Highly Reversible Cuprous Mediated Cathode Chemistry for Magnesium Batteries

Amorphous Cobalt Polyselenides with Hyperbranched Polymer Additive as High-Capacity Magnesium Storage Cathode Materials Through Cationic and Anionic Co-Redox Mechanism

Rechargeable magnesium batteries (RMBs) are a promising energy-storage technology with low cost and high reliability, while the lack of high-performance cathodes is impeding the development. Herein, a series of amorphous cobalt polyselenides (CoSex, x>2) is synthesized with the assistance of organic amino-terminal hyperbranched polymer (AHP) additive and investigated as cathodes for RMBs. The coordination of cobalt cations with the amino groups of AHP leads to the formation of amorphous CoSex rather than crystalline CoSe2. The amorphous structure is favorable for magnesium-storage reaction kinetics, and the polyselenide anions provide extra capacities besides the redox of cobalt cations. Moreover, the organic AHP molecules retained in CoSex-AHP provide an elastic matrix to accommodate the volume change of conversion reaction. With a moderate x value (2.73) and appropriate AHP content (11.58%), CoSe2.7-AHP achieves a balance between capacity and cycling stability. Amorphous CoSe2.7-AHP provides high capacities of 246.6 and 94 mAh g‒1, respectively, at 50 and 2000 A g‒1, as well as a capacity retention rate of 68.5% after 300 cycles. The mechanism study demonstrates CoSex-AHP undergoes reversible redox of Co2+/3+↔Co0 and Sen 2‒↔Se2‒. The present study demonstrates amorphous polyselenides with cationic-anionic redox activities is as a feasible strategy to construct high-capacity cathode materials for RMBs.

Catalyzing Desolvation at Cathode-Electrolyte Interface Enabling High-Performance Magnesium-Ion Batteries

Magnesium ion batteries (MIBs) are expected to be the promising candidates in the post-lithium-ion era with high safety, low cost and almost dendrite-free nature. However, the sluggish diffusion kinetics and strong solvation capability of the strongly polarized Mg2+ are seriously limiting the specific capacity and lifespan of MIBs. In this work, catalytic desolvation is introduced into MIBs for the first time by modifying vanadium pentoxide (V2O5) with molybdenum disulfide quantum dots (MQDs), and it is demonstrated via density function theory (DFT) calculations that MQDs can effectively lower the desolvation energy barrier of Mg2+, and therefore catalyze the dissociation of Mg2+-1,2-Dimethoxyethane (Mg2+-DME) bonds and release free electrolyte cations, finally contributing to a fast diffusion kinetics within the cathode. Meanwhile, the local interlayer expansion can also increase the layer spacing of V2O5 and speed up the magnesiation/demagnesiation kinetics. Benefiting from the structural configuration, MIBs exhibit superb reversible capacity (≈300 mAh g-1 at 50 mA g-1) and unparalleled cycling stability (15 000 cycles at 2 A g-1 with a capacity of ≈70 mAh g-1). This approach based on catalytic reactions to regulate the desolvation behavior of the whole interface provides a new idea and reference for the development of high-performance MIBs.

Mo3S13 Cluster-Based Cathodes for Rechargeable Magnesium Batteries: Reversible Magnesium Association/Dissociation at the Bridging Disulfur along with Sulfur-Sulfur Bond Break/Formation

Multivalent cation batteries are attracting increasing attention in energy-storage applications, but reversible storage of highly polarizing multivalent cations is a major difficulty for the electrode materials. In the present study, charge-delocalizing Mo3S13 cluster-based materials (crystalline (NH4)2Mo3S13 and amorphous MoSx) are designed and investigated as cathodes for rechargeable magnesium batteries. Both of the cathodes show high magnesium storage capacities (296 and 302 mAh g-1 at 100 mA g-1) and superior rate performances (76 and 80 mAh g-1 at 15 A g-1). A high area loading of 3.0 mg cm-2 could be achieved. These performances are of the highest level compared with those of reported magnesium storage materials. Further mechanism study and theoretical computation demonstrate the magnesium storage active sites are the bridging disulfur groups of the Mo3S13 cluster. The valence state of bridging disulfur decreases/increases largely during magnesiation/demagnesiation along with breaking/formation of the sulfur-sulfur bond, which makes the Mg-association/dissociation highly reversible. The sulfur-sulfur bond breaking and formation provides high reversible capacities. Prominently, the valence state increase and sulfur-sulfur bond formation of the bridging disulfur during charge weakens the bonding with Mg2+, significantly assisting the magnesium dissociation. The present study not only develops high-performance magnesium storage cathode materials but also demonstrates the importance of constructing favorable magnesium storage active sites in the high-performance cathode materials design. The findings presented herein are of great significance for the development of electrode materials for the storage of multivalent cations.

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.

