Reversible S 0 /MgS x Redox Chemistry in a MgTFSI 2 /MgCl 2 /DME Electrolyte for Rechargeable Mg/S Batteries

Mitigating Interfacial Capacity Fading in Vanadium Pentoxide by Sacrificial Vanadium Sulfide Encapsulation for Rechargeable Mg-Ion Batteries

Rechargeable Mg-ion Batteries (RMB) containing a Mg metal anode offer the promise of higher specific volumetric capacity, energy density, safety, and economic viability than lithium-ion battery technology, but their realization is challenging. The limited availability of suitable inorganic cathodes compatible with electrolytes relevant to Mg metal anode restricts the development of RMBs. Despite the promising capability of some oxides to reversibly intercalate Mg+2 ions at high potential, its lack of stability in chloride-containing ethereal electrolytes, relevant to Mg metal anode hinders the realization of a full practical RMB. Here the successful in situ encapsulation of monodispersed spherical V2O5 (≈200 nm) is demonstrated by a thin layer of VS2 (≈12 nm) through a facile surface reduction route. The VS2 layer protects the surface of V2O5 particles in RMB electrolyte solution (MgCl2 + MgTFSI in DME). Both V2O5 and V2O5@VS2 particles demonstrate high initial discharge capacity. However, only the V2O5@VS2 material demonstrates superior rate performance, Coulombic efficiency (100%), and stability (138 mA h g-1 discharge capacity after 100 cycles), signifying the ability of the thin VS2 layer to protect the V2O5 cathode and facilitate the Mg+2 ion intercalation/deintercalation into V2O5.

Theoretical study on the role of magnesium chloride complexes induced by different magnesium-to-chlorine ratios in magnesium-sulfur batteries

In magnesium-sulfur batteries, electrolyte exploration is vital for developing high-energy-density, safe, and reliable batteries. This study focused on cyclic THF and chain DME, representative solvents in ether electrolytes. MgCl2, an ideal anionic salt, forms mono-nuclear (MgCl2(DME)2), bi-nuclear ([Mg2(μ-Cl)2(DME)4]2+), and tri-nuclear ([Mg3(μ-Cl)4(DME)5]2+) complexes in DME. With increasing salt concentration, these complexes sequentially form. Under lower salt concentrations, THF and MgCl2 form mono-nuclear complexes ([MgCl2(THF)4]) and continue to form bi-nuclear complexes ([Mg2(μ-Cl)3(THF)6]+). However, at higher salt concentrations, bi-nuclear complexes ([Mg2(μ-Cl)3(THF)6]+) directly form in THF. Comparing HOMO-LUMO values, [Mg(DME)3]2+ is easily oxidized. Energy gaps decrease with Cl- ion addition, enhancing solution conductivity. Ratios of Mg2+ and Cl- in S-reduction complexes differ, suggesting DME is better at a low Mg/Cl ratio, and THF at a high Mg/Cl ratio. This study contributes to understanding complexes and enhancing Mg-S battery performance.

A High-Energy Aqueous All-Sulfur Battery

Rechargeable aqueous batteries are promising energy storage devices because of their high safety and low cost. However, their energy densities are generally unsatisfactory due to the limited capacities of ion-inserted electrode materials, prohibiting their widespread applications. Herein, a high-energy aqueous all-sulfur battery was constructed via matching S/Cu2 S and S/CaSx redox couples. In such batteries, both cathodes and anodes undergo the conversion reaction between sulfur/metal sulfides redox couples, which display high specific capacities and rational electrode potential difference. Furthermore, during the charge/discharge process, the simultaneous redox of Cu2+ ion charge-carriers also takes place and contributes to a more two-electron transfer, which doubles the capacity of cathodes. As a result, the assembled aqueous all-sulfur batteries deliver a high discharge capacity of 447 mAh g-1 based on total mass of sulfur in cathode and anode at 0.1 A g-1 , contributing to an enhanced energy density of 393 Wh kg-1 . This work will widen the scope for the design of high-energy aqueous batteries.

Chloride-Free Electrolyte Based on Tetrabutylammonium Triflate Additive for Extended Anodic Stability in Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have been proposed as a promising alternative to currently commercialized lithium-ion batteries. However, Mg anode passivation in conventional electrolytes necessitates the use of highly corrosive Cl- ions in the electrolyte. Herein for the first time, we design a chloride-free electrolyte for RMBs with magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) and magnesium triflate (Mg(OTf)2) as the main salts and tetrabutylammonium triflate (TBAOTf) as an additive. The TBAOTf additive improved the dissolution of Mg salts, consequently enhancing the charge-carrying species in the electrolyte. COMSOL studies further revealed desirable Mg growth in our modulated electrolyte, substantiated by homogeneous electric flux distribution across the electrolyte-electrode interface. Post-mortem chemical composition analysis uncovered a MgF2-rich solid electrolyte interphase (SEI) that facilitated exceptional Mg deposition/dissolution reversibility. Our study illustrates a highly promising strategy for synthesizing a corrosion-free and reversible Mg battery electrolyte with a widened anodic stability window of up to 4.43 V.

