Understanding the Cathode-Electrolyte Interfacial Chemistry in Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have garnered significant attention due to their potential to provide high energy density, utilize earth-abundant raw materials, and employ metal anode safely. Currently, the lack of applicable cathode materials has become one of the bottleneck issues for fully exploiting the technological advantages of RMBs. Recent studies on Mg cathodes reveal divergent storage performance depending on the electrolyte formulation, posing interfacial issues as a previously overlooked challenge. This minireview begins with an introduction of representative cathode-electrolyte interfacial phenomena in RMBs, elaborating on the unique solvation behavior of Mg2+, which lays the foundation for interfacial chemistries. It is followed by presenting recently developed strategies targeting the promotion of Mg2+ desolvation in the electrolyte and alternative cointercalation approaches to circumvent the desolvation step. In addition, efforts to enhance the cathode-electrolyte compatibility via electrolyte development and interfacial engineering are highlighted. Based on the abovementioned discussions, this minireview finally puts forward perspectives and challenges on the establishment of a stable interface and fast interfacial chemistry for RMBs.

Interlayer Engineering of VS2 Nanosheets via In Situ Aniline Intercalative Polymerization toward Long-Cycling Magnesium-Ion Batteries

Rechargeable magnesium batteries (RMBs) show great potential in large-scale energy storage systems, due to Mg2+ with high polarity leading to strong interactions within the cathode lattice, and the limited discovery of functional cathode materials with rapid kinetics of Mg2+ diffusion and desirable cyclability retards their development. Herein, we innovatively report the confined synthesis of VS2/polyaniline (VS2/PANI) hybrid nanosheets. The VS2/PANI hybrids with expanded interlayer spacing are successfully prepared through the exfoliation of VS2 and in situ polymerization between VS2 nanosheets and aniline. The intercalated PANI increases the interlayer spacing of VS2 from 0.57 to 0.95 nm and improves its electronic conductivity, leading to rapid Mg-ion diffusivity of 10-10-10-12 cm2 s-1. Besides, the PANI sandwiched between layers of VS2 is conducive to maintaining the structural integrity of electrode materials. Benefiting from the above advantages, the VS2/PANI-1 hybrids present remarkable performance for Mg2+ storage, showing high reversible discharge capacity (245 mA h g-1 at 100 mA g-1) and impressive long lifespan (91 mA h g-1 after 2000 cycles at 500 mA g-1). This work provides new perspectives for designing high-performance cathode materials based on layered materials for RMBs.

Unusual Hybrid Magnesium Storage Mechanism in a New Type of Bi2O2CO3 Anode

Bismuth and bismuth-based compounds have been extensively studied as anodes as prospective candidates for rechargeable magnesium batteries (rMBs). However, the unsatisfactory magnesium-storage capability caused by the typical alloying reaction mechanism severely restricts the practical option for anodes in rMBs. Herein, polyaniline intercalated Bi2O2CO3 nanosheets are prepared by an effective interlayer engineering strategy to fine-tune the layer structure of Bi2O2CO3, achieving enhanced magnesium-storage capacity, rate performance, as well as long cycle life. Excitedly, a stepwise insertion-conversion-alloying reaction is aroused to stabilize the performance, which is elucidated by in/ex situ investigations. Moreover, first-principles calculations confirm that the coupling of Bi2O2CO3 and polyaniline not only increases the conductivity induced by the strong density of states and the interior self-built-in electric field but also significantly reduces the energy barrier of Mg shuttles. Our findings shed light on exploring new electrode materials with an appropriate working mechanism toward high-performance rechargeable batteries.

