Decoupled aqueous batteries using pH-decoupling electrolytes

Practical issues toward high-voltage aqueous rechargeable batteries

This review offers a critical and exhaustive examination of the current state and innovative advances in high-voltage Li, Na, K, and Zn aqueous rechargeable batteries, an area poised for significant technological breakthroughs in energy storage systems. The practical issues that have traditionally hampered the development of aqueous batteries, such as limited operating potential windows, challenges in stable solid-electrolyte interphase (SEI) formation, the need for active materials optimized for aqueous environments, the misunderstood intercalation chemistry, the unreliable assessment techniques, and the overestimated performance and underestimated physicochemical and electrochemical drawbacks, are highlighted. We believe that this review not only brings together existing knowledge but also pushes the boundaries by providing a roadmap for future research and development efforts aimed at overcoming the longstanding challenges faced by the promising aqueous rechargeable batteries.

Strengthened O-H Bonds in an Aqueous Electrolyte for Nonflammable Li-Ion Batteries

Aqueous lithium-ion batteries (ALIBs) have attracted increasing attention for their inherent safety and environmental benefits. However, the narrow electrochemical stability window of water (1.23 V) imposed by hydrogen evolution reaction (HER) and high melting point (0 °C) critically limits the energy density and low-temperature operation. In this work, we report an electrolyte design principle that water activity can be suppressed by strengthening the O-H bonds between water molecules and regulating the pH. A strong polar solvent N,N-dimethylformamide (DMF) interacts with H2O through intermolecular hydrogen bonds, thereby reinforcing the covalent O-H bonds of water. Meanwhile, KOH renders the electrolyte alkaline, inhibiting water dissociation kinetics and consequently raising the HER overpotential. The designed LD1.7W0.6-KOH electrolyte exhibits a freezing point below -82 °C and expanded electrochemical stability window of 4.3 V. The 2.5 V aqueous LiMn2O4/Li4Ti5O12 full cell delivers 2000 cycles at a rate of 6 C with a Coulombic efficiency of 99% at 30 °C and over 1950 cycles at 0 °C. This work offers atomic-level insights into alkaline HER while providing guidance for advancing low-temperature aqueous batteries by tuning the electrolyte structures.

Pre-Established Ion Transport Pathways Through Electrolyte Initiator for High-Efficiency Polymer Interface Enabling Ultra-Stable Aqueous Zinc-Metal Anodes

Achieving stable zinc-metal anodes is pivotal to realizing high-performance aqueous zinc-metal batteries (AZMBs). The construction of a functional polymer interface layer on the zinc-metal anode surface is confirmed as an effective strategy for mitigating dendrite growth and side reactions, thereby significantly enhancing the stability of zinc-metal anode. However, polymers capable of withstanding electrolyte environments over the long term typically suffer from elevated interfacial impedance, which hinders Zn2+ transport. Here, a pioneering zinc-metal anode enabled by a functional polymer interface layer with high-efficiency ion transport is introduced. This polymer layer is polymerized in situ on the zinc-metal anode surface through an innovative redox initiation system, where zinc trifluoromethanesulfonate (Zn(OTf)2) salts function as both reductant and ion transport pre-pathways, ensuring high-efficiency ion transport. The resultant interface layer achieves an ideal balance of ionic conductivity, water resistance, adhesion, and mechanical properties, effectively suppressing dendrite growth and side reactions. Symmetric cells assembled with this interface layer deliver an impressive lifespan of 8800 and 1600 h under 1 and 5 mA cm-2, respectively. This interface layer further demonstrates exceptional feasibility and versatility in Zn-NVO and Zn-PANI batteries. This work provides groundbreaking insights into the strategic design of high-performance polymer interface layers for AZMBs.

Unveil the Failure of Alkali Ion-Sulfur Aqueous Batteries: Resolving Water Migration by Coordination Regulation

Sulfur aqueous battery (SAB) is promising owing to its high theoretical capacity and cost competitiveness. Although decoupled electrolyte design has successfully endowed transition metal ion-SABs with customizability to achieve high energy density, its effectiveness in alkali ion-SABs remains problematic. Here, we identify for the first time an intractable phenomenon of alkali-ion-driven water migration between decoupled electrolytes through ex situ NMR, which is recognized as the origin of the irreversible sulfur redox reactions. To address the challenge, we propose an alkali-ion-H2O-poor coordination strategy to effectively regulate water migration by incorporating low molecular polarity index (MPI) anions. In situ Raman, synchrotron spectroscopy, and molecule dynamic simulations reveal that the repulsion of low MPI anions to water effectively disrupts the hydration patterns around the alkali cations, and thereby minimizes the concomitant water migration. The elaborated Na+-SAB achieved an ultrahigh capacity of 1634 mAh g-1 (97.7% sulfur utilization) and prolonged stability over 500 cycles. Furthermore, the versatility of the alkali-ion-H2O-poor coordination strategy is further substantiated in Li+-SAB and K+-SAB batteries, boosting the scope of the following SAB systems.