A triple-synergistic small-molecule sulfur cathode promises energetic Cu-S electrochemistry

Metal-sulfur batteries have received great attention for electrochemical energy storage due to high theoretical capacity and low cost, but their further development is impeded by low sulfur utilization, poor electrochemical kinetics, and serious shuttle effect of the sulfur cathode. To avoid these problems, herein, a triple-synergistic small-molecule sulfur cathode is designed by employing N, S co-doped hierarchical porous bamboo charcoal as a sulfur host in an aqueous Cu-S battery. Expect the enhanced conductivity and chemisorption induced by N, S synergistic co-doping, the intrinsic synergy of macro-/meso-/microporous triple structure also ensures space-confined small-molecule sulfur as high utilization reactant and effectively alleviates the volume expansion during conversion reaction. Under a further joint synergy between hierarchical structure and heteroatom doping, the resulting sulfur cathode endows the Cu-S battery with outstanding electrochemical performance. Cycled at 5 A g-1, it can deliver a high reversible capacity of 2,509.8 mAh g-1 with a good capacity retention of 97.9% after 800 cycles. In addition, a flexible hybrid pouch cell built by a small-molecule sulfur cathode, Zn anode, and gel electrolytes can firmly deliver high average operating voltage of about 1.3 V with a reversible capacity of over 2,500 mAh g-1 under various destructive conditions, suggesting that the triple-synergistic small-molecule sulfur cathode promises energetic metal-sulfur batteries.

Research development on electrolytes for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are considered strong candidates for next-generation energy-storage systems owing to their high theoretical capacity, divalent nature and the natural abundancy of magnesium (Mg) resources on Earth. However, the development of MIBs has been mainly limited by the incompatibility of Mg anodes with several Mg salts and conventional organic-liquid electrolytes. Therefore, one major challenge faced by MIBs technology lies on developing safe electrolytes, which demonstrate appropriate electrochemical voltage window and compatibility with Mg anode. This review discusses the development of MIBs from the point-of-view of the electrolyte syntheses. A systematic assessment of promising electrolyte design strategies is proposed including liquid and solid-state electrolytes. Liquid-based electrolytes have been largely explored and can be categorized by solvent-type: organic solvent, aqueous solvent, and ionic-liquids. Organic-liquid electrolytes usually present high electrochemical and chemical stability but are rather dangerous, while aqueous electrolytes present high ionic conductivity and eco-friendliness but narrow electrochemical stability window. Some ionic-liquid electrolytes have proved outstanding performance but are fairly expensive. As alternative to liquid electrolytes, solid-state electrolytes are increasingly attractive to increase energy density and safety. However, improving the ionic conductivity of Mg ions in these types of electrolytes is extremely challenging. We believe that this comprehensive review will enable researchers to rapidly grasp the problems faced by electrolytes for MIBs and the electrolyte design strategies proposed to this date.

Review of Design Routines of MXene Materials for Magnesium-Ion Energy Storage Device

Renewable energy storage using electrochemical storage devices is extensively used in various field applications. High-power density supercapacitors and high-energy density rechargeable batteries are some of the most effective devices, while lithium-ion batteries (LIBs) are the most common. Due to the scarcity of Li resources and serious safety concerns during the construction of LIBs, development of safer and cheaper technologies with high performance is warranted. Magnesium is one of the most abundant and replaceable elements on earth, and it is safe as it does not generate dendrite following cycling. However, the lack of suitable electrode materials remains a critical issue in developing electrochemical energy storage devices. 2D MXenes can be used to construct composites with different dimensions, owing to their suitable physicochemical properties and unique magnesium-ion adsorption structure. In this study, the construction strategies of MXene in different dimensions, including its physicochemical properties as an electrode material in magnesium ion energy storage devices are reviewed. Research advancements of MXene and MXene-based composites in various kinds of magnesium-ion storage devices are also analyzed to understand its energy storage mechanisms. Finally, current opportunities, challenges, and future prospects are also briefly discussed to provide crucial information for future research.