An Amorphous Molybdenum Polysulfide Cathode for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) attract research interest owing to the low cost and high reliability, but the design of cathode materials is the major difficulty of their development. The bivalent magnesium cation suffers from a strong interaction with the anion and is difficult to intercalate into traditional magnesium intercalation cathodes. Herein, an amorphous molybdenum polysulfide (a-MoSx ) is synthesized via a simple one-step solvothermal reaction and used as the cathode material for RMBs. The a-MoSx cathode provides a high capacity (185 mAh g-1 ) and a good rate performance (50 mAh g-1 at 1000 mA g-1 ), which are much superior compared with crystalline MoS2 and demonstrate the privilege of amorphous RMB cathodes. A mechanism study demonstrates both of molybdenum and sulfur undergo redox reactions and contribute to the capacity. Further optimizations indicate low-temperature synthesis would favor the magnesium storage performance of a-MoSx .

Anion-Incorporated Mg-Ion Solvation Modulation Enables Fast Magnesium Storage Kinetics of Conversion-Type Cathode Materials

Rechargeable magnesium batteries (RMB) have emerged as one of the most promising alternatives to lithium-ion batteries due to the prominent advantages of magnesium metal anodes. Nevertheless, their application is hindered by sluggish Mg-ion storage kinetics in cathodes, although various structural modifications of cathode materials have been performed. Herein, an electrolyte design using an anion-incorporated Mg-ion solvation structure is developed to promote the Mg-ion storage reactions of conversion-type cathode materials. The addition of the trifluoromethanesulfonate anion (OTf^- ) in the ether-based Mg-ion electrolyte modulates the solvation structure of Mg²⁺ from [Mg(DME)₃ ]²⁺ to [Mg(DME)_2.5 OTf]⁺ (DME = dimethoxy ethane), which facilitates Mg-ion desolvation and thus significantly expedites the charge transfer of the cathode material. As a result, the as-prepared CuSe cathode material on copper current collector exhibits a considerable increase in magnesium storage capacity from 61% (228 mAh g^-1 ) to 95% (357 mAh g^-1 ) of the theoretical capacity at 0.1 A g^-1 and a more than twofold capacity increase at a high current density of 1.0 A g^-1 . This work provides an efficient strategy via electrolyte modulation to achieve high-rate conversion-type cathode materials for RMBs. The incorporation of the trifluoromethanesulfonate anion in the Mg-ion solvation structure of the borate-based Mg-ion electrolyte enables the fast magnesium storage kinetics of the conversion-type cathode materials. The as-prepared copper selenide cathode achieved a more than twofold capacity increase at a high rate and the highest reversible capacities compared to those of the previously reported metal selenide cathodes.

In Situ Formed Magnesiophilic Sites Guiding Uniform Deposition for Stable Magnesium Metal Anodes

Owing to its high volumetric capacity and natural abundance, magnesium (Mg) metal has attracted tremendous attention as an ideal anode material for rechargeable Mg batteries. Despite Mg deposition playing an integral role in determining the cycling lifespan, its exact behavior is not clearly understood yet. Herein, for the first time, we introduce a facile approach to build magnesiophilic In/MgIn sites in situ on a Mg metal surface using InCl₃ electrolyte additive for rechargeable Mg batteries. These magnesiophilic sites can regulate Mg deposition behaviors by homogenizing the distributions of Mg-ion flux and electric field at the electrode-electrolyte interphase, allowing flat and compact Mg deposition to inhibit short-circuiting. The as-designed Mg metal batteries achieve a stable cycling lifespan of 340 h at 1.0 mA cm^-2 and 1.0 mAh cm^-2 using Celgard separators, while the full cell coupled with Mo₆S₈ cathode maintains a high capacity retention of 95.5% over 800 cycles at 1 C.

