Anticatalytic Strategies to Suppress Water Electrolysis in Aqueous Batteries

MOF-based nanomaterials for advanced aqueous-ion batteries

Metal-organic frameworks (MOFs)-based nanomaterials have great potential in the field of electrochemical energy storage due to their abundant pore size, high specific surface area, controllable structure and porosity, and homogeneous metal center. MOFs complexes and derivatives not only inherit the original morphology characteristics of MOFs but also provide excellent electrochemical performance. Batteries operating in aqueous electrolytes are cheaper, safer, and have higher ionic conductivity than those operating in conventional organic electrolytes. Therefore, it is useful to summarize the MOFs that should be used for aqueous electrochemical energy storage devices. This manuscript firstly introduces the composition and energy storage mechanism of aqueous Li/Na/Zn ion batteries. In addition, a detailed review of the development of MOFs-based nanomaterials and their commonly used characterization under aqueous conditions is presented. The relationship between the structure and composites of MOFs-based nanomaterials and electrochemical performance is highlighted. The applications of MOFs composites in aqueous batteries in terms of electrode materials and electrolytes are presented and summarized. Finally, research directions and perspectives for MOFs-based nanomaterials in advanced aqueous batteries are presented.

Concentrated Chloride Electrolytes Enable High-Efficiency, Long-Cycling, and Dendrite-Free Aqueous Trivalent Antimony Batteries

Aqueous trivalent metal batteries are promising energy storage systems, which can leverage unique three-electron redox reactions to deliver high capacity and high energy. Among them, antimony (Sb) stands out with a high capacity (660 mAh g-1), abundant availability, and low cost. However, the severe Sb3+ hydrolysis reaction drastically hinders the development of aqueous antimony batteries. Herein, we address this issue by employing a concentrated lithium chloride electrolyte, which stabilizes reactive Sb3+ ions via forming robust antimony-chloride complexes. This approach effectively mitigates hydrolysis and achieves highly reversible Sb plating behavior, leading to high efficiency (99.7%-99.8%), long lifespan (7300 h, 10 months), and uniform spherical deposition morphology. When paired with a manganese dioxide (MnO2) cathode, the Sb‖MnO2 battery demonstrates a high capacity of 309 mAh g-1 and exceptional cycling stability of 50 000 cycles (∼70% retention). Additionally, Sb shows promise as a high-capacity cathode, which can integrate with low-potential zinc into novel dual-metal plating batteries with long cycling life (4,000 h). This work not only deepens our fundamental understanding of trivalent Sb3+ redox chemistry but also opens new opportunities to stabilize hydrolysable and high-charge-density cations for multivalent battery applications.

Achieving a Highly Reversible Four-Electron Redox of S/Cu2S for Aqueous Zn/S─Cu Battery

The combination of sulfur (S) cathode with Cu2+/Cu+ redox carriers has been considered as a promising cathode for the next-generation aqueous energy storage device due to the high specific capacity (two-step four-electron conversion), intrinsic safety, and low cost. Nevertheless, the unsatisfactory cycling stability of a sulfur-copper (S-Cu) cathode hinders its practical application. Herein, the two-step four-electron conversions are first decoupled in our study, identifying the inferior conversion reversibility between the intermedia CuS and the final product S as the primary cause for deteriorating cycling capability. Concerning the different hydrated dimensions of the Cu(H2O)6 2+ and Cu(H2O)2 +, the strategy of "spatial confinement" is proposed to alter the oxidation path of Cu2S and prevent the formation of the intermedia CuS. To ensure efficient S incorporation into the spatially confined carbon matrix, selenium (Se) is employed to "cut" the S8 into short-chain molecules. Consequently, the well-designed S-Cu cathode achieves a one-step four-electron reaction and exhibits superior electrochemical stability with an average capacity attenuation of 0.034% during 700 cycles. Our study provides an in-depth understanding of the conversion mechanism for the S-Cu cathode within the spatial-confinement environment and renders valuable insights for developing advanced conversion-type cathodes in aqueous energy-storage device.

Interface Preconstruction Enables Robust Passivation of the Ah-Level Aqueous Li-ion Batteries

The solid electrolyte interphase (SEI) offers effective passivation on the anode for aqueous lithium-ion batteries (ALIBs). Conventional passivation in ALIBs mainly relies on the LiF-contained SEI originating from anion reduction in the electrolyte. However, such SEI formation is a competitive reaction negatively impacted by the parasitic hydrogen evolution reaction (HER), resulting in high Li+ irreversible consumption and imperfect bare flaws. To address this issue, we propose preconstructing an artificial interphase by introducing a multifunctional interface additive CsF to build superior passivation on the anode in ALIBs. CsF first undergoes a displacement reaction with LiTFSI from the fresh electrolyte to form the LiF in situ on the interface of the anode before the cycles, avoiding the extra Li+ irreversible consumption. Meanwhile, we uncover that the dissolving Cs+ in the electrolyte can destroy the hydrogen bond network of the water to lower water activity on the anode interface and strongly interact with TFSI- to form the cation-anion complex, facilitating the anion proximity to the anode interface. The anion reduction based on the artificial interphase can finally help achieve the robust SEI in ALIBs. Such passivation stabilizes the aqueous electrolyte, significantly suppressing the side reaction of the HER that allows ALIBs to obtain a superior long life above 2000 cycles. The ampere-hour-level (Ah-level) pouch cell achieves an energy density of 57 Wh/kg and 176 Wh/L with high energy efficiency (∼94%).

Rechargeable Cadmium Metal Batteries Enabled by an Aqueous CdSO4 Electrolyte

Rechargeable aqueous cadmium (Cd) metal batteries enabled by the Cd plating and stripping behaviors of anode show great promise for next-generation energy storage applications due to the superior corrosion resistance, high specific capacity (476.5 mAh g-1), suitable redox potential (-0.4 V vs standard hydrogen electrode), and cost-effectiveness of Cd anode. However, this field is still in its infancy, with limited scientific exploration and numerous unresolved challenges. Therefore, to bridge the existing research gap in the field of Cd metal batteries, this study conducted a comprehensive investigation into the electrochemical performance of the Cd metal, utilizing a stable and low-cost aqueous CdSO4 solution as an electrolyte. It is revealed that the solvation structure of Cd2+ transitions from [Cd(H2O)9]2+ to [Cd(H2O)6(SO4)3]4- as the CdSO4 concentration increases from 0.5 to 3 M. This transition reduces the activity of H2O around Cd2+, promotes charge transfer, and enhances desolvation/solvation kinetics during Cd plating/stripping processes. Consequently, when the concentration of CdSO4 reached nearly saturated 3 M, the Cd anode presents optimal corrosion-resistant and dendrite-free capabilities, reflecting in durable Cd plating/stripping performance for over 2000 h with a high Coulombic efficiency of 99.7%. Furthermore, the Cd//V2O5 full cell achieves an ultralong cycle life, maintaining stable performance for 30,000 cycles with minimal capacity degradation. The high reversibility of the Cd anode and the exceptional performance of the full cell affirm the potential of aqueous CdSO4 electrolyte as a foundation for the future development of Cd metal batteries, offering profound insights and guidance for advancing aqueous energy storage technology.