Hydrogen Sensing Mechanism of WS2 Gas Sensors Analyzed with DFT and NAP-XPS

Nanostructured tungsten disulfide (WS₂) is one of the most promising candidates for being used as active nanomaterial in chemiresistive gas sensors, as it responds to hydrogen gas at room temperature. This study analyzes the hydrogen sensing mechanism of a nanostructured WS₂ layer using near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT). The W 4f and S 2p NAP-XPS spectra suggest that hydrogen makes physisorption on the WS₂ active surface at room temperature and chemisorption on tungsten atoms at temperatures above 150 °C. DFT calculations show that a hydrogen molecule physically adsorbs on the defect-free WS₂ monolayer, while it splits and makes chemical bonds with the nearest tungsten atoms on the sulfur point defect. The hydrogen adsorption on the sulfur defect causes a large charge transfer from the WS₂ monolayer to the adsorbed hydrogen. In addition, it decreases the intensity of the in-gap state, which is generated by the sulfur point defect. Furthermore, the calculations explain the increase in the resistance of the gas sensor when hydrogen interacts with the WS₂ active layer.

Reversible Magnesium Metal Cycling in Additive-Free Simple Salt Electrolytes Enabled by Spontaneous Chemical Activation

Rechargeable magnesium (Mg) batteries can offer higher volumetric energy densities and be safer than their conventional counterparts, lithium-ion batteries. However, their practical implementation is impeded due to the passivation of the Mg metal anode or the severe corrosion of the cell parts in conventional electrolyte systems. Here, we present a chemical activation strategy to facilitate the Mg deposition/stripping process in additive-free simple salt electrolytes. By exploiting the simple immersion-triggered spontaneous chemical reaction between reactive organic halides and Mg metal, the activated Mg anode exhibited an overpotential below 0.2 V and a Coulombic efficiency as high as 99.5% in a Mg(TFSI)₂ electrolyte. Comprehensive analyses reveal simultaneous evolution of morphology and interphasial chemistry during the activation process, through which stable Mg cycling over 990 cycles was attained. Our activation strategy enabled the efficient cycling of Mg full-cell candidates using commercially available electrolytes, thereby offering possibilities of building practical Mg batteries.

The Proof-of-Concept of Anode-Free Rechargeable Mg Batteries

The desperate pursuit of high gravimetric specific energy leads to the ignorance of volumetric energy density that is one of the basic requirements for batteries. Due to the high volumetric capacity, less-prone formation of dendrite, and low reduction potential of Mg metal, rechargeable Mg batteries are considered with innately high volumetric energy density. Nevertheless, the substantial elevation in energy density is compromised by extremely excessive Mg metal anode. Herein, the proof-of-concept of anode-free Mg₂ Mo₆ S₈ -MgS/Cu batteries is proposed, in which MgS as the premagnesiation additive constantly decomposes to replenish Mg loss by electrolyte corrosion over cycling, while both Mg₂ Mo₆ S₈ and MgS acts as the active material to reversibly provide high capacities. Besides, Mg₂ Mo₆ S₈ shows superior catalytic activity on the decomposition of MgS and provides the strong affinity to polysulfides to restrain their dissolution. Consequently, the anode-free Mg₂ Mo₆ S₈ -MgS/Cu batteries deliver a high reversible capacity of 190 mAh g^-1 with the capacity retention of 92% after 100 cycles, corresponding to the highly competitive energy density of 420 Wh L^-1 . The proposed anode-free Mg battery here spotlights the great promise of Mg batteries in achieving high volumetric energy densities, which will significantly expedite the advances of Mg batteries in practice.

A Passivation-Free Solid Electrolyte Interface Regulated by Magnesium Bromide Additive for Highly Reversible Magnesium Batteries

Highly reversible Mg battery chemistry demands a suitable electrolyte formulation highly compatible with currently available electrodes. In general, conventional electrolytes form a passivation layer on the Mg anode, requiring the use of MgCl₂ additives that lead to severe corrosion of cell components and low anodic stability. Herein, for the first time, we conducted a comparative study of a series of Mg halides as potential electrolyte additives in conventional magnesium bis(hexamethyldisilazide)-based electrolytes. A novel electrolyte formulation that includes MgBr₂ showed unprecedented performance in magnesium plating/stripping, with an average Coulombic efficiency of 99.26% over 1000 cycles at 0.5 mA/cm² and 0.5 mAh/cm². Further analysis revealed the in situ formation of a robust Mg anode-electrolyte interface, which leads to dendrite-free Mg deposition and stable cycling performance in a Mg-Mo₆S₈ battery over 100 cycles. This study demonstrates the rational formulation of a novel MgBr₂-based electrolyte with high anodic stability of 3.1 V for promising future applications.