Ultrathin CuF2 -Rich Solid-Electrolyte Interphase Induced by Cation-Tailored Double Electrical Layer toward Durable Sodium Storage

Solid-electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high-capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3-4 nm) induced by Cu⁺ -tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium-ion batteries. Unique EDL with SO₃ CF₃ -Cu complex absorbing on CuS in NaSO₃ CF₃ /diglyme electrolyte is demonstrated by in situ surface-enhanced Raman, Cyro-TEM and theoretical calculation, in which SO₃ CF₃ -Cu could be reduced to CuF₂ -rich SEI. Dispersed CuF₂ and F-containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na⁺ transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g^-1 after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high-stability electrode.

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.

An aqueous copper battery enabled by Cu2+/Cu+ and Cu3+/Cu2+ redox conversion chemistry

We propose an aqueous copper battery via Cu²⁺/Cu⁺ and Cu³⁺/Cu²⁺ redox conversion chemistry on an activated carbon (AC) electrode enabled by a 30 m ChCl + 1 m CuCl₂ electrolyte, where Cu³⁺/Cu²⁺ redox promotes the discharge capacity by ∼50 mA h g^-1 at ∼1.0 V vs. Ag/AgCl with stable cycling.

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.

Core-Shell-Structured Carbon Nanotube@VS4 Nanonecklaces as a High-Performance Cathode Material for Magnesium-Ion Batteries

As a typical layered transition metal chalcogenide, VS₄ is considered as a promising cathode material for advanced magnesium-ion batteries. However, the poor electronic conductivity and severe polarization effect restrict its practical applications. Herein, we report a betaine-assisted solvothermal strategy to coat VS₄ nanoblocks on the surface of carbon nanotubes (CNTs), obtaining unique core-shell-structured CNT@VS₄ nanonecklaces. As a result of the morphology-controlling effect of betaine, VS₄ exhibits an unusual nanoblock morphology, which renders abundant active sites and promotes the contact between the electrode and electrolyte. CNTs serve as a highly conductive skeleton, combining with the VS₄ nanoblocks and ensuring their uniform distribution. As a benefit from the synergistic effect of abundant active sites and electron-conductive highways, the as-synthesized CNT@VS₄ nanonecklaces manifest remarkable performance for magnesium storage, including a large reversible capacity of 170 mAh g^-1 at 0.1 A g^-1, outstanding cycle stability (76.3 mAh g^-1 after 800 cycles at 0.5 A g^-1), and superior rate performance (77.2 mAh g^-1 at 2 A g^-1).

Size-Controllable Nickel Sulfide Nanoparticles Embedded in Carbon Nanofibers as High-Rate Conversion Cathodes for Hybrid Mg-Based Battery

The integration of highly-safe Mg anode and fast Li+ kinetics endows hybrid Mg2+ /Li+ batteries (MLIBs) a promising future, but the practical application is circumvented by the lack of appropriate cathodes that enable the realization of an enough participation of Mg2+ in the reactions, resulting in a high dependence on Li+ . Herein, the authors develop a series of size-controllable nickel sulfide nanoparticles embedded in carbon nanofibers (NiS@C) with synergistic effect of particle diameter and carbon content as the cathode material for MLIBs. The optimized particle size is designed to maximize the utilization of the active material and remit internal stress, and appropriate carbon encapsulation efficiently inhibiting the pulverization of particles and accelerates the ability of conducting ions and electrons. In consequence, the representative NiS@C delivers superior electrochemical performance with a highest discharge capacity of 435 mAh g-1 at 50 mA g-1 . Such conversion cathode also exhibits excellent rate performance and remarkable cycle life. Significantly, the conversion mechanism of NiS in MLIBs is unambiguously demonstrated for the first time, affirming the corporate involvement of both Mg2+ and Li+ at the cathodic side. This work underlines a guide for developing conversion-type materials with high rate capability and cyclic performance for energy storage applications.

Ether-Water Hybrid Electrolyte Contributing to Excellent Mg Ion Storage in Layered Sodium Vanadate

Magnesium ion batteries have potential for large-scale energy storage. However, the high charge density of Mg²⁺ ions establishes a strong intercalation energy barrier in host materials, causing sluggish diffusion kinetics and structural degradation. Here, we report that the kinetic and dissolution issues connected to cathode materials can be resolved simultaneously using a tetraethylene glycol dimethyl ether (TEGDME)-water hybrid electrolyte. The lubricating and shielding effect of water solvent could boost the swift transport of Mg²⁺, contributing to a high diffusion coefficient within the sodium vanadate (NaV₈O₂₀·nH₂O) cathode. Meanwhile, the organic TEGDME component can coordinate with water to diminish its activity, thus providing the hybrid electrolyte with a broad electrochemical window of 3.9 V. More importantly, the TEGDME preferentially amassed at the interface, leading to a robust cathode electrolyte interface layer that suppresses the dissolution of vanadium species. Consequently, the NaV₈O₂₀·nH₂O cathode achieved a specific capacity of 351 mAh g^-1 at 0.3 A g^-1 and a long cycle life of 1000 cycles in this hybrid electrolyte. A mechanism study revealed the reversible interaction of Mg²⁺ during cycles. This organic water hybrid electrolyte is effective for overcoming the difficulty of multivalent ion storage.