Cooperative Cationic and Anionic Redox Reactions in Ultrathin Polyvalent Metal Selenide Nanoribbons for High-Performance Electrochemical Magnesium-Ion Storage

Rechargeable magnesium batteries (RMBs) are considered as potential energy storage devices due to their high volumetric specific capacity, good safety, as well as source abundance. Despite extensive efforts devoted to constructing an efficient magnesium battery system, the sluggish Mg²⁺ diffusion in conventional cathode materials often leads to slow rate kinetics, low capacity, and poor cycling lifespan. Although transition metal selenides with soft anion frameworks have attracted extensive attention, their Mg²⁺ storage mechanism still needs to be clarified. Herein, we demonstrate that the ultrathin CoSe₂ nanoribbons can be used as a robust cathode material for RMBs and reveal a novel Mg²⁺ storage mechanism based on cooperative cationic (Co) and anionic (Se) redox processes via systematic ex-situ characterizations. Compared to other metal selenide cathodes based on conversion reactions of solely metal cations, the cooperative cationic-anionic redox reactions of the CoSe₂ cathode contribute to obtaining an enhanced specific capacity and boosted electrochemical kinetics. Moreover, on one hand, the ultrathin nanoribbon structure enables effective contact between the electrode material and electrolyte and on the other hand significantly reduces the length and time consumption of Mg²⁺ diffusion, leading to dominated surface-driven capacitance-controlled Mg²⁺ storage behavior and rapid Mg²⁺ storage kinetics. As a result, the ultrathin CoSe₂ nanoribbon cathode exhibits a reversible discharge capacity of ∼130 mAh g^-1 at 100 mA g^-1, good rate capability (116 mAh g^-1 at 300 mA g^-1), and long cyclability over 600 cycles. This finding confirms the development potentiality of polyvalent metal selenide cathode materials based on a cooperative cationic-anionic redox mechanism for the construction of next-generation multivalent secondary batteries.

Basal Planes Unlocking and Interlayer Engineering Endows Proton Doped-MoO2.8F0.2 with Fast and Stable Magnesium Storage

Molybdenum trioxide has served as a promising cathode material of rechargeable magnesium batteries (RMBs), because of its rich valence states and high theoretical capacity; yet, it still suffers from sluggish (de)intercalation kinetics and inreversible structure change for highly polarized Mg²⁺ in the interlayer and intralayer of structure. Herein, F^- substitutional and H⁺ interstitial doping is proposed for α-MoO₃ materials (denoted HMoOF) by the intralayer/interlayer engineering strategy to boost the performance of RMBs. F^- substitutional doping generates molybdenum vacancies along the Mo-O-□ or Mo-F-□ configurations (where □ represents the cationic vacancy) for unlocking the inactive basal plane of the layered crystal structure, and it further accelerates Mg²⁺ diffusion along the b-axis. Interstitial-doped H⁺ can expand interlayer spacing for reducing Mg²⁺ energy barrier along the ac plane and serve as a "pillar" to stabilize the interlayer structure. Moreover, anion and cation dual doping trigger shallow impurity levels (acceptors levels and donor levels), which helps to easily acquire the electrons from the valence band and donate the electrons to the conduction band. Consequently, the HMoOF electrode exhibits a high reversible capacity (241 mA h g^-1 at 0.1 A g^-1), an excellent rate capability (137.4 mAh g^-1 at 2 A g^-1), and a long cycling stability (capacity retention of 98% after 800 cycles at 1 A g^-1) in RMBs. This work affords meaningful insights in layered materials for developing high-kinetics and long-life RMBs.

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.

Simply Prepared Magnesium Vanadium Oxides as Cathode Materials for Rechargeable Aqueous Magnesium Ion Batteries

Vanadium-oxide-based materials exist with various vanadium oxidation states having rich chemistry and ability to form layered structures. These properties make them suitable for different applications, including energy conversion and storage. Magnesium vanadium oxide materials obtained using simple preparation route were studied as potential cathodes for rechargeable aqueous magnesium ion batteries. Structural characterization of the synthesized materials was performed using XRD and vibrational spectroscopy techniques (FTIR and Raman spectroscopy). Electrochemical behavior of the materials, observed by cyclic voltammetry, was further explained by BVS calculations. Sluggish Mg²⁺ ion kinetics in MgV₂O₆ was shown as a result of poor electronic and ionic wiring. Complex redox behavior of the studied materials is dependent on phase composition and metal ion inserted/deinserted into/from the material. Among the studied magnesium vanadium oxides, the multiphase oxide systems exhibited better Mg²⁺ insertion/deinsertion performances than the single-phase ones. Carbon addition was found to be an effective dual strategy for enhancing the charge storage behavior of MgV₂O₆.

