Current Design Strategies for Rechargeable Magnesium-Based Batteries

Superionic conduction in a Mg2+-containing covalent organic framework at intermediate temperature

We report superionic conduction in a Mg2+-containing covalent organic framework (COF) at intermediate temperature in the absence of guest vapors. A COF containing Mg2+ carriers and polyethylene oxide (PEO) chains in its channels (TPB-PEO-9-COF-Mg) was synthesized. TPB-PEO-9-COF-Mg showed superionic conductivity above 10-4 S cm-1 under dry N2 at 160 °C.

High-Capacity and Ultra-Long-Life Mg-Metal Batteries Enabled by Intercalation-Conversion Hybrid Cathode Materials

The advancement of rechargeable Mg-metal batteries (RMBs) is severely impeded by the lack of suitable cathode materials. Despite the good cyclic stability of intercalation-type compounds, their specific capacity is relatively low. Conversely, the conversion-type cathodes can deliver a higher capacity but often suffer from poor cycling reversibility and stability. Herein, a WSe2/Se intercalation-conversion hybrid material with elemental Se uniformly distributed into WSe2 nanosheets is fabricated via a simple solvothermal method for high-performance RMBs. The uniformly introduced Se confined in WSe2 nanosheets can not only efficiently improve the conductivity of the hybrid cathodes, facilitating the fast electron transport and ion diffusion, but also provide additional specific capacity. Besides, the WSe2 can effectively inhibit the detrimental Se dissolution and polyselenide shuttle, thereby activating the activity of Se and improving its utilization. Consequently, the synergy of intercalation and conversion mechanisms endows WSe2/Se hybrids with superior reversible capacity of 252 mAh g-1 at 0.1 A g-1 and ultra-long cyclability of up to 5000 cycles at 2.0 A g-1 with capacity retention of 78.1%. This work demonstrates the feasibility of the strategy by integrating intercalation and conversion mechanisms for developing high-performance cathode materials for RMBs.

Progress and Challenges of Vanadium Oxide Cathodes for Rechargeable Magnesium Batteries

Among the challenges related to rechargeable magnesium batteries (RMBs) still not resolved are positive electrode materials with sufficient charge storage and rate capability as well as stability and raw material resources. Out of the materials proposed and studied so far, vanadium oxides stand out for these requirements, but significant further improvements are expected and required. They will be based on new materials and an improved understanding of their mode of operation. This report provides a critical review focused on this material, which is embedded in a brief overview on the general subject. It starts with the main strategic ways to design layered vanadium oxides cathodes for RMBs. Taking these examples in more detail, the typical issues and challenges often missed in broader overviews and reviews are discussed. In particular, issues related to the electrochemistry of intercalation processes in layered vanadium oxides; advantageous strategies for the development of vanadium oxide composite cathodes; their mechanism in aqueous, "wet", and dry non-aqueous aprotic systems; and the possibility of co-intercalation processes involving protons and magnesium ions are considered. The perspectives for future development of vanadium oxide-based cathode materials are finally discussed and summarized.

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.

Enhancing Reversibility and Stability of Mg Metal Anodes: High-Exposure (002) Facets and Nanosheet Arrays for Superior Mg Plating/Stripping

Magnesium metal batteries (MMBs), recognized as promising contenders for post-lithium battery technologies, face challenges such as uneven magnesium (Mg) plating and stripping behaviors, leading to uncontrollable dendrite growth and irreversible structural damage. Herein, we have developed a Mg foil featuring prominently exposed (002) facets and an architecture of nanosheet arrays (termed (002)-Mg), created through a one-step acid etching method. Specifically, the prominent exposure of Mg (002) facets, known for their inherently low surface and adsorption energies with Mg atoms, not only facilitates smooth nucleation and dense deposition but also significantly mitigates side reactions on the Mg anode. Moreover, the nanosheet arrays on the surface evenly distribute the electric field and Mg ion flux, enhancing Mg ion transfer kinetics. As a result, the fabricated (002)-Mg electrodes exhibit unprecedented long-cycle performance, lasting over 6000 h (>8 months) at a current density of 3 mA cm-2 for a capacity of 3 mAh cm-2. Furthermore, the corresponding pouch cells equipped with various electrolytes and cathodes demonstrate remarkable capacity and cycling stability, highlighting the superior electrochemical compatibility of the (002)-Mg electrode. This study provides new insights into the advancement of durable MMBs by modifying the crystal structure and morphology of Mg.

