Electrochemical and spectroscopic analysis of Mg2+ intercalation into thin film electrodes of layered oxides: V2O5 and MoO3

Alkali-Ion-Assisted Activation of ε-VOPO4 as a Cathode Material for Mg-Ion Batteries

Rechargeable multivalent-ion batteries are attractive alternatives to Li-ion batteries to mitigate their issues with metal resources and metal anodes. However, many challenges remain before they can be practically used due to the low solid-state mobility of multivalent ions. In this study, a promising material identified by high-throughput computational screening is investigated, ε-VOPO4, as a Mg cathode. The experimental and computational evaluation of ε-VOPO4 suggests that it may provide an energy density of >200 Wh kg-1 based on the average voltage of a complete cycle, significantly more than that of well-known Chevrel compounds. Furthermore, this study finds that Mg-ion diffusion can be enhanced by co-intercalation of Li or Na, pointing at interesting correlation dynamics of slow and fast ions.

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

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

First-Principles and Experimental Investigation of ABO4 Zircons as Calcium Intercalation Cathodes

Identifying next-generation batteries with multivalent ions, such as Ca2+ is an active area of research to meet the increasing demand for large-scale, renewable energy storage solutions. Despite the promise of higher energy densities with multivalent batteries, one of their main challenges is addressing the sluggish kinetics in cathodes that arise from stronger electrostatic interactions between the multivalent ion and host lattice. In this paper, zircons are theoretically and experimentally evaluated as Ca cathodes. A migration barrier as low as 113 meV is computationally found in YVO4, which is the lowest Ca2+ barrier reported to date. Low barriers are confirmed across 18 zircon compositions, which are related to the low coordination change and reduced interstitial site preference of Ca2+ along the diffusion pathway. Among the four materials (BiVO4, YVO4, EuCrO4, and YCrO4) that were synthesized, characterized, and electrochemically cycled, the highest initial capacity of 81 mA h/g and the most reversible capacity of 65 mA h/g were achieved in YVO4 and BiVO4, respectively. Despite the facile migration of multivalent ions in zircons, density functional theory predictions of the unstable, discharged structures at higher Ca2+ concentrations (Cax>0.25ABO4), the low dimensionality of the migration pathway, and the defect analysis of the B site atom can rationalize the limited intercalation observed upon electrochemical cycling.

Dual-Ion Intercalation Chemistry Enabling Hybrid Metal-Ion Batteries

To outline the role of dual-ion intercalation chemistry to reach sustainable energy storage, the present Review aimed to compare two types of batteries: widely accepted dual-ion batteries based on cationic and anionic co-intercalation versus newly emerged hybrid metal-ion batteries using the co-intercalation of cations only. Among different charge carrier cations, the focus was on the materials able to co-intercalate monovalent ions (such Li⁺ and Na⁺ , Li⁺ and K⁺ , Na⁺ and K⁺ , etc.) or couples of mono- and multivalent ions (Li⁺ and Mg²⁺ , Na⁺ and Mg²⁺ , Na⁺ and Zn²⁺ , H⁺ and Zn²⁺ , etc.). Furthermore, the Review was directed on co-intercalation materials composed of environmentally benign and low-cost transition metals (e. g., Mn, Fe, etc.). The effect of the electrolyte on the co-intercalation properties was also discussed. The summarized knowledge on dual-ion energy storage could stimulate further research so that the hybrid metal-ion batteries become feasible in near future.

