A four-electron Zn-I(2) aqueous battery enabled by reversible I(-)/I(2)/I(+) conversion

Dynamical Janus Interface Design for Reversible and Fast-Charging Zinc-Iodine Battery under Extreme Operating Conditions

Aqueous zinc (Zn) iodine (I2) batteries have emerged as viable alternatives to conventional metal-ion batteries. However, undesirable Zn deposition and irreversible iodine conversion during cycling have impeded their progress. To overcome these concerns, we report a dynamical interface design by cation chemistry that improves the reversibility of Zn deposition and four-electron iodine conversion. Due to this design, we demonstrate an excellent Zn-plating/-stripping behavior in Zn||Cu asymmetric cells over 1000 cycles with an average Coulombic efficiency (CE) of 99.95%. Moreover, the Zn||I2 full cells achieve a high-rate capability (217.1 mA h g-1 at 40 A g-1; C rate of 189.5C) at room temperature and enable stable cycling with a CE of more than 99% at -50 °C at a current density of 0.05 A g-1. In situ spectroscopic investigations and simulations reveal that introducing tetraethylammonium cations as ion sieves can dynamically modulate the electrode-electrolyte interface environment, forming the unique water-deficient and chloride ion (Cl-)-rich interface. Such Janus interface accounts for the suppression of side reactions, the prevention of ICl decomposition, and the enrichment of reactants, enhancing the reversibility of Zn-stripping/-plating and four-electron iodine chemistry. This fundamental understanding of the intrinsic interplay between the electrode-electrolyte interface and cations offers a rational standpoint for tuning the reversibility of iodine conversion.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

A Versatile Redox-Active Electrolyte for Solid Fixation of Polyiodide and Dendrite-Free Operation in Sustainable Aqueous Zinc-Iodine Batteries

Practical utilization of zinc-iodine (Zn-I2) batteries is hindered by significant challenges, primarily stemming from the polyiodide shuttle effect on the cathode and dendrite growth on the anode. Herein, a feasible redox-active electrolyte has been introduced with tetraethylammonium iodide as an additive that simultaneously addresses the above mentioned challenges via polyiodide solidification on the cathode and the electrostatic shielding effect on the anode. The tetraethylammonium (TEA+) captures water-soluble polyiodide intermediates (I3 -, I5 -), forming a solid complex at the cathode, thereby suppressing capacity loss during charge/discharge. Furthermore, the TEA+ mitigates dendrite growth on the Zn anode via the electrostatic shielding effect, promoting uniform and compact Zn deposition at the anode. Consequently, the Zn||Zn symmetric cell demonstrates superior cycling stability during Zn plating/stripping over 4,200 h at 1 mA cm-2 and 1 mAh cm-2. The Zn||NiNC full-cell exhibits a stable capacity retention of 98.4% after 20 000 cycles (>5 months) with near-unity Coulombic efficiency at 1 A g-1. The study provides novel insights for establishing a new direction for low-cost, sustainable, and long-lifespan Zn-I2 batteries.

Aqueous Electrolyte With Weak Hydrogen Bonds for Four-Electron Zinc-Iodine Battery Operates in a Wide Temperature Range

In the pursuit of high-performance energy storage systems, four-electron zinc-iodine aqueous batteries (4eZIBs) with successive I-/I2/I+ redox couples are appealing for their potential to deliver high energy density and resource abundance. However, susceptibility of positive valence I+ to hydrolysis and instability of Zn plating/stripping in conventional aqueous electrolyte pose significant challenges. In response, polyethylene glycol (PEG 200) is introduced as co-solvent in 2 m ZnCl2 aqueous solution to design a wide temperature electrolyte. Through a comprehensive investigation combining spectroscopic characterizations and theoretical simulations, it is elucidated that PEG disrupts the intrinsic strong H-bonds of water by global weak PEG-H2O interaction, which strengthens the O─H covalent bond of water and intensifies the coordination with Zn2+. This synergistic effect substantially reduces water activity to restrain the I+ hydrolysis, facilitating I-/I2/I+ redox kinetics, mitigating I3 - formation and smoothening Zn deposition. The 4eZIBs in the optimized hybrid electrolyte not only deliver superior cyclability with a low fading rate of 0.0009% per cycle over 20 000 cycles and a close-to-unit coulombic efficiency but also exhibit stable performance in a wide temperature range from 40 °C to -40 °C. This study offers valuable insights into the rational design of electrolytes for 4eZIBs.

Precisely Constructing Orbital-Coupled Fe─Co Dual-atom Sites for High-Energy-Efficiency Zn-Air/Iodide Hybrid Batteries

Rechargeable Zn-air batteries (ZABs) are promising for energy storage and conversion. However, the high charging voltage and low energy efficiency hinder their commercialization. Herein, these challenges are addressed by employing precisely constructed multifunctional Fe-Co diatomic site catalysts (FeCo-DACs) and integrating iodide/iodate redox into ZABs to create Zinc-air/iodide hybrid batteries (ZAIHBs) with highly efficient multifunctional catalyst. The strong coupling between the 3d orbitals of Fe and Co weakens the excessively strong binding strength between active sites and intermediates, enhancing the catalytic activities for oxygen reduction/evolution reaction and iodide/iodate redox. Consequently, FeCo-DACs exhibit outstanding bifunctional oxygen catalytic activity with a small potential gap (ΔE = 0.66 V) and outstanding stability. Moreover, an outstanding catalytic performance toward iodide/iodate redox is obtained. Therefore, FeCo-DAC-based ZAIHBs exhibit high energy efficiency of up to 75% at 10 mA cm-2 and excellent cycling stability (72% after 500 h). This research offers critical insights into the rational design of DACs and paves the way for high-energy efficiency energy storage devices.

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.

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.

Synergistic effect between S and Te enhancing the electrochemical behavior of heteroatomic TeS-x cathodes in aqueous Zn-TeS batteries

Aqueous Zn-S batteries (AZSBs) have garnered increasing attention in the energy storage field owing to their high capacity, energy density, and cost effectiveness. Nevertheless, sulfur (S) cathodes face challenges, primarily stemming from sluggish reaction kinetics and the formation of an irreversible byproduct (SO42-) during the charge, hindering the progress of AZSBs. Herein, Te-S bonds within S-based cathodes were introduced to enhance electron and ion transport and facilitate the conversion reaction from zinc sulfide (ZnS) to S. This was achieved by constructing heteroatomic TeS-x@Ketjen black composite cathodes (HM-TeS-x@KB, where x  = 36, 9, and 4). The HM-TeS-9@KB electrode exhibits long-term cycling stability, maintaining a capacity decay rate of 0.1 % per cycle over 450 cycles at a current density of 10 A g-1. Crucially, through a combination of experimental data analysis and theoretical calculations, the impact mechanism of Te on the charge and discharge of S active materials within the HM-TeS-9@KB cathode in AZSBs was investigated. The presence of Te-S bonds boost the intrinsic conductivity and wettability of the HM-TeS-9@KB cathode. Furthermore, during the charge, the interaction of preferentially oxidized Te with S atoms within ZnS promotes the oxidation reaction from ZnS to S and suppresses the irreversible side reaction between ZnS and H2O. These findings indicate that the heteroatomization of chalcogen S molecules represents a promising approach for enhancing the electrochemical performance of S cathodes in AZSBs.

