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

Ion-Docking Effect Enabling Rechargeable High-Voltage Magnesium-Iodine/Chlorine Battery

Rechargeable magnesium (Mg) batteries represent a promising energy storage system by offering low cost and dendrite-less propensity. However, the limited selection of cathode materials, and often with low voltage and capacity, constrain Mg batteries. Herein, by exploiting the ion-docking effect between two halogen species-iodine cations (I+) and chlorine anions (Cl-)-we activate the cathodic activity of halogens and develop a magnesium-iodine/chlorine (Mg-I/Cl) battery prototype with high energy and power density. The ion-docking effect enables I+ and Cl- to mutually balance and disperse their charges, and weakens the coordination strength between Cl- and Mg2+ while enhancing the stability of I+, thus facilitating the multi-electron (2 + 1/3) redox reactions of halogens. We also find the solvation state of iodine species determines the reaction process of the I0/I3 -/I- redox couples. The here-developed magnesium-iodine/chlorine battery features an impressively high discharge plateau of up to 3.0 V with a high capacity exceeding 400 mAh g-1, and demonstrates a stable lifespan for 500 cycles, with the ability of ultra-fast charging at 20C and low-temperature cycling under -30 °C. These findings may provide new insights for developing high-energy-density Mg battery systems.

Cation-driven phase transition and anion-enhanced kinetics for high energy efficiency zinc-interhalide complex batteries

Aqueous Zn-halogen batteries, valued for high safety, large capacity, and low cost, suffer from the polyhalide shuttle effect and chaotic zinc electrodeposition, reducing energy efficiency and lifespan. Here we show a cation-driven positive electrode phase transition to suppress the shuttle effect and achieve uniform zinc electrodeposition, along with an anion kinetic enhancement strategy to improve energy efficiency and lifespan. Taking tetramethylammonium halide (TMAX, X = F, Cl, Br) as a subject, TMA+ promotes oriented zinc (101) deposition on the negative electrode through electrostatic shielding, significantly extending cycling life. Concurrently, it captures I3- on the positive electrode, forming a stable solid-phase interhalide complex that enhances coulombic efficiency. Compared to I3- and TMAI3, X- anions lower the Gibbs free energy differences of I- → I2X- and I2X- → TMAI2X, accelerating I-/I2X-/TMAI2X conversions and improving voltage efficiency. In TMAF-modified electrolytes, zinc interhalide complex batteries achieve a high energy efficiency of 95.2% at 0.2 A g-1 with good reversibility, showing only 0.1% capacity decay per cycle over 1000 cycles. At 1 A g-1, they show a low decay rate of 0.1‰ per cycle across 10,000 cycles. This study provides insights into enhancing energy efficiency and long-term stability for sustainable energy storage.

Constructing Matching Interfaces by Amorphous Engineering for Enhanced Lithium Ion Transport in Quasi-Solid-State Lithium-Iodine Batteries

Quasi-solid-state lithium-iodine (Li-I2) batteries have shown prospects as their high theoretical capacity, high safety, and abundant iodine resources. However, the interface between the crystalline filler and the flexible polymer skeleton of composite solid electrolytes exhibits inadequate bonding, leading to higher interface energy and sluggish migration dynamics of Li+. In this work, a continuous interface solid electrolyte is designed by combining the atomic structure rearrangement of metal-organic framework (MOF) to achieve interface coupling between MOF and aramid fiber. Based on the experimental results and theoretical calculations, the amorphous engineering promotes Li+ migration and polyiodide confinement effects for Li-I2 batteries. The batteries show a high capacity of 170.7 mAh g-1 at 5 C and achieve a capacity retention rate of 97.8% after 450 cycles. More impressively, the batteries achieve a long life of 3000 cycles at the high current density of 20 C with a good capacity retention of 94.1%. This work reveals the mechanism of coupled interface with structure matching in Li+ migration and polyiodide integration process, providing guidance for the design of novel composite solid electrolytes to achieve high-performance Li-I2 batteries.

Multiple Protophilic Redox-Active Sites in Reticular Organic Skeletons for Superior Proton Storage

Protons (H+) with the smallest size and fastest redox kinetics are regarded as competitive charge carriers in the booming Zn-organic batteries (ZOBs). Developing new H+-storage organic cathode materials with multiple ultralow-energy-barrier protophilic sites and super electron delocalization routes to propel superior ZOBs is crucial but still challenging. Here we design multiple protophilic redox-active reticular organic skeletons (ROSs) for activating better proton storage, triggered by intermolecular H-bonding and π-π stacking interactions between 2,6-diaminoanthraquinone and 2,4,6-triformylphloroglucinol nanofibrous polymer. ROSs expose reticular electron delocalization geometries to fully access build-in protophilic carbonyl sites and promote ultrarapid H+ migration with an ultralow activation energy (0.13 vs. 0.29 eV of Zn2+ ions), thus delivering high capacity (359 mAh g-1) and large-current survivability (100 A g-1). Moreover, the extended interconnected reticular structures strengthen the anti-dissolution of ROSs in aqueous electrolytes, affording long-lasting proton-storage activity in ZOBs to a superior level (60,000 cycles at 20 A g-1). Systematic studies identify the source of excellent charge storage as high-kinetics H+-coupled five-electron redox process of carbonyl motifs in superstable ROSs. These findings can be of importance for evoking superior proton activity in multiple redox organics to build advanced Zn-organic batteries.