A Hydrogel Electrolyte-Based Zn-CO2 Battery with Improved Life

Environmental-friendly aqueous Zn-CO2 batteries present bifunctional potentials of achieving carbon neutrality and energy storage. Nonetheless, anode corrosions derived from H2O molecules and high risks of volatilization and leakage hinder the advancement of Zn-CO2 batteries. In this work, polyvinyl alcohol (PVA)-based hydrogel electrolyte with fast ion diffusion kinetics, high mechanical strength, and flexibility is developed to replace liquid electrolyte. Since hydroxyl radicals in the polymer chain can interact with H2O and Zn2+, electrode corrosion from free H2O and active H2O around Zn2+ is significantly inhibited, facilitating the uniform deposition of Zn2+ cations. The introduction of an ionic liquid plasticizer further enhances the interaction between Zn2+ and polymer backbone, as well as amorphous extent of the electrolyte. The hydrogel electrolyte possesses adequate self-healing ability, whose ionic conductivity reaches 7.95 × 10-3 S cm-1. The symmetric Zn metal batteries containing the electrolyte remain steady for >2000 h under different current densities. Furthermore, the Zn-CO2 battery based on Ru nanoparticles cathode and hydrogel electrolyte realizes a discharge capacity of 6028 mAh g-1 and stable cyclicity for 90 times. The reaction path of hydrogel electrolyte-based Zn-CO2 battery is that CO2 is reduced to ZnCO3 and C species followed by reversible decomposition of discharge products on recharge.

Strong Metal-Support Interaction to Invert Hydrogen Evolution Overpotential of Cu Coating for High-Coulombic-Efficiency Stable Zn Anode in Aqueous Zn-Ion Batteries

Cu exhibits strong zincophilic properties but suffers from a much lower hydrogen evolution reaction (HER) overpotential compared to Zn, which significantly undermines the coulombic efficiency and stability of the Zn anode. Consequently, Cu is regarded as an unsuitable coating for Zn anode protection. In this work, the HER overpotential of Cu versus Zn is inverted through strong metal-support interaction (SMSI) to modify the electronic structure of Cu. This interaction facilitates electron transfer, enriching positive charge and slowing down the adsorption kinetics of H+ on the Cu surface. As a result, at very low current densities of 0.2 and 2 mA cm⁻2, the Cu-coated-Zn||Cu cell achieves exceptionally high coulombic efficiencies of 99.11% and 99.91% over 2500 and 1600 h of cycling (100% depth of discharge (DOD)), which remarkably surpasses the performance of Zn anode protective coatings all reported. Moreover, a 1 Ah soft-packed full battery is not bulged and retains 94.7% of its initial capacity after 150 cycles. This study overturns the conventional concept by leveraging SMSI to tune the electronic structure, reverses the HER overpotential, and expands the range of viable metals for anode protection in aqueous metal batteries.

Ionic Liquid-Based Electrolyte with Multiple Hydrogen Bonding Network Enabling High-Voltage Stable Proton Batteries Across Wide Temperature Range

Proton batteries are strong contender for next-generation energy storage due to their high safety and rapid response. However, the narrow electrochemical window of acidic aqueous electrolytes limits their energy density and stability. Here, an ionic liquid (IL)-based electrolyte (EMImOTf-H3PO4) containing H3PO4 in polar IL solvent 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMImOTf) is developed for stable high-voltage energy storage. H3PO4 serving as a proton source interacts with both EMIm+ and OTf-, forming an intricate hydrogen bonding network that effectively prevents electrolyte decomposition at high voltage. The half-cell in EMImOTf-H3PO4 electrolyte and pre-protonated vanadium hexacyanoferrate (H-VHCF) cathode demonstrates a 126% improvement in Coulombic efficiency over aqueous electrolytes at a current density of 1 A g-1. The fabricated PTCDA/MXene//EMImOTf-H3PO4//H-VHCF full battery achieves an operating voltage of 2 V at room temperature, surpassing currently reported values for proton batteries. After 30 000 cycles at 5 A g-1, the battery retains 86.1% of its initial capacity. It delivers an energy density of 87.5 Wh kg-1 and a power density of 30.6 kW kg-1 at room temperature, and can maintain stable operation across a temperature range of 110 °C (-60 ∼ 50 °C). These findings present new possibilities for proton batteries in all-weather grid-scale energy storage applications.

Molecular Micellar Aggregate Electrolytes Enable Durable Electrochemical Proton Storage

Proton electrochemistry holds eminent potential for developing high capacity and rate energy storage devices in the post-lithium era. However, the decomposition of water in acidic aqueous electrolytes causes electrode corrosion, leading to capacity fading. Herein, we report a judicious design of molecular micellar aggregates as non-aqueous electrolytes for stable and high-voltage electrochemical proton storage. The key to our strategy lies in introducing cetyltrimethylammonium bromide (CTAB), forming micelles to improve the miscibility of acetonitrile (ACN) and H3PO4, afford channel for proton transport, and electrostatically interact with phosphate ions of H3PO4 to further promote proton transport. Such aggregates impart rapid and stable electrochemical proton storage with a widened operating voltage (1.8 V vs. 1.5 V in aqueous electrolyte). By optimizing CTAB content, proton transport can be enhanced. Asymmetric full proton battery using the optimal CTAB electrolyte achieves a maximum energy density of 102.8 Wh kg-1 and a maximum power density of 10.1 kW kg-1. Our simple yet robust route to micellar aggregate electrolytes enables stable proton storage, underscoring its potential for grid-scale energy storage, emergency power supplies, and portable electronics.

Unraveling the Versatility of Carbon Black - Polylactic Acid (CB/PLA) 3D-Printed Electrodes via Sustainable Electrochemical Activation

Commercially available conductive filaments are not designed for electrochemical applications, resulting in 3D printed electrodes with poor electrochemical behavior, restricting their implementation in energy and sensing technologies. The proper selection of an activation method can unlock their use in advanced applications. In this work, rectangular electrodes made from carbon black - polylactic acid (CB/PLA) filament are 3D printed with different layouts (grid and compact) and then activated using a highly reproducible eco-compatible electrochemical (EC) treatment. The electrodes are characterized for their morphological, structural, and electrochemical features to obtain insights into the material properties and functionality. Furthermore, the influence of the electrode layout as well as the activation conditions are studied aiming to provide a better understanding of the mechanism driving the electrochemical behavior of the electrodes. The EC activation enhances the electrochemical performance, provides a uniform electrochemical activity in the electrode's interface and allows the manipulation of the electrochemical properties of 3D printed electrodes by adjusting the duration of the treatment. CB/PLA electrodes offer a wide stable potential window that benefits their use in water-based electrochemical applications. Thus, their suitability for Zn-ion batteries and electrochemical sensing is explored, followed by their application in hydroquinone determination in water samples.