Porous CuO Microspheres as Long-Lifespan Cathode Materials for Aqueous Zinc-Ion Batteries

Cathode materials with conversion mechanisms for aqueous zinc-ion batteries (AZIBs) have shown a great potential as next-generation energy storage materials due to their high discharge capacity and high energy density. However, improving their cycling stability has been the biggest challenge plaguing researchers. In this study, CuO microspheres were prepared using a simple hydrothermal reaction, and the morphology and crystallinity of the samples were modulated by controlling the hydrothermal reaction time. The as-synthesized materials were used as cathode materials for AZIBs. The electrochemical experiments showed that the CuO-4h sample, undergoing a hydrothermal reaction for 4 h, had the longest lifecycle and the best rate of capability. A discharge capacity of 131.7 mAh g-1 was still available after 700 cycles at a current density of 500 mA g-1. At a high current density of 1.5 A g-1, the maintained capacity of the cell is 85.4 mA h g-1. The structural evolutions and valence changes in the CuO-4h cathode material were carefully explored by using ex situ XRD and ex situ XPS. CuO was reduced to Cu2O and Cu after the initial discharge, and Cu was oxidized to Cu2O instead of CuO during subsequent charging processes. We believe that these findings could introduce a novel approach to exploring high-performance cathode materials for AZIBs.

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

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

Armoring the cathode with starch gel enables Shuttle-Free Zinc-Iodine batteries

Zinc-iodine batteries (ZIBs) have been recognized as a promising energy storage device due to their high energy density, low cost and environmental friendliness. However, the development of ZIBs is hindered by the shuttle effect of polyiodides which results in capacity degradation and poor cycling performance. Inspired by the ability of starch to form inclusion compounds with iodine, we propose to use a starch gel on the cathode to suppress the shuttle of polyiodides. Herein, porous carbon is utilized as a host for iodine species and provides an excellent conductive network, while starch gel is used as another host to suppress polyiodides shuttle, resulting in improved battery performance. The test results demonstrate that the conversion between I-/I2/I3- in the cathode and the effective inclusion role of starch suppress the shuttle of polyiodides during the charging process. Meanwhile, based on the electrochemical tests and theoretical DFT calculations, it is found that starch has a stronger ability to adsorb polyiodides compared to carbon materials, which enables effective confinement of polyiodides. The ZIBs used the cathode with starch gel exhibit high coulombic efficiency (>95 % at 0.2 A/g) and low self-discharge (86.8 % after resting for 24 h). This strategy is characterized by its simplicity, low cost and high applicability, making it significant for the advancement of high-performance ZIBs.

Recent Progress on Rechargeable Zn-X (X=S, Se, Te, I2, Br2) Batteries

Recently, aqueous Zn-X (X=S, Se, Te, I2, Br2) batteries (ZXBs) have attracted extensive attention in large-scale energy storage techniques due to their ultrahigh theoretical capacity and environmental friendliness. To date, despite tremendous research efforts, achieving high energy density in ZXBs remains challenging and requires a synergy of multiple factors including cathode materials, reaction mechanisms, electrodes and electrolytes. In this review, we comprehensively summarize the various reaction conversion mechanism of zinc-sulfur (Zn-S) batteries, zinc-selenium (Zn-Se) batteries, zinc-tellurium (Zn-Te) batteries, zinc-iodine (Zn-I2) batteries, and zinc-bromine (Zn-Br2) batteries, along with recent important progress in the design and electrolyte of advanced cathode (S, Se, Te, I2, Br2) materials. Additionally, we investigate the fundamental questions of ZXBs and highlight the correlation between electrolyte design and battery performance. This review will stimulate an in-deep understanding of ZXBs and guide the design of conversion batteries.

2D Tungsten Borides Induced Interfacial Modulation Engineering Toward High-Rate Performance Zinc-Iodine Battery

Aqueous zinc-iodine batteries are promising candidates for large-scale energy storage due to their high energy density and low cost. However, their development is hindered by several drawbacks, including zinc dendrites, anode corrosion, and the shuttle of polyiodides. Here, the design of 2D-shaped tungsten boride nanosheets with abundant borophene subunits-based active sites is reported to guide the (002) plane-dominated deposition of zinc while suppressing side reactions, which facilitates interfacial nucleation and uniform growth of zinc. Meanwhile, the interfacial d-band orbits of tungsten sites can further enhance the anchoring of polyiodides on the surface, to promote the electrocatalytic redox conversion of iodine. The resulting tungsten boride-based I2 cathodes in zinc-iodine cells exhibit impressive cyclic stability after 5000 cycles at 50 C, which accelerates the practical applications of zinc-iodine batteries.

Practical four-electron zinc-iodine aqueous batteries enabled by orbital hybridization induced adsorption-catalysis

The successive I-/I0/I+ redox couples in the four-electron zinc-iodine aqueous battery (4eZIB) is plagued by the instability of the electrophilic I+ species, which could either be hydrolyzed or be neutralized by the I3- redox intermediates. We present an adsorption-catalysis approach that effectively suppresses the hydrolysis of ICl species and also provides an enhanced reaction kinetics to surpass the formation of triiodide ions. We elucidate that the improved stability is attributed to the pronounced orbital hybridization between the d orbitals of Fe-N4 moieties (atomic Fe supported on nitrogen doped carbon) and the p orbitals of iodine species (I2 and ICl). Such d-p orbital hybridization leads to enhanced adsorption for iodine species, increased energy barrier for proton detachment from the ICl·HOH intermediate during hydrolysis, and efficient catalysis of the iodine redox reactions with high conversion efficiency. The proposed 4eZIB demonstrates practical areal capacity (>3 mAh cm-2) with a near-unity coulombic efficiency, high energy density of 420 Wh kg-1 (based on cathode mass), and long-term stability (over 10,000 cycles). Even at -20 °C, the battery exhibits stable performance for over 1000 cycles with high iodine utilization ratio.

