Anion chemistry in energy storage devices

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 Dual Salt/Dual Solvent Electrolyte Enables Ultrahigh Utilization of Zinc Metal Anode for Aqueous Batteries

Rechargeable aqueous zinc batteries are promising in next-generation sustainable energy storage. However, the low zinc (Zn) metal anode reversibility and utilization in aqueous electrolytes due to Zn corrosion and poor Zn2+ deposition kinetics significantly hinder the development of Zn-ion batteries. Here, a dual salt/dual solvent electrolyte composed of Zn(BF4)2/Zn(Ac)2 in water/TEGDME (tetraethylene glycol dimethyl ether) solvents to achieve reversible Zn anode at an ultrahigh depth of discharge (DOD) is developed. An "inner co-salt and outer co-solvent" synergistic effect in this unique dual salt/dual solvent system is revealed. Experimental results and theoretical calculations provide evidence that the ether co-solvent inhibits water activity by forming hydrogen bonding with the water and coordination effects with the proton in the outer Zn2+ solvation structure. Meanwhile, the anion of zinc acetate co-salt enters the inner Zn2+ solvation structure, thereby accelerating the desolvation kinetics. Strikingly, based on the electrolyte design, the zinc anode shows high reversibility at an ultrahigh utilization of 60% DOD with 99.80% Coulombic efficiency and 9.39 mAh cm-2 high capacity. The results far exceed the performance reported in electrolyte design work recently. The work provides fundamental insights into inner co-salt and outer co-solvent synergistic regulation in multifunctional electrolytes for reversible aqueous metal-ion batteries.

Crown Ether Electrolyte Additive Enables High-Rate and Stable Polyviologen Cathode Material for Chloride Ion Batteries

A variety of inorganic and inorganic cathode materials for chloride ion storage are reported. However, their application in chloride ion batteries (CIB) is hindered by poor rate capability and cycling stability. Herein, an organic poly(butyl viologen dichloride) (PBVCl2) cathode material with significantly enhanced rate and cycling performance in the CIB is achieved using a crown ether (18-Crown-6) additive in the tributylmethylammonium chloride-based electrolyte. The as-prepared PBVCl2 cathodes exhibit impressive capacity increases from 149.4 to 179.1 mAh g-1 at 0.1 C and from 57.8 to 111.9 mAh g-1 at 10 C, demonstrating the best rate performance with the highest energy density among those of various reported cathodes for CIBs. This impressive performance improvement is a result of the great boosts in charge transfer, ion transport, and interface stability of the battery by the use of 18-Crown-6, which also contributes to a more than twofold increase in capacity retention after 120 cycles. The electrode reaction mechanism of the CIB based on highly reversible chloride ion transfer is revealed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.

Fe3O4-doped mesoporous carbon cathode with a plumber's nightmare structure for high-performance Li-S batteries

Shuttling of lithium polysulfides and slow redox kinetics seriously limit the rate and cycling performance of lithium-sulfur batteries. In this study, Fe3O4-dopped carbon cubosomes with a plumber's nightmare structure (SP-Fe3O4-C) are prepared as sulfur hosts to construct cathodes with high rate capability and long cycling life for Li-S batteries. Their three-dimensional continuous mesochannels and carbon frameworks, along with the uniformly distributed Fe3O4 particles, enable smooth mass/electron transport, strong polysulfides capture capability, and fast catalytic conversion of the sulfur species. Impressively, the SP-Fe3O4-C cathode exhibits top-level comprehensive performance, with high specific capacity (1303.4 mAh g-1 at 0.2 C), high rate capability (691.8 mAh gFe3O41 at 5 C), and long cycling life (over 1200 cycles). This study demonstrates a unique structure for high-performance Li-S batteries and opens a distinctive avenue for developing multifunctional electrode materials for next-generation energy storage devices.

Fast Interfacial Defluorination Kinetics Enables Stable Cycling of Low-Temperature Lithium Metal Batteries

The development of low-temperature lithium metal batteries (LMBs) encounters significant challenges because of severe dendritic lithium growth during the charging/discharging processes. To date, the precise origin of lithium dendrite formation still remains elusive due to the intricate interplay between the highly reactive lithium metal anode and organic electrolytes. Herein, we unveil the critical role of interfacial defluorination kinetics of localized high-concentration electrolytes (LHCEs) in regulating lithium dendrite formation, thereby determining the performance of low-temperature LMBs. We investigate the impact of solvation structures of LHCEs on low-temperature LMBs by employing tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-MeTHF) as comparative solvents. The combination of comprehensive characterizations and theoretical simulations reveals that the THF-based LHCE featured with a strong solvation strength exhibits fast interfacial defluorination reaction kinetics, thus leading to the formation of an amorphous and inorganic-rich solid-electrolyte interphase (SEI) that can effectively suppress the growth of lithium dendrites. As a result, the highly reversible Li metal anode achieves an exceptional Coulombic efficiency (CE) of up to ∼99.63% at a low temperature of -30 °C, thereby enabling stable cycling of low-temperature LMB full cells. These findings underscore the crucial role of electrolyte interfacial reaction kinetics in shaping SEI formation and provide valuable insights into the fundamental understanding of electrolyte chemistry in LMBs.