Toward High-Performance Mg-S Batteries via a Copper Phosphide Modified Separator

Magnesium-sulfur (Mg-S) batteries are emerging as a promising alternative to lithium-ion batteries, due to their high energy density and low cost. Unfortunately, current Mg-S batteries typically suffer from the shuttle effect that originates from the dissolution of magnesium polysulfide intermediates, leading to several issues such as rapid capacity fading, large overcharge, severe self-discharge, and potential safety concern. To address these issues, here we harness a copper phosphide (Cu3P) modified separator to realize the adsorption of magnesium polysulfides and catalyzation of the conversion reaction of S and Mg2+ toward stable cycling of Mg-S cells. The bifunctional layer with Cu3P confined in a carbon matrix is coated on a commercial polypropylene membrane to form a porous membrane with high electrolyte wettability and good thermal stability. Density functional theory (DFT) calculations, polysulfide permeability tests, and post-mortem analysis reveal that the catalytic layer can adsorb polysulfides, effectively restraining the shuttle effect and facilitating the reversibility of the Mg-S cells. As a result, the Mg-S cells can achieve a high specific capacity, fast rates (449 mAh g-1 at 0.1 C and 249 mAh g-1 at 1.0 C), and a long cycle life (up to 500 cycles at 0.5 C) and operate even at elevated temperatures.

Organic Electrolyte Design for Rechargeable Batteries: From Lithium to Magnesium

Rechargeable magnesium batteries (RMBs) have been considered as one of the most viable battery chemistries amongst the "post" lithium-ion battery (LIB) technologies owing to their high volumetric capacity and the natural abundance of their key elements. The fundamental properties of Mg-ion conducting electrolytes are of essence to regulate the overall performance of RMBs. In this Review, the basic electrochemistry of Mg-ion conducting electrolytes batteries is discussed and compared to that of the Li-ion conducting electrolytes, and a comprehensive overview of the development of different Mg-ion conducting electrolytes is provided. In addition, the remaining challenges and possible solutions for future research are intensively discussed. The present work is expected to give an impetus to inspire the discovery of key electrolytes and thereby improve the electrochemical performances of RMBs and other related emerging battery technologies.

Solid-State Electrolytes for Rechargeable Magnesium-Ion Batteries: From Structure to Mechanism

Rechargeable magnesium (Mg)-ion batteries have received growing attention as a next-generation battery system owing to their advantages of sufficient reserves, lower cost, better safety, and higher volumetric energy density than lithium-ion batteries. However, Mg as an anode can be easily passivated during charging/discharging by most common solvents, which are inconducive for magnesium deposition/stripping. Based on this, the development of Mg-ion solid-state electrolytes in the last decades led to the formulization of several concepts beyond previously reported designs. These exciting studies have once again sparked an interest in all-solid-state magnesium-ion batteries. In this review, Mg solid-state electrolytes, including inorganic (oxides, hydrides, and chalcogenides) and organic (metal-organic frameworks and polymers) materials are classified and summarized in detail. Moreover, the structural characteristics and the migration mechanism of Mg²⁺ ions are also discussed with a focus on pending questions and future prospects.

A High Potential Polyanion Cathode Material for Rechargeable Mg-Ion Batteries

The development of Mg-ion batteries is hindered by the lack of cathode materials that allow facile and reversible Mg²⁺ intercalation at high potential. Herein, the authors present a polyanion cathode material of K₂ (VO)₂ (HPO₄ )₂ (C₂ O₄ )⋅4.5H₂ O (KVPCH) for Mg-ion batteries. The inductive effect of polyanions ensures the high redox potential of the vanadium centers. In addition, the material contains structural water located between the layers. It helps with Mg²⁺ desolvation at the electrode-electrolyte interface and facilitates its diffusion in the structure, as confirmed by experimental analysis and theoretical calculations. Thanks to those factors, the KVPCH electrode presents excellent Mg storage capability at room temperature. It delivers 121 mAh g^-1 capacity at 1C with a high average discharge potential of 0.16 V versus Ag/Ag⁺ (3.2 V vs Mg/Mg²⁺ ). A capacity retention of 87% is realized after 1500 cycles at 5C. A rocking-chair Mg-ion full cell is also demonstrated with the KVPCH cathode and a MoO_x anode, which achieves 46 mAh g^-1 capacity (based on the total active material mass of two electrodes). This work would bring effective paths for the design of cathode materials for Mg-ion batteries.