A Fluorine-Free Organic/Inorganic Interphase for Highly Reversible Aqueous Zinc Batteries

Construction of robust solid electrolyte interphases (SEIs) has proved effective in mitigating dendrite growth and side reactions of zinc (Zn) anodes in aqueous electrolytes. Fluorinated SEIs, in particular, have garnered significant attention due to their exceptional electrochemical stability and high Zn2+ conductivity. However, the formation of such SEIs typically relies on the use of fluorine (F)-containing precursors, which inadvertently raise environmental and biological concerns because they show high resistance to degradation in natural environments. Herein, we develop an F-free organic/inorganic hybrid SEI for aqueous Zn batteries using a low-cost N-acetyl-D-glucosamine (NAG) electrolyte additive. The NAG additive not only modulates the solvation structure of Zn2+ but also preferentially adsorbs on the Zn anode to promote the in situ formation of a robust organic (Zn chelates)/inorganic (ZnS and ZnCO3) hybrid SEI layer, thereby enhancing Zn2+ de-solvation kinetics and Zn plating/stripping reversibility. Consequently, the Zn anode exhibits a long-term cycling over 6500 h at 0.5 mA cm‒2, a high average Coulombic efficiency of 99.6% at 1 mA cm‒2, and greatly extended cycling stability in full cells (up to 2000 cycles). Our electrolyte design paves a promising avenue toward practical Zn batteries that combine performance, cost-effectiveness, and eco-friendliness.

Water-in-Acid Strategy for Corrosion-Free Proton Storage: Phosphoric Acid Electrolyte Engineering Toward Sustainable Aqueous Batteries

Aqueous proton batteries, leveraging the intrinsic advantages of protons such as minimal hydrated radius, natural abundance, and rapid transport kinetics, have emerged as promising candidates for next-generation energy storage. However, conventional strong acid electrolytes like H2SO4 suffer from critical limitations including electrode dissolution and incompatibility with battery components. To circumvent these challenges, weak acids (e.g., HCOOH and H3PO4) have been strategically selected as alternative electrolytes due to their non-corrosive characteristics. Particularly, the implementation of high-concentration "water-in-acid" (WIA) effectively suppresses undesirable interactions between electrode materials and free water molecules. Through electrolyte engineering, we developed a 9.5 M H3PO4 WIA system that synergizes with a molybdenum trioxide electrode, achieving remarkable electrochemical performance: a high reversible capacity of 229.8 mAh g-1 at 3 A g-1 and exceptional cycling stability with 83.86% capacity retention after 1000 cycles at 5 A g-1, surpassing conventional H₂SO₄-based systems by both capacity and cyclability. This innovative approach establishes a new paradigm for developing high-performance aqueous energy storage systems through acid-dominated electrolyte design.

Unlocking the Potential of Aqueous Zinc-Ion Batteries: Hybrid SEI Construction through Bifunctional Regulator-Assisted Electrolyte Engineering

Aqueous zinc-ion batteries (AZIBs) represent promising candidates for energy storage devices, because of their inherent high safety and cost efficiency. However, challenges such as uneven zinc ion deposition during electrochemical reduction and anode interface side reactions pose significant obstacles to their advancement and practical deployment. Herein, a medium-concentration aqueous electrolyte combined with a bifunctional regulator (aspartame) is developed to address these issues. Practical validation experiments and theoretical calculations demonstrate that the medium-concentration Zn(OTf)2 aqueous electrolyte containing Aspartame can form a robust hybrid solid electrolyte interface (SEI) containing ZnF2 and ZnS by simultaneously modulating the Zn2+ solvation structure and optimizing the metal-molecule interface, thereby enabling dense Zn deposition. It achieves dendrite-free Zn plating and stripping and excellent Zn reversibility. Significantly, the Zn||V2O5 full cell exhibits an average capacity of 240 mAh g-1 over 8000 cycles at 5 A g-1. This work provides new insight into solvation and interface design for high-performance AZIBs.

Strategic Nitrogen Site Alignment in Covalent Organic Frameworks for Enhanced Performance in Aqueous Zinc-Iodide Batteries

Aqueous zinc-iodine batteries (AZIBs) are gaining attention as next-generation energy storage systems due to their high theoretical capacity, enhanced safety, and cost-effectiveness. However, their practical application is hindered by challenges such as slow reaction kinetics and the persistent polyiodide shuttle effect. To address these limitations, we developed a novel class of covalent organic frameworks (COFs) featuring electron-rich nitrogen sites with varied density and distribution (N1-N4) along the pore walls. These nitrogen sites enhance iodine species confinement and mass transport. Our experimental and theoretical studies reveal that the continuous and optimized distribution of nitrogen sites within the COF structure significantly reduces internal resistance and boosts redox activity. Moreover, the N4-COF demonstrates superior performance compared to other porous materials, due to its high density and strategic alignment of active sites. The I2@N4-COF cathode achieves a remarkable specific capacity of 348 mAh g-1 at 1 C, almost 1.8 times greater than that of the I2@N1-COF, while also maintaining excellent cycling stability. This integration of a porous framework with aligned nitrogen sites in the N4-COF structure not only enhances iodine redox behavior but also offers a promising design strategy for developing high-performance AZIB electrodes.