Rational Design Strategy of Novel Energy Storage Systems: Toward High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising candidates to replace currently commercialized lithium-ion batteries (LIBs) in large-scale energy storage applications owing to their merits of abundant resources, low cost, high theoretical volumetric capacity, etc. However, the development of RMBs is still facing great challenges including the incompatibility of the electrolyte and the lack of suitable cathode materials with high reversible capacity and fast kinetics of Mg²⁺ . While tremendous efforts have been made to explore compatible electrolytes and appropriate electrode materials, the rational design of unconventional Mg-based battery systems is another effective strategy for achieving high electrochemical performance. This review specifically discusses the recent research progress of various Mg-based battery systems. First, the optimization of electrolyte and electrode materials for conventional RMBs is briefly discussed. Furthermore, various Mg-based battery systems, including Mg-chalcogen (S, Se, Te) batteries, Mg-halogen (Br₂ , I₂ ) batteries, hybrid-ion batteries, and Mg-based dual-ion batteries are systematically summarized. This review aims to provide a comprehensive understanding of different Mg-based battery systems, which can inspire latecomers to explore new strategies for the development of high-performance and practically available RMBs.

Anionic Te-Substitution Boosting the Reversible Redox in CuS Nanosheet Cathodes for Magnesium Storage

The conversion-type copper chalcogenide cathode materials hold great promise for realizing the competitive advantages of rechargeable magnesium batteries among next-generation energy storage technologies; yet, they suffer from sluggish kinetics and low redox reversibility due to large Coulombic resistance and ionic polarization of Mg²⁺ ions. Here we present an anionic Te-substitution strategy to promote the reversible Cu⁰/Cu⁺ redox reaction in Te-substituted CuS_1-xTe_x nanosheet cathodes. X-ray absorption fine structure analysis demonstrates that Te dopants occupy the anionic sites of sulfur atoms and result in an improved oxidation state of the Cu species. The kinetically favored CuS_1-xTe_x (x = 0.04) nanosheets deliver a specific capacity of 446 mAh g^-1 under a 20 mA g^-1 current density and a good long-life cycling stability upon 1500 repeated cycles with a capacity decay rate of 0.0345% per cycle at 1 A g^-1. Furthermore, the CuS_1-xTe_x (x = 0.04) nanosheets can also exhibit an enhanced rate capability with a reversible specific capacity of 100 mAh g^-1 even under a high current density of 1 A g^-1. All the obtained electrochemical characteristics of CuS_1-xTe_x nanosheets significantly exceed those of pristine CuS nanosheets, which can contribute to the improved redox reversibility and favorable kinetics of CuS_1-xTe_x nanosheets. Therefore, anionic Te-substitution demonstrates a route for purposeful cathode chemistry regulation in rechargeable magnesium batteries.

Evaluation of Mg[B(HFIP)4]2-Based Electrolyte Solutions for Rechargeable Mg Batteries