Charge-Compensation in a Displacement Mg2+ Storage Cathode through Polyselenide-Mediated Anion Redox

Chalcogenides have been viewed as important conversion-type Mg²⁺ -storage cathodes to fulfill the high volumetric energy density promise of magnesium (Mg) batteries. However, the low initial Columbic efficiency and the rapid capacity degradation remain challenges for the chalcogenide cathodes, as the clear Mg²⁺ -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of the Cu_2-x Se cathode is a reversible displacement reaction along with a polyselenide (PSe) mediated solution process of anion-compensation. The unique anion redox improves charge storage, while the dissolution of PSe also leads to performance degradation. To address this issue, we introduce Mo₆ S₈ into the Cu_2-x Se cathode to immobilize PSe, which significantly improves performance, especially the reversible capacity (from 140 mAh g^-1 to 220 mAh g^-1 ). This work provides inspiration for the modification of the Mg²⁺ -storage cathode, which is a milestone for high-performance Mg batteries.

Negative sulfur-based electrodes and their application in battery cells: Dual-ion batteries as an example

In this work, a cell concept comprising of an anion intercalating graphite-based positive electrode (cathode) and an elemental sulfur-based negative electrode (anode) is presented as a transition metal- and in a specific concept even Li-free cell setup using a Li-ion containing electrolyte or a Mg-ion containing electrolyte. The cell achieves discharge capacities of up to 37 mAh g−1 and average discharge cell voltages of up to 1.9 V. With this setup, more than 100 cycles with a high capacity retention (> 90% of the highest achieved value) and Coulombic efficiencies up to 95% could be achieved, which opens a broad new field for energy storage approaches.

Chemistry-performance relationships of polymer gel-electrolytes for Mg-S and Li-S batteries: Influence of network cation solvation capacity on polymer-polysulfide interactions

Metal-sulfur batteries are a promising next-generation energy storage technology, offering high theoretical energy densities with low cost and good sustainability. An active area of research is the development of electrolytes that address unwanted migration of sulfur and intermediate species known as polysulfides during operation of metal-sulfur batteries, a phenomenon that leads to low energy efficiency and short life-spans. A particular class of electrolytes, gel polymer electrolytes, are especially attractive for their ability to repel polysulfides on the basis of structure, electrostatics, and other polymer properties. Here, within the context of magnesium- and lithium-sulfur batteries, we investigate the impact of gel polymer electrolyte cation solvation capacity, a property related to network dielectric constant and chemistry, on sulfur/polysulfide-polymer interactions, an understudied property-performance relationship. Polymers with lower cation solvation capacity are found to permanently absorb less polysulfide active material, which increases sulfur utilization for Li-S batteries and significantly increases charge efficiency and life-span for Li-S and Mg-S batteries.

Role of a Multivalent Ion-Solvent Interaction on Restricted Mg2+ Diffusion in Dimethoxyethane Electrolytes

The diffusion behavior of Mg²⁺ in electrolytes is not as readily accessible as that from Li⁺ or Na⁺ utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting ²⁵Mg NMR. In MgTFSI₂/DME solutions, "bound" DME (coordinating to Mg²⁺) and "free" DME (bulk) are distinguishable from ¹H NMR. With the exchange rates between them obtained from 2D ¹H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg²⁺) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65-70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C-H bond length. Comparison of the diffusion behaviors of Mg²⁺, TFSI^-, DME, and Li⁺ reveals a relative restriction to Mg²⁺ diffusion that is caused by the long-range interaction between Mg²⁺ and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.

Current Design Strategies for Rechargeable Magnesium-Based Batteries

As a next-generation electrochemical energy storage technology, rechargeable magnesium (Mg)-based batteries have attracted wide attention because they possess a high volumetric energy density, low safety concern, and abundant sources in the earth's crust. While a few reviews have summarized and discussed the advances in both cathode and anode materials, a comprehensive and profound review focusing on the material design strategies that are both representative of and peculiar to the performance improvement of rechargeable Mg-based batteries is rare. In this mini-review, all nine of the material design strategies and approaches to improve Mg-ion storage properties of cathode materials have been comprehensively examined from both internal and external aspects. Material design concepts are especially highlighted, focusing on designing "soft" anion-based materials, intercalating solvated or complex ions, expanding the interlayer of layered cathode materials, doping heteroatoms into crystal lattice, size tailoring, designing metastable-phase materials, and developing organic materials. To achieve a better anode, strategies based on the artificial interlayer design, efficient electrolyte screening, and alternative anodes exploration are also accumulated and analyzed. The strategy advances toward Mg-S and Mg-Se batteries are summarized. The advantages and disadvantages of all-collected material design strategies and approaches are critically discussed from practical application perspectives. This mini-review is expected to provide a clear research clue on how to rationally improve the reliability and feasibility of rechargeable Mg-based batteries and give some insights for the future research of Mg-based batteries as well as other multivalent-ion battery chemistries.