Investigating the role of structural water on the electrochemical properties of α-V2O5 through density functional theory

The α polymorph of V₂O₅ is one of the few known cathodes capable of reversibly intercalating multivalent ions such as Mg, Ca, Zn and Al, but suffers from sluggish diffusion kinetics. The role of H₂O within the electrolyte and between the layers of the structure in the form of a xerogel/aerogel structure, though, has been shown to lower diffusion barriers and lead to other improved electrochemical properties. This density functional theory study systematically investigates how and why the presence of structural H₂O within α-V₂O₅ changes the resulting structure, voltage, and diffusion kinetics for the intercalation of Li, Na, Mg, Ca, Zn, and Al. We found that the coordination of H₂O molecules with the ion leads to an improvement in voltage and energy density for all ions. This voltage increase was attributed to the extra host sites for electrons present with H₂O, thus leading to a stronger ionization of the ion and a higher voltage. We also found that the increase in interlayer distance and a potential "charge shielding" effect drastically changes the electrostatic environment and the resulting diffusion kinetics. For Mg and Ca, this resulted in a decrease in diffusion barrier from 1.3 eV and 2.0 eV to 0.89 eV and 0.4 eV, respectively. We hope that our study motivates similar research regarding the role of water in both V₂O₅ xerogels/aerogels and other layered transition metal oxides.

Insights into the Reaction Mechanisms of Nongraphitic High-Surface Porous Carbons for Application in Na- and Mg-Ion Batteries

The fabrication of low-cost carbon materials and high-performance sodium- and magnesium-ion batteries comprising hierarchical porous electrodes and superior electrolytes is necessary for complementing Li-ion energy storage. In this work, nongraphitic high-surface porous carbons (NGHSPCs) exhibited an unprecedented formation of n-stages (stage-1 and stage-2) due to the co-intercalation of sodium (Na(dgm)₂C₂₀) with diglyme. X-ray diffraction patterns, Patterson diagram, Raman spectra, and IR spectra suggested the presence of n-stages. This phenomenon implies an increase of the initial capacity (∼200 mAh g^-1) and good Na-ion diffusion (2.97 × 10^-13 cm² s^-1), employing diglyme as compared to standard electrolytes containing propylene carbonate and fluoroethylene carbonate. Additionally, the current approach is scalable to full Na- and Mg-ion cells by using t-Na₅V(PO₄)₂F₂ and MgMnSiO₄ cathodes, respectively, reaching 250 and 110 W h kg^-1 based on the anode mass. The simultaneous Mg (de)insertion from/into MgMnSiO₄ and the adsorption/desorption of bistriflimide ions on the NGHSPC surface is responsible for capacity enhancement.

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

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

Investigation of Ca Insertion into α-MoO3 Nanoparticles for High Capacity Ca-Ion Cathodes

Calcium-ion batteries (CIBs) are a promising alternative to lithium-ion batteries (LIBs) due to the low redox potential of calcium metal and high abundance of calcium compounds. Due to its layered structure, α-MoO₃ is regarded as a promising cathode host lattice. While studies have reported that α-MoO₃ can reversibly intercalate Ca ions, limited electrochemical activity has been noted, and its reaction mechanism remains unclear. Here, we re-examine Ca insertion into α-MoO₃ nanoparticles with a goal to improve reaction kinetics and clarify the storage mechanism. The α-MoO₃ electrodes demonstrated a specific capacity of 165 mA h g^-1 centered near 2.7 V vs Ca²⁺/Ca, stable long-term cycling, and good rate performance at room temperature. This work demonstrates that, under the correct conditions, layered oxides can be a promising host material for CIBs and renews prospects for CIBs.

Life-Related Hazards of Materials Applied to Mg–S Batteries

Nowadays, rechargeable batteries utilizing an S cathode together with an Mg anode are under substantial interest and development. The review is made from the point of view of materials engaged during the development of the Mg–S batteries, their sulfur cathodes, magnesium anodes, electrolyte systems, current collectors, and separators. Simultaneously, various hazards related to the use of such materials are discussed. It was found that the most numerous groups of hazards are posed by the material groups of cathodes and electrolytes. Such hazards vary widely in type and degree of danger and are related to human bodies, aquatic life, flammability of materials, or the release of flammable or toxic gases by the latter.