One of the greatest challenges toward rechargeable magnesium batteries is the development of noncorrosive electrolyte solutions with high anodic stability that can support reversible Mg deposition/dissolution. In the last few years, magnesium electrolyte solutions based on Cl-free fluorinated alkoxyborates were investigated for Mg batteries due to their high anodic stability and ionic conductivity and the possibility of reversible deposition/dissolution in ethereal solvents. Here, the electrochemical performance of Mg[B(hexafluoroisopropanol)₄]₂/dimethoxyethane (Mg[B(HFIP)₄]₂/DME) solutions was examined. These electrolyte solutions require a special "conditioning" pretreatment that removes undesirable active moieties. Such a process was developed and explored, and basic scientific issues related to the mechanism by which it affects Mg deposition/dissolution were addressed. The chemical changes that occur during the conditioning process were examined. Mg[B(HFIP)₄]₂/DME solutions were found to enable reversible Mg deposition, albeit with a relatively low Coulombic efficiency of 95% during the first cycles. Prolonged deposition/dissolution cycling tests demonstrate a stable behavior of magnesium electrodes. Overall, this system presents a reasonable electrolyte solution and can serve as a basis for future efforts to develop chlorine-free alternatives for secondary magnesium batteries. It is clear that such a conditioning process is mandatory, as it removes reactive contaminants that lead to unavoidable passivation and deactivation of Mg electrodes from the solution.

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.

A Pyrite Iron Disulfide Cathode with a Copper Current Collector for High-Energy Reversible Magnesium-Ion Storage

Owing to its low cost, high theoretical capacity, and environmentally friendly characteristics, pyrite FeS₂ demonstrates promise as a cathode material for high-energy metal-anode-based rechargeable batteries. When it is used in a rechargeable magnesium battery (RMB), the electrode couple exhibits an extremely low theoretical volume change upon full discharge. However, its electrochemical Mg-ion storage is considerably hindered by slow reaction kinetics. In this study, a high-performance FeS₂ cathode for RMBs using a copper current collector is reported, which is involved in cathode reactions via a reversible redox process between copper and cuprous sulfide. This phase transformation with the formation of copper nanowires during discharge activates the redox reactions of FeS₂ via a two-step and four-electron Mg-ion transfer that dominates the cathode reactions. As a result, the as-prepared FeS₂ nanomaterial cathode delivers a significantly enhanced reversible capacity of 679 mAh g^-1 at 50 mA g^-1 . The corresponding energy density of 714 Wh kg^-1 is superior to those of all previously reported metal chalcogenide cathodes in RMBs or hybrid batteries using a Mg metal anode. Notably, the as-assembled FeS₂ -Mg battery can operate over 1000 cycles with a good capacity retention at 400 mA g^-1 .

Facile Preparation of CuCo2 S4 /Cu7.2 S4 Nanocomposites as High-Performance Cathode Materials for Rechargeable Magnesium Batteries*

Rechargeable magnesium batteries (RMBs) have been considered a promising energy-storage device due to their high energy density and high safety, but they still suffer from a lack of high-rate performance and cycle performance of the cathode. Nanosized CuCo₂ S₄ /Cu_7.2 S₄ composites have been synthesized for the first time by a facile solvothermal method. Herein, the magnesium ion storage behavior when applied in the cathode for RMBs is discussed. Electrochemical results demonstrated that the CuCo₂ S₄ /Cu_7.2 S₄ composites exhibit a high initial discharge capacity of 256 mAh g^-1 at 10 mA g^-1 and 123 mAh g^-1 at 300 mA g^-1 at room temperature and an outstanding long-term cyclic stability over 300 cycles at 300 mA g^-1 . Furthermore, the electrochemical storage mechanism demonstrated that the storage process of magnesium ion in the CuCo₂ S₄ /Cu_7.2 S₄ cathode is mainly driven by strong pseudocapacitive effects.

Effects of Conjugated Structure on the Magnesium Storage Performance of Dianhydrides

Inorganic cathodes of rechargeable Mg batteries suffer from limited selections, while organic materials provide more options. Herein, three conjugated dianhydrides, pyromellitic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride and 3,4,9,10-perylenetetracarboxylic dianhydride are comparatively investigated to elucidate the effects of conjugated structure on the Mg²⁺ storage performances. It is observed that the reversible Mg²⁺ storage capacity is more dependent on the conjugated structure than carbonyl numbers. Ex-situ mechanism study illustrates that the extended conjugated structure delocalizes the electron density, hence enhancing carbonyl enolization and increasing the Mg²⁺ storage capacity. Furthermore, the largely conjugated structure buffers the charge density change during repeated magnesiation/demagnesiation resulting in better cyclability. Prominently, 3,4,9,10-perylenetetracarboxylic dianhydride shows a high Mg²⁺ storage capacity (160 mAh g^-1 ) and a good cycling stability (80 % capacity retention after 100 cycles) with the largest conjugated structure. This work provides a low-cost cathode for rechargeable Mg batteries that can be utilized for designing high-performance organic Mg battery cathodes.