Anchored Weakly-Solvated Electrolytes for High-Voltage and Low-Temperature Lithium-ion Batteries

Electrolytes endowed with high oxidation/reduction interfacial stability, fast Li-ion desolvation process and decent ionic conductivity over wide temperature region are known critical for low temperature and fast-charging performance of energy-dense batteries, yet these characteristics are rarely satisfied simultaneously. Here, we report anchored weakly-solvated electrolytes (AWSEs), that are designed by extending the chain length of polyoxymethylene ether electrolyte solvent, can achieve the above merits at moderate salt concentrations. The -O-CH2-O- segment in solvent enables the weak four-membered ring Li+ coordination structure and the increased number of segments can anchor the solvent by Li+ without largely sacrificing the ionic dissociation ability. Therefore, the single salt/single solvent AWSEs enable solvent co-intercalation-free behavior towards graphite (Gr) anode and high oxidation stability towards high-nickel cathode (LiNi0.8Co0.1Mn0.1O2-NCM811), as well as the formation of inorganic rich electrode/electrolyte interphase on both of them due to the anion-rich solvation shells. The capacity retention of Gr||NCM811 Ah-class pouch cell can reach 70.85 % for 1000 cycles at room-temperature and 75.86 % for 400 cycles at -20 °C. This work points out a promising path toward the molecular design of electrolyte solvents for high-energy/power battery systems that are adaptive for extreme conditions.

Intercalation in 2D materials and in situ studies

Intercalation of atoms, ions and molecules is a powerful tool for altering or tuning the properties - interlayer interactions, in-plane bonding configurations, Fermi-level energies, electronic band structures and spin-orbit coupling - of 2D materials. Intercalation can induce property changes in materials related to photonics, electronics, optoelectronics, thermoelectricity, magnetism, catalysis and energy storage, unlocking or improving the potential of 2D materials in present and future applications. In situ imaging and spectroscopy technologies are used to visualize and trace intercalation processes. These techniques provide the opportunity for deciphering important and often elusive intercalation dynamics, chemomechanics and mechanisms, such as the intercalation pathways, reversibility, uniformity and speed. In this Review, we discuss intercalation in 2D materials, beginning with a brief introduction of the intercalation strategies, then we look into the atomic and intrinsic effects of intercalation, followed by an overview of their in situ studies, and finally provide our outlook.

Constructing π-π Superposition Effect of Tetralithium Naphthalenetetracarboxylate with Electron Delocalization for Robust Dual-Ion Batteries

Organics are gaining significance as electrode materials due to their merits of multi-electron reaction sites, flexible rearrangeable structures and redox reversibility. However, organics encounter finite electronic conductivity and inferior durability especially in organic electrolytes. To circumvent above barriers, we propose a novel design strategy, constructing conductive network structures with extended π-π superposition effect by manipulating intermolecular interaction. Tetralithium 1,4,5,8-naphthalenetetracarboxylate (LNTC) interwoven by carbon nanotubes (CNTs) forms LNTC@CNTs composite firstly for Li-ion storage, where multiple conjugated carboxyls contribute sufficient Li-ion storage sites, the unique network feature enables electrolyte and charge mobility conveniently combining electron delocalization in π-conjugated system, and the enhanced π-π superposition effect between LNTC and CNTs endows laudable structural robustness. Accordingly, LNTC@CNTs maintain an excellent Li-ion storage capacity retention of 96.4 % after 400 cycles. Electrochemical experiments and theoretical simulations elucidate the fast reaction kinetics and reversible Li-ion storage stability owing to the electron delocalization and π-π superposition effect, while conjugated carboxyls are reversibly rearranged into enolates during charging/discharging. Consequently, a dual-ion battery combining this composite anode and expanded graphite cathode exhibits a peak specific capacity of 122 mAh g-1 and long cycling life with a capacity retention of 84.2 % after 900 cycles.