Opportunities and Limitations of Ionic Liquid- and Organic Carbonate Solvent-Based Electrolytes for Mg-Ion-Based Dual-Ion Batteries

Dual-ion batteries (DIBs) offer a great alternative to state-of-the-art lithium-ion batteries, based on their high promises due to the absence of transition metals and the use of low-cost materials, which could make them economically favorable targeting stationary energy storage applications. In addition, they are not limited by certain metal cations, and DIBs with a broad variety of utilized ions could be demonstrated over the last years. Herein, a systematic study of different electrolyte approaches for Mg-ion-based DIBs was conducted. A side-by-side comparison of Li- and Mg-ion-based electrolytes using activated carbon as negative electrode revealed the opportunities but also limitations of Mg-ion-based DIBs. Ethylene sulfite was successfully introduced as electrolyte additive and increased the specific discharge capacity significantly up to 93±2 mAh g-1 with coulombic efficiencies over 99 % and an excellent capacity retention of 88 % after 400 cycles. In addition, and for the first time, highly concentrated carbonate-based electrolytes were employed for Mg-ion-based DIBs, showing adequate discharge capacities and high coulombic efficiencies.

Using a Chloride-Free Magnesium Battery Electrolyte to Form a Robust Anode-Electrolyte Nanointerface

Magnesium bis(hexamethyldisilazide) (Mg(HMDS)₂)-based electrolytes are compelling candidates for rechargeable magnesium batteries due to their high compatibility with magnesium metal anode. However, the usual combination of Mg(HMDS)₂ with chloride salts limits their practical application due to severe corrosion of cell components and low anodic stability. Herein, we report for the first time, a chloride-free Mg(HMDS)₂-based electrolyte in 1,2-dimethoxyethane. By chemically controlling the moisture content using tetrabutylammonium borohydride as a moisture scavenger, the electrolyte demonstrates outstanding electrochemical performance in magnesium plating/stripping, with an average Coulombic efficiency of 98.3% over 150 cycles, and is noncorrosive to cell components. Surface analysis and depth profiling of the magnesium metal anode reveals the formation of a robust solid electrolyte interphase at the anode-electrolyte nanointerface, which allows magnesium plating/stripping to occur reversibly. The electrolyte also demonstrates good compatibility with a copper sulfide nanomaterial cathode, which exhibits a high initial discharge capacity of 261.5 mAh g^-1.

Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg-S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg-S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag₂S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg²⁺ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S₈ influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.

Performance Study of MXene/Carbon Nanotube Composites for Current Collector- and Binder-Free Mg-S Batteries

The realization of sustainable and cheap Mg-S batteries depends on significant improvements in cycling stability. Building on the immense research on cathode optimization from Li-S batteries, for the first time a beneficial role of MXenes for Mg-S batteries is reported. Through a facile, low-temperature vacuum-filtration technique, several novel current collector- and binder-free cathode films were developed, with either dipenthamethylene thiuram tetrasulfide (PMTT) or S8 nanoparticles as the source of redox-active sulfur. The importance of combining MXene with a high surface area co-host material, such as carbon nanotubes, was demonstrated. A positive effect of MXenes on the average voltage and reduced self-discharge was also discovered. Ascribed to the rich polar surface chemistry of Ti3 C2 Tx MXene, an almost doubling of the discharge capacity (530 vs. 290 mA h g-1 ) was achieved by using MXene as a polysulfide-confining interlayer, obtaining a capacity retention of 83 % after 25 cycles.

Material design strategies to improve the performance of rechargeable magnesium-sulfur batteries

Beyond current lithium-ion technologies, magnesium-sulfur (Mg-S) batteries represent one of the most attractive battery chemistries that utilize low cost, sustainable, and high capacity materials. In addition to high gravimetric and volumetric energy densities, Mg-S batteries also enable safer operation due to the lower propensity for magnesium dendrite growth compared to lithium. However, the development of practical Mg-S batteries remains challenging. Major problems such as self-discharge, rapid capacity loss, magnesium anode passivation, and low sulfur cathode utilization still plague these batteries, necessitating advanced material design strategies for the cathode, anode, and electrolyte. This review critically appraises the latest research and design principles to address specific issues in state-of-the-art Mg-S batteries. In the process, we point out current limitations and open-ended questions, and propose future research directions for practical realization of Mg-S batteries and beyond.

Interfaces in rechargeable magnesium batteries

This minireview provides a concise overview on the development of electrolytes for rechargeable magnesium (Mg) batteries. It elucidates the intrinsic driving force of the evolution from Grignard-based electrolytes to electrolytes based on simple Mg salts. Additional discussion includes the key electrochemical processes at the interfaces in Mg electrolytes, with a focus on unaddressed issues and future research directions.