Polyelectrolyte Membrane Enables Highly Reversible Zinc Battery Chemistry via Immobilizing Anion and Stabilizing Water

The integration of water-based electrolytes into zinc-ion batteries encounters challenges due to the limited voltage window of water, interfacial side reactions of mobile counterions, and the growth of zinc metal (Zn0) dendrites during charge. In this study, we introduce a nonfluorinated, cation-conducting polyelectrolyte membrane (PEM) designed to alleviate these challenges by suppressing the reactivities of both water and counterions. This PEM forms hydrogen bonds with water molecules through its proton-accepting side chains, thus shifting the lowest unoccupied molecular orbital (LUMO) energy of water from -0.37 to -0.14 eV and inducing a negative shift in the onset potential for hydrogen evolution by 110 mV. Additionally, it immobilizes the counteranions onto the polymer backbones via covalent bonding, hence making the Zn2+ transference number nearly unity (0.96). Meanwhile, the high modulus PEM establishes a solid-state diffusion barrier to homogenize the interfacial Zn2+ flux, leading to 3D in-plane interfacial Zn2+ diffusion and compact Zn0 plating within the (002) plane. Atomic resolution scanning transmission electron microscopy (STEM) reveals corrosion-free Zn0 deposition without electrolyte degradation, while operando transition X-ray microscopy (TXM) further illustrates the real-time dendrite-free Zn0 plating process at 5 mA/cm2. Consequently, the unique properties of this water-binding and anion-tethering PEM enable enhanced electrochemical performance without employing highly fluorinated and expensive anions. This PEM demonstrates a durability of 3800 h in Zn0-Zn0 symmetric cells and a lifetime of 6000 cycles in Zn0-LiV3O8 full cells.

Accelerating Ion Desolvation via Bioinspired Ion Channel Design in Nonconcentrated Aqueous Electrolytes

In aqueous-based electrochemical energy storage devices, uncontrolled hydrolysis of water at the electrochemical interfaces limits the application of such aqueous batteries or supercapacitors in business. The "water-in-salt" design is a valid strategy to broaden the electrochemical stability window in aqueous electrolytes, but drawbacks such as high manufacturing cost, high electrolyte viscosity, etc., also hinder its development. Here, inspired by biological ion channels in cell membranes, we propose an effective approach to engineer the electrode surface, inducing the desolvation of hydrated ions at the electrochemical interface and inhibiting water decomposition in nonconcentrated electrolytes. The biological engineering strategy enables the induction of controlled desolvation and accelerates the transportation of hydrated ions, e.g., potassium. The subnanometer design (0.8 nm) forces the hydrated potassium ions to shed their solvation shell with a hydration number of only 0.3, while the electrostatic interactions between the pore groups and the potassium ions facilitate their transport. The Zn||Zn cells demonstrate a stable cycling lifespan of over 1000 h at 1 mA cm-2/10 mAh cm-2. This work sheds new light on regulating the electrochemical interfaces in low-concentration aqueous electrolytes for designing aqueous-based energy storage devices.

Towards Highly Stable Sn2+ Electrolyte for Aqueous Tin Batteries Using Hydroquinone Antioxidant

Sn redox chemistry in aqueous acidic electrolyte was characterized with high reversibility and kinetics, which is considered as competitive anode material for aqueous batteries. Unfortunately, divalent Sn2+ is unstable in aqueous electrolyte. It was revealed that Sn2+ is easy to be oxidized to tetravalent Sn4+ by dissolved oxygen and then forms precipitate through hydrolysis process, leading to serious performance decay. Here, hydroquinone (HQ) was employed as antioxidant to prevent Sn2+ oxidation. With addition of HQ, Sn2+ electrolyte can maintain transparent and colorless after long-time exposure under air atmosphere, while large amounts of sediment were observed in the electrolyte without HQ. Moreover, HQ can adsorb on electrode surface to regulate Sn deposition and suppress Sn dendrite formation. With the difunctional HQ additive, Sn anode shows a stable cycling performance more than 1500 cycles with a high average coulombic efficiency (CE) of 99.9 %. And the organic||Sn cell shows high cycling stability for 10,000 cycles with 71 % capacity retention. The hydroquinone antioxidant strategy provides a facile and cost-effective way to develop highly stable Sn2+-based electrolyte.

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.

Band Engineering of Mn-P Alloy Enables HER-suppressed Aqueous Manganese Ion Batteries

Aqueous manganese ion batteries hold potential for stationary storage applications owing to their merits in cost, energy density, and environmental sustainability. However, the formidable challenge is the instability of metallic manganese (Mn) anodes in aqueous electrolytes due to severe hydrogen evolution reaction (HER), which is more serious than the commonly studied Zn metal anodes. Moreover, the mechanism of HER side reactions has remained unclear. Herein, we design a series of Mn-P alloying anodes by precisely regulating their energy band structures to mitigate the HER issue. It is found that the serious HER primarily originates from the spontaneous Mn-H2O reaction driven by the excessively high HOMO energy level of Mn, rather than electrocatalytic water splitting. Owing to a reduced HOMO energy level and enhanced electron escape work function, the MnP anode achieves an evidently enhanced cycle durability (over 1000 hours at a high current density of 5 mA cm-2). The MnP||AgVO full cell with an N/P ratio of 4 exhibits better rate capability and extended cycle life (7000 cycles) with minimal capacity degradation than the cell using metallic Mn anode (less than 100 cycles). This study provides a practical approach for developing highly durable aqueous Mn ion batteries.

Theoretical insights and design of MXene for aqueous batteries and supercapacitors: status, challenges, and perspectives

Aqueous batteries and supercapacitors are promising electrochemical energy storage systems (EESSs) due to their low cost, environmental friendliness, and high safety. However, aqueous EESS development faces challenges like narrow electrochemical windows, irreversible dendrite growth, corrosion, and low energy density. Recently, two-dimensional (2D) transition metal carbide and nitride (MXene) have attracted more attention due to their excellent physicochemical properties and potential applications in aqueous EESSs. Understanding the atomic-level working mechanism of MXene in energy storage through theoretical calculations is necessary to advance aqueous EESS development. This review comprehensively summarizes the theoretical insights into MXene in aqueous batteries and supercapacitors. First, the basic properties of MXene, including structural composition, experimental and theoretical synthesis, and advantages in EESSs are introduced. Then, the energy storage mechanism of MXene in aqueous batteries and supercapacitors is summarized from a theoretical calculation perspective. Additionally, the theoretical insights into the side reactions and stability issues of MXene in aqueous EESSs are emphasized. Finally, the prospects of designing MXene for aqueous EESSs through computational methods are given.

An Mn-Enriched Interfacial Layer for Reversible Aqueous Mn Metal Batteries

Aqueous manganese metal batteries have emerged as promising candidates for stationary storage due to their natural abundance, safety, and high energy density. However, the high chemical reactivity and sluggish migration kinetics of the Mn metal anode induce a severe hydrogen evolution reaction (HER) and dendrite formation, respectively. The situation deteriorates in the low-concentration electrolyte especially. Here, we propose a novel approach to construct an Mn-enriched interfacial layer (Mn@MIL) on the Mn metal anode surface to address these challenges simultaneously. The Mn@MIL acts as a physical barrier to not only suppress HER but also accelerate the Mn2+ diffusion kinetics through the Mn2+ saturated interfacial layer to inhibit dendrite growth. Therefore, in the low-concentration electrolyte (1 M MnCl2), the Mn||Mn symmetric cells and Mn||V2O5 full cells with high mass loading demonstrate promising cycling stability with minimal polarization and parasitic reactions, making them more suitable for practical applications in a smart grid.