A Rocking-Chair Type Aqueous Nickel-Organic Battery with Azobenzene Anode

Aqueous nickel-organic batteries have the potential for grid-scale energy storage due to their high safety and sustainability merits. However, organic anodes generally store charge by coordinating with alkaline metal cations, which could cause electrolyte consumption. Here, azobenzene (AZO) is screened out from carbonyl, imine, and azo compounds to serve as anodes, combining it with Ni(OH)2 cathodes to construct a "rocking-chair" type battery system. Qualitative and quantitative analyses demonstrate the N═N group acts as the active center, while protons serve as charge carriers during the electrochemical reaction. Benefiting from the small ionic radius and fast ions transport of protons, this battery not only delivers an excellent rate performance, with a capacity of 281.5 mAh g-¹ at a current density of 1C (0.3 A g-¹) and maintains 274.4 mAh g-¹ at 100C, but also exhibits remarkable long-term cycling stability, retaining 92.5% of its initial capacity after 10 000 cycles. Additionally, a pouch cell with a discharge capacity of 1.36 Ah is also assembled, yielding an energy density of 64.3 Wh kg-¹ (based on the total mass). This work expands the range of organic anode materials, and inspires the development of aqueous nickel-organic batteries with a proton "rocking-chair" mechanism.

Stimulating the Potential of Zn Anodes to Operate in Low pH and Harsh Environments forHighly Sustainable Zn Batteries

Compared to near-neutral electrolytes (pH=3-6), Zn||Mn batteries in acidic environments can achieve voltages up to ~2 V. However, high proton concentrations raise concerns about Zn anode stability. Current strategies for inhibiting hydrogen evolution corrosion (HEC) on the anode in Zn-based batteries mainly focus on the near-neutral electrolytes. To supplement this gap, we developed a conversion-type interphase strategy using phosphate, sulfate precipitation, and phytic acid modification layers for Zn anodes to demonstrate the potential of Zn anode to operate in acidic electrolytes. This approach enables stable Zn stripping/plating at pH=2.2 for over 3,600 h and 400 h at 1 mA cm-2/0.5 mAh cm-2 and 20 mA cm-2/10 mAh cm-2. Benefiting from stable Zn electrodes, the electrolytic Zn||Mn batteries can operate at 1.90 V. To show more harsh scenarios, the seawater-based 0.25 Ah-scale Zn||Mn pouch cells can be assembled with a practical energy density of 57.4 Wh  kg-1 cell. Significantly, we analyze and emphasize that seawater holds promise as an alternative to deionized water for electrolyte solvents due to its energy and economic effectiveness. This strategy has motivated to expand the working pH range of metal anodes and provides the rational design for grid-scale energy storage technologies.

A Rechargeable Urea-Assisted Zn-Air Battery With High Energy Efficiency and Fast-Charging Enabled by Engineering High-Energy Interfacial Structures

Electrochemical urea oxidation reaction (UOR) offers a promising alternative to the oxygen evolution reaction (OER) in clean energy conversion and storage systems. Nickel-based catalysts are regarded as highly promising electrocatalysts for the UOR. However, their effectiveness is significantly hindered by the unavoidable self-oxidation reaction of nickel species during UOR. To address this challenge, we proposed an interface chemistry modulation strategy to boost UOR kinetics by creating a high-energy interfacial heterostructure. This heterostructure incorporates Ag at the CoOOH@NiOOH heterojunction interface, where strong interactions significantly promote the electron exchanges at the heterojunction interface between -OH and -O groups. Consequently, the improved electron delocalization leads to the formation of stronger bonds between Co sites and urea CO(NH2)2, promoting a preference for urea to occupy Co active sites over OH*. The resulting catalyst, Ag-CoOOH@NiOOH, demonstrates ultrahigh UOR activity with a low potential of 1.33 V at 100 mA cm-2. The fabricated catalyst exhibits a mass activity over 11.9 times greater than the initial cobalt oxyhydroxide. The rechargeable urea-assisted zinc-air batteries (ZABs) achieve a record-breaking energy efficiency of 74.56 % at 1 mA cm-2, remarkable durability (1000 hours at a current density of 50 mA cm-2), and quick charge performances.