Reducing Dead Species by Electrochemically-Densified Cathode-Interface-Reaction Layer towards High-Rate-Endurable Zn||I-Br Batteries

Interhalogen-involved aqueous Zn||halogen batteries (AZHBs) are latent high-energy systems for grid-level energy storage, yet usually suffer from poor high-rate endurability caused by the formation of "dead species". Herein, via an electrochemically-densified cathode-interface-reaction layer (CIRL), Zn||I-Br batteries involving interhalogen reactions between the I2 cathode and Br- from the electrolytes are initially achieved with excellent high-rate endurability. Different from that in diluted electrolytes, the CIRL formed in Br--concentrated electrolyte is denser and water-lean, which enables halogen species conversion with a more rapid charge transfer and lower activation energy. More importantly, the CIRL robustly affords a decent I2 conservation by accelerated conversion kinetics and limited species diffusion, thereby endowing the Zn||I-Br batteries with an ultralong high-rate lifespan. The electrochemical mechanism is sufficiently verified by multiple spectral characterizations. Consequently, Zn||I-Br batteries in Br--concentrated (20 m) electrolytes exhibit an overwhelming rate capability and lifespan to those in Br--diluted (2 m) electrolytes. Typically, when cycled at a large current density of 10 A g-1, an ultralong lifespan of over 25,000 cycles is achieved with a high retention of 98.3 %. This study provides new insight into the CIRL-dictated active species conservation for high-rate endurable AZHBs, which could apply to other high-energy interhalogen batteries.

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.

Electrode/Electrolyte Optimization-Induced Double-Layered Architecture for High-Performance Aqueous Zinc-(Dual) Halogen Batteries

Aqueous zinc-halogen batteries are promising candidates for large-scale energy storage due to their abundant resources, intrinsic safety, and high theoretical capacity. Nevertheless, the uncontrollable zinc dendrite growth and spontaneous shuttle effect of active species have prohibited their practical implementation. Herein, a double-layered protective film based on zinc-ethylenediamine tetramethylene phosphonic acid (ZEA) artificial film and ZnF2-rich solid electrolyte interphase (SEI) layer has been successfully fabricated on the zinc metal anode via electrode/electrolyte synergistic optimization. The ZEA-based artificial film shows strong affinity for the ZnF2-rich SEI layer, therefore effectively suppressing the SEI breakage and facilitating the construction of double-layered protective film on the zinc metal anode. Such double-layered architecture not only modulates Zn2+ flux and suppresses the zinc dendrite growth, but also blocks the direct contact between the metal anode and electrolyte, thus mitigating the corrosion from the active species. When employing optimized metal anodes and electrolytes, the as-developed zinc-(dual) halogen batteries present high areal capacity and satisfactory cycling stability. This work provides a new avenue for developing aqueous zinc-(dual) halogen batteries.

Bifunctionally Electrocatalytic Bromine Redox Reaction by Single-Atom Catalysts for High-Performance Zinc Batteries

Aqueous zinc-bromine (Zn||Br2) batteries are regarded as one of the most promising energy storage devices due to their high safety, theoretical energy density, and low cost. However, the sluggish bromine redox kinetics and the formation of a soluble tribromide (Br3 -) hinder their practical applications. Here, it is proposed dispersed single iron atom coordinated with nitrogen atoms (FeN5) in a mesoporous carbon framework (FeSAC-CMK) as a conductive catalytic bromine host, which possesses porous structure and electrocatalytic functionality of FeN5 species for enhanced confinement and electrocatalytic effect. The active FeN5 species can fix the bromine (Br0) species to suppress the formation of Br3 - effectively and bifunctionally catalyze the bromide (Br-)/Br° conversion. These free up 1/3 Br- locked by Br3 - complexing agent for enhanced bromine utilization efficiency and conversion reversibility. Accordingly, the Zn||Br2 battery with FeSAC-CMK delivers an impressive specific capacity of 344 mAh g-1 at 0.2 A g-1 and superior rate capability with 164 mAh g-1 achieved even at 20 A g-1, much higher than that of inactive CMK (262 mAh g-1 at 0.2 A g-1; 6 mAh g-1 at only 8 A g-1). Furthermore, the battery demonstrates excellent cycling performance of 88% capacity retention after 2000 cycles.