Large-Scale Integration of the Ion-Reinforced Phytic Acid Layer Stabilizing Magnesium Metal Anode

Rechargeable magnesium batteries (RMBs) have garnered significant attention for their potential in large-scale energy storage applications. However, the commercial development of RMBs has been severely hampered by the rapid failure of large-sized Mg metal anodes, especially under fast and deep cycling conditions. Herein, a concept proof involving a large-scale ion-reinforced phytic acid (PA) layer (100 cm × 7.5 cm) with an excellent water-oxygen tolerance, high Mg2+ conductivity, and favorable electrochemical stability is proposed to enable rapid and uniform plating/stripping of Mg metal anode. Guided by even distributions of Mg2+ flux and electric field, the as-prepared large-sized PA-Al@Mg electrode (5.8 cm × 4.5 cm) exhibits no perforation and uniform Mg plating/stripping after cycling. Consequently, an ultralong lifespan (2400 h at 3 mA cm-2 with 1 mAh cm-2) and high current tolerance (300 h at 9 mA cm-2 with 1 mAh cm-2) of the symmetric cell using the PA-Al@Mg anode could be achieved. Notably, the PA-Al@Mg//Mo6S8 full cell demonstrates exceptional stability, operating for 8000 cycles at 5 C with a capacity retention of 99.8%, surpassing that of bare Mg (3000 cycles, 74.7%). Moreover, a large-sized PA-Al@Mg anode successfully contributes to the stable pouch cell (200 and 750 cycles at 0.1 and 1 C), further confirming its significant potential for practical utilization. This work provides valuable theoretical insights and technological support for the practical implementation of RMBs.

Cost-Effective Rechargeable Magnesium Battery Based on a Fluorinated Alkoxyaluminate Electrolyte and a Carbonyl Polymer Cathode

Rechargeable magnesium batteries (RMBs) are one of the most promising "post-lithium" battery technologies, but the electrochemical performance is still far from expectation due to the sluggish reaction kinetics of divalent Mg2+ ions. Herein, we report a low-cost, high-performance Mg-organic battery based on the combination of a fluorinated alkoxyaluminate electrolyte and a carbonyl polymer cathode material. First, the one-pot synthesized Mg[Al(HFIP)4]2 (HFIP = hexafluoro-2-propanol) is proved superior to the Mg[B(HFIP)4]2 analogue in both Mg anode compatibility and electrochemical window, as the electrolyte salt in the G2-DME (G2 = diethylene glycol dimethyl ether; DME = 1,2-dimethoxyethane) mixture solvent. Second, a simple wet grinding method is proposed to effectively improve the dispersion uniformity of the poly(benzoquinone-pyrrole) (PBQPy) active material in the cathode. Third, the elaborate Mg-PBQPy battery exhibits superior electrochemical performance within 0.4-3.0 V, including a high reversible capacity of 197 mA h g-1, a high average discharge voltage of 1.6 V, and a high capacity retention of 71% after 500 cycles. Finally, based on various electrochemical analysis and ex situ characterization results, we propose a general microscopic structure evolution model to reveal the electrochemical behaviors of carbonyl polymer cathode in RMBs, including the swelling of polymer active material, trapping of Mg2+ ions, and reversible redox reaction.

Binary Mg-1 at%Gd alloy anode for high-performance rechargeable magnesium batteries

Rechargeable magnesium batteries (RMBs) become a highly promising candidate for the large-scale energy storage system by right of the high volumetric capacity, intrinsic safety and abundant resources of Mg anode. However, the uneven Mg stripping and large overpotential will cause a severe pitting perforation and the followed failure of Mg anode. Herein, we proposed a high-performance binary Mg-1 at% Gd alloy anode prepared by the melting and hot extrusion. The introduction of 1 at% Gd element can effectively reduce the Mg2+ diffusion energy barrier (0.34 eV) on alloy surface and induces the formation of a robust and low-resistance electrolyte/anode interphase, thus enabling a uniform and fast Mg plating/stripping. As a result, the Mg-1 at.% Gd anode displays a largely enhanced life of 220 h and a low overpotential of 213 mV at a high current density of 5.0 mA cm-2 with 2.5 mAh cm-2 . Moreover, the assembled Mg-1 at.% Gd//Mo6 S8 full cell delivers a high rate performance (73.5 mAh g-1 at 5 C) and ultralong cycling stability of 8000 cycles at 5 C. This work brings new insights to design the new-type and practical Mg alloy anodes for commercial RMBs.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.