Constructing static two-electron lithium-bromide battery

Despite their potential as conversion-type energy storage technologies, the performance of static lithium-bromide (SLB) batteries has remained stagnant for decades. Progress has been hindered by the intrinsic liquid-liquid redox mode and single-electron transfer of these batteries. Here, we developed a high-performance SLB battery based on the active bromine salt cathode and the two-electron transfer chemistry with a Br-/Br+ redox couple by electrolyte tailoring. The introduction of NO3- improved the reversible single-electron transition of Br-, and more impressively, the coordinated Cl- anions activated the Br+ conversion to provide an additional electron transfer. A voltage plateau was observed at 3.8 V, and the discharge capacity and energy density were increased by 142 and 159% compared to the one-electron reaction benchmark. This two-step conversion mechanism exhibited excellent stability, with the battery functioning for 1000 cycles. These performances already approach the state of the art of currently established Li-halogen batteries. We consider the established two-electron redox mechanism highly exemplary for diversified halogen 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.

Advanced cathodes for aqueous Zn batteries beyond Zn2+ intercalation

Aqueous zinc (Zn) batteries have attracted global attention for energy storage. Despite significant progress in advancing Zn anode materials, there has been little progress in cathodes. The predominant cathodes working with Zn2+/H+ intercalation, however, exhibit drawbacks, including a high Zn2+ diffusion energy barrier, pH fluctuation(s) and limited reproducibility. Beyond Zn2+ intercalation, alternative working principles have been reported that broaden cathode options, including conversion, hybrid, anion insertion and deposition/dissolution. In this review, we report a critical assessment of non-intercalation-type cathode materials in aqueous Zn batteries, and identify strengths and weaknesses of these cathodes in small-scale batteries, together with current strategies to boost material performance. We assess the technical gap(s) in transitioning these cathodes from laboratory-scale research to industrial-scale battery applications. We conclude that S, I2 and Br2 electrodes exhibit practically promising commercial prospects, and future research is directed to optimizing cathodes. Findings will be useful for researchers and manufacturers in advancing cathodes for aqueous Zn batteries beyond Zn2+ intercalation.

Aqueous Rechargeable Zn-Iodine Batteries: Issues, Strategies and Perspectives

The static aqueous rechargeable Zn-Iodine batteries (ARZiBs) have been studied extensively because of their low-cost, high-safety, moderate voltage output, and other unique merits. Nonetheless, the poor electrical conductivity and thermodynamic instability of the iodine cathode, the complicated conversion mechanism, and the severe interfacial reactions at the Zn anode side induce their low operability and unsatisfactory cycling stability. This review first clarifies the typical configuration of ARZiBs with a focus on the energy storage mechanism and uncovers the issues of the ARZiBs from a fundamental point of view. After that, it categorizes the recent optimization strategies into cathode fabrication, electrolyte modulation, and separator/anode modification; and summarizes and highlights the achieved progress of these strategies in advanced ARZiBs. Given that the ARZiBs are still at an early stage, the future research outlook is provided, which hopefully may guide the rational design of advanced ARZiBs.

Development of Inverse-Opal-Structured Charge-Deficient Co9S8@nitrogen-Doped-Carbon to Catalytically Enable High Energy and High Power for the Two-Electron Transfer I+/I- Electrode

The iodine (I) electrode involving two-electron transfer chemistry by converting between I+ and I-, has the potential to deliver theoretically doubled capacity and higher working voltage platforms, thus achieving higher energy density. However, owing to the slow kinetics of the cascade two-electron transfer reactions, the system suffers from large overpotentials and low power density, especially at high working currents and low temperatures. Here, an inverse-opal-structured cobalt sulfide@nitrogen-doped-carbon (Co9S8@NC) catalyst with unique charge-deficient states is developed to promote the reaction kinetics of the I-/I+ electrode. The charge-deficient Co9S8@NC catalyst not only enables strong physicochemical adsorption with the iodine species but also significantly reduces the activation energy and interfacial charge transfer resistance of the cascade I+/I0/I- conversion reaction. Consequently, the prototypical Zn‖I+/I0/I- battery equipped with the Co9S8@NC catalyst can deliver a high energy density of 554 Wh kg-1 and a stable cycle life of 5000 cycles at 30 °C. Moreover, at a subzero temperature of -30 °C, the battery can exhibit enhanced kinetics and a high power density of 1514 W kg-1, high energy density of 485 Wh kg-1.

Halogen-powered static conversion chemistry

Halogen-powered static conversion batteries (HSCBs) thrive in energy storage applications. They fall into the category of secondary non-flow batteries and operate by reversibly changing the chemical valence of halogens in the electrodes or/and electrolytes to transfer electrons, distinguishing them from the classic rocking-chair batteries. The active halide chemicals developed for these purposes include organic halides, halide salts, halogenated inorganics, organic-inorganic halides and the most widely studied elemental halogens. Aside from this, various redox mechanisms have been discovered based on multi-electron transfer and effective reaction pathways, contributing to improved electrochemical performances and stabilities of HSCBs. In this Review, we discuss the status of HSCBs and their electrochemical mechanism-performance correlations. We first provide a detailed exposition of the fundamental redox mechanisms, thermodynamics, conversion and catalysis chemistry, and mass or electron transfer modes involved in HSCBs. We conclude with a perspective on the challenges faced by the community and opportunities towards practical applications of high-energy halogen cathodes in energy-storage devices.

Extended Battery Compatibility Consideration from an Electrolyte Perspective

The performance of electrochemical batteries is intricately tied to the physicochemical environments established by their employed electrolytes. Traditional battery designs utilizing a single electrolyte often impose identical anodic and cathodic redox conditions, limiting the ability to optimize redox environments for both anode and cathode materials. Consequently, advancements in electrolyte technologies are pivotal for addressing these challenges and fostering the development of next-generation high-performance electrochemical batteries. This review categorizes perspectives on electrolyte technology into three key areas: additives engineering, comprehensive component analysis encompassing solvents and solutes, and the effects of concentration. By summarizing significant studies, the efficacy of electrolyte engineering is highlighted, and the review advocates for further exploration of optimized component combinations. This review primarily focuses on liquid electrolyte technologies, briefly touching upon solid-state electrolytes due to the former greater vulnerability to electrode and electrolyte interfacial effects. The ultimate goal is to generate increased awareness within the battery community regarding the holistic improvement of battery components through optimized combinations.