Interfacial Engineering of Polymer Membranes with Intrinsic Microporosity for Dendrite-Free Zinc Metal Batteries

Metallic zinc has emerged as a promising anode material for high-energy battery systems due to its high theoretical capacity (820 mAh g-1), low redox potential for two-electron reactions, cost-effectiveness and inherent safety. However, current zinc metal batteries face challenges in low coulombic efficiency and limited longevity due to uncontrollable dendrite growth, the corrosive hydrogen evolution reaction (HER) and decomposition of the aqueous ZnSO4 electrolyte. Here, we report an interfacial-engineering approach to mitigate dendrite growth and reduce corrosive reactions through the design of ultrathin selective membranes coated on the zinc anodes. The submicron-thick membranes derived from polymers of intrinsic microporosity (PIMs), featuring pores with tunable interconnectivity, facilitate regulated transport of Zn2+-ions, thereby promoting a uniform plating/stripping process. Benefiting from the protection by PIM membranes, zinc symmetric cells deliver a stable cycling performance over 1500 h at 1 mA/cm2 with a capacity of 0.5 mAh while full cells with NaMnO2 cathode operate stably at 1 A g-1 over 300 cycles without capacity decay. Our work represents a new strategy of preparing multi-functional membranes that can advance the development of safe and stable zinc metal batteries.

Organic Electrolyte Additives for Aqueous Zinc Ion Batteries: Progress and Outlook

Aqueous zinc ion batteries (AZIBs) are considered one of the most prospective new-generation electrochemical energy storage devices with the advantages of high specific capacity, good safety, and high economic efficiency. Nevertheless, the enduring problems of low Coulombic efficiency (CE) and inadequate cycling stability of zinc anodes, originating from dendrites, hydrogen precipitation and passivation, are closely tied to their thermodynamic instability in aqueous electrolytes, which significantly shortens the cycle life of the battery. Electrolyte additives can solve the above difficulties and are important for the advancement of affordable and reliable AZIBs. Organic electrolyte additives have attracted widespread attention due to their unique properties, however, there is a lack of systematic discussion on the performance and mechanism of action of organic electrolyte additives. In this review, a comprehensive overview of the application of organic electrolyte additives in AZIBs is presented. The role of organic electrolyte additives in stabilizing zinc anodes is described and evaluated. Finally, further potential directions and prospects for improving and directing organic electrolyte additives for AZIBs are presented.

Designing a Novel C3-Fe-N Interface Local Coordination Microenvironment for Efficient Electrocatalytic Water Splitting

Developing single-atomic electrocatalysts (SACs) with high activity and stability for electrocatalytic water-splitting has been challenging. Moreover, the practical utilization of SACs is still far from meeting the the theoretical prediction. Herein a facile and easy scale-up fabrication method is proposed for designing a novel carbon-iron-nitrogen (C-Fe-N) electrocatalyst with a single atom electron bridge (C-Fe-N SAEBs), which exhibits lower overpotential and impedance than previously reported electrocatalysts. 0.8-C-Fe-N SAEBs exhibits significant activity and excellent stability in the bi-functional decomposition of water. The excellent performance of the C-Fe-N SAEBs electrocatalyst can be attributed to the strong coupling effect at the interface owing to the formation of a single atom C3-Fe-N local coordination microenvironment at the interface, which enhance the exposure of active sites and charge transfer, and reduced the adsorption energy barrier of intermediates. Theoretical calculation and synchrotron radiation analysis are performed to understand the mechanistic insights behind the experimental results. The results reveal that the active C3-Fe-N local coordination microenvironment at the interface not only improves water-splitting behavior but also provides a deeper understanding of local-interface geometry/electronic structure for improving the electrocatalytic activity. Thus, the proposed electrocatalyst, as well as the mechanistic insights into its properties, presents a significant stride toward practical application.

High-Energy Aqueous S-MnO2 Batteries with Redox Charge Carriers

The energy densities of conventional aqueous batteries are often unsatisfactory due to the limited capacities of electrode materials. Therefore, the design of creative aqueous batteries has to be considered. Herein, aqueous S-MnO2 batteries are constructed by matching S/Cu2S redox couples and MnO2 deposition/dissolution. In such batteries, S/Cu2S redox couples undergo the solid-solid conversion reaction with four-electron transfer, ensuring a high specific capacity of 2220 mAh g-1 in S anodes. Furthermore, the conversion reaction of S/Cu2S redox couples can take place stably in acidic electrolyte that is essential for the MnO2 deposition/dissolution. As a result, the S/Cu2S redox couples can match MnO2 deposition/dissolution well, which endow the batteries with a membrane-free configuration. As a proof of concept, Ah-level prismatic and single-flow batteries were assembled and could operate stably for over 1000 h, demonstrating their great potential for large-scale energy storage. This work broadens the horizons of aqueous batteries beyond metal-manganese chemistry.

