High-Rate Aqueous Aluminum-Ion Batteries Enabled by Confined Iodine Conversion Chemistry

Capitalizing on the Iodometric Reaction for Energetic Aqueous Energy Storage

Iodometric and iodimetric titrations represent a prevailing technique to determine the concentration of Cu2+ ions in aqueous solutions; However, their utilization in electrochemical energy storage has been overlooked due to the poor reversibility between CuI and Cu2+ related to the shuttling effect of I3- species. In this work, we developed a 4A zeolite separator capable of suppressing the free shuttling of I3- ions, thus achieving a record-high capacity retention of 95.7% upon 600 cycles. Theoretical and experimental studies reveal that the negatively charged zeolite can effectively impede the approach and penetration of I3- ions, as a result of electrostatic interaction between them. To explore the practical potential, a hybrid cell of Zn∥I2 consisting of Cu2+ redox agent has been assembled with a discharge capacity of 356 mA h g-1. The cell affords a specific energy of 443 W h kg-1 based on I2, or 193 W h kg-1 based on both electrodes. This work offers insight on the energy utilization of the iodometric reactions and advocates a Cu2+-mediated cell design that could potentially double the capacity and energy of conventional aqueous battery systems.

Understanding the Role of Zinc Hydroxide Sulfate and its Analogues in Mildly Acidic Aqueous Zinc Batteries: A Review

Mildly acidic aqueous zinc batteries (AZBs) have attracted tremendous attention for grid storage applications with the expectation to tackle the issues of Li-ion batteries on high cost and poor safety. However, the performance, particularly energy density and cycle stability of AZBs are still unsatisfactory when compared with LIBs. To help the development of AZBs, a lot of effort have been made to understand the battery reaction mechanisms and precedent microscopic and spectroscopic analyses have shown flake-like large particles of zinc hydroxide sulfate (ZHS) and its analogues formed on the surfaces of cathodes and anodes in sulfate and other electrolyte systems during cycling. However, because of the complexity of the thermodynamics and kinetics of aqueous reactions to understand different battery conditions, controversies still exist. This article will review the roles of ZHS discussed in recent representative references aiming to shine light on the fundamental mechanisms of AZBs and pave ways to further improve the battery performance.

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.

MBene with Redox-Active Terminal Groups for an Energy-Dense Cascade Aqueous Battery

Two-dimensional (2D) transition metal borides (MBenes), new members of the 2D materials family, hold great promise for use in the electrocatalytic and energy storage fields because of their high specific area, high chemical activity, and fast charge carrier mobility. Although various types of MBenes are reported, layered MBenes featuring redox-active terminal groups for high energy output are not yet produced. A facile and energy-efficient method for synthesizing MBenes equipped with redox-active terminal groups for cascade Zn||I2 batteries is presented. Layered MBenes have ordered metal vacancies and ─Br terminal groups, enabling the sequential reactions of I-/I0 and Br-/Br0. The I2-hosting MBene-Br cathode results in a specific energy as high as 485.8 Wh kg-1 at 899.7 W kg-1 and a specific power as high as 6007.7 W kg-1 at 180.2 Wh kg-1, far exceeding the best records for Zn||I2 batteries. The results of this study demonstrate that the challenges of MBene synthesis can be overcome and reveal an efficient path for producing high-performance redox-active electrode materials for energy-dense cascade aqueous batteries.

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.

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 .

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.

Next-generation magnesium-ion batteries: The quasi-solid-state approach to multivalent metal ion storage

Mg-ion batteries offer a safe, low-cost, and high-energy density alternative to current Li-ion batteries. However, nonaqueous Mg-ion batteries struggle with poor ionic conductivity, while aqueous batteries face a narrow electrochemical window. Our group previously developed a water-in-salt battery with an operating voltage above 2 V yet still lower than its nonaqueous counterpart because of the dominance of proton over Mg-ion insertion in the cathode. We designed a quasi-solid-state magnesium-ion battery (QSMB) that confines the hydrogen bond network for true multivalent metal ion storage. The QSMB demonstrates an energy density of 264 W·hour kg-1, nearly five times higher than aqueous Mg-ion batteries and a voltage plateau (2.6 to 2.0 V), outperforming other Mg-ion batteries. In addition, it retains 90% of its capacity after 900 cycles at subzero temperatures (-22°C). The QSMB leverages the advantages of aqueous and nonaqueous systems, offering an innovative approach to designing high-performing Mg-ion batteries and other multivalent metal ion batteries.