Heat-Resistant Covalent Organic Framework (COF) PVA-Hybridized Gel Electrolyte for the Preparation of Dendrite-Free Zinc-Ion Batteries

High-performance zinc-ion batteries (ZIBs) have attracted a great deal of attention due to their high theoretical capacity and high level of safety. Herein, we propose a covalent organic framework (COF) hybrid poly(vinyl alcohol) (PVA)-based gel electrolyte, which can induce the uniform deposition of Zn2+ and achieve dendrite-free formation. By grafting sulfonic acid groups on the surface of COFs to absorb Zn2+, we can strengthen the interaction between strong ions and dipoles in the electrolyte, improve the ionic conductivity, and achieve stable Zn2+ electroplating and stripping. Due to the good thermal stability of the COF material itself, the gel electrolyte hybrid with the PVA hydrogel shows high mechanical strength and good heat resistance and is capable of stable cycling for >1000 h at 50 °C, with a capacity retention rate of ≤75%. This study provides a new approach for developing high-temperature-resistant and highly stable dendrite-free ZIBs.

A Bio-Inspired Multifunctional Hydrogel Network with Toughly Interfacial Chemistry for Dendrite-Free Flexible Zinc Ion Battery

Flexible and high-performance aqueous zinc-ion batteries (ZIBs), coupled with low cost and safe, are considered as one of the most promising energy storage candidates for wearable electronics. Hydrogel electrolytes present a compelling alternative to liquid electrolytes due to their remarkable flexibility and clear advantages in mitigating parasitic side reactions. However, hydrogel electrolytes suffer from poor mechanical properties and interfacial chemistry, which limits them to suppressed performance levels in flexible ZIBs, especially under harsh mechanical strains. Herein, a bio-inspired multifunctional hydrogel electrolyte network (polyacrylamide (PAM)/trehalose) with improved mechanical and adhesive properties was developed via a simple trehalose network-repairing strategy to stabilize the interfacial chemistry for dendrite-free and long-life flexible ZIBs. As a result, the trehalose-modified PAM hydrogel exhibits a superior strength and stretchability up to 100 kPa and 5338 %, respectively, as well as strong adhesive properties to various substrates. Also, the PAM/trehalose hydrogel electrolyte provides superior anti-corrosion capability for Zn anode and regulates Zn nucleation/growth, resulting in achieving a high Coulombic efficiency of 98.8 %, and long-term stability over 2400 h. Importantly, the flexible Zn//MnO2 pouch cell exhibits excellent cycling performance under different bending conditions, which offers a great potential in flexible energy-related applications and beyond.

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

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

Regulation of Molecular Microheterogeneity in Electrolytes Enables Ampere-Hour-Level Aqueous LiMn2O4||Li4Ti5O12 Pouch Cells

Aqueous batteries are attractive due to their high safety and fast reaction kinetics, but the narrow electrochemical stability window of H2O limits their applications. It is a big challenge to broaden the electrochemical operation window of aqueous electrolytes while retaining fast reaction kinetics. Here, a new organic aqueous mixture electrolyte of manipulatable (3D) molecular microheterogeneity with H2O-rich and H2O-poor domains is demonstrated. H2O-poor domains molecularly surround the reformed microclusters of H2O molecules through interfacial H-bonds, which thus not only inhibit the long-range transfer of H2O but also allow fast and consecutive Li+ transport. This new design enables low-voltage anodes reversibly cycling with aqueous-based electrolytes and high ionic conductivity of 4.5 mS cm-1. LiMn2O4||Li4Ti5O12 full cells demonstrate excellent cycling stability over 1000 cycles under various C rates and a low temperature of -20 °C. 1 Ah pouch cell delivers a high energy density of 79.3 Wh kg-1 and high Coulombic efficiency of 99.4% at 1 C over 200 cycles. This work provides new insights into the design of electrolytes based on the molecular microheterogeneity for rechargeable batteries.

An anti-corrosive cellulose nanocrystal/carbon nanotube derived Zn anode interface for dendrite-free aqueous Zn-ion batteries

Notorious zinc dendrite growth and hydrogen precipitation reactions disrupt the galvanic/stripping process at the electrolyte/electrode interface, which seriously affects the cycling stability of zinc anodes in aqueous zinc ion batteries. To improve the stability and reversibility of zinc anodes, we report an artificial SEI consisting of hydrophobic carbon nanocrystals and highly conductive carbon nanotube networks. This interfacial hydrophobicity effectively excludes free water from the surface of the zinc anode, which prevents water erosion and reduces the interfacial side reactions, resulting in a significant improvement in the cycling stability and coulombic efficiency of Zn plating/stripping. Benefiting from the reversible proton storage and fast desolvation kinetic behavior of the CNC/CNT interlayer, the stable cycling time of Zn/Zn symmetric batteries exceeds 700 h even at a high current density of 5 mA cm-2. The assembled CNC/CNT@Zn‖V2O5 full cell maintains a high capacity of 101.1 mA h g-1 after 5000 cycles (1.0 mA g-1). This study opens up a new area for expanding the use of organic compounds in zinc anode protection and offers a promising strategy for accelerating the development of aqueous zinc-ion batteries.

Mastering Proton Activities in Aqueous Batteries

Advanced aqueous batteries are promising solutions for grid energy storage. Compared with their organic counterparts, water-based electrolytes enable fast transport kinetics, high safety, low cost, and enhanced environmental sustainability. However, the presence of protons in the electrolyte, generated by the spontaneous ionization of water, may compete with the main charge-storage mechanism, trigger unwanted side reactions, and accelerate the deterioration of the cell performance. Therefore, it is of pivotal importance to understand and master the proton activities in aqueous batteries. This Perspective comments on the following scientific questions: Why are proton activities relevant? What are proton activities? What do we know about proton activities in aqueous batteries? How do we better understand, control, and utilize proton activities?

Flexible Aqueous Cr-Ion Hybrid Supercapacitors with Remarkable Electrochemical Properties in all Climates

The integration of hostless battery-like metal anodes for hybrid supercapacitors is a realistic design method for energy storage devices with promising future applications. With significant Cr element deposits on Earth, exceptionally high theoretical capacity (1546 mAh g-1), and accessible redox potential (-0.74 V vs. reversible hydrogen electrode) of Cr metals, the design of Cr anodes has rightly come into our focus. This work presents a breakthrough design of a flexible Cr-ion hybrid supercapacitor (CHSC) based on a porous graphitized carbon fabric (PGCF) substrate prepared by K2FeO4 activation. In the CHSC device, PGCF acts as both a current collector and cathode material due to its high specific surface area and superior conductivity. The use of a highly concentrated LiCl-CrCl3 electrolyte with high Cr plating/stripping efficiency and excellent antifreeze properties enables the entire PGCF-based CHSC to achieve well-balanced performance in terms of energy density (up to 1.47 mWh cm-2), power characteristics (reaching 9.95 mW cm-2) and durability (95.4 % capacity retention after 30,000 cycles), while realizing it to work well under harsh conditions of -40 °C. This work introduces a new concept for low-temperature energy storage technology and confirms the potential application of Cr anodes in hybrid supercapacitors.