Reconfiguring the Hydrogen Networks of Aqueous Electrolyte to Stabilize Iron Hexacyanoferrate for High-Voltage pH-Decoupled Cell

Prussian blue analogs (PBAs) are promising insertion-type cathode materials for different types of aqueous batteries, capable of accommodating metal or non-metal ions. However, their practical application is hindered by their susceptibility to dissolution, which leads to a shortened lifespan. Herein, we have revealed that the dissolution of PBAs primarily originates from the locally elevated pH of electrolytes, which is caused by the proton co-insertion during discharge. To address this issue, the water-locking strategy has been implemented, which interrupts the generation and Grotthuss diffusion of protons by breaking the well-connected hydrogen bonding network in aqueous electrolytes. As a result, the hybrid electrolyte enables the iron hexacyanoferrate to endure over 1000 cycles at a 1 C rate and supports a high-voltage pH-decoupled cell with an average voltage of 1.95 V. These findings provide insights for mitigating the dissolution of electrode materials, thereby enhancing the viability and performance of aqueous batteries.

Electrode and Electrolyte Co-Energy-Storage Electrochemistry Enables High-Energy Zn-S Decoupled Batteries

In the search for next-generation green energy storage solutions, Cu-S electrochemistry has recently gained attraction from the battery community owing to its affordability and exceptionally high specific capacity of 3350 mAh gs -1. However, the inferior conductivity and substantial volume expansion of the S cathode hinder its cycling stability, while the low output voltage limits its energy density. Herein, a hollow carbon sphere (HCS) is synthesized as a 3D conductive host to achieve a stable S@HCS cathode, which enables an outstanding cycling performance of 2500 cycles (over 9 months). To address the latter, a Zn//S@HCS alkaline-acid decoupled cell is configured to increase the output voltage from 0.18 to 1.6 V. Moreover, an electrode and electrolyte co-energy storage mechanism is proposed to offset the reduction in energy density resulting from the extra electrolyte required in Zn//S decoupled cells. When combined, the Zn//S@HCS alkaline-acid decoupled cell delivers a record energy density of 334 Wh kg-1 based on the mass of the S cathode and CuSO4 electrolyte. This work tackles the key challenges of Cu-S electrochemistry and brings new insights into the rational design of decoupled batteries.

Activating the Intrinsic Zincophilicity of PAM Hydrogel to Stabilize the Metal-Electrolyte Dynamic Interface for Stable and Long-Life Zinc Metal Batteries

As a potential material to solve rampant dendrites and hydrogen evolution reaction (HER) problem of aqueous zinc metal batteries (AZMB), hydrogel electrolytes usually require additional additives or multi-molecular network strategies to solve existing problems of ionic conductivity, mechanical properties and interface stability. However, the intrinsic zincophilic properties of the gel itself are widely neglected leading to the addition of additional molecules and the complexity of the preparation process. In this work, we innovatively utilize the characteristics of acrylamide's high zincophilic group density, activating the intrinsic zincophilic properties of PAM gel through a simple concentration control strategy which reconstructs a novel zinc-electrolyte interface different from conventional PAM electrolyte. The activated novel gel electrolyte with intrinsic zincophilic properties has high ionic conductivity and effectively suppresses water activity, thereby inhibiting HER corrosion. Meanwhile, it induces uniform deposition of (002) crystal planes, leading to excellent deposition kinetics and long cycle life, thereby ensuring high interfacial stability. Compared with conventional PAM gel electrolytes, the activated zincophilic group-rich hydrogel maintained excellent cycling stability (1 mA/cm2, 1 mAh/cm2) over 2250 hours; The Zn//MnO₂ coin cell using novel zincophilic group -rich hydrogel still retains a high specific capacity of more than 170 mAh/g at 0.5 A/g after 1000 cycles.

Layered CrO2·nH2O as a cathode material for aqueous zinc-ion batteries: ab initio study

Aqueous zinc-ion batteries are considered potential large-scale energy storage systems due to their low cost, environmentally friendly nature, and high safety. However, the development of high energy density cathode materials and uncertain reaction mechanisms remains a major challenge. In this work, the reaction mechanism, discharge voltage and diffusion properties of layered CrO2 as a cathode material for aqueous zinc-ion batteries were studied using first-principles calculations, and the effect of pre-intercalated structural water on the electrochemical performance of CrO2 electrodes is also discussed. The results show that CrO2 exhibits high average discharge voltages (2.65 V for H insertion (pH = 7) and 1.97 V for Zn insertion) and medium theoretical capacities (319 mA h g-1 (H and Zn)). The H intercalation voltage strongly depends on the pH value of the electrolyte. The H/Zn co-insertion mechanism occurs at low hydrogen concentrations (c(H) ≤ 0.125), where the initial insertion of H reduces the total amount of subsequent Zn insertion. For the substrate containing structured water (CrO2·nH2O, n ≥ 0.5), the average voltage of Zn insertion is significantly increased, while the average voltage of H slightly decreases. In addition, the pre-intercalated water strategy significantly improved the diffusion properties of H and Zn. This study shows that layered CrO2·nH2O is a promising cathode material for aqueous zinc-ion batteries, and also provides theoretical guidance for the development of high-performance cathode materials for aqueous zinc-ion batteries.