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.

Rechargeable Mg2+/Li+, Mg2+/Na+, and Mg2+/K+ Hybrid Batteries Based on Layered VS2

Rechargeable Mg batteries have great potential in next-generation scalable energy-storage applications, but the electrochemical performance is limited by the Mg-intercalation cathodes. Hybrid batteries based on dual-cation (Mg²⁺ and alkali metal cations) electrolytes would not only improve the electrochemical performance but also induce the co-intercalation of Mg²⁺ with alkali metal cations. As previous reports overwhelmingly focus on Mg²⁺/Li⁺ hybrid batteries, in this work, Mg²⁺/Na⁺ and Mg²⁺/K⁺ hybrid batteries are constructed using a typical layered VS₂ cathode and studied in comparison with Mg²⁺/Li⁺ batteries. It is observed that Mg²⁺ could co-intercalate into VS₂ with Li⁺, Na⁺, or K⁺. However, Mg-intercalation is irreversible in the Mg²⁺/Li⁺ system, and co-intercalation of Mg²⁺ and K⁺ would cause a collapse of VS₂. Comparatively, the co-intercalation of Mg²⁺ and Na⁺ into VS₂ exhibits the highest reversibility, and the Mg²⁺/Na⁺ hybrid battery shows the best cycling stability without capacity fading within 1000 cycles. Our work highlights the co-intercalation reversibility of a non-pre-expanded layered disulfide cathode and delivers insights for the development of high-performance rechargeable Mg metal batteries.

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.

Progress and Perspective on Rechargeable Magnesium-Sulfur Batteries

Rechargeable magnesium-sulfur (Mg-S) batteries are emerging as a promising candidate for next-generation energy storage technologies owing to their prominent advantages in terms of high volumetric energy density, low cost, and enhanced safety. However, their practical implementation is facing great challenges in finding electrolytes that can fulfill a multitude of rigorous requirements along with efficient sulfur cathodes and magnesium anodes. This review highlights electrolyte design for reliable Mg-S batteries in terms of efficient Mg-based salt construction (cation/anion design of organomagnesium salt-based electrolytes, optimization of all inorganic salt-based electrolytes and choosing of simple salt-based electrolytes), suitable solvent selection, and strategies for confronting corrosivity of Mg electrolytes. Before the comprehensive overview of the research status of Mg-based electrolytes, the understanding of Mg-S electrochemistry and views on the recent progress and potential strategies for high-performance S-based cathode and Mg anode are also provided for a holistic insight into Mg-S systems. At the end, the perspectives on the possible research directions for constructing high performance practical Mg-S batteries are also shared.

A Self-Conditioned Metalloporphyrin as a Highly Stable Cathode for Fast Rechargeable Magnesium Batteries

Development of practical rechargeable Mg batteries (RMBs) is impeded by their limited cycle life and rate performance of cathodes. As demonstrated herein, a copper-porphyrin with meso-functionalized ethynyl groups is capable of reversible two- and four-electron storage at an extremely fast rate (tested up to 53 C). The reversible four-electron redox process with cationic-anionic contributions resulted in a specific discharge capacity of 155 mAh g-1 at the high current density of 1000 mA g-1 . Even at 4000 mA g-1 , it still delivered >70 mAh g-1 after 500 cycles, corresponding to an energy density of >92 Wh kg-1 at a high power of >5100 W kg-1 . The ability to provide such high-rate performance and long-life opens the way to the development of practical cathodes for multivalent metal batteries.

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 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.

Mg storage properties of hollow copper selenide nanocubes

Rechargeable Mg batteries are thought to be suitable for scalable energy-storage applications because of their high safety and low cost. However, the bivalent Mg2+ cations suffer from sluggish solid-state diffusion kinetics. Herein, a hollow morphological approach is introduced to design copper selenide cathodes for rechargeable Mg batteries. Hollow Cu2-xSe nanocubes are fabricated via a solution reaction and their Mg-storage properties are investigated in comparison to simple nanoparticles. The hollow structures accommodate the volume change during magnesiation/demagnesiation and maintain material integrity, and thus a remarkable cycling stability of over 200 cycles is achieved. A kinetic study demonstrates that a hollow structure favors solid-phase Mg2+ diffusion, and therefore the hollow Cu2-xSe nanocubes exhibit a high capacity of 250 mA h g-1 at 100 mA g-1 as well as a superior rate capability. Mechanism investigation indicates that Cu2-xSe experiences a structure conversion during which a phase transformation occurs. This work develops a facile method for the preparation of hollow copper selenides and highlights the advantages of hollow structures in the design of high-performance Mg-storage materials.

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.