From Inorganic to Organic Iodine: Stabilization of I+ Enabling High-Energy Lithium-Iodine Battery

Organic materials have been considered a class of promising cathodes for metal-ion batteries because of their sustainability in preparation and source. However, organic batteries with high energy density and application potential require high discharge voltage, multielectron transfer, and long cycling performance. Here, we report an exceptional lithium-iodine (Li//I2) battery, in which the organic iodine (BPD-HI) cathode formed by the Lewis acid-base coordination between hydroiodic acid (HI) and 4,4'-bipyridine (BPD) allows 2e- transfer via the I-/I0 and I0/I+ redox couples. The I+ stabilized by BPD exhibits a high discharge voltage plateau at ∼3.4 V. Remarkably, from inorganic to organic iodine, it realizes a 2-fold increase in the achieved capacity, up to ∼400 mA h gI-1 (Theor. 422 mA h gI-1 and 245.6 mA h g-1 based on the mass of BPD-HI), and an over 2-fold energy density, reaching 1160 W h kgI-1 (Theor. 1324 W h kgI-1). More importantly, a capacity retention rate of 85% over 850 cycles is attained for the Li//BPD-HI battery at a current density of 2 A gI-1. This facile strategy enables positively charged I+ to be electrochemically active in a rechargeable lithium battery. The new redox chemistry discovered provides new insights for developing organic batteries with high energy density.

Suppressing the Shuttle Effect of Aqueous Zinc-Iodine Batteries: Progress and Prospects

Aqueous zinc-iodine batteries are considered to be one of the most promising devices for future electrical energy storage due to their low cost, high safety, high theoretical specific capacity, and multivalent properties. However, the shuttle effect currently faced by zinc-iodine batteries causes the loss of cathode active material and corrosion of the zinc anodes, limiting the large-scale application of zinc-iodine batteries. In this paper, the electrochemical processes of iodine conversion and the zinc anode, as well as the induced mechanism of the shuttle effect, are introduced from the basic configuration of the aqueous zinc-iodine battery. Then, the inhibition strategy of the shuttle effect is summarized from four aspects: the design of cathode materials, electrolyte regulation, the modification of the separator, and anode protection. Finally, the current status of aqueous zinc-iodine batteries is analyzed and recommendations and perspectives are presented. This review is expected to deepen the understanding of aqueous zinc-iodide batteries and is expected to guide the design of high-performance aqueous zinc-iodide batteries.

Low-current-density stability of vanadium-based cathodes for aqueous zinc-ion batteries

Vanadium-based cathodes have received widespread attention in the field of aqueous zinc-ion batteries, presenting a promising prospect for stationary energy storage applications. However, the rapid capacity decay at low current densities has hampered their development. In particular, capacity stability at low current densities is a requisite in numerous practical applications, typically encompassing peak load regulation of the electricity grid, household energy storage systems, and uninterrupted power supplies. Despite possessing notably high specific capacities, vanadium-based materials exhibit severe instability at low current densities. Moreover, the issue of stabilizing electrode reactions at these densities for vanadium-based materials has been explored insufficiently in existing research. This review aims to investigate the matter of stability in vanadium-based materials at low current densities by concentrating on the mechanisms of capacity fading and optimization strategies. It proposes a comprehensive approach that includes electrolyte optimization, electrode modulation, and electrochemical operational conditions. Finally, we presented several crucial prospects for advancing the practical development of vanadium-based aqueous zinc-ion batteries.

Rational electrolyte design and electrode regulation for boosting high-capacity Zn-iodine fiber-shaped batteries with four-electron redox reactions

Aqueous Zn ion-based fiber-shaped batteries (AZFBs) with the merits of high flexibility and safety have received much attention for powering wearable electronic devices. However, the relatively low specific capacity provided by cathode materials limits their practical application. Herein, we first propose a simple strategy for fabricating high-capacity Zn-iodine fiber-shaped batteries with a high concentration electrolyte and a reduced graphene oxide fiber (GF) cathode. It was found that oxygen functional groups in the graphene sheet demonstrate strong interaction with polyiodides but hinder electron conductivity; thus, the optimal balance between the specific capacity and coulombic efficiency of the GF electrode can be a function of the surface properties at different hydrothermal temperatures. Besides, the regulated high concentration electrolyte effectively suppresses the diffusion of polyiodides, which is attributed to the constrained freedom of water. More importantly, a four-electron redox mechanism was experimentally revealed through in situ Raman spectra. As a result, this fiber-shaped battery delivers a superior high reversible capacity of 390 mA h cm-3 at 1 A cm-3, an excellent rate performance of 125.7 mA h cm-3 at a high current density of 8 A cm-3 and outstanding cycling life with 82% capacitance retention after 2500 cycles.

Asymmetric Anion Zinc Salt Derived Solid Electrolyte Interphase Enabled Long-Lifespan Aqueous Zinc Bromine Batteries

Organic additives with high-reduction potentials are generally applied in aqueous electrolytes to stabilize the Zn anode, while compromise safety and environmental compatibility. Highly concentrated water-in-salt electrolytes have been proposed to realize the high reversibility of Zn plating/stripping; however, their high cost and viscosity hinder their practical applications. Therefore, exploring low-concentration Zn salts, that can be used directly to stabilize Zn anodes, is of primary importance. Herein, we developed an asymmetric anion group, bi(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (DFTFSI- )-based novel zinc salt, Zn(DFTFSI)2 , to obtain a high ionic conductivity and a highly stable dendrite-free Zn anode. Experimental tests and theoretical calculations verified that DFTFSI- in the Zn2+ solvation sheath and inner Helmholtz plane would be preferentially reduced to construct layer-structured SEI films, inhibiting hydrogen evolution and side reactions. Consequently, the Zn| | ${||}$ Zn symmetric cell with 1M Zn(DFTFSI)2 aqueous electrolyte delivers an ultralong cycle life for >2500 h outperforming many other conventional Zn salt electrolytes. The Zn| | ${||}$ Br2 battery also exhibits a long lifespan over 1200 cycles at ~99.8 % Coulombic efficiency with a high capacity retention of 92.5 %. Furthermore, this outstanding performance translates well to a high-areal-capacity Zn| | ${||}$ Br2 battery (~5.6 mAh ⋅ cm-2 ), cycling over 320 cycles with 95.3 % initial capacity retained.