In Situ Visualization of Ion Transport Processes in Aqueous Batteries

Aqueous rechargeable batteries are regarded as one of the most reliable solutions for electrochemical energy storage, and ion (e.g., H+ or OH-) transport is essential for their electrochemical performance. However, modeling and numerical simulations often fall short of depicting the actual ion transport characteristics due to deviations in model assumptions from reality. Experimental methods, including laser interferometry, Raman, and nuclear magnetic resonance imaging, are limited by the complexity of the system and the restricted detection of ions, making it difficult to detect specific ions such as H+ and OH-. Herein, in situ visualization of ion transport is achieved by innovatively introducing laser scanning confocal microscopy. Taking neutral Zn-air batteries as an example and using a pH-sensitive probe, real-time dynamic pH changes associated with ion transport processes are observed during battery operation. The results show that after immersion in the zinc sulfate electrolyte, the pH near the Zn electrode changes significantly and pulsation occurs, which demonstrates the intense self-corrosion hydrogen evolution reaction and the periodic change in the reaction intensity. In contrast, the change in the pH of the galvanized electrode plate is weak, proving its significant corrosion inhibition effect. For the air electrode, the heterogeneity of ion transport during the discharging and charging process is presented. With an increase of the current density, the ion transport characteristics gradually evolve from diffusion dominance to convection-diffusion codominance, revealing the importance of convection in the ion transport process inside batteries. This method opens up a new approach of studying ion transport inside batteries, guiding the design for performance enhancement.

2D Conjugated Metal-Organic Frameworks Bearing Large Pore Apertures and Multiple Active Sites for High-Performance Aqueous Dual-Ion Batteries

2D conjugated metal-organic frameworks (2D c-MOFs) with large pore sizes and high surface areas are advantageous for adsorbing iodine species to enhance the electrochemical performance of aqueous dual-ion batteries (ADIBs). However, most of the reported 2D c-MOFs feature microporous structures, with few examples exhibiting mesoporous characteristics. Herein, we developed two mesoporous 2D c-MOFs, namely PA-TAPA-Cu-MOF and PA-PyTTA-Cu-MOF, using newly designed arylimide based multitopic catechol ligands (6OH-PA-TAPA and 8OH-PA-PyTTA). Notably, PA-TAPA-Cu-MOF exhibits the largest pore sizes (3.9 nm) among all reported 2D c-MOFs. Furthermore, we demonstrated that these 2D c-MOFs can serve as promising cathode host materials for polyiodides in ADIBs for the first time. The incorporation of triphenylamine moieties in PA-TAPA-Cu-MOF resulted in a higher specific capacity (423.4 mAh g-1 after 100 cycles at 1.0 A g-1) and superior cycling performance, retaining 96 % capacity over 1000 cycles at 10 A g-1 compared to PA-PyTTA-Cu-MOF. Our comparative analysis revealed that the increased number of N anchoring sites and larger pore size in PA-TAPA-Cu-MOF facilitate efficient anchoring and conversion of I3 -, as supported by spectroscopic electrochemistry and density functional theory calculations.

Fully-Printed Flexible Aqueous Rechargeable Sodium-Ion Batteries

The flexible aqueous rechargeable sodium-ion batteries (ARSIBs) are a promising portable energy storage system that can meet the flexibility and safety requirements of wearable electronic devices. However, it faces huge challenges in mechanical stability and facile manufacturing processes. Herein, the first fully-printed flexible ARSIBs with appealing mechanical performance by screen-printing technique is prepared, which utilizes Na3V2(PO4)2F3/C (NVPF/C) as the cathode and 2% nitrogenous carbon-loaded Na3MnTi(PO4)3/C (NMTP/C/NC) as the anode. In particular, the organic co-solvent ethylene glycol (EG) is cleverly added to 17 m (mol kg-1) NaClO4 electrolyte to prepare a 17 m NaClO4-EG mixed electrolyte. This mixed electrolyte can withstand low temperatures of -20 °C in practical applications. Encouragingly, the fully-printed flexible ARSIBs (NMTP/C/NC//NVPF/C) exhibit a discharge capacity of 40.1 mAh g-1, an energy density of 40.1 Wh kg-1, and outstanding cycle performance. Moreover, these batteries with various shapes can be used as an energy wristband for an electronic watch in the bending states. The fully-printed flexible ARSIBs in this work are expected to shed light on the development of energy for wearable electronics.