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.

Pillar-Structured Ti3 C2 Tx MXene with Engineered Interlayer Spacing for High-Performance Magnesium Batteries

Two-dimentional (2D) Ti3 C2 Tx MXene has attracted significant attention in non-lithium-ion batteries due to its excellent electrical conductivity, high volumetric capacity, and ability to accommodate intercalants. Rechargeable magnesium batteries with Mg metal anodes are noted for their high theoretical energy density, potential safety, earth abundance, dendrite-free Mg2+ plating/stripping mechanism on the anode side, and low cost. Nevertheless, owing to the large polarity of divalent Mg2+ ions, the insertion of Mg2+ into the MXene layers suffers from sluggish kinetics, limiting the performance for storage of Mg2+ ions. Herein, a simple self-assembly strategy is demonstrated to achieve high magnesium ion storage capability with pillar-structured Ti3 C2 Tx MXene by intercalating a hyperbranched polyethylene ionomer containing quaternary ammonium ions. The ionomer intercalation/modification leads to the expansion of interlayer spacing of the MXene and, meanwhile, improves its affinity to low-polarity THF-based electrolyte. The delaminated ionomer-modified MXene shows significantly improved electrochemical performance as a cathode material for Mg batteries. It shows a promising cycling stability with a capacity retention of 86% after 400 cycles at 200 mA g-1 , as well as outstanding high-rate performance with a capacity of 110 mAh g-1 retained at 1,000 mA g-1 relative to 213 mAh g-1 at 20 mA g-1 .

Potassium ion pre-intercalated MnO2 for aqueous multivalent ion batteries

Manganese dioxide (MnO2), as a cathode material for multivalent ion (such as Mg2+ and Al3+) storage, is investigated due to its high initial capacity. However, during multivalent ion insertion/extraction, the crystal structure of MnO2 partially collapses, leading to fast capacity decay in few charge/discharge cycles. Here, through pre-intercalating potassium-ion (K+) into δ-MnO2, we synthesize a potassium ion pre-intercalated MnO2, K0.21MnO2·0.31H2O (KMO), as a reliable cathode material for multivalent ion batteries. The as-prepared KMO exhibits a high reversible capacity of 185 mAh/g at 1 A/g, with considerable rate performance and improved cycling stability in 1 mol/L MgSO4 electrolyte. In addition, we observe that aluminum-ion (Al3+) can also insert into a KMO cathode. This work provides a valid method for modification of manganese-based oxides for aqueous multivalent ion batteries.

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.

A two-dimensional VO2/VS2 heterostructure as a promising cathode material for rechargeable Mg batteries: a first principles study

Rechargeable magnesium batteries (RMBs) are considered as highly promising energy storage systems. However, the lack of cathode materials with fast Mg2+ diffusion kinetics and high energy density severely hinders the development of RMBs. Herein, a two-dimensional (2D) VO2/VS2 heterostructure as a RMB cathode material is proposed by introducing an O-V-O layer in VS2 to improve the discharge voltage and specific capacity while keeping the fast Mg2+ diffusion kinetics. Based on first principle calculations, the geometric structures, electronic characteristics of the VO2/VS2 heterostructure, and the adsorption properties and diffusion behaviors of Mg2+ in VO2/VS2 are systematically studied. The metallic properties of VO2/VS2 and a relatively low diffusion barrier of Mg2+ (0.6 eV) in VO2/VS2 enable a large potential in delivering high rate performance in actual RMBs. Compared with traditional VS2 materials (1.25 V), the average discharge platform of VO2/VS2 could be increased to 1.7 V. The theoretical capacities of the layered VS2 and VO2/VS2 are calculated as 233 and 301 mA h g-1, respectively. Thus, the VO2/VS2 heterostructure exhibits a high theoretical energy density of 511.7 W h kg-1, significantly surpassing that of VS2 (291.3 W h kg-1). This work provides important guidance for designing high-energy and high-rate 2D heterostructure cathode materials for RMBs and other multivalent ion 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 .