In Situ Induced Interface Engineering in Hierarchical Fe3O4 Enhances Performance for Alkaline Solid-State Energy Storage

Rechargeable aqueous batteries adopting Fe-based materials are attracting widespread attention by virtue of high-safety and low-cost. However, the present Fe-based anodes suffer from low electronic/ionic conductivity and unsatisfactory comprehensive performance, which greatly restrict their practicability. Concerning the principle of physical chemistry, fabricating electrodes that could simultaneously achieve ideal thermodynamics and fast kinetics is a promising issue. Herein, hierarchical Fe3O4@Fe foam electrode with enhanced interface/grain boundary engineering is fabricated through an in situ self-regulated strategy. The electrode achieves ultrahigh areal capacity of 31.45 mA h cm-2 (50 mA cm-2), good scale application potential (742.54 mA h for 25 cm2 electrode), satisfied antifluctuation capability, and excellent cycling stability. In/ex situ characterizations further validate the desired thermodynamic and kinetic properties of the electrode endowed with accurate interface regulation, which accounts for salient electrochemical reversibility in a two-stage phase transition and slight energy loss. This work offers a suitable strategy in designing high-performance Fe-based electrodes with comprehensive inherent characteristics for high-safety large-scale energy storage.

Triflate anion chemistry for enhanced four-electron zinc-iodine aqueous batteries

I+ hydrolysis, sluggish iodine redox kinetics and the instability of Zn anodes are the primary challenges for aqueous four-electron zinc-iodine batteries (4eZIBs). Herein, the OTf- anion chemistry in aqueous electrolyte is essential for developing advanced 4eZIBs. It is elucidated that OTf- anions establish weak hydrogen bonds (H bonds) with water to stabilize I+ species while optimizing a water-lean Zn2+ coordination structure to mitigate Zn dendrites and corrosion. Moreover, the interaction of the OTf- anions with the iodine species results in an increased equilibrium average intermolecular bond length of the iodine species, facilitating the 4e redox kinetics of iodine with improved reversibility.

Biomass Solid-State Electrolyte with Abundant Ion and Water Channels for Flexible Zinc-Air Batteries

Flexible zinc-air batteries are the leading candidates as the next-generation power source for flexible/wearable electronics. However, constructing safe and high-performance solid-state electrolytes (SSEs) with intrinsic hydroxide ion (OH-) conduction remains a fundamental challenge. Herein, by adopting the natural and robust cellulose nanofibers (CNFs) as building blocks, the biomass SSEs with penetrating ion and water channels are constructed by knitting the OH--conductive CNFs and water-retentive CNFs together via an energy-efficient tape casting. Benefiting from the abundant ion and water channels with interconnected hydrated OH- wires for fast OH- conduction under a nanoconfined environment, the biomass SSEs reveal the high water-uptake, impressive OH- conductivity of 175 mS cm-1 and mechanical robustness simultaneously, which overcomes the commonly existed dilemma between ion conductivity and mechanical property. Remarkably, the flexible zinc-air batteries assemble with biomass SSEs deliver an exceptional cycle lifespan of 310 h and power density of 126 mW cm-2. The design methodology for water and ion channels opens a new avenue to design high-performance SSEs for batteries.

Investigation of Fe-Ni Battery/Module for Grid Service Duty Cycles

Iron-nickel (Fe-Ni) batteries are renowned for their durability and resilience against overcharging and operating temperatures. However, they encounter challenges in achieving widespread adoption for energy storage applications due to their low efficiency and the need for regular maintenance and electrolyte replacement, which adds to maintenance costs. This study evaluates and demonstrates the capabilities of Fe-Ni batteries for participating in grid energy storage applications. Stable performance was observed frequency regulation (FR) testing at 100% and 50% state of charge (SOC)s, while at 50% SOC, there was a 14% increase in efficiency compared to 100% SOC. Although 25% SOC achieved higher efficiency, limited cyclability was observed due to reaching the discharge cutoff voltage. Optimal SOC selection, battery monitoring, maintenance, and appropriate charging strategies of Fe-Ni batteries seem to be crucial for their FR applications. Fe-Ni batteries exhibit stable peak shaving (PS) results, indicating their suitability and reliability under various load conditions for PS testing. Extended cycling tests confirm their potential for long-term grid-scale energy storage, enhancing their appeal for PS and FR applications.

Recent Advances in Aqueous Non-Metallic Ion Batteries with Organic Electrodes

Aqueous non-metallic ion batteries have attracted much attention in recent years owing to their fast kinetics, long cycle life, and low manufacture cost. Organic compounds with flexible structural designability are promising electrode materials for aqueous non-metallic ion batteries. In this review, the recent progress of organic electrode materials is systematically summarized for aqueous non-metallic ion batteries with the focus on the interaction between non-metallic ion charge carriers and organic electrode host materials. Both the cations (proton, ammonium ion, and methyl viologen ions) and anions (chloridion, sulfate ion, perchlorate ion, trifluoromethanesulfonate and trifluoromethanesulfonimide ion) storage are discussed. Moreover, the design strategies toward improving the comprehensive performance of organic electrode materials in aqueous non-metallic ion batteries will be summarized. More organic electrode materials with new reaction mechanisms need to be explored to meet the diverse demands of aqueous non-metallic ion batteries with different charge carriers in the future. This review provides insights into developing high-performance organic electrodes for aqueous non-metallic ion batteries.

Stabilizing Zinc Anodes by a Uniform Nucleation Process with Cysteine Additive

Zinc anodes in aqueous batteries face challenges such as dendrite growth and interfacial instability. This study investigates the use of cysteine as an electrolyte additive to address these issues. By establishing the correlation between the size of zinc nuclei and the surface tensions/contact angle at the electrolyte-anode interface, it is demonstrated that the addition of cysteine in the electrolyte alters the surface tensions/contact angle at the electrolyte-anode interface and the nucleation process of zinc. This alteration results in the formation of smaller and more dispersed nuclei, as opposed to the formation of larger island grains. This has a profound impact on the subsequent deposition growth process, enabling smooth and uniform zinc electrodeposition without the formation of dendrites. Additionally, cysteine molecules create a stable interface during zinc plating and stripping, effectively preventing corrosion from side reactions. The incorporation of cysteine in the electrolyte significantly enhances cycling stability and extends the lifespan of zinc anodes in aqueous batteries.