Building Smarter Aqueous Batteries

Amidst the global trend of advancing renewable energies toward carbon neutrality, energy storage becomes increasingly critical due to the intermittency of renewables. As an alternative to lithium-ion batteries (LIBs), aqueous batteries have received growing attention for large-scale energy storage due to their economical and safe features. Despite the fruitful achievements at the material level, the reliability and lifetime of aqueous batteries are still far from satisfactory. Alike LIBs, integrating smartness is essential for more reliable and long-life aqueous batteries via operando monitoring and automatic response to extreme abuses. In this review, recent advances in sensing techniques and multifunctional battery-sensor systems together with self-healing methods in aqueous batteries is summarized. The significant role of artificial intelligence in designing and optimizing aqueous batteries with high efficiency is also highlighted. Ultimately, it is extrapolated toward the future and present the humble perspective for building smarter aqueous batteries.

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.

Polymer hetero-electrolyte enabled solid-state 2.4-V Zn/Li hybrid batteries

The high redox potential of Zn0/2+ leads to low voltage of Zn batteries and therefore low energy density, plaguing deployment of Zn batteries in many energy-demanding applications. Though employing high-voltage cathode like spinel LiNi0.5Mn1.5O4 can increase the voltages of Zn batteries, Zn2+ ions will be immobilized in LiNi0.5Mn1.5O4 once intercalated, resulting in irreversibility. Here, we design a polymer hetero-electrolyte consisting of an anode layer with Zn2+ ions as charge carriers and a cathode layer that blocks the Zn2+ ion shuttle, which allows separated Zn and Li reversibility. As such, the Zn‖LNMO cell exhibits up to 2.4 V discharge voltage and 450 stable cycles with high reversible capacity, which are also attained in a scale-up pouch cell. The pouch cell shows a low self-discharge after resting for 28 days. The designed electrolyte paves the way to develop high-voltage Zn batteries based on reversible lithiated cathodes.

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-.

Regulating Interlayer-Spacing of Vanadium Phosphates for High-Capacity and Long-Life Aqueous Iron-Ion Batteries

Although the research on aqueous batteries employing metal as the anode is still mainly focused on the aqueous zinc-ion battery, aqueous iron-ion batteries are considered as promising aqueous batteries owing to the lower cost, higher specific capacity, and better stability. However, the sluggish Fe2+ (de)intercalation leads to unsatisfactory specific capacity and poor electrochemical stability, which makes it difficult to find cathode materials with excellent electrochemical properties. Herein, phenylamine (PA)-intercalated VOPO4 materials with expanded interlayer spacing are synthesized and applied successfully in aqueous iron-ion batteries. Owing to enough diffusion space from the expanded interlayer, which can boost fast Fe2+ diffusion, the aqueous iron-ion battery shows a high specific capacity of 170 mAh g-1 at 0.2 A g-1 , excellent rate performance, and cycle stability (96.2% capacity retention after 2200 cycles). This work provides a new direction for cathode material design in the development of aqueous iron-ion batteries.

Decoupled Design for Highly Efficient Perchlorate Anion Intercalation and High-Energy Rechargeable Aqueous Zn-Graphite Batteries

Seeking new cathode chemistry with high onset potential and compatibility with electrolytes has become a challenge for aqueous Zn ion batteries. Anion intercalation in graphite (4.5 V vs Li+ /Li) possesses the potentiality but usually shows a competitive relationship with oxygen evolution reaction (OER) in aqueous solutions. Herein, a decoupled design is proposed to optimize a full utilization of perchlorate ion intercalation in graphite cathode by pH adjustment. Benefiting from the decoupled design, high Coulombic efficiency is obtained by decelerating the kinetic of OER in acidic media. The decoupled Zn-graphite battery exhibits a wide potential window of 2.01 V, as well as an attractive energy density of 231 Wh kg-1 . In addition, a Zn-graphite battery withSO 4 2 - ${\mathrm{SO}}_4^{2 - }$ insertion is assembled, which demonstrates the capability of the proposed decoupled strategy to integrate novel electrode chemistries for high-performance aqueous Zn-based energy storage systems.

Alkaline-based aqueous sodium-ion batteries for large-scale energy storage

Aqueous sodium-ion batteries are practically promising for large-scale energy storage, however energy density and lifespan are limited by water decomposition. Current methods to boost water stability include, expensive fluorine-containing salts to create a solid electrolyte interface and addition of potentially-flammable co-solvents to the electrolyte to reduce water activity. However, these methods significantly increase costs and safety risks. Shifting electrolytes from near neutrality to alkalinity can suppress hydrogen evolution while also initiating oxygen evolution and cathode dissolution. Here, we present an alkaline-type aqueous sodium-ion batteries with Mn-based Prussian blue analogue cathode that exhibits a lifespan of 13,000 cycles at 10 C and high energy density of 88.9 Wh kg-1 at 0.5 C. This is achieved by building a nickel/carbon layer to induce a H3O+-rich local environment near the cathode surface, thereby suppressing oxygen evolution. Concurrently Ni atoms are in-situ embedded into the cathode to boost the durability of batteries.