Implanting CuS Quantum Dots into Carbon Nanorods for Efficient Magnesium-Ion Batteries

Magnesium-ion batteries (MIBs) are emerging as potential next-generation energy storage systems due to high security and high theoretical energy density. Nevertheless, the development of MIBs is limited by the lack of cathode materials with high specific capacity and cyclic stability. Currently, transition metal sulfides are considered as a promising class of cathode materials for advanced MIBs. Herein, a template-based strategy is proposed to successfully fabricate metal-organic framework-derived in-situ porous carbon nanorod-encapsulated CuS quantum dots (CuS-QD@C nanorods) via a two-step method of sulfurization and cation exchange. CuS quantum dots have abundant electrochemically active sites, which facilitate the contact between the electrode and the electrolyte. In addition, the tight combination of CuS quantum dots and porous carbon nanorods increases the electronic conductivity while accelerating the transport speed of ions and electrons. With these architectural and compositional advantages, when used as a cathode material for MIBs, the CuS-QD@C nanorods exhibit remarkable performance in magnesium storage, including a high reversible capacity of 323.7 mAh g-1 at 100 mA g-1 after 100 cycles, excellent long-term cycling stability (98.5 mAh g-1 after 1000 cycles at 1.0 A g-1 ), and satisfying rate performance (111.8 mA g-1 at 1.0 A g-1 ).

Hierarchical BiOCl flowerlike microspheres via a room-temperature solid-state chemical reaction as a new anode for rechargeable magnesium-ion batteries

Rechargeable magnesium-ion batteries (MIBs) have received much attention in recent years, but their development remains limited due to a lack of anode materials with high capacity and fast diffusion kinetics. Herein, for the first time, hierarchical BiOX (X = Cl, Br, I) flowerlike microspheres composed of interleaved nanosheets are constructed via a simple room-temperature solid-state chemical reaction as the anode for MIBs. Among them, BiOCl flowerlike microspheres deliver good cycling stability (110 mA h g^-1 after 100 cycles) and a superior rate capacity (134 mA h g^-1 at 500 mA g^-1). This is attributed to their unique flowerlike microsphere structure that not only accommodates a volume change to maintain their structural integrity but also shortens the ion-transport path to improve the diffusion rate. Importantly, ex situ tests were carried out to clarify the phase and structure evolution of the BiOCl flowerlike microspheres during cycling. The results show that BiOCl is first transformed to Bi and then alloyed to Mg₃Bi₂ in the discharging process, and Mg₃Bi₂ is turned back to Bi in the charging process. Besides, the initial microsphere structure is essentially maintained during the discharging/charging process, indicating the better stability of the structure. The current study demonstrates that the structural design of flowerlike microspheres is an effective strategy to develop promising anode materials for MIBs.

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.

Solvent Induced Activation Reaction of MoS2 Nanosheets within Nitrogen/Sulfur-Codoped Carbon Network Boosting Sodium Ion Storage

MoS₂ , as a classical 2D material, becomes a capable anode candidate for sodium-ion batteries. However, MoS₂ presents a disparate electrochemical performance in the ether-based and ester-based electrolyte with unclear mechanism. Herein, tiny MoS₂ nanosheets embedded in nitrogen/sulfur-codoped carbon (MoS₂ @NSC) networks are designed and fabricated through an uncomplicated solvothermal method. Thanks to the ether-based electrolyte, the MoS₂ @NSC shows a unique capacity growth in the original stage of cycling. But in the ester-based electrolyte, MoS₂ @NSC shows a usual capacity decay. The increasing capacity puts down to the gradual transformation from MoS₂ to MoS₃ with the structure reconstruction. Based on the above mechanism, MoS₂ @NSC demonstrates an excellent recyclability and the specific capacity keeps around 286 mAh g^-1 at 5 A g^-1 after 5000 cycles with an ultralow capacity fading rate of only 0.0034% per cycle. In addition, a MoS₂ @NSC‖Na₃ V₂ (PO₄ )₃ full cell with ether-based electrolyte is assembled and demonstrates a capacity of 71 mAh g^-1 , suggesting the potential application of MoS₂ @NSC. Here the electrochemical conversion mechanism of MoS₂ is revealed in the ether-based electrolyte and significance of the electrolyte design on the promoting Na ion storage behavior is highlighted.