Rechargeable Mg-Br2 Battery with Ultrafast Bromine Chemistry

A sustainable society necessitates the support of diversified energy storage systems. Magnesium metal batteries, known for the environmental friendliness, safety of dendrite-less, cost-effective, and high volumetric capacity of magnesium metal, exhibit promising prospects. However, the high charge density of the magnesium ion leads to sluggish ion diffusion in cathodes, posing challenges for developing magnesium metal battery systems with high power and high energy density. Here, inspired by the Hard-Soft-Acid-Base theory, we propose a soft-anion-induced bond weakening strategy to address the diffusion difficulty. The bulky and broadly electron-distributed succinimide ion (SN-) in SN-Mg-Br significantly weakens the Mg-Br bond, promoting rapid magnesium ion transport and enabling ultrafast bromine chemistry, thus realizing a highly rechargeable Mg-Br2 battery prototype. Benefiting from the solubilization of SN-, the Mg-Br2 batteries achieve a high discharge plateau of 2.7 V, a remarkable specific capacity of 326 mAh gBr-1, and an impressive lifespan of 400 cycles. Attributed to the half-half diffusion/adsorption-desorption control process mechanism, the batteries can be well cycled under high-rate charging at 10 C and ultralow temperatures down to -55 °C. This bond weakening strategy may stimulate the development of battery systems with similar high charge density to magnesium ion, toward high power and high energy density, paving the way for sustainable energy storage systems.

Rechargeable alkali metal-chlorine batteries: advances, challenges, and future perspectives

The emergence of Li-SOCl2 batteries in the 1970s as a high-energy-density battery system sparked considerable interest among researchers. However, limitations in the primary cell characteristics have restricted their potential for widespread adoption in today's sustainable society. Encouragingly, recent developments in alkali/alkaline-earth metal-Cl2 (AM-Cl2) batteries have shown impressive reversibility with high specific capacity and cycle performance, revitalizing the potential of SOCl2 batteries and becoming a promising technology surpassing current lithium-ion batteries. In this review, the emerging AM-Cl2 batteries are comprehensively summarized for the first time. The development history and advantages of Li-SOCl2 batteries are traced, followed by the critical working mechanisms for achieving high rechargeability. The design concepts of electrodes and electrolytes for AM-Cl2 batteries as well as key characterization techniques are also demonstrated. Furthermore, the current challenges and corresponding strategies, as well as future directions regarding the battery are systematically discussed. This review aims to deepen the understanding of the state-of-the-art AM-Cl2 battery technology and accelerate the development of practical AM-Cl2 batteries for next-generation high-energy storage systems.

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.

Activating and Stabilizing a Reversible four Electron Redox Reaction of I-/I+ for Aqueous Zn-Iodine Battery

Low capacity and poor cycle stability greatly inhibit the development of zinc-iodine batteries. Herein, a high-performance Zn-iodine battery has been reached by designing and optimizing both electrode and electrolyte. The Br- is introduced as the activator to trigger I+, and coupled with I+ forming interhalogen to stabilize I+ to achieve a four-electron reaction, which greatly promotes the capacity. And the Ni-Fe-I LDH nanoflowers serve as the confinement host to enable the reactions of I-/I+ occurring in the layer due to the spacious and stable interlayer spacing of Ni-Fe-I LDH, which effectively suppresses the iodine-species shuttle ensuring high cycling stability. As a result, the electrochemical performance is greatly enhanced, especially in specific capacity (as high as 350 mAh g-1 at 1 A g-1 far higher than two-electron transfer Zn-iodine batteries) and cycling performance (94.6 % capacity retention after 10000 cycles). This strategy provides a new way to realize high capacity and long-term stability of Zn-iodine batteries.

A Pyrophosphate Bifunctional Cathode with Inductive Effect for High-Voltage and Self-Charging Zinc Ion Battery

With the growing demand for new energy storage devices, rechargeable aqueous zinc ion batteries (ZIBs) have attracted widespread attention due to their low cost and high safety. Among the cathode materials for ZIBs, polyanionic-based cathode materials with high voltage, high stability, and low cost have great potential. In this paper, tetragonal Na2VOP2O7 was prepared using a simple sol-gel method. The discharge platform voltage amounted to 1.8 V and had good rate and cycle performance due to the inductive effect of pyrophosphate. Then, a protective layer of Zn-hydroxyapatite (ZnHAP) modification was applied to the cathode surface, which can inhibit the hydrolysis of vanadium ions. The capacity was enhanced by 19 % after modification and the capacity retention after 100 cycles was also higher. Interestingly, the Na2VOP2O7 cathode also possesses a self-charging effect, recovering to 48 % of its initial capacity with an open-circuit voltage (OCV) of 1.1 V within a certain period, and light exposure can reduce the self-charging time by 83 %. These beneficial results indicate that the pyrophosphate bifunctional cathode with inductive effect has a great potential to construct high-voltage and multifunctional zinc ion battery.