Highly reversible zinc metal anode enabled by strong Brønsted acid and hydrophobic interfacial chemistry

Uncontrollable zinc (Zn) plating and hydrogen evolution greatly undermine Zn anode reversibility. Previous electrolyte designs focus on suppressing H2O reactivity, however, the accumulation of alkaline byproducts during battery calendar aging and cycling still deteriorates the battery performance. Here, we present a direct strategy to tackle such problems using a strong Brønsted acid, bis(trifluoromethanesulfonyl)imide (HTFSI), as the electrolyte additive. This approach reformulates battery interfacial chemistry on both electrodes, suppresses continuous corrosion reactions and promotes uniform Zn deposition. The enrichment of hydrophobic TFSI- anions at the Zn|electrolyte interface creates an H2O-deficient micro-environment, thus inhibiting Zn corrosion reactions and inducing a ZnS-rich interphase. This highly acidic electrolyte demonstrates high Zn plating/stripping Coulombic efficiency up to 99.7% at 1 mA cm-2 ( > 99.8% under higher current density and areal capacity). Additionally, Zn | |ZnV6O9 full cells exhibit a high capacity retention of 76.8% after 2000 cycles.

Zn-based batteries for sustainable energy storage: strategies and mechanisms

Batteries play a pivotal role in various electrochemical energy storage systems, functioning as essential components to enhance energy utilization efficiency and expedite the realization of energy and environmental sustainability. Zn-based batteries have attracted increasing attention as a promising alternative to lithium-ion batteries owing to their cost effectiveness, enhanced intrinsic safety, and favorable electrochemical performance. In this context, substantial endeavors have been dedicated to crafting and advancing high-performance Zn-based batteries. However, some challenges, including limited discharging capacity, low operating voltage, low energy density, short cycle life, and complicated energy storage mechanism, need to be addressed in order to render large-scale practical applications. In this review, we comprehensively present recent advances in designing high-performance Zn-based batteries and in elucidating energy storage mechanisms. First, various redox mechanisms in Zn-based batteries are systematically summarized, including insertion-type, conversion-type, coordination-type, and catalysis-type mechanisms. Subsequently, the design strategies aiming at enhancing the electrochemical performance of Zn-based batteries are underscored, focusing on several aspects, including output voltage, capacity, energy density, and cycle life. Finally, challenges and future prospects of Zn-based batteries are discussed.

A Water-in-Salt Electrolyte for Room-Temperature Fluoride-Ion Batteries Based on a Hydrophobic-Hydrophilic Salt

Realizing room-temperature, efficient, and reversible fluoride-ion redox is critical to commercializing the fluoride-ion battery, a promising post-lithium-ion battery technology. However, this is challenging due to the absence of usable electrolytes, which usually suffer from insufficient ionic conductivity and poor (electro)chemical stability. Herein we report a water-in-salt (WIS) electrolyte based on the tetramethylammonium fluoride salt, an organic salt consisting of hydrophobic cations and hydrophilic anions. The new WIS electrolyte exhibits an electrochemical stability window of 2.47 V (2.08-4.55 V vs Li+/Li) with a room-temperature ionic conductivity of 30.6 mS/cm and a fluoride-ion transference number of 0.479, enabling reversible (de)fluoridation redox of lead and copper fluoride electrodes. The relationship between the salt property, the solvation structure, and the ionic transport behavior is jointly revealed by computational simulations and spectroscopic analysis.

Starch-mediated colloidal chemistry for highly reversible zinc-based polyiodide redox flow batteries

Aqueous Zn-I flow batteries utilizing low-cost porous membranes are promising candidates for high-power-density large-scale energy storage. However, capacity loss and low Coulombic efficiency resulting from polyiodide cross-over hinder the grid-level battery performance. Here, we develop colloidal chemistry for iodine-starch catholytes, endowing enlarged-sized active materials by strong chemisorption-induced colloidal aggregation. The size-sieving effect effectively suppresses polyiodide cross-over, enabling the utilization of porous membranes with high ionic conductivity. The developed flow battery achieves a high-power density of 42 mW cm-2 at 37.5 mA cm-2 with a Coulombic efficiency of over 98% and prolonged cycling for 200 cycles at 32.4 Ah L-1posolyte (50% state of charge), even at 50 °C. Furthermore, the scaled-up flow battery module integrating with photovoltaic packs demonstrates practical renewable energy storage capabilities. Cost analysis reveals a 14.3 times reduction in the installed cost due to the applicability of cheap porous membranes, indicating its potential competitiveness for grid energy storage.

The Influence of Ions on the Electrochemical Stability of Aqueous Electrolytes

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.

Outer Sphere Electron Transfer Enabling High-Voltage Aqueous Electrolytes

Aqueous electrolytes with a low voltage window (1.23 V) and prone side reactions, such as hydrogen evolution reaction and cathode dissolution, compromise the advantages of high safety and low cost of aqueous metal-ion batteries. Herein, introducing catechol (CAT) into the aqueous electrolyte, an outer sphere electron transfer mechanism is initiated to inhibit the water reactivity, achieving an electrochemical window of 3.24 V. In a typical Zn-ion battery, the outer sphere electrons jump from CAT to Zn2+-H2O at a geometrically favorable situation and between the solvation molecules without breaking or forming chemical bonds as that of the inner sphere electron transfers. The excited state π-π stacking further leads to the outer sphere electron transfer occurring at the electrolyte/electrode interface. This high-voltage electrolyte allows achieving an operating voltage two times higher than that of the usual aqueous electrolytes and provides almost the highest energy density and power density for the V2O5-based aqueous Zn-ion full batteries. The Zn//Zn symmetric battery delivers a 4000 h lifespan, and the Zn//V2O5 full battery achieves a ∼380 W h kg-1 energy density and a 92% capacity retention after 3000 cycles at 1 A g-1 and a 2.4 V output voltage. This outer sphere electron transfer strategy paves the way for designing high-voltage aqueous electrolytes.

Electrolyte and Interphase Engineering of Aqueous Batteries Beyond "Water-in-Salt" Strategy

Aqueous batteries are promising alternatives to non-aqueous lithium-ion batteries due to their safety, environmental impact, and cost-effectiveness. However, their energy density is limited by the narrow electrochemical stability window (ESW) of water. The "Water-in-salts" (WIS) strategy is an effective method to broaden the ESW by reducing the "free water" in the electrolyte, but the drawbacks (high cost, high viscosity, poor low-temperature performance, etc.) also compromise these inherent superiorities. In this review, electrolyte and interphase engineering of aqueous batteries to overcome the drawbacks of the WIS strategy are summarized, including the developments of electrolytes, electrode-electrolyte interphases, and electrodes. First, the main challenges of aqueous batteries and the problems of the WIS strategy are comprehensively introduced. Second, the electrochemical functions of various electrolyte components (e.g., additives and solvents) are summarized and compared. Gel electrolytes are also investigated as a special form of electrolyte. Third, the formation and modification of the electrolyte-induced interphase on the electrode are discussed. Specifically, the modification and contribution of electrode materials toward improving the WIS strategy are also introduced. Finally, the challenges of aqueous batteries and the prospects of electrolyte and interphase engineering beyond the WIS strategy are outlined for the practical applications of aqueous batteries.