Fast kinetics of monoclinic VO2(B) bulk upon magnesiation via DFT+U calculations

Although magnesium rechargeable batteries (MRBs) have gained considerable attention, research relating to MRBs is still in its infancy. One issue is that magnesium ions are difficult to reversibly (de)intercalate in most electrode materials. Among various available cathodes, VO₂(B) is a promising layered cathode material for use in MRBs. Totally different from monolayer VO₂, the magnesiation mechanism in monoclinic bulk VO₂(B) has not been clearly clarified to this day. For the first time, we systematically investigated the influence of magnetism and van der Waals (vdW) forces on the electronic structure and diffusion kinetics of magnesium in bulk VO₂(B) using a series of DFT+U calculations. The Mg diffusivity can reach a high value of 1.62 × 10^-7 cm² s^-1 at 300 K, which is comparable to Li⁺. These results demonstrate that VO₂(B) is a potential host material with high mobility and fast kinetics.

Advancing towards a Practical Magnesium Ion Battery

A post-lithium battery era is envisaged, and it is urgent to find new and sustainable systems for energy storage. Multivalent metals, such as magnesium, are very promising to replace lithium, but the low mobility of magnesium ion and the lack of suitable electrolytes are serious concerns. This review mainly discusses the advantages and shortcomings of the new rechargeable magnesium batteries, the future directions and the possibility of using solid electrolytes. Special emphasis is put on the diversity of structures, and on the theoretical calculations about voltage and structures. A critical issue is to select the combination of the positive and negative electrode materials to achieve an optimum battery voltage. The theoretical calculations of the structure, intercalation voltage and diffusion path can be very useful for evaluating the materials and for comparison with the experimental results of the magnesium batteries which are not hassle-free.

Solvation sheath reorganization enables divalent metal batteries with fast interfacial charge transfer kinetics

[Figure: see text].

Amorphous Redox-Rich Polysulfides for Mg Cathodes

The lack of appropriate cathodes is restraining the advances of Mg batteries. Crystalline cathode materials suffer from sluggish reaction kinetics and low-capacity delivery. The finite type of crystalline structure further confines the rational design of cathode materials. Herein, we proposed amorphization and anion enrichment as a brand-new strategy to not only enhance the solid-state ion diffusion and provide more ion-storage sites in amorphous structure but also contribute to the local transfer of multiple electrons through the additional anionic redox centers. Accordingly, a series of amorphous titanium polysulfides (a-TiS x , x = 2, 3, and 4) were designed, which significantly outperformed their crystalline counterparts and achieved a highly competitive energy density of ∼260 Wh/kg. The unique Mg2+ storage mechanism involves the dissociation/formation of S-S bonds and changes in the coordination number of Ti, namely, a mixture of conversion and intercalation reaction, accompanied by the joint cationic (Ti) and anionic (S) redox-rich chemistry. Our proposed amorphous and redox-rich design philosophy might provide an innovative direction for developing high-performance cathode materials for multivalent-ion batteries.

Layered Iron Vanadate as a High-Capacity Cathode Material for Nonaqueous Calcium-Ion Batteries

Calcium-ion batteries represent a promising alternative to the current lithium-ion batteries. Nevertheless, calcium-ion intercalating materials in nonaqueous electrolytes are scarce, probably due to the difficulties in finding suitable host materials. Considering that research into calcium-ion batteries is in its infancy, discovering and characterizing new host materials would be critical to further development. Here, we demonstrate FeV3O9∙1.2H2O as a high-performance calcium-ion battery cathode material that delivers a reversible discharge capacity of 303 mAh g−1 with a good cycling stability and an average discharge voltage of ~2.6 V (vs. Ca/Ca2+). The material was synthesized via a facile co-precipitation method. Its reversible capacity is the highest among calcium-ion battery materials, and it is the first example of a material with a capacity much larger than that of conventional lithium-ion battery cathode materials. Bulk intercalation of calcium into the host lattice contributed predominantly to the total capacity at a lower rate, but became comparable to that due to surface adsorption at a higher rate. This stimulating discovery will lead to the development of new strategies for obtaining high energy density calcium-ion batteries.