Stabilized four-electron aqueous zinc-iodine batteries by quaternary ammonium complexation

Four-electron aqueous zinc-iodine batteries (4eZIBs) leveraging the I-/I0/I+ redox couple have garnered attention for their potential high voltage, capacity, and energy density. However, the electrophilic I+ species is highly susceptible to hydrolysis due to the nucleophilic attack by water. Previous endeavors to develop 4eZIBs primarily relied on highly concentrated aqueous electrolytes to mitigate the hydrolysis issue, nonetheless, it introduced challenges associated with dissolution, high electrolyte viscosity, and sluggish electrode kinetics. In this work, we present a novel complexation strategy that capitalizes on quaternary ammonium salts to form solidified compounds with I+ species, rendering them impervious to solubilization and hydrolysis in aqueous environments. The robust interaction in this complexation chemistry facilitates a highly reversible I-/I0/I+ redox process, significantly improving reaction kinetics within a conventional ZnSO4 aqueous electrolyte. The proposed 4eZIB exhibits a superior rate capability and an extended lifespan of up to 2000 cycles. This complexation chemistry offers a promising pathway for the development of advanced 4eZIBs.

Advancements in aqueous zinc-iodine batteries: a review

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

Mechanically Interlocked Interphase with Energy Dissipation and Fast Li-Ion Transport for High-Capacity Lithium Metal Batteries

Constructing an artificial solid electrolyte interphase (ASEI) on Li metal anodes (LMAs) is a potential strategy for addressing the dendrite issues. However, the mechanical fatigue of the ASEI caused by stress accumulation under the repeated deformation from the Li plating/stripping is not taken seriously. Herein, this work introduces a mechanically interlocked [an]daisy chain network (DC MIN) into the ASEI to stabilize the Li metal/ASEI interface by combining the functions of energy dissipation and fast Li-ion transport. The DC MIN featured by large-range molecular motions is cross-linked via efficient thiol-ene click chemistry; thus, the DC MIN has flexibility and excellent mechanical properties. As an ASEI, the crown ether units in DC MIN not only interact with the dialkylammonium of a flexible chain, forming the energy dissipation behavior but also coordinate with Li ion to support the fast Li-ion transport in DC MIN. Therefore, a stable 2800 h-symmetrical cycling (1 mA cm-2 ) and an excellent 5 C-rate (full cell with LiFePO4 ) performance are achieved by DC MIN-based ASEI. Furthermore, the 1-Ah pouch cell (LiNi0.88 Co0.09 Mn0.03 O2 cathode) with DC MIN-coated LMA exhibits improved capacity retention (88%) relative to the Control. The molecular design of DC MIN provides new insights into the optimization of an ASEI for high-energy LMAs.

Adaptive Ion Channels Formed in Ultrathin and Semicrystalline Polymer Interphases for Stable Aqueous Batteries

Aqueous Zn batteries have recently emerged as promising candidates for large-scale energy storage, driven by the need for a safe and cost-effective technology with sufficient energy density and readily accessible electrode materials. However, the energy density and cycle life of Zn batteries have been limited by inherent chemical, morphological, and mechanical instabilities at the electrode-electrolyte interface where uncontrolled reactions occur. To suppress the uncontrolled reactions, we designed a crystalline polymer interphase for both electrodes, which simultaneously promotes electrode reversibility via fast and selective Zn transport through the adaptive formation of ion channels. The interphase comprises an ultrathin layer of crystalline poly(1H,1H,2H,2H-perfluorodecyl acrylate), synthesized and applied as a conformal coating in a single step using initiated chemical vapor deposition (iCVD). Crystallinity is optimized to improve interphase stability and Zn-ion transport. The optimized interphase enables a cycle life of 9500 for Zn symmetric cells and over 11,000 for Zn-MnO2 full-cell batteries. We further demonstrate the generalizability of this interphase design using Cu and Li as examples, improving their stability and achieving reversible cycling in both. The iCVD method and molecular design unlock the potential of highly reversible and cost-effective aqueous batteries using earth-abundant Zn anode materials, pointing to grid-scale energy storage.

Facile preparation of urchin-like morphology V2O3-VN nano-heterojunction cathodes for high-performance Aqueous zinc ion battery

Rechargeable aqueous zinc ion batteries (RAZIBs) are of interest for energy storage in smart grids. However, slow Zn2+ diffusion kinetics, insufficient active sites, and poor intrinsic conductivity are always challenging to exploit the huge potential of the batteries. Here, we prepare V2O3-VN nano-heterojunction composites with sea urchin-like morphology as the cathode for AZIBs. The electrode achieves high capacities (e.g., 0.1 A g-1, 532.6 mAh g-1), good rate and cycle performance (263.4 mAh g-1 at 5 A g-1 current density with 90.8% capacity retention). Detailed structural analyses suggest that the V2O3-VN composite is composed of different crystal planes of V2O3 and VN, which form an efficient heterogeneous interfacial network in the bulk electrode, accounting for its good electrochemical properties. Theoretical calculations reveal that compared with V2O3, VN and physically mixed V2O3/VN, the V2O3-VN heterostructure exhibits good cation adsorption and electrode conductivity, thereby accelerating the charge carrier mobility and electrochemical activity of the electrode. Moreover, ex-situ characterization techniques are utilized to investigate the zinc storage mechanism in detail, providing new ideas for the development of AZIBs cathode materials through the construction of heterojunction structures.

Perovskite Cathodes for Aqueous and Organic Iodine Batteries Operating Under One and Two Electrons Redox Modes

Although conversion-type iodine-based batteries are considered promising for energy storage systems, stable electrode materials are scarce, especially for high-performance multi-electron reactions. The use of tin-based iodine-rich 2D Dion-Jacobson (DJ) ODASnI4 (ODA: 1,8-octanediamine) perovskite materials as cathode materials for iodine-based batteries is suggested. As a proof of concept, organic lithium-perovskite and aqueous zinc-perovskite batteries are fabricated and they can be operated based on the conventional one-electron and advanced two-electron transfer modes. The active elemental iodine in the perovskite cathode provides capacity through a reversible I- /I+ redox pair conversion at full depth, and the rapid electron injection/extraction leads to excellent reaction kinetics. Consequently, high discharge plateaus (1.71 V vs Zn2+ /Zn; 3.41 V vs Li+ /Li), large capacity (421 mAh g-1 I ), and a low decay rate (1.74 mV mAh-1 g-1 I ) are achieved for lithium and zinc ion batteries, respectively. This study demonstrates the promising potential of perovskite materials for high-performance metal-iodine batteries. Their reactions based on the two-electron transfer mechanism shed light on similar battery systems aiming for decent operational stability and high energy density.