Design of Single-Atom Catalysts for E lectrocatalytic Nitrogen Fixation

The Electrochemical nitrogen reduction reaction (ENRR) can be used to solve environmental problems as well as energy shortage. However, ENRR still faces the problems of low NH3 yield and low selectivity. The NH3 yield and selectivity in ENRR are affected by multiple factors such as electrolytic cells, electrolytes, and catalysts, etc. Among these catalysts are at the core of ENRR research. Single-atom catalysts (SACs) with intrinsic activity have become an emerging technology for numerous energy regeneration, including ENRR. In particular, regulating the microenvironment of SACs (hydrogen evolution reaction inhibition, carrier engineering, metal-carrier interaction, etc.) can break through the limitation of intrinsic activity of SACs. Therefore, this Review first introduces the basic principles of NRR and outlines the key factors affecting ENRR. Then a comprehensive summary is given of the progress of SACs (precious metals, non-precious metals, non-metallic) and diatomic catalysts (DACs) in ENRR. The impact of SACs microenvironmental regulation on ENRR is highlighted. Finally, further research directions for SACs in ENRR are discussed.

Critical Solvation Structures Arrested Active Molecules for Reversible Zn Electrochemistry

Aqueous Zn-ion batteries (AZIBs) have attracted increasing attention in next-generation energy storage systems due to their high safety and economic. Unfortunately, the side reactions, dendrites and hydrogen evolution effects at the zinc anode interface in aqueous electrolytes seriously hinder the application of aqueous zinc-ion batteries. Here, we report a critical solvation strategy to achieve reversible zinc electrochemistry by introducing a small polar molecule acetonitrile to form a "catcher" to arrest active molecules (bound water molecules). The stable solvation structure of [Zn(H2O)6]2+ is capable of maintaining and completely inhibiting free water molecules. When [Zn(H2O)6]2+ is partially desolvated in the Helmholtz outer layer, the separated active molecules will be arrested by the "catcher" formed by the strong hydrogen bond N-H bond, ensuring the stable desolvation of Zn2+. The Zn||Zn symmetric battery can stably cycle for 2250 h at 1 mAh cm-2, Zn||V6O13 full battery achieved a capacity retention rate of 99.2% after 10,000 cycles at 10 A g-1. This paper proposes a novel critical solvation strategy that paves the route for the construction of high-performance AZIBs.

Sustainable Aqueous Batteries Based on Bipolar Dissociation of Aluminum Hydroxyacetate Electrolyte

Rechargeable aqueous batteries are potential systems for large-scale energy storage due to their high safety and low cost. However, developing aqueous batteries with high sustainability, affordability, and reversibility is urgent and challenging. Here we report an amphoteric aluminum hydroxyacetate (AlAc(OH)2) electrolyte with the ability of bipolar ionization of H+ and OH-, which facilitates the redox reactions at both the anthraquinone (AQ) anode and nickel hydroxide (Ni(OH)2) cathode. The bipolar ionization ability of the AlAc(OH)2(H2O)3 solvation structure results from the strong polarization ability of Al3+ and OH-. The H+/OH- dissociation ability with a dissociation constant of 5.0/3.0 is stronger than that of water (14.0), which boosts the simultaneous stable redox reactions of electrodes. Specifically, H+ uptake prevents the AQ anode from the formation of an ionic bond, suppressing the electrode dissolution, whereas OH- provides the local alkaline environment for the stable conversion reaction of the Ni(OH)2 cathode. The AQ anode in the designed AQ||Ni(OH)2 battery delivers a discharge capacity of 243.9 mAh g-1 and a capacity retention of 78.2% after 300 cycles with high reversibility. Moreover, a pouch cell with a discharge capacity of 0.90 Ah was assembled, exhibiting an energy density of 44.7 Wh kg-1 based on the total mass of the battery. This work significantly widens the types of aqueous batteries and represents a design philosophy of bipolar electrolytes and distinct electrochemical reactions with H+ and OH-.

Water-Ion Interaction Determines the Mobility of Ions in Highly Concentrated Aqueous Electrolytes

Solvation engineering plays a critical role in tailoring the performance of batteries, particularly through the use of highly concentrated electrolytes, which offer heterogeneous solvation structures of mobile ions with distinct electrochemical properties. In this study, we employed spectroscopic techniques and molecular dynamics simulations to investigate mixed-cation (Li+/K+) acetate aqueous electrolytes. Our research unravels the pivotal role of water in facilitating ion transport within a highly viscous medium. Notably, Li+ cations primarily form ion aggregates, predominantly interacting with acetate anions, while K+ cations emerge as the principal charge carriers, which is attributed to their strong interaction with water molecules. Intriguingly, even at a concentration as high as 40 m, a substantial amount of water molecules persistently engages in hydrogen bonding with one another, creating mobile regions rich in K+ ions. Our observations of a redshift of the OH stretching band of water suggest that the strength of the hydrogen bond alone cannot account for the expansion of the electrochemical stability window. These findings offer valuable insights into the cation transfer mechanism, shedding light on the contribution of water-bound cations to both the ion conductivity and the electrochemical stability window of aqueous electrolytes for rechargeable batteries. Our comprehensive molecular-level understanding of the interplay between cations and water provides a foundation for future advances in solvation engineering, leading to the development of high-performance batteries with improved energy storage and safety profiles.

A 2.4 V Aqueous Zinc-Ion Battery Enabled by the Photoelectrochemical Effect of a Modified BiOI Photocathode: Shattering the Shackle of the Electrochemical Window of an Aqueous Electrolyte

Aqueous zinc-ion batteries emerge as a promising energy storage system with merits of high security, abundance, and being environmentally benign. But the low operating voltages of aqueous electrolytes restrict their energy densities. Previous reports have mostly focused on modifying the electrolytes to enlarge the operating voltages of aqueous zinc-ion batteries. However, either extra-expensive salts or potential safety hazards of organic additives are considered to be adverse for practical large-scale applications. Here, a proof-of-concept to enlarge the operating voltage of an aqueous zinc-ion battery by incorporating a well-designed semiconductor photocathode is proposed, which produces a photovoltage (Vph) across the semiconductor/liquid junction (SCLJ) interface to elevate the output voltage of zinc-ion battery under irradiation. The operating voltage of an aqueous zinc-ion battery can be markedly raised from 1.78 (thermodynamic limit) to 2.4 V when a BiOI nanoflake array photocathode with good surface modification is introduced, achieving a round-trip efficiency of 114.3% and a 34.8% increase of energy density compared to the theoretical value. The successive ionic layer adsorption and reaction modified surface effectively passivates surface trap defects of the BiOI photocathode and thus enlarges its Vph from 60 to 240 mV under irradiation. This study provides a design to enlarge the output voltages of aqueous zinc-ion batteries and other energy storage systems, providing insight into widening the voltage window, which is that the operating voltages are determined by photocathode under irradiation and not restricted by the electrochemical stability window of dilute aqueous electrolytes.