Hybrid MgCl2/AlCl3/Mg(TFSI)2 Electrolytes in DME Enabling High-Rate Rechargeable Mg Batteries

Rechargeable magnesium batteries (RMBs) are considered as one of the most promising next-generation secondary batteries due to their low cost, safety, dendrite-free nature, as well as high volumetric energy density. However, the lack of suitable cathode material and electrolyte is the greatest challenge facing practical RMBs. Herein, a hybrid electrolyte MgCl₂/AlCl₃/Mg(TFSI)₂ (MACT) in dimethyl ether (DME) is developed and exhibits excellent electrochemical performance. The high ionic conductivity (6.82 mS cm^-1) and unique solvation structure of [Mg₂(μ-Cl)₂(DME)₄]²⁺ promote the fast Mg kinetics and favorable thermodynamics in hybrid Mg salts and DME electrolyte, accelerating mass transport and the charge transfer process. Therefore, the great rate capability can be realized both in symmetric Mg/Mg cell and in CuS/Mg full cell.

Designing Nanostructured Metal Chalcogenides as Cathode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are regarded as promising candidates for beyond-lithium-ion batteries owing to their high energy density. Moreover, as Mg metal is earth-abundant and has low propensity for dendritic growth, RMBs have the advantages of being more affordable and safer than the currently used lithium-ion batteries. However, the commercial viability of RMBs has been negatively impacted by slow diffusion kinetics in most cathode materials due to the high charge density and strongly polarizing nature of the Mg²⁺ ion. Nanostructuring of potential cathode materials such as metal chalcogenides offers an effective means of addressing these challenges by providing larger surface area and shorter migration routes. In this article, a review of recent research on the design of metal chalcogenide nanostructures for RMBs' cathode materials is provided. The different types and structures of metal chalcogenide cathodes are discussed, and the synthetic strategies through which nanostructuring of these materials can be achieved are described. An organized summary of their electrochemical performance is also presented, along with an analysis of the current challenges and future directions. Although particular focus is placed on RMBs, many of the nanostructuring concepts that are discussed here can be carried forward to other next-generation energy storage systems.

Ultralong-Lifespan Magnesium Batteries Enabled by the Synergetic Manipulation of Oxygen Vacancies and Electronic Conduction

As a potential next-generation energy storage system, rechargeable magnesium batteries (RMBs) have been receiving increasing attention due to their excellent safety performance and high energy density. However, the sluggish kinetics of Mg²⁺ in the cathode has become one of the main bottlenecks restricting the development of RMBs. Here, we introduce oxygen vacancies to spherical NaV₆O₁₅ cross-linked with carbon nanotubes (CNTs) (denoted as SNVO_X-CNT) as a cathode material to achieve an impressive long-term cycle life of RMBs. The introduction of oxygen vacancies can improve the electrochemical performance of the NaV₆O_15-X cathode material. Besides, owing to the introduction of CNTs, excellent internal/external electronic conduction paths can be built inside the whole electrode, which further achieves excellent electrochemical performance. Moreover, such a unique structure can efficiently improve the diffusion kinetics of Mg²⁺ (ranging from 1.28 × 10^-12 to 7.21 × 10^-12 cm²·s^-1). Simulation calculations further prove that oxygen vacancies can cause Mg²⁺ to be inserted in NaV₆O_15-X. Our work proposes a strategy for the synergistic effect of oxygen vacancies and CNTs to improve the diffusion coefficient of Mg²⁺ in NaV₆O₁₅ and enhance the electrochemical performance of RMBs.

Recent Progress on Layered Cathode Materials for Nonaqueous Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising candidates for next-generation energy storage systems owing to their high safety and the low cost of magnesium resources. One of the main challenges for RMBs is to develop suitable high-performance cathode materials. Layered materials are one of the most promising cathode materials for RMBs due to their relatively high specific capacity and facile synthesis process. This review focuses on recent progress on layered cathode materials for RMBs, including layered oxides, sulfides, selenides, and other layered materials. In addition, effective strategies to improve the electrochemical performance of layered cathode materials are summarized. Moreover, future perspectives about the application of layered materials in RMBs are also discussed. This review provides some significant guidance for the further development of layered materials for RMBs.

Recent Progress and Challenges in the Optimization of Electrode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have been regarded as one of the promising electrochemical energy storage systems to complement Li-ion batteries owing to the low-cost and high safety characteristics. However, the various challenges including the sluggish solid-state diffusion of highly polarizing Mg²⁺ ions in hosts, and the formation of blocking layers on Mg metal surface have seriously impeded the development of high-performance RMBs. In order to solve these problems toward practical applications of RMBs, a tremendous amount of work on electrodes and electrolytes has been conducted in the last few decades. Creative optimization strategies including the modification of cathodes and anodes such as shielding the charges of divalent Mg²⁺ , expanding the layers of host materials, and optimizing the interface of electrode-electrolyte are raised to promote the technology. In this review, the detailed description of innovative approaches, representative examples, and facing challenges for developing high-performance electrodes are presented. Based on the review of these strategies, guidelines are provided for future research directions on improving the overall battery performance, especially on the electrodes.

Cuprous Self-Doping Regulated Mesoporous CuS Nanotube Cathode Materials for Rechargeable Magnesium Batteries

Copper sulfides are broadly explored as the possible cathode materials for rechargeable magnesium batteries on account of their high theoretical capacity of 560 mAh g^-1. However, the CuS cathodes usually suffer from serious capacity decay caused by structure collapse during the repeated magnesiation/demagnesiation process. Herein, we present a cuprous self-doping strategy to synthesize mesoporous CuS nanotubes with robust structural stability for rechargeable magnesium batteries and regulate their electrochemical magnesium storage behavior. Electrochemical results show that the mesoporous CuS nanotubes can exhibit high specific capacity, remarkable cycling performance, and good rate capability. The observed discharge capacity of the mesoporous CuS nanotubes could reach about 281.2 mAh g^-1 at 20 mA g^-1 and 168.9 mAh g^-1 at 500 mA g^-1. Furthermore, a remarkable ultralong-term cyclic stability with a reversible capacity of 72.5 mAh g^-1 at 1 A g^-1 is obtained after 550 cycles. These results demonstrate that the mesoporous nanotube structure and the simple cuprous self-doping effect could promote the practical application of copper sulfide cathode materials for rechargeable magnesium batteries.

A Stable Solid Electrolyte Interphase for Magnesium Metal Anode Evolved from a Bulky Anion Lithium Salt

Rechargeable magnesium (Mg) metal batteries are a promising candidate for "post-Li-ion batteries" due to their high capacity, high abundance, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the formation of passivating surface film rather than Mg²⁺ -conducting solid electrolyte interphase (SEI) on Mg anode surface has always restricted the development of rechargeable Mg batteries. A stable SEI is constructed on the surface of Mg metal anode by the partial decomposition of a pristine Li electrolyte in the electrochemical process. This Li electrolyte is easily prepared by dissolving lithium tetrakis(hexafluoroisopropyloxy)borate (Li[B(hfip)₄ ]) in dimethoxyethane. It is noteworthy that Mg²⁺ can be directly introduced into this Li electrolyte during the initial electrochemical cycles for in situ forming a hybrid Mg²⁺ /Li⁺ electrolyte, and then the cycled electrolyte can conduct Mg-ion smoothly. The existence of this as-formed SEI blocks the further parasitic reaction of Mg metal anode with electrolyte and enables this electrolyte enduring long-term electrochemical cycles stably. This approach of constructing superior SEI on Mg anode surface and exploiting novel Mg electrolyte provides a new avenue for practical application of high-performance rechargeable Mg batteries.

Anionic Se-Substitution toward High-Performance CuS1- x Sex Nanosheet Cathode for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (rMBs) are promising as the most ideal further energy storage systems but lack competent cathode materials due to sluggish redox reaction kinetics. Herein, developed is an anionic Se-substitution strategy to improve the rate capability and the cycling stability of 2D CuS_{1- x} Se_x nanosheet cathodes through an efficient microwave-induced heating method. The optimized CuS_{1- x} Se_x (X = 0.2) nanosheet cathode can exhibit high reversible capacity of 268.5 mAh g^-1 at 20 mA g^-1 and good cycling stability (140.4 mAh g^-1 at 300 mA g^-1 upon 100 cycles). Moreover, the CuS_{1- x} Se_x (X = 0.2) nanosheet cathode can deliver remarkable rate capability with a reversible capacity of 119.2 mAh g^-1 at 500 mA g^-1 , much higher than the 21.7 mAh g^-1 of pristine CuS nanosheets. The superior electrochemical performance can be ascribed to the enhanced reaction kinetics, enriched cation storage active sites, and shortened ion diffusion pathway of the CuS_{1- x} Se_x nanosheet. Therefore, tuning anionic chemical composition demonstrates an effective strategy to develop novel cathode materials for rMBs.

Microstructure Characteristics of Cathode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) that use pure Mg or Mg alloy as anode and materials allowing Mg ions to insert/extract as cathode have many advantages such as high energy density, environmental friendliness, low cost, and safety of handling. RMBs are regarded as a promising candidate for portable power sources and heavy load energy devices. However, there are still some technological issues impeding their commercial application. The most important issue is the absence of applicable cathode materials because of the high charge density, strong polarization effect, and very slow insertion/extraction speed of Mg²⁺ ions. In recent years, the research reports on the cathode materials of RMBs have increased significantly. Here, an extensive number of research papers are reviewed in terms of the microstructure characteristics of cathode materials for RMBs. The status and issues of cathode materials are analyzed and discussed in detail. The future development directions and perspectives are prospected for providing an understanding of the related research activities on RMBs.

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.

Microwave-Assisted Synthesis of CuS Hierarchical Nanosheets as the Cathode Material for High-Capacity Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (rMBs) have been recognized as one of most promising next-generation energy storage devices with high energy and power density. However, the development of rMBs has been hampered by the lack of usable cathode materials with high capacity and cycling stability. Herein, we report an ultra-rapid, cost-effective, and scalable synthesis of ultrathin CuS hierarchical nanosheets by a one-step microwave-assisted preparation. Benefiting from the exceptional structural configuration, when used as the cathode material for rMBs at room temperature, the CuS hierarchical nanosheets deliver a high reversible discharge capacity of 300 mA h g^-1 at 20 mA g^-1, remarkable rate capability (256.5 mA h g^-1 at 50 mA g^-1 and 237.5 mA h g^-1 at 100 mA g^-1), and excellent cycling stability (135 mA h g^-1 at 200 mA g^-1 over 200 cycles). To date, the obtained excellent electrochemical performances are superior to most results ever reported for cathode materials of rMBs.

Atomic Substitution Enabled Synthesis of Vacancy-Rich Two-Dimensional Black TiO2- x Nanoflakes for High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg²⁺ ions, the insertion of Mg²⁺ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO_{2- x} (B-TiO_{2- x}) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS₂ nanoflakes as precursors. We find out that the presence of OVs in B-TiO_{2- x} electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg²⁺ ion storage. The vacancy-rich B-TiO_{2- x} nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.