Deploying Cationic Cellulose Nanofiber Confinement to Enable High Iodine Loadings Towards High Energy and High-Temperature Zn-I2 Battery

High iodine loading and high-temperature adaptability of the iodine cathode are prerequisites to achieving high energy density at full battery level and promoting the practical application for the zinc-iodine (Zn-I2 ) battery. However, it would aggravate the polyiodide shuttle effect when employing high iodine loading and working temperature. Here, a sustainable cationic cellulose nanofiber (cCNF) was employed to confine the active iodine species through strong physiochemical adsorption to enlarge the iodine loading and stabilize it even at high temperatures. The cCNF could accommodate dual-functionality by enlarging the iodine loading and suppressing the polyiodide shuttle effect, owing to the unique framework structure with abundant surface positive charges. As a result, the iodine cathode based on the cCNF could deliver high iodine mass loading of 14.1 mg cm-2 with a specific capacity of 182.7 mAh g-1 , high areal capacity of 2.6 mAh cm-2 , and stable cycling over 3000 cycles at 2 A g-1 , thus enabling a high energy density of 34.8 Wh kg-1 and the maximum power density of 521.2 W kg-1 at a full Zn-I2 battery level. In addition, even at a high temperature of 60 °C, the Zn-I2 battery could still deliver a stable cycling.

Six-Electron-Redox Iodine Electrodes for High-Energy Aqueous Batteries

Iodine (I2 ) shows great promising as the active material in aqueous batteries due to its distinctive merits of high abundance in ocean and low cost. However, in conventional aqueous I2 -based batteries, the energy storage mechanism of I- /I2 conversion is only two-electron redox reaction, limiting their energy density. Herein, six-electron redox chemistry of I2 electrodes is achieved via the synergistic effect of redox-ion charge-carriers and halide ions in electrolytes. The redox-active Cu2+ ions in electrolytes induce the conversion between Cu2+ ions and I2 to CuI at low potential. Simultaneously, the Cl- ions in electrolytes activate the I2 /ICl redox couple at high potential. As a result, in our case, I2 -based battery system with six-electron redox is developed. Such energy storage mechanism with six-electron redox leads to high discharge potential and capacity, excellent rate capability, as well as stable cycling behavior of I2 electrodes. Impressively, six-electron-redox I2 cathodes can match various aqueous metal (e.g. Zn, Mn and Fe) anodes to construct metal||I2 hybrid batteries. These hybrid batteries not only deliver enhanced capacities, but also exhibit higher operate voltages, which contributes to superior energy densities. Therefore, this work broadens the horizon for the design of high-energy aqueous I2 -based batteries.

Crowding Effect-Induced Zinc-Enriched/Water-Lean Polymer Interfacial Layer Toward Practical Zn-Iodine Batteries

Although the meticulous design of functional diversity within the polymer interfacial layer holds paramount significance in mitigating the challenges associated with hydrogen evolution reactions and dendrite growth in zinc anodes, this pursuit remains a formidable task. Here, a large-scale producible zinc-enriched/water-lean polymer interfacial layer, derived from carboxymethyl chitosan (CCS), is constructed on zinc anodes by integration of electrodeposition and a targeted complexation strategy for highly reversible Zn plating/stripping chemistry. Zinc ions-induced crowding effect between CCS skeleton creates a strong hydrogen bonding environment and squeezes the moving space for water/anion counterparts, therefore greatly reducing the number of active water molecules and alleviating cathodic I3- attack. Moreover, the as-constructed Zn2+-enriched layer substantially facilitate rapid Zn2+ migration through the NH2-Zn2+-NH2 binding/dissociation mode of CCS molecule chain. Consequently, the large-format Zn symmetry cell (9 cm2) with a Zn-CCS electrode demonstrates excellent cycling stability over 1100 h without bulging. When coupled with an I2 cathode, the assembled Zn-I2 multilayer pouch cell displays an exceptionally high capacity of 140 mAh and superior long-term cycle performance of 400 cycles. This work provides a universal strategy to prepare large-scale production and high-performance polymer crowding layer for metal anode-based battery, analogous outcomes were veritably observed on other metals (Al, Cu, Sn).

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

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

A Reversible Six-Electron Transfer Cathode for Advanced Aqueous Zinc Batteries

The electrochemical reactions for the storage of Zn2+ while embracing more electron transfer is a foundation of the future high-energy aqueous zinc batteries. Herein, we report a six-electron transfer electrochemistry of nano-sized TeO2 /C (n-TeO2 /C) cathode by facilitating the reversible conversion of TeO2 ↔Te and Te↔ZnTe. Benefitting from the integrated conductive nanostructure and the proton-rich environment in providing optimized electrochemical kinetics (facilitated Zn2+ uptake and high electronic conductivity) and feasible thermodynamic process (low Gibbs free energy change), the as-prepared n-TeO2 /C with stable cycling performance exhibits a superior reversible capacity of over 800 mAh g-1 at 0.1 A g-1 . A precise understanding of the reaction mechanism via ex situ and in situ characterizations presents that the reversible six-electron transfer reaction is proton-dependent, and a proton generating and consuming mechanism of three-phase conversion n-TeO2 /C in the weakly acidic electrolyte is thoroughly revealed.

Establishing Ultralow Self-Discharge Zn-I2 Battery by Optimizing ZnSO4 Electrolyte Concentration

As one of promising candidates for large-scale energy-storage systems, Zn-I2 aqueous battery exhibits multifaceted advantages including low cost, high energy/powder density, and intrinsic operational safety, but also suffers from fast self-discharge and short cycle/shelf lifespan associating with I3 - shuttle, Zn dendrite growth, and corrosion. In this paper, the battery's self-discharge rate is successfully suppressed down to an unprecedent level of 17.1% after an ultralong shelf-time of 1 000 h (i.e., 82.9% capacity retention after 41 days open-circuit storage), by means of manipulating solvation structures of traditional ZnSO4 electrolyte via simply adjusting electrolyte concentration. Better yet, the optimized 2.7 m ZnSO4 electrolyte further prolongs the cycle lifespan of the battery up to >10 000 and 43 000 cycles at current density of 1 and 5 A g-1 , respectively, thanks to the synthetic benefits from reduced free water content, modified solvation structure and lowered I2 dissolution in the electrolyte. With both long lifespan and ultralow self-discharge, this reliable and affordable Zn-I2 battery may provide a feasible alternative to the centuries-old lead-acid battery.