Acidified Nitrogen Self-Doped Porous Carbon with Superprotonic Conduction for Applications in Solid-State Proton Battery

Solid proton electrolytes play a crucial role in various electrochemical energy storage and conversion devices. However, the development of fast proton conducting solid proton electrolytes at ambient conditions remains a significant challenge. In this study, a novel acidified nitrogen self-doped porous carbon material is presented that demonstrates exceptional superprotonic conduction for applications in solid-state proton battery. The material, designated as MSA@ZIF-8-C, is synthesized through the acidification of nitrogen-doped porous carbon, specifically by integrating methanesulfonic acid (MSA) into zeolitic imidazolate framework-derived nitrogen self-doped porous carbons (ZIF-8-C). This study reveals that MSA@ZIF-8-C achieves a record-high proton conductivity beyond 10-2  S cm-1 at ambient condition, along with good long-term stability, positioning it as a cutting-edge alternative solid proton electrolyte to the default aqueous H2 SO4 electrolyte in proton batteries.

Component regulation on ternary FeCoNi nano-bundles as efficient electrocatalysts for driving water oxidation

Metal organic frameworks (MOFs) are considered as promising electrocatalytic materials due to their tunable porosity, functional organic ligands, and large specific surface area for oxygen evolution reaction (OER). Recently, most reported electrocatalysts focus on the establishing heterogeneous structures by thermal treatments to improve OER performance. However, the thermal treatments are accompanied by the complex synthetic process and destruction of the MOFs structure. Therefore, improving the catalytic performance of pristine MOFs remains a challenge. Here, a series of trimetallic MaMbMc-MOFs (M represents metal element) were synthesized by one-pot method. Modulating the Co/Ni ratio not only adjusts the morphology of FeCoNi-MOFs, but also effectively optimizes the electronic structure. The composition-optimized FeCo0.5Ni2.5-MOF nano-bundles (FeCo0.5Ni2.5-NBs) only required a low overpotential of 273 mV to achieve the current density of 10 mA cm-2 in alkaline solution, with a Tafel slope of 51.1 mV dec-1, lower than other FeCoNi-MOFs and commercial RuO2 catalyst. The two-electrode couple FeCo0.5Ni2.5-NBs || Pt/C achieved the cell voltage of 1.55 V, delivering current density of 10 mA cm-2 for overall water splitting.

Ionic Competition between Na+ and H+ in Aqueous Sodium-Ion Battery Electrolytes

Aqueous electrolytes have become a research hotspot because of their high safety and low cost, while the inevitable ionization phenomenon of water in aqueous solution leads to the existence of competitive ions (H+) except the active ions. In this article, we take aqueous Na base electrolyte as an example to clear the ion competition behavior by modeling, simulating together with experimental verification. First, the reaction tendency of the two ions (Na+ and H+) is obtained by calculating the Gibbs energy change of the reaction. Furthermore, the properties of electrolytes with different concentrations including transportation are obtained by modeling. After that, relevant experiments are also proceeded to verify the simulation results. Then, the ion competition behavior is analyzed by in situ observation by controlling the constant concentration of Na+: the high concentration of Na+ can reduce the proportion of H+ and reduce the competitiveness of H+; a high concentration of Na+ causes the increased viscosity and reduces the ion diffusion. Based on this, the correlation between ion competitiveness and ion ratio is also confirmed by keeping the concentration of Na+ unchanged and adjusting the concentration of H+ (adjusting pH). The influence of the ion competition phenomenon (Na+ and H+) is the reaction characteristics of the substance itself and the ratio of ion concentration. Finally, the electrochemical performance is further verified in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI) symmetric cells and in full-cells with vanadium phosphate sodium (NVP) as the cathode and PTCDI as the anode.

Electrolyte Interphases in Aqueous Batteries

The narrow electrochemical stability window of water poses a challenge to the development of aqueous electrolytes. In contrast to non-aqueous electrolytes, the products of water electrolysis do not contribute to the formation of a passivation layer on electrodes. As a result, aqueous electrolytes require the reactions of additional components, such as additives and co-solvents, to facilitate the formation of the desired solid electrolyte interphase (SEI) on the anode and cathode electrolyte interphase (CEI) on the cathode. This review highlights the fundamental principles and recent advancements in generating electrolyte interphases in aqueous batteries.

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.

Electrolyte Additives for Stable Zn Anodes

Zn-ion batteries are regarded as the most promising batteries for next-generation, large-scale energy storage because of their low cost, high safety, and eco-friendly nature. The use of aqueous electrolytes results in poor reversibility and leads to many challenges related to the Zn anode. Electrolyte additives can effectively address many such challenges, including dendrite growth and corrosion. This review provides a comprehensive introduction to the major challenges in and current strategies used for Zn anode protection. In particular, an in-depth and fundamental understanding is provided of the various functions of electrolyte additives, including electrostatic shielding, adsorption, in situ solid electrolyte interphase formation, enhancing water stability, and surface texture regulation. Potential future research directions for electrolyte additives used in aqueous Zn-ion batteries are also discussed.

Asymmetric Electrolytes Design for Aqueous Multivalent Metal Ion Batteries

With the rapid development of portable electronics and electric road vehicles, high-energy-density batteries have been becoming front-burner issues. Traditionally, homogeneous electrolyte cannot simultaneously meet diametrically opposed demands of high-potential cathode and low-potential anode, which are essential for high-voltage batteries. Meanwhile, homogeneous electrolyte is difficult to achieve bi- or multi-functions to meet different requirements of electrodes. In comparison, the asymmetric electrolyte with bi- or multi-layer disparate components can satisfy distinct requirements by playing different roles of each electrolyte layer and meanwhile compensates weakness of individual electrolyte. Consequently, the asymmetric electrolyte can not only suppress by-product sedimentation and continuous electrolyte decomposition at the anode while preserving active substances at the cathode for high-voltage batteries with long cyclic lifespan. In this review, we comprehensively divide asymmetric electrolytes into three categories: decoupled liquid-state electrolytes, bi-phase solid/liquid electrolytes and decoupled asymmetric solid-state electrolytes. The design principles, reaction mechanism and mutual compatibility are also studied, respectively. Finally, we provide a comprehensive vision for the simplification of structure to reduce costs and increase device energy density, and the optimization of solvation structure at anolyte/catholyte interface to realize fast ion transport kinetics.

Enhancing the Performance of Reversible Zn Deposition by Ultrathin Polyelectrolyte Coatings

Modifying the surfaces of zinc and other metallic substrates is considered an effective strategy to enhance the reversibility of the zinc deposition and stripping processes. While a variety of surface modification strategies have been explored, their ability to be practically implemented is not always trivial due to the associated high costs and complexity of the proposed techniques. In this study, we showcase a straightforward method for preparing ultrathin polyelectrolyte coatings using polydiallyldimethylammonium chloride (PDDA) and polyethylenimine (PEI). The coatings, characterized by their electrostatic charge and hydrophobicity, suppress side reactions and even out the electrodeposition process across the substrate surface. The PDDA-coated anodes demonstrate significantly reduced voltage hysteresis, uniform zinc morphology, improved self-discharge rates, and an impressive Coulombic efficiency exceeding 99% over prolonged cycling. Our findings highlight the potential that such cost-effective and straightforward surface treatments could be widely applied in Zn metal-based batteries.