Tungsten-oxygen bond pre-introduced VO2(B) nanoribbons enable fast and stable zinc ion storage ability

The tunnel structure of the bronze phase vanadium dioxide (VO2(B)) can be used as the zinc ion storage active sites. However, the intense charge repulsion of divalent Zn2+ causes a sluggish reaction kinetics in the tunnel VO2(B). Here, a tungsten-oxygen bond pre-introduced (TOBI) approach is proposed to modulate the tunnel structure of VO2(B). The VO2(B) cathodes with TOBI of 0.5 at% to 3.0 at% have been controllably synthesized by a simple hydrothermal method. The results from structural analysis uncover that the pre-introduced W6+ replaces the V4+ in VO2(B) to form WO6 octahedra. Benefiting from the rapid diffusion kinetics, enhanced structural stability and improved conductivity enabled by the TOBI, the optimal VO2(B) nanoribbons with 1.5 at% shows a high reversible capacity of 265 mAh g-1, a high rate-performance of up-to 10 A g-1 and a long cycling stability of 2000 cycles. Moreover, a pseudo-capacitive dominated Zn2+ intercalation/de-intercalation behavior is solidly determined by the electrochemical kinetics testing and structural characterizations. This TOBI method is referential for developing other multivalent ion battery cathodes with outstanding performances.

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.

Challenges and Perspectives of Organic Multivalent Metal-Ion Batteries

Rechargeable organic multivalent metal-ion batteries (MMIBs) have attracted a surge of interest as promising alternatives for large-scale energy storage applications because they can combine the advantages of both organic electrodes and multivalent metal-ion batteries. However, the development of organic MMIBs is hampered by many factors, which mean they lag far behind organic alkali-metal- (e.g., Li-, Na-, and K-) ion batteries. Herein, the challenges that are specifically faced by organic MMIBs are analyzed and the strategies that can probably solve such challenges are then discussed. As a special challenge that organic MMIBs are facing, the charge-storage mechanism is particularly underlined to deeply understand the structure-property relationships for guiding the future design of high-performance organic electrodes for MMIBs. The perspectives are thereby elaborated in this review with the outlook of practical applications of organic MMIBs.

Status Quo on Graphene Electrode Catalysts for Improved Oxygen Reduction and Evolution Reactions in Li-Air Batteries

Reduced global warming is the goal of carbon neutrality. Therefore, batteries are considered to be the best alternatives to current fossil fuels and an icon of the emerging energy industry. Voltaic cells are one of the power sources more frequently employed than photovoltaic cells in vehicles, consumer electronics, energy storage systems, and medical equipment. The most adaptable voltaic cells are lithium-ion batteries, which have the potential to meet the eagerly anticipated demands of the power sector. Working to increase their power generating and storage capability is therefore a challenging area of scientific focus. Apart from typical Li-ion batteries, Li-Air (Li-O₂) batteries are expected to produce high theoretical power densities (3505 W h kg^-1), which are ten times greater than that of Li-ion batteries (387 W h kg^-1). On the other hand, there are many challenges to reaching their maximum power capacity. Due to the oxygen reduction reaction (ORR) and oxygen evolution reaction (OES), the cathode usually faces many problems. Designing robust structured catalytic electrode materials and optimizing the electrolytes to improve their ability is highly challenging. Graphene is a 2D material with a stable hexagonal carbon network with high surface area, electrical, thermal conductivity, and flexibility with excellent chemical stability that could be a robust electrode material for Li-O₂ batteries. In this review, we covered graphene-based Li-O₂ batteries along with their existing problems and updated advantages, with conclusions and future perspectives.

Electrolytes for Multivalent Metal-Ion Batteries: Current Status and Future Prospect

Electrochemical energy storage has experienced unprecedented advancements in recent years and extensive discussions and reviews on the progress of multivalent metal-ion batteries have been made mainly from the aspect of electrode materials, but relatively little work comprehensively discusses and provides an outlook on the development of electrolytes in these systems. Under this circumstance, this Review will initially introduce different types of electrolytes in current multivalent metal-ion batteries and explain the basic ion conduction mechanisms, preparation methods, and pros and cons. On this basis, we will discuss in detail the research and development of electrolytes for multivalent metal-ion batteries in recent years, and finally, critical challenges and prospects for the application of electrolytes in multivalent metal-ion batteries will be put forward.