A Comprehensive Evaluation of Battery Technologies for High-Energy Aqueous Batteries

Aqueous batteries have garnered significant attention in recent years as a viable alternative to lithium-ion batteries for energy storage, owing to their inherent safety, cost-effectiveness, and environmental sustainability. This study offers a comprehensive review of recent advancements, persistent challenges, and the prospects of aqueous batteries, with a primary focus on energy density compensation of various battery engineering technologies. Additionally, cutting-edge high-energy aqueous battery designs are emphasized as a reference for future endeavors in the pursuit of high-energy storage solutions. Finally, a dual-compatibility battery configuration perspective aimed at concurrently optimizing cycle stability, redox potential, capacity utilization for both anode and cathode materials, as well as the selection of potential electrode candidates, is proposed with the ultimate goal of achieving cell-level energy densities exceeding 400 Wh kg-1 .

Toward Sustainable Metal-Iodine Batteries: Materials, Electrochemistry and Design Strategies

Due to the natural abundance of iodine, cost-effective, and sustainability, metal-iodine batteries are competitive for the next-generation energy storage systems with high energy density, and large power density. However, the inherent properties of iodine such as electronic insulation and shuttle behavior of soluble iodine species affect negatively rate performance, cyclability, and self-discharge behavior of metal-iodine batteries, while the dendrite growth and metal corrosion on the anode side brings potential safety hazards and inferior durability. These problems of metal-iodine system still exist and need to be solved urgently. Herein, we summarize the research progress of metal-iodine batteries in the past decades. Firstly, the classification, design strategy and reaction mechanism of iodine electrode are briefly outlined. Secondly, the current development and protection strategy of conventional metal anodes in metal-iodine batteries are highlighted, and some potential anode materials and their design strategies are proposed. Thirdly, the key electrochemical parameters of state-of-art metal-iodine batteries are compared and analyzed to solve critical issues for realizing next-generation iodine-based energy storage systems. Therefore, the aim of this review is to promote the development of metal-iodine batteries and provide guidelines for their design.

Porous Cyclodextrin Polymer Enables Dendrite-Free and Ultra-Long Life Solid-State Zn-I2 Batteries

Zn-I2 batteries have attracted attention due to their low cost, safety, and environmental friendliness. However, their performance is still limited by the irreversible growth of Zn dendrites, hydrogen evolution reactions, corrosion, and shuttle effect of polyiodide. In this work, we have prepared a new porous polymer (CD-Si) by nucleophilic reaction of β-cyclodextrin with SiCl4 , and CD-Si is applied to the solid polymer electrolyte (denoted PEO/PVDF/CD-Si) to solve above-mentioned problems. Through the anchoring of the CD-Si, a conductive network with dual transmission channels was successfully constructed. Due to the non-covalent anchoring effect, the ionic conductivity of the solid polymer electrolytes (SPE) can reach 1.64×10-3  S cm-1 at 25 °C. The assembled symmetrical batteries can achieve highly reversible dendrite-free galvanizing/stripping (stable cycling for 7500 h at 5 mA cm-2 and 1200 h at 20 mA cm-2 ). The solid-state Zn-I2 battery shows an ultra-long life of over 35,000 cycles at 2 A g-1 . Molecular dynamics simulations are performed to elucidate the working mechanism of CD-Si in the polymer matrix. This work provides a novel strategy towards solid electrolytes for Zn-I2 batteries.

Amino-Functionalized Interfacial Layer Enables an Ultra-Uniform Amorphous Solid Electrolyte Interphase for High-Performance Aqueous Zinc-Based Batteries

Aqueous rechargeable zinc-based batteries (ZBBs) are emerging as desirable energy storage systems because of their high capacity, low cost, and inherent safety. However, the further application of ZBBs still faces many challenges, such as the issues of uncontrolled dendrite growth and severe parasitic reactions occurring at the Zn anode. Herein, an amino-grafted bacterial cellulose (NBC) film is prepared as artificial solid electrolyte interphase (SEI) for the Zn metal anodes, which can significantly reduce zinc nucleation overpotential and lead to the dendrite-free deposition of Zn metal along the (002) crystal plane more easily without any external stimulus. More importantly, the chelation between the modified amino groups and zinc ions can promote the formation of an ultra-even amorphous SEI upon cycling, reducing the activity of hydrate ions, and inhibiting the water-induced side reactions. As a result, the Zn||Zn symmetric cell with NBC film exhibits lower overpotential and higher cyclic stability. When coupled with the V2 O5 cathode, the practical pouch cell achieves superior electrochemical performance over 1000 cycles.

Hetero-Polyionic Hydrogels Enable Dendrites-Free Aqueous Zn-I2 Batteries with Fast Kinetics

Rechargeable aqueous Zn-I2 batteries (ZIB) are regarded as a promising energy storage candidate. However, soluble polyiodide shuttling and rampant Zn dendrite growth hamper its commercial implementation. Herein, a hetero-polyionic hydrogel is designed as the electrolyte for ZIBs. On the cathode side, iodophilic polycationic hydrogel (PCH) effectively alleviates the shuttle effect and facilitates the redox kinetics of iodine species. Meanwhile, polyanionic hydrogel (PAH) toward Zn metal anode uniformizes Zn2+ flux and prevents surface corrosion by electrostatic repulsion of polyiodides. Consequently, the Zn symmetric cells with PAH electrolyte demonstrate remarkable cycling stability over 3000 h at 1 mA cm-2 (1 mAh cm-2 ) and 800 h at 10 mA cm-2 (5 mAh cm-2 ). Moreover, the Zn-I2 full cells with PAH-PCH hetero-polyionic hydrogel electrolyte deliver a low-capacity decay of 0.008 ‰ per cycle during 18 000 cycles at 8 C. This work sheds light on hydrogel electrolytes design for long-life conversion-type aqueous batteries.

Unlocking the Capacity of Bismuth Oxide by a Redox Mediator Strategy for High-Performance Aqueous Zn-Ion Batteries

Many cathode materials store zinc ions based on the intercalation reaction mechanism in neutral aqueous Zn-ion batteries, and the structural design of the cathodes has been stuck in the curing mode by extending the ion diffusion channel. Here, we first develop halide ions to unlock the electrochemical activity of conversion-type Bi2O3 in aqueous Zn-ion batteries. Notably, the iodide ion shows the best performance compatibility with the Bi2O3 cathode. The electrochemical reaction mechanism studies show that iodide ions can be regarded as a redox medium to reduce the charge-transfer activation energy and motivate the conversion of Bi2O3 from Bi3+ to Bi0 during the cycle. Unsurprising, the discharge-specific capacity can reach 436.8 mAh g-1 at 0.5 A g-1 and achieve a cyclic lifespan of 6000 cycles at a current density of 3 A g-1. The activation of the Bi2O3 conversion reaction by iodide ions is of great significance for broadening the research range of ZIB cathode materials.