Gel polymer electrolytes for rechargeable batteries toward wide-temperature applications

Rechargeable batteries, typically represented by lithium-ion batteries, have taken a huge leap in energy density over the last two decades. However, they still face material/chemical challenges in ensuring safety and long service life at temperatures beyond the optimum range, primarily due to the chemical/electrochemical instabilities of conventional liquid electrolytes against aggressive electrode reactions and temperature variation. In this regard, a gel polymer electrolyte (GPE) with its liquid components immobilized and stabilized by a solid matrix, capable of retaining almost all the advantageous natures of the liquid electrolytes and circumventing the interfacial issues that exist in the all-solid-state electrolytes, is of great significance to realize rechargeable batteries with extended working temperature range. We begin this review with the main challenges faced in the development of GPEs, based on extensive literature research and our practical experience. Then, a significant section is dedicated to the requirements and design principles of GPEs for wide-temperature applications, with special attention paid to the feasibility, cost, and environmental impact. Next, the research progress of GPEs is thoroughly reviewed according to the strategies applied. In the end, we outline some prospects of GPEs related to innovations in material sciences, advanced characterizations, artificial intelligence, and environmental impact analysis, hoping to spark new research activities that ultimately bring us a step closer to realizing wide-temperature rechargeable batteries.

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.

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.

The Construction of Anion-Induced Solvation Structures in Low-concentration Electrolyte for Stable Zinc Anodes

Aqueous zinc-ion batteries (ZIBs) are promising large-scale energy storage devices because of their low cost and high safety. However, owing to the high activity of H2O molecules in electrolytes, hydrogen evolution reaction and side reactions usually take place on Zn anodes. Herein, additive-free PCA-Zn electrolyte with capacity of suppressing the activity of free and solvated H2O molecules was designed by selecting the cationophilic and solventophilic anions. In such electrolyte, contact ion-pairs and solvent-shared ion-pairs were achieved even at low concentration, where PCA- anions coordinate with Zn2+ and bond with solvated H2O molecules. Simultaneously, PCA- anions also induce the construction of H-bonds between free H2O molecules and them. Therefore, the activity of free and solvated H2O molecules is effectively restrained. Furthermore, since PCA- anions possess a strong affinity with metal Zn, they can also adsorb on Zn anode surface to protect Zn anode from the direct contact of H2O molecules, inhibiting the occurrence of water-triggered side reactions. As a result, plating/stripping behavior of Zn anodes is highly reversible and the coulombic efficiency can reach to 99.43 % in PCA-Zn electrolyte. To illustrate the feasibility of PCA-Zn electrolyte, the Zn||PANI full batteries were assembled based on PCA-Zn electrolyte and exhibited enhanced cycling performance.

Water-stable boroxine structure with dynamic covalent bonds

Boroxines are significant structures in the production of covalent organic frameworks, anion receptors, self-healing materials, and others. However, their utilization in aqueous media is a formidable task due to hydrolytic instability. Here we report a water-stable boroxine structure discovered from 2-hydroxyphenylboronic acid. We find that, under ambient environments, 2-hydroxyphenylboronic acid undergoes spontaneous dehydration to form a dimer with dynamic covalent bonds and aggregation-induced enhanced emission activity. Intriguingly, upon exposure to water, the dimer rapidly transforms into a boroxine structure with excellent pH stability and water-compatible dynamic covalent bonds. Building upon these discoveries, we report the strong binding capacity of boroxines toward fluoride ions in aqueous media, and develop a boroxine-based hydrogel with high acid-base stability and reversible gel-sol transition. This discovery of the water-stable boroxine structure breaks the constraint of boroxines not being applicable in aqueous environments, opening a new era of researches in boroxine chemistry.

Sustainable Dual-Ion Batteries beyond Li

The limitations of resources used in current Li-ion batteries may hinder their widespread use in grid-scale energy storage systems, prompting the search for low-cost and resource-abundant alternatives. "Beyond-Li cation" batteries have emerged as promising contenders; however, they confront noteworthy challenges due to the scarcity of suitable host materials for these cations. In contrast, anions, the other crucial component in electrolytes, demonstrate reversible intercalation capacity in specific materials like graphite. The convergence of anion and cation storage has given rise to a new battery technology known as dual-ion batteries (DIBs). This comprehensive review presents the current status, advancements, and future prospects of sustainable DIBs beyond Li. Notably, most DIBs exhibit similar cathode reaction mechanisms involving anion intercalation, while the distinguishing factor lies in the cation types functioning at the anode. Accordingly, the review is organized into sections by various cation types, including Na-, K-, Mg-, Zn-, Ca-, Al-, NH4 + -, and proton-based DIBs. Moreover, a perspective on these novel DIBs is presented, along with proposed protocols for investigating DIBs and promising future research directions. It is envisioned that this review will inspire fresh concepts, ideas, and research directions, while raising important questions to further tailor and understand sustainable DIBs, ultimately facilitating their practical realization.