High-Sulfur Loading and Single Ion-Selective Membranes for High-Energy and Durable Decoupled Aqueous Batteries

The decoupled battery design is promising for breaking the energy density limit of traditional aqueous batteries. However, the complex battery configuration and low-selective separator membranes restrict their energy output and service time. Herein, a zinc-sulfur decoupled aqueous battery is achieved by designing a high-mass loading sulfur electrode and single ion-selective membrane (ISM). A vertically assembled nanosheet network constructed with the assistance of a magnetic field enables facile electron and ion conduction in thick sulfur electrodes, which is conducive to boosting the cell-level energy output. For the tailored ISM, the Na ions anchored on its skeleton effectively prevent the crossover of OH- or Cu2+ , facilitating the transport of Na+ and ensuring structural and mechanical stability. Consequently, the Zn-S aqueous battery achieves a reversible energy density of 3988 Wh kgs -1 (by sulfur mass), stable operation over 300 cycles, and an energy density of 53.2 mWh cm-2 . The sulfur-based decoupled system may be of immediate benefit toward safe, reliable, and affordable static energy storage.

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.

Aqueous batteries: from laboratory to market

This perspective discusses the fundamental benefits and drawbacks of aqueous batteries and the challenges of the development of such battery technology from laboratory scale to industrial applications.

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

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

Thin Film Composite Membranes with Regulated Crossover and Water Migration for Long-Life Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are promising for large-scale long-duration energy storage owing to their inherent safety, decoupled power and energy, high efficiency, and longevity. Membranes constitute an important component that affects mass transport processes in RFBs, including ion transport, redox-species crossover, and the net volumetric transfer of supporting electrolytes. Hydrophilic microporous polymers, such as polymers of intrinsic microporosity (PIM), are demonstrated as next-generation ion-selective membranes in RFBs. However, the crossover of redox species and water migration through membranes are remaining challenges for battery longevity. Here, a facile strategy is reported for regulating mass transport and enhancing battery cycling stability by employing thin film composite (TFC) membranes prepared from a PIM polymer with optimized selective-layer thickness. Integration of these PIM-based TFC membranes with a variety of redox chemistries allows for the screening of suitable RFB systems that display high compatibility between membrane and redox couples, affording long-life operation with minimal capacity fade. Thickness optimization of TFC membranes further improves cycling performance and significantly restricts water transfer in selected RFB systems.

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.

Highly Reversible Zinc Metal Anode in a Dilute Aqueous Electrolyte Enabled by a pH Buffer Additive

Aqueous zinc-ion batteries have drawn increasing attention due to the intrinsic safety, cost-effectiveness and high energy density. However, parasitic reactions and non-uniform dendrite growth on the Zn anode side impede their application. Herein, a multifunctional additive, ammonium dihydrogen phosphate (NHP), is introduced to regulate uniform zinc deposition and to suppress side reactions. The results show that the NH4 + tends to be preferably absorbed on the Zn surface to form a "shielding effect" and blocks the direct contact of water with Zn. Moreover, NH4 + and (H2 PO4 )- jointly maintain pH values of the electrode-electrolyte interface. Consequently, the NHP additive enables highly reversible Zn plating/stripping behaviors in Zn//Zn and Zn//Cu cells. Furthermore, the electrochemical performances of Zn//MnO2 full cells and Zn//active carbon (AC) capacitors are improved. This work provides an efficient and general strategy for modifying Zn plating/stripping behaviors and suppressing side reactions in mild aqueous electrolyte.

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

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

Construction of Atomic Metal-N2 Sites by Interlayers of Covalent Organic Frameworks for Electrochemical H2 O2 Synthesis

Electrosynthesis of H₂ O₂ is a promising alternative to the anthraquinone oxidation process because of its low energy utilization and cost-effectiveness. Heteroatom-doped carbons-based catalysts have been widely developed for H₂ O₂ synthesis. However, their doping degree, defective degree, and location of active sites are difficult to be preciously controlled at molecular level. Herein, a dioxin-linked covalent organic framework (COF) is used as the template to preciously construct different metal-N₂ sites along the porous walls for H₂ O₂ synthesis. By tuning the metal centers, the catalyst with Ca-N₂ sites enables to catalyze H₂ O₂ production with selectivity over 95% from 0.2 to 0.6 V versus RHE, while the H₂ O₂ yields for Co sites or Ni sites are 20% and 60% in the same potential range. In addition, the turnover frequency (TOF) values for Ca-N₂ sites are 11.63 e^-1 site^-1 s^-1 , which are 58 and 20 times higher than those of Co and Ni sites (0.20 and 0.57 e^-1 site^-1 s^-1 ). The theoretical calculations further reveal that the OOH* desorption on Ca sites is easier than those on Co or Ni sites, and thus catalyzes the oxygen reduction reaction in the 2e^- pathway with high efficiency.