Halide Exchange in Perovskites Enables Bromine/Iodine Hybrid Cathodes for Highly Durable Zinc Ion Batteries

With the increasing need for reliable storage systems, the conversion-type chemistry typified by bromine cathodes attracts considerable attention due to sizeable theoretical capacity, cost efficiency, and high redox potential. However, the severe loss of active species during operation remains a problem, leading researchers to resort to concentrated halide-containing electrolytes. Here, profiting from the intrinsic halide exchange in perovskite lattices, a novel low-dimensional halide hybrid perovskite cathode, TmdpPb2[IBr]6, which serves not only as a halogen reservoir for reversible three-electron conversions but also as an effective halogen absorbent by surface Pb dangling bonds, C─H…Br hydrogen bonds, and Pb─I…Br halogen bonds, is proposed. As such, the Zn||TmdpPb2[IBr]6 battery delivers three remarkable discharge voltage plateaus at 1.21 V (I0/I-), 1.47 V (I+/I0), and 1.74 V (Br0/Br-) in a typical halide-free electrolyte; meanwhile, realizing a high capacity of over 336 mAh g-1 at 0.4 A g-1 and high capacity retentions of 88% and 92% after 1000 cycles at 1.2 A g-1 and 4000 cycles at 3.2 A g-1, respectively, accompanied by a high coulombic efficiency of ≈99%. The work highlights the promising conversion-type cathodes based on metal-halide perovskite materials.

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.

Organocatalytic Lithium Chloride Oxidation by Covalent Organic Frameworks for Rechargeable Lithium-Chlorine Batteries

Rechargeable Li-Cl2 battery is a promising high energy density battery system. However, reasonable cycle life could only be achieved under low specific capacities due to the sluggish oxidation of LiCl to Cl2 . Herein, we propose an amine-functionalized covalent organic framework (COF) with catalytic activity, namely COF-NH2 , that significantly decreases the oxidation barrier of LiCl and accelerates the oxidation kinetics of LiCl in Li-Cl2 cell. The resulting Li-Cl2 cell using COF-NH2 (Li-Cl2 @COF-NH2 ) simultaneously exhibits low overpotential, ultrahigh discharge capacity up to 3500 mAh/g and a promoted utilization ratio of deposited LiCl at the first cycle (UR-LiCl) of 81.4 %, which is one of the highest reported values to date. Furthermore, the Li-Cl2 @COF-NH2 cell could be stably cycled for over 200 cycles when operating at a capacity of 2000 mAh/g at -20 °C with a Coulombic efficiency (CE) of ≈100 % and a discharge plateau of 3.5 V. Our superior Li-Cl2 batteries enabled by organocatalyst enlighten an arena towards high-energy storage applications.

Bidirectional manipulation of iodine redox kinetics in aqueous Fe-I2 electrochemistry

Catalyzing conversion is a promising approach to unlock the theoretical potentials of the I2/I- redox couple in aqueous Fe-I2 electrochemistry. However, most reported results only obtain one-directional efficient iodine conversion and cannot realize a balance of full reduction and reoxidation, thereby resulting in rapid capacity decay and/or low coulombic efficiency. Herein, the concept of bidirectional catalysis based on a core-shell structured composite cathode design, which accelerates the formation and the decomposition of FeI2 simultaneously during battery dynamic cycling, is proposed to regulate the Fe-I2 electrochemical reactions. Notably, the functional matrix integrates N, P co-doping and FeP nanocrystals into a carbon shell to achieve bidirectional catalysis. More specifically, the carbon shell acts as a physical barrier to effectively capture active species within its confined environment, N, P heteroatoms function better in directing the iodine reduction and FeP facilitates the decomposition of FeI2. As confirmed with in situ and ex situ analysis, the Fe-I2 cell operates a one-step but reversible I2/FeI2 pair with enhanced kinetics. Consequently, the composite cathode exhibits a reversible Fe2+ storage capability of 202 mA h g-1 with a capacity fading rate of 0.016% per cycle over 500 cycles. Further, a stable pouch cell was fabricated and yielded an energy density of 146 W h kgiodine-1. Moreover, postmortem analysis reveals that the capacity decay of the Fe-I2 cell originates from anodic degradation rather than the accumulation of inactive iodine. This study represents a promising direction to manipulate iodine redox in rechargeable metal-iodine batteries.