limits its discharge capacity and redox potential. In this study, we activate a CuS cathode to form a high-valence Cu2+&S compound using a deep-eutectic-solvent (DES)-based electrolyte. The presence of Cl- in the DES-based electrolyte is crucial to the reversibility of the redox chemistry, and the liquid-phase-involved electrochemical process facilitates redox kinetics. A four-electron transfer pathway involving five reaction steps is identified for the CuS electrode, which unleashes the full electrochemical activity of the S element. Consequently, the full cell delivers a large discharge capacity of ∼800 mAh g-1 at 0.2 A g-1 and yields a high discharge plateau starting at 1.58 V, contributing to energy densities of up to 650 Wh kg-1 (based on CuS). This work offers a promising approach to developing high-energy zinc batteries.

Zn-ion Batteries: Charge Storing Mechanism and Development Challenges

Improving the energy share of renewable energy technologies is the only solution to reduce greenhouse gas emissions and air pollution. The high-performing green battery energy storage technologies are critical for storing energy to address the intermittent nature of renewable energy resources. In recent years, aqueous batteries, particularly Zn-ion batteries (ZIBs), have achieved and shown great potential for stationary energy storage systems owing to their low cost and safer operation. However, the practical applications of the ZIBs have significantly been impeded due to the gap between the breakthroughs achieved in academic research and industrial developments. The present review discusses the ZIB's advantages, possibilities, and shortcomings for stationary energy storage systems. The Review begins with a brief introduction to the ZIBs and their charge storage mechanisms based on the structural properties of cathode materials. The scientific and technical challenges that obstruct the commercialization of the ZIBs are discussed in detail concerning their impact on accelerating the utilization of the ZIBs for real-life applications. The final section highlights the outlook on research in this flourishing field.

Wide-Potential-Window Bimetallic Hydrated Eutectic Electrolytes with High-Temperature Resistance for Zinc-Ion Hybrid Capacitors

Aqueous zinc-ion hybrid capacitors (ZHCs) are considered ideal energy-storage devices. However, the common aqueous Zn2+ -containing electrolytes used in ZHCs often cause parasitic reactions during charging-discharging owing to free water molecules. Hydrated eutectic electrolytes (HEEs) that bind water molecules through solvation shells and hydrogen bonds can be applied at high temperatures and within a wide potential window. This study reports a novel bimetallic HEE (ZnK-HEE), consisting of zinc chloride, potassium chloride, ethylene glycol, and water, which enhances the capacity and electrochemical reaction kinetics of ZHCs. The bimetallic solvation shell in ZnK-HEE is studied by molecular dynamics and density functional theory, confirming its low step-by-step desolvation energy. A Zn//activated carbon ZHC in ZnK-HEE shows a high operating voltage of 2.1 V, along with an ultrahigh capacity of 326.9 mAh g-1 , power density of 2099.7 W kg-1 , and energy density of 343.2 Wh kg-1 at 100 °C. The reaction mechanisms of charging-discharging process are investigated by ex situ X-ray diffraction. This study reports a promising electrolyte for high-performance ZHCs, which exhibits high-temperature resistance and is operable within a wide potential window.

Conversion-Type Cathode Materials for Aqueous Zn Metal Batteries in Nonalkaline Aqueous Electrolytes: Progress, Challenges, and Solutions

Aqueous Zn metal batteries are attractive as safe and low-cost energy storage systems. At present, due to the narrow window of the aqueous electrolyte and the strong reliance of the Zn2+ ion intercalated reaction on the host structure, the current intercalated cathode materials exhibit restricted energy densities. In contrast, cathode materials with conversion reactions can promise higher energy densities. Especially, the recently reported conversion-type cathode materials that function in nonalkaline electrolytes have garnered increasing attention. This is because the use of nonalkaline electrolytes can prevent the occurrence of side reactions encountered in alkaline electrolytes and thereby enhance cycling stability. However, there is a lack of comprehensive review on the reaction mechanisms, progress, challenges, and solutions to these cathode materials. In this review, four kinds of conversion-type cathode materials including MnO2 , halogen materials (Br2 and I2 ), chalcogenide materials (O2 , S, Se, and Te), and Cu-based compounds (CuI, Cu2 O, Cu2 S, CuO, CuS, and CuSe) are reviewed. First, the reaction mechanisms and battery structures of these materials are introduced. Second, the fundamental problems and their corresponding solutions are discussed in detail in each material. Finally, future directions and efforts for the development of conversion-type cathode materials for aqueous Zn batteries are proposed.

Challenges and Strategies in the Development of Zinc-Ion Batteries

Although promising, the practical use of zinc-ion batteries (ZIBs) remains plagued with uncontrollable dendrite growth, parasitic side reactions, and the high intercalation energy of divalent Zn²⁺ ions. Hence, much work has been conducted to alleviate these issues to maximize the energy density and cyclic life of the cell. In this holistic review, the mechanisms and rationale for the stated challenges shall be summarized, followed by the corresponding strategies employed to mitigate them. Thereafter, a perspective on present research and the outlook of ZIBs would be put forth in hopes to enhance their electrochemical properties in a multipronged approach.