Tuning the Electrical Conductivity of a Flexible Fabric-Based Cu-HHTP Film through a Novel Redox Interaction between the Guest-Host System

Integration of metal-organic frameworks (MOFs) and flexible fabrics has been recently considered as a promising strategy applied in wearable electronic devices. We synthesized a flexible fabric-based Cu-HHTP film consisted of Cu²⁺ ions and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) via a self-sacrificial template method. The obtained Cu-HHTP film displays an outstanding nanostructured surface and uniformity. Iodine molecules are first introduced into the pores of Cu-HHTP to investigate the influence of guest molecules on electrical conductivity in a 2D guest-host system. After doping, the conductivity of the Cu-HHTP film shows an increased dependent on the doping time, and the maximum value is more than 30 times that of the original MOFs. The enhanced electrical conductivity results from an intriguing redox interaction occurred between the confined iodine molecules and the framework. The organic ligands are oxidized by iodine molecules, and generating new ions allows for subsequent participation in the regulation of the mixed valence bands of copper ions in MOFs, changing the ratio of Cu²⁺/Cu⁺, promoting the charge transport of the framework, and then synergistically enhancing the electronic conductivity. This study successfully prepared a flexible fabric-based conductive I₂@Cu-HHTP film and presented insights into revealing the behavior of iodine molecules after entering the Cu-HHTP pores, expanding the possibilities of Cu-HHTP used in flexible wearable electronics.

Development of long lifespan high-energy aqueous organic||iodine rechargeable batteries

Rechargeable aqueous metal||I₂ electrochemical energy storage systems are a cost-effective alternative to conventional transition-metal-based batteries for grid energy storage. However, the growth of unfavorable metallic deposition and the irreversible formation of electrochemically inactive by-products at the negative electrode during cycling hinder their development. To circumvent these drawbacks, herein we propose 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) as negative electrode active material and a saturated mixed KCl/I₂ aqueous electrolyte solution. The use of these components allows for exploiting two sequential reversible electrochemical reactions in a single cell. Indeed, when they are tested in combination with an active carbon-enveloped I₂ electrode in a glass cell configuration, we report an initial specific discharge capacity of 900 mAh g⁻¹ (electrode mass of iodine only) and an average cell discharge voltage of 1.25 V at 40 A g⁻¹ and 25\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}±1 °C. Finally, we also report the assembly and testing of a PTCDI|KCl-I₂|carbon paper multilayer pouch cell prototype with a discharge capacity retention of about 70% after 900 cycles at 80 mA and 25\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document}±1 °C.

Aqueous I₂-based batteries are a promising system for cost-effective and environmentally-friendly electricity storage. Here, the authors propose a high-capacity and long-lasting aqueous I₂ battery system using an electrochemically active organic molecule at the negative electrode.

Elucidating the regulation mechanism of the photoelectrochemical effect of photocathodes on battery discharge voltages: a case study of aqueous zinc-iodine batteries

We demonstrated a method to increase the discharge voltages of zinc-iodine batteries by introducing a p-type semiconductor photocathode to trigger the photoelectrochemical reduction reaction of the cathode redox. Accordingly, the photogenerated voltage across the semiconductor/liquid junction interface would be added to the discharge voltages of zinc-iodine batteries, realizing a discharge voltage (1.49 V) exceeding the theoretical value (1.30 V) under illumination, which is equivalent to an energy density increase of 31% compared to that of zinc-iodine batteries under dark conditions.

Long-Life Aqueous Zn-I2 Battery Enabled by a Low-Cost Multifunctional Zeolite Membrane Separator

Aqueous zinc iodide (Zn-I₂) batteries are promising large-scale energy-storage devices. However, the uncontrollable diffuse away/shuttle of soluble I₃^- leads to energy loss (low Coulombic efficiency, CE), and poor reversibility (self-discharge). Herein, we employ an ordered framework window within a zeolite molecular sieve to restrain I₃^- crossover and prepare zeolite molecular sieve particles into compact, large-scale, and flexible membranes at the engineering level. The as-prepared membrane can confine I₃^- within the catholyte region and restrain its irreversible escape, which is proved via space-resolution and electrochemical in situ time-resolution Raman technologies. As a result, overcharge/self-discharge and Zn corrosion are effectively controlled by zeolite separator. After replacing the typically used glass fiber separator to a zeolite membrane, the CE of Zn-I₂ battery improves from 78.9 to 98.6% at 0.2 A/g. Besides, after aging at the fully charged state for 5.0 h, self-discharge is restrained and CE is enhanced from 44.0 to 85.65%. Moreover, the Zn-I₂ cell maintains 91.0% capacity over 30,000 cycles at 4.0 A/g.