Atomic-scale unveiling of multiphase evolution during hydrated Zn-ion insertion in vanadium oxide

An initial crystalline phase can transform into another phases as cations are electrochemically inserted into its lattice. Precise identification of phase evolution at an atomic level during transformation is thus the very first step to comprehensively understand the cation insertion behavior and subsequently achieve much higher storage capacity in rechargeable cells, although it is sometimes challenging. By intensively using atomic-column-resolved scanning transmission electron microscopy, we directly visualize the simultaneous intercalation of both H₂O and Zn during discharge of Zn ions into a V₂O₅ cathode with an aqueous electrolyte. In particular, when further Zn insertion proceeds, multiple intermediate phases, which are not identified by a macroscopic powder diffraction method, are clearly imaged at an atomic scale, showing structurally topotactic correlation between the phases. The findings in this work suggest that smooth multiphase evolution with a low transition barrier is significantly related to the high capacity of oxide cathodes for aqueous rechargeable cells, where the crystal structure of cathode materials after discharge differs from the initial crystalline state in general.

The detailed understanding of the structural variations during cycling in cathodes for Zn-ion aqueous rechargeable batteries is still limited. Here, the authors utilize atomic-column-resolved scanning transmission electron microscopy to elucidate multiphase evolution during hydrated Zn-Ion insertion in vanadium oxide.

Magnesium Ion Storage Properties in a Layered (NH4)2V6O16·1.5H2O Nanobelt Cathode Material Activated by Lattice Water

Magnesium ion batteries have attracted increasing attention as a promising energy storage device due to the high safety, high volumetric capacity, and low cost of Mg. However, the strong Coulombic interactions between Mg²⁺ ions and cathode materials seriously hinder the electrochemical performance of the batteries. To seek a promising cathode material for magnesium ion batteries, in this work, (NH₄)₂V₆O₁₆·1.5H₂O and water-free (NH₄)₂V₆O₁₆ materials are synthesized by a one-step hydrothermal method. The effects of NH₄⁺ and lattice water on the Mg²⁺ storage properties in these kinds of layered cathode materials are investigated by experiments and first-principles calculations. Lattice water is demonstrated to be of vital importance for Mg²⁺ storage, which not only stabilizes the layered structure of (NH₄)₂V₆O₁₆·1.5H₂O but also promotes the transport kinetics of Mg²⁺. Electrochemical experiments of (NH₄)₂V₆O₁₆·1.5H₂O show a specific capacity of 100 mA·h·g^-1 with an average discharge voltage of 2.16 V vs Mg²⁺/Mg, highlighting the potential of (NH₄)₂V₆O₁₆·1.5H₂O as a high-voltage cathode material for magnesium ion batteries.

Critical Role of the Interphase at Magnesium Electrodes in Chloride-Free, Simple Salt Electrolytes

Magnesium (Mg) batteries are a potential beyond lithium-ion technology but currently suffer from poor cycling performance, partly due to the interphase formed when magnesium electrodes react with electrolytes. The use of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) electrolytes would enable high-voltage intercalation cathodes, but many reports identify poor Mg plating/stripping in the electrolyte solution due to a passivating interphase. Here, we have assessed the Mg plating/stripping mechanism at bulk Mg electrodes in a Mg(TFSI)₂-based electrolyte by cyclic voltammetry, ex situ Fourier-transform infrared spectroscopy, and electron microscopy and compared this to the cycling of a Grignard-based electrolyte. Our studies indicate a nontypical cycling mechanism at Mg surfaces in Mg(TFSI)₂-based electrolytes that occurs through Mg deposits rather than the bulk electrode. Fourier-transform infrared spectroscopy demonstrates an evolution in the interphase chemistry during conditioning (repeated cycling) and that this is a critical step