Hybrid Electrolyte Design for High-Performance Zinc-Sulfur Battery

Rechargeable aqueous Zn/S batteries exhibit high capacity and energy density. However, the long-term battery performance is bottlenecked by the sulfur side reactions and serious Zn anode dendritic growth in the aqueous electrolyte medium. This work addresses the problem of sulfur side reactions and zinc dendrite growth simultaneously by developing a unique hybrid aqueous electrolyte using ethylene glycol as a co-solvent. The designed hybrid electrolyte enables the fabricated Zn/S battery to deliver an unprecedented capacity of 1435 mAh g^-1 and an excellent energy density of 730 Wh kg^-1 at 0.1 Ag^-1 . In addition, the battery exhibits capacity retention of 70% after 250 cycles even at 3 Ag^-1 . Moreover, the cathode charge-discharge mechanism studies demonstrate a multi-step conversion reaction. During discharge, the elemental sulfur is sequentially reduced by Zn to S^2- ( S 8 → S x 2 - → S 2 2 - + S 2 - ) ${{\rm{S}}_8}{\bm{ \to }}{\rm{S}}_{\rm{x}}^{2{\bm{ - }}}{\bm{ \to }}{\rm{S}}_2^{2{\bm{ - }}}{\bm{ + }}{{\rm{S}}^{2{\bm{ - }}}})$ , forming ZnS. On charging, the ZnS and short-chain polysulfides will oxidize back to elemental sulfur. This electrolyte design strategy and unique multi-step electrochemistry of the Zn/S system provide a new pathway in tackling both key issues of Zn dendritic growth and sulfur side reactions, and also in designing better Zn/S batteries in the future.

Interface Engineering of Zinc Electrode for Rechargeable Alkaline Zinc-Based Batteries

Rechargeable aqueous zinc-based batteries have gained considerable interest because of their advantages of high theoretical capacity, being eco-friendly, and cost effectiveness. In particular, zinc-based batteries with alkaline electrolyte show great promise due to their high working voltage. However, there remain great challenges for the commercialization of the rechargeable alkaline zinc-based batteries, which are mainly impeded by the limited reversibility of the zinc electrode. The critical problems refer to the dendrites growth, electrode passivation, shape change, and side reactions, affecting discharge capacity, columbic efficiency, and cycling stability of the battery. All the issues are highly associated with the interfacial properties, including both electrons and ions transport behavior at the electrode interface. Herein, this work concentrates on the fundamental electrochemistry of the challenges in the zinc electrode and the design strategies for developing high-performance zinc electrodes with regard to optimizing the interfaces between host and active materials as well as electrode and electrolyte. In addition, potential directions for the investigation of electrodes and electrolytes for high-performance zinc-based batteries are presented, aiming at promoting the development of rechargeable alkaline zinc-based batteries.

Eutectic Electrolytes Chemistry for Rechargeable Zn Batteries

Rechargeable zinc batteries (RZBs) have proved to be promising candidates as an alternative to lithium-ion batteries due to their low cost, inherent safety, and environmentally benign features. While designing cost-effective electrolyte systems with excellent compatibility with electrode materials, high energy/power density as well as long life-span challenge their further application as grid-scale energy storage devices. Eutectic electrolytes as a novel class of electrolytes have been extensively reported and explored taking advantage of their feasible preparation and high tunability. Recently, some perspectives have summarized the development and application of eutectic electrolytes in metal-based batteries, but their infancy requires further attention and discussion. This review systematically presents the fundamentals and definitions of eutectic electrolytes. Besides, a specific classification of eutectic electrolytes and their recent progress and performance on RZB fields are introduced as well. Significantly, the impacts of various composing eutectic systems are disserted for critical RZB chemistries including attractive features at electrolyte/electrode interfaces and ions/charges transport kinetics. The remaining challenges and proposed perspectives are ultimately induced, which deliver opportunities and offer practical guidance for the novel design of advanced eutectic electrolytes for superior RZB scenarios.

Zn-Based Deep Eutectic Solvent as the Stabilizing Electrolyte for Zn Metal Anode in Rechargeable Aqueous Batteries

Owing to its low cost and high safety, metallic zinc has received considerable attention as an anode material for zinc aqueous batteries (ZIBs). However, the Zn metal instability as a result ultrafast of obstinate dendrite formation, free-water-induced parasite reactions, and corrosive electrolytes has detrimental effects on the implementation of ZIBs. We present an alternative stable electrolyte for ZIBs based on a zinc chloride/ethylene glycol deep eutectic solvent (DES). This electrolyte consists of abundant low-cost materials and a utilizable Zn2+ concentration of approximately 1 M. It combines the advantages of the aqueous and DES media to provide safe and reversible Zn plating/stripping with a two-fold increase in the cycling life compared to that of conventional aqueous electrolytes. With these advantages, the Zn symmetric cell operates at 0.2 mA cm-2 for 300 h. Due to its high efficiency and compositional versatility, this electrolyte enables the investigation of a non-aqueous electrolyte family for ZIBs that fulfill grid-scale electrical energy storage requirements.