Insight into the Role of Fluoroethylene Carbonate on the Stability of Sb||Graphite Dual-Ion Batteries in Propylene Carbonate-Based Electrolyte

Sodium dual-ion batteries (Na-DIBs) have attracted increasing attention due to their high operative voltages and low-cost raw materials. However, the practical applications of Na-DIBs are still hindered by the issues, such as low capacity and poor Coulombic efficiency, which is highly correlated with the compatibility between electrode and electrolyte but rarely investigated. Herein, fluoroethylene carbonate (FEC) is introduced into the electrolyte to regulate cation/anion solvation structure and the stability of cathode/anode-electrolyte interphase of Na-DIBs. The FEC modulates the environment of PF6 - solvation sheath and facilitates the interaction of PF6 - on graphite. In addition, the NaF-rich interphase caused by the preferential decomposition of FEC effectively inhibits side reactions and pulverization of anodes with the electrolyte. Consequently, Sb||graphite full cells in FEC-containing electrolyte achieve an improved capacity, cycling stability and Coulombic efficiency. This work elucidates the underlying mechanism of bifunctional FEC and provides an alternative strategy of building high-performance dual ion batteries.

Comprehensive Understandings of Hydrogen Bond Chemistry in Aqueous Batteries

Aqueous batteries are emerging as highly promising contenders for large-scale grid energy storage because of uncomplicated assembly, exceptional safety, and cost-effectiveness. The unique aqueous electrolyte with a rich hydrogen bond (HB) environment inevitably has a significant impact on the electrode materials and electrochemical processes. While numerous reviews have focused on the materials design and assembly of aqueous batteries, the utilization of HB chemistry is overlooked. Herein, instead of merely compiling recent advancements, this review presents a comprehensive summary and analysis of the profound implication exerted by HB on all components of the aqueous batteries. Intricate links between the novel HB chemistry and various aqueous batteries are ingeniously constructed within the critical aspects, such as self-discharge, structural stability of electrode materials, pulverization, solvation structures, charge carrier diffusion, corrosion reactions, pH sensitivity, water splitting, polysulfides shuttle, and H2 S evolution. By adopting a vantage point that encompasses material design, binder and separator functionalization, electrolyte regulation, and HB optimization, a critical examination of the key factors that impede electrochemical performance in diverse aqueous batteries is conducted. Finally, insights are rendered properly based on HB chemistry, with the aim of propelling the advancement of state-of-the-art aqueous batteries.

Physicochemical properties and density functional theory calculation of octahedral UiO-66 with Bis(Trifluoromethanesulfonyl)imide ionic liquids

In this study, the physicochemical properties and molecular interactions between zirconium-based metal-organic framework (UiO-66) and three different ionic liquids based on bis(trifluoromethanesulfonyl)imide anion (EMIM+, BMIM+ and OMIM+) was performed via a combined experimental and computational approach. The ionic liquid loaded UiO-66 or IL@UiO-66 was synthesized and characterized to understand the host-guest interaction. Density functional theory calculation was performed to analyse the electronic structure of IL@UiO-66 to provide molecular insight on the dominant interactions occurred in the hybrid material. Results showed that all ILs were successfully incorporated into the micropores of UiO-66. The 3D framework was retained even after loaded with ILs as analyzed from XRD pattern. FTIR spectrum reveals that interactions of ILs with UiO-66 influenced by the alkyl chain length of the cation. The anion has a profound affinity with the UiO-66 due to the presence of electronegative atoms. Phase transition study from DSC suggested that the incorporation of ILs has stabilized the framework of UiO-66 by shifting the endothermic peak to a higher state. These findings were further elaborated with DFT calculation. Geometrical optimizations confirmed the structural parameter changes of UiO-66 when loaded with ILs. These was mainly contributed by the non-covalent interactions which was confirmed by the reduced density gradient scattered plot. Another important findings are the strength of hydrogen bonding at the host-guest interface was influenced by the alkyl chain length. The molecular orbital analysis also shows that the size of alkyl chain influence the reactivity of the hybrid material. The present study provides fundamental insights on the molecular interaction of UiO-66 and ILs as a hybrid material, which can open new possibilities for advanced material for metal-organic framework applications in energy storage system, catalysis, gas storage and medicinal chemistry.