Three-Electron Transfer-Based High-Capacity Organic Lithium-Iodine (Chlorine) Batteries

Conversion-type batteries apply the principle that more charge transfer is preferable. The underutilized electron transfer mode within two undermines the electrochemical performance of halogen batteries. Here, we realised a three-electron transfer lithium-halogen battery based on I- /I+ and Cl- /Cl0 couples by using a common commercial electrolyte saturated with Cl- anions. The resulting Li||tetrabutylammonium triiodide (TBAI3 ) cell exhibits three distinct discharging plateaus at 2.97, 3.40, and 3.85 V. Moreover, it has a high capacity of 631 mAh g-1 I (265 mAh g-1 electrode , based on entire mass loading) and record-high energy density of up to 2013 Wh kg-1 I (845 Wh kg-1 electrode ). To support these findings, experimental characterisations and density functional theory calculations were conducted to elucidate the redox chemistry involved in this novel interhalogen strategy. We believe our paradigm presented here has a foreseeable inspiring effect on other halogen batteries for high-energy-density pursuit.

Transforming a Primary Li-SOCl2 Battery into a High-Power Rechargeable System via Molecular Catalysis

Li-SOCl2 batteries possess ultrahigh energy densities and superior safety features at a wide range of operating temperatures. However, the Li-SOCl2 battery system suffers from poor reversibility due to the sluggish kinetics of SOCl2 reduction during discharging and the oxidation of the insulating discharge products during charging. To achieve a high-power rechargeable Li-SOCl2 battery, herein we introduce the molecular catalyst I2 into the electrolyte to tailor the charging and discharging reaction pathways. The as-assembled rechargeable cell exhibits superior power density, sustaining an ultrahigh current density of 100 mA cm-2 during discharging and delivering a reversible capacity of 1 mAh cm-2 for 200 cycles at a current density of 2 mA cm-2 and 6 mAh cm-2 for 50 cycles at a current density of 5 mA cm-2. Our results reveal the molecular catalyst-mediated reaction mechanisms that fundamentally alter the rate-determining steps of discharging and charging in Li-SOCl2 batteries and highlight the viability of transforming a primary high-energy battery into a high-power rechargeable system, which has great potential to meet the ever-increasing demand of energy-storage systems.

Zinc-dual-halide complexes suppressing polyhalide formation for rechargeable aqueous zinc-halogen batteries

Aqueous zinc-halogen batteries suffer from poor coulombic efficiency and short cycle life owing to the formation and dissolution of polyhalides in electrolytes. Herein, we apply a zinc-dual-halide complex strategy to confine free halides and suppress polyhalide formation. The high stabilities of zinc-dual-halide complexes are identified to be essential for effective confinement. The resulting Zn-Br2 and Zn-I2 cells deliver excellent rate capability and cycling stability, as well as high coulombic efficiency and energy efficiency.

Low-cost and Non-flammable Eutectic Electrolytes for Advanced Zn-I2 Batteries

As a burgeoning electrolyte system, eutectic electrolytes based on ZnCl2 /Zn(CF3 SO3 )2 /Zn(TFSI)2 have been widely proposed in advanced Zn-I2 batteries; however, safety and cost concerns significantly limit their applications. Here, we report new-type ZnSO4 -based eutectic electrolytes that are both safe and cost-effective. Their universality is evident in various solvents of polyhydric alcohols, in which multiple -OH groups not only involve in Zn2+ solvation but also interact with water, resulting in the high stability of electrolytes. Taking propylene glycol-based hydrated eutectic electrolyte as an example, it features significant advantages in non-flammability and low price that is <1/200 cost of Zn(CF3 SO3 )2 /Zn(TFSI)2 -based eutectic electrolytes. Moreover, its effectiveness in confining the shuttle effects of I2 cathode and side reactions of Zn anodes is evidenced, resulting in Zn-I2 cells with high reversibility at 1 C and 91.4 % capacity remaining under 20 C. After scaling up to the pouch cell with a record mass loading of 33.3 mg cm-2 , super-high-capacity retention of 96.7 % is achieved after 500 cycles, which exceeds other aqueous counterparts. This work significantly broadens the eutectic electrolyte family for advanced Zn battery design.

Tellurium with Reversible Six-Electron Transfer Chemistry for High-Performance Zinc Batteries

Chalcogens, especially tellurium (Te), as conversion-type cathodes possess promising prospects for zinc batteries (ZBs) with potential rich valence supply and high energy density. However, the conversion reaction of Te is normally restricted to the Te2-/Te0 redox with a low voltage plateau at ∼0.59 V (vs Zn2+/Zn) rather than the expected positive valence conversion of Te0 to Ten+, inhibiting the development of Te-based batteries toward high output voltage and energy density. Herein, the desired reversible Te2-/Te0/Te2+/Te4+ redox behavior with up to six-electron transfer was successfully activated by employing a highly concentrated Cl--containing electrolyte (Cl- as strong nucleophile) for the first time. Three flat discharge plateaus located at 1.24, 0.77, and 0.51 V, respectively, are attained with a total capacity of 802.7 mAh g-1. Furthermore, to improve the stability of Ten+ products and enhance the cycling stability, a modified ionic liquid (IL)-based electrolyte was fabricated, leading to a high-performance Zn∥Te battery with high areal capacity (7.13 mAh cm-2), high energy density (542 Wh kgTe-1 or 227 Wh Lcathdoe+anode-1), excellent cycling performance, and a low self-discharge rate based on 400 mAh-level pouch cell. The results enhance the understanding of tellurium chemistry in batteries, substantially promising a remarkable route for advanced ZBs.

A twelve-electron conversion iodine cathode enabled by interhalogen chemistry in aqueous solution

The battery chemistry aiming for high energy density calls for the redox couples that embrace multi-electron transfer with high redox potential. Here we report a twelve-electron transfer iodine electrode based on the conversion between iodide and iodate in aqueous electrolyte, which is six times than that of the conventional iodide/iodine redox couple. This is enabled by interhalogen chemistry between iodine (in the electrode) and bromide (in the acidic electrolyte), which provides an electrochemical-chemical loop (the bromide-iodate loop) that accelerates the kinetics and reversibility of the iodide/iodate electrode reaction. In the deliberately designed aqueous electrolyte, the twelve-electron iodine electrode delivers a high specific capacity of 1200 mAh g-1 with good reversibility, corresponding to a high energy density of 1357 Wh kg-1. The proposed iodine electrode is substantially promising for the design of future high energy density aqueous batteries, as validated by the zinc-iodine full battery and the acid-alkaline decoupling battery.