Origin of Excellent Charge Storage Properties of Defective Tin Disulphide in Magnesium/Lithium-Ion Hybrid Batteries

Lithium-ion batteries (LIBs) are excellent electrochemical energy sources, albeit with existing challenges, including high costs and safety concerns. Magnesium-ion batteries (MIBs) are one of the potential alternatives. However, the performance of MIBs is poor due to their sluggish solid-state Mg²⁺ diffusion kinetics and severe electrode polarizability. Rechargeable magnesium-ion/lithium-ion (Mg²⁺/Li⁺) hybrid batteries (MLHBs) with Mg²⁺ and Li⁺ as the charge carriers create a synergy between LIBs and MIBs with significantly improved charge transport kinetics and reliable safety features. However, MLHBs are yet to reach a reasonable electrochemical performance as expected. This work reports a composite electrode material with highly defective two-dimensional (2D) tin sulphide nanosheets (SnS_x) encapsulated in three-dimensional (3D) holey graphene foams (HGF) (SnS_x/HGF), which exhibits a specific capacity as high as 600 mAh g^-1 at 50 mA g^-1 and a compelling specific energy density of ~ 330 Wh kg^-1. The excellent electrochemical performance surpasses previously reported hybrid battery systems based on intercalation-type cathode materials under comparable conditions. The role played by the defects in the SnS_x/HGF composite is studied to understand the origin of the observed excellent electrochemical performance. It is found that it is closely related to the defect structure in SnS_x, which offers percolation pathways for efficient ion transport and increased internal surface area assessable to the charge carriers. The defective sites also absorb structural stress caused by Mg²⁺ and Li⁺ insertion. This work is an important step towards realizing high-capacity cathode materials with fast charge transport kinetics for hybrid 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).

Revealing the reaction and fading mechanism of FeSe2 cathode for rechargeable magnesium batteries

Rechargeable Mg batteries (RMBs) are advantageous large-scale energy-storage devices because of the high abundance and high safety, but exploring high-performance cathodes remains the largest difficulty for the development. Compared with oxides and sulfides, selenides show better Mg-storage performance because the weaker interaction with the Mg2+ cation favors fast kinetics. Herein, nanorods-like FeSe2 was synthesized and investigated as cathodes for RMBs. Compared with microspheres and microparticles, nanorods exhibits higher capacity and better rate capability with the smaller particle size. The FeSe2 nanorods show a high capacity of 191 mAh g-1 at 50 mA g-1 and a good rate performance of 39 mAh g-1 at 1000 mA g-1. Ex-situ characterizations demonstrate the Mg2+ intercalation mechanism for FeSe2, and slight conversion reaction occurs on the surface of the particles. The capacity fading is mainly because of the dissolution of Fe2+, which is caused by the reaction between Fe2+ and Cl- of the electrolyte during the charge process on the surface of the particles. The surface of FeSe2 is mainly selenium after long cycling, which may also dissolve in the electrolyte during cycling. The present work develops a new type of Mg2+ intercalation cathode for RMBs. More importantly, the fading mechanism revealed herein has considered the specificity of Mg battery electrolyte and would assist a better understanding of selenide cathodes for RMBs.

Storing Mg Ions in Polymers: A Perspective

The electrochemical performance of rechargeable Mg batteries (RMBs) is primarily determined by the cathodes. However, the strong interaction between highly polarized Mg²⁺ and the host lattice is a big challenge for inorganic cathode materials. While endowed with weak interaction with Mg²⁺ , organic polymers are capable of fast reaction kinetics. Besides, with the advantages of light weight, abundance, low cost, and recyclability, polymers are deemed as ideal cathode materials for RMBs. Although polymer cathodes have remarkably progressed in recent years, there are still significant challenges to overcome before reaching practical application. In this perspective, the challenges faced by polymer cathodes are critically focused, followed by the retrospection of efforts devoted to design polymers. Some feasible strategies are proposed to explore new structures and chemistries for the practical application of polymer cathodes in 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.