Rechargeable Calcium-Sulfur Batteries Enabled by an Efficient Borate-Based Electrolyte

Rechargeable metal-sulfur batteries show great promise for energy storage applications because of their potentially high energy and low cost. The multivalent-metal based electrochemical system exhibits the particular advantage of the feasibility of dendrite-free metal anode. Calcium (Ca) represents a promising anode material owing to the low reductive potential, high capacity, and abundant natural resources. However, calcium-sulfur (Ca-S) battery technology is in an early R&D stage, facing the fundamental challenge to develop a suitable electrolyte enabling reversible electrochemical Ca deposition, and at the same time, sulfur redox reactions in the system. Herein, a study of a room-temperature Ca-S battery by employing a stable and efficient calcium tetrakis(hexafluoroisopropyloxy) borate Ca[B(hfip)₄ ]₂ electrolyte is presented. The Ca-S batteries exhibit a cell voltage of ≈2.1 V (close to its thermodynamic value) and good reversibility. The mechanistic studies hint at a redox chemistry of sulfur with polysulfide/sulfide species involved in the Ca-based system.

Highly Efficient Non-Nucleophilic Mg(CF3SO3)2-Based Electrolyte for High-Power Mg/S Battery

The application of high-energy Mg/S batteries was obstructed by the insufficiency of low-cost and non-nucleophilic electrolyte. In this work, a non-nucleophilic electrolyte was prepared by facilely dissolving Mg(CF₃SO₃)₂, MgCl₂, and AlCl₃ in 1,2-dimethoxyethane (DME). The equilibrium species of the MTB electrolyte mainly comprise [Mg₂(μ-Cl)₂(DME)₄]²⁺ and [(CF₃SO₃)AlCl₃]^-, which are generated by dehalodimerization reaction and Lewis acid-base reaction, respectively. The electrolyte exhibits a highly efficient reversible Mg deposition/dissolution, low overpotential of around 250 mV, and good oxidative stability up to 3.5 V after conditioning. The conditioning process that the active [Mg₂(μ-Cl)₂(DME)₄]²⁺ species transforms into [Mg₃(μ₃-Cl)(μ₂-Cl)₂(DME)₇]³⁺ was demonstrated, owing to the irreversible deposition of Al³⁺ on Mg foil. More importantly, this electrolyte exhibited good compatibility and kinetics for reversible Mg/MgS_x redox, resulting in a high specific capacity of 866 mAh g^-1 at 200 mA g^-1 and high power density of 550 W kg^-1. This work offers a new direction for low-cost, non-nucleophilic electrolyte and paves the way to explore high-power Mg/S batteries.

Sequestration of Sulphide from Biogas by thermal-treated iron nanoparticles synthesized using tea polyphenols

Dark tea-iron nanoparticles (DT-Fe NPs) were prepared using extracts of dark tea leaves as a reducing agent, and further underwent thermal treatment in air. The H₂S removal performances of thermal-treated DT-Fe NPs for biogas were further evaluated using a custom-designed fixed-bed reactor (reaction temperature of 250°C, H₂S content of 1%). Significant morphology and chemical composition differences were observed when DT-Fe NPs were treated at different temperatures (300-800^oC). X-ray diffractometer analysis revealed that a phase transition from γ-Fe₂O₃ to α-Fe₂O₃ occurred under heat treatment. When the thermal treatment temperature was 300°C, only α-Fe₂O₃ was detected. Both α-Fe₂O₃ and γ-Fe₂O₃ were present in the sample treated at 400°C. When the thermal treatment temperature was 500-800°C, γ-Fe₂O₃ in the sample was completely converted to α-Fe₂O₃. The H₂S removal capacity is 14.72 mg H₂S/g for DT-Fe NPs without treatment. However, the value increased significantly to 408.30 mg H₂S/g after 400°C thermal treatment, which can be explained by the formation of highly active γ-Fe₂O₃. The reaction product of thermal-treated DT-Fe NPs at 400°C and H₂S were further characterized by X-ray diffractometer and X-ray photoelectron spectroscopy. The results showed that it is composed of FeS₂ and FeS, in which 72.6% of the sulphur existed as disulphide and 27.4% as monosulphide.

A Metal-Organic Framework Thin Film for Selective Mg2+ Transport

The incompatibility between the anode and the cathode chemistry limits the used of Mg as an anode. This issue may be addressed by separating the anolyte and the catholyte with a membrane that only allows for Mg²⁺ transport. Mg-MOF-74 thin films were used as the separator for this purpose. It was shown to meet the needs of low-resistance, selective Mg²⁺ transport. The uniform MOF thin films supported on Au substrate with thicknesses down to ca. 202 nm showed an intrinsic resistance as low as 6.4 Ω cm² , with the normalized room-temperature ionic conductivity of ca. 3.17×10^-6  S cm^-1 . When synthesized directly onto a porous anodized aluminum oxide (AAO) support, the resulting films were used as a standalone membrane to permit stable, low-overpotential Mg striping and plating for over 100 cycles at a current density of 0.05 mA cm^-2 . The film was effective in blocking solvent molecules and counterions from crossing over for extended period of time.

A Four-Electron Sulfur Electrode Hosting a Cu2+ /Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu₂ S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g^-1 based on the sulfur mass or 609 mAh g^-1 based on the mass of Cu₂ S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg^-1 based on the mass of the Cu₂ S/carbon composite cathode, and stable cycling over 110 cycles.

Reversible intercalation of methyl viologen as a dicationic charge carrier in aqueous batteries

The interactions between charge carriers and electrode structures represent one of the most important considerations in the search for new energy storage devices. Currently, ionic bonding dominates the battery chemistry. Here we report the reversible insertion of a large molecular dication, methyl viologen, into the crystal structure of an aromatic solid electrode, 3,4,9,10-perylenetetracarboxylic dianhydride. This is the largest insertion charge carrier when non-solvated ever reported for batteries; surprisingly, the kinetic properties of the (de)insertion of methyl viologen are excellent with 60% of capacity retained when the current rate is increased from 100 mA g⁻¹ to 2000 mA g⁻¹. Characterization reveals that the insertion of methyl viologen causes phase transformation of the organic host, and embodies guest-host chemical bonding. First-principles density functional theory calculations suggest strong guest-host interaction beyond the pure ionic bonding, where a large extent of covalency may exist. This study extends the boundary of battery chemistry to large molecular ions as charge carriers and also highlights the electrochemical assembly of a supramolecular system.

Rechargeable batteries are governed by reversible intercalation reactions. Here the authors expand the intercalation chemistry to include methyl viologen as a large dicationic charge carrier in aqueous batteries. The bonding between guest and aromatic host is not pure ionic but shows strong covalent character.

Current Status and Future Prospects of Metal-Sulfur Batteries

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

In Situ X-ray Absorption Spectroscopic Investigation of the Capacity Degradation Mechanism in Mg/S Batteries

The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge-discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS)₂-AlCl₃ electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg₃S₈, through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS₂ as a catalyst to activate the irreversible reaction of low-order MgS _x and MgS, which results in an increased discharging capacity up to 900 mAh·g^-1 and a longer cycling life.

High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes for Magnesium-Sulfur Batteries

The shortage of high-performance and easily prepared electrolyte has hindered the progress of rechargeable magnesium-sulfur (Mg-S) batteries. In this paper, we develop a new electrolyte based on Mg(CF₃SO₃)₂-AlCl₃ dissolved in tetrahydrofuran and tetraglyme mixed solvents. Mg(SO₃CF₃)₂ as an Mg²⁺ source is nonnucleophilic, easy to handle, and much cheaper than Mg(TFSI)₂ (TFSI = bis(trifluoromethanesulfonyl)imide). After modification with anthracene (π stabilizing agent) as a coordinating ligand to stabilize the Mg²⁺ ions and MgCl₂ to improve the interface properties by accelerating the reaction of Mg(CF₃SO₃)₂ with AlCl₃, the electrolyte exhibits a low overpotential for overall Mg deposition and dissolution, moderate anodic stability (3.25 V on Pt, 2.5 V on SS, 2.0 V on Cu, and 1.85 V on Al, respectively), and a suitable ionic conductivity (1.88 mS cm^-1). More importantly, this electrolyte modulated by Li-salt additives exhibits good compatibility with S cathode and can be applicable for Mg-S batteries. The rational formulation of the new electrolyte could provide a new avenue for simply prepared Mg electrolytes of Mg-S and rechargeable magnesium batteries.

Improving a Mg/S Battery with YCl3 Additive and Magnesium Polysulfide

Rechargeable magnesium/sulfur (Mg/S) batteries are widely regarded as one of the alternatives to lithium-ion batteries. However, a key factor restricting their application is the lack of suitable electrolyte. Herein, an electrolyte additive that can reduce the polarization voltage is developed and 98.7% coulombic efficiency is realized. The as-prepared Mg-ion electrolyte exhibits excellent Mg plating/stripping performance with a low overpotential of 0.11 V for plating process, and high anodic stability up to 3.0 V (vs Mg/Mg2+). When it is coupled with magnesium polysulfide, which has high reactivity and is homogeneously distributed on carbon matrix, the Mg/S cells deliver a good cycling stability with a high discharge capacity over 1000 mAh g-1 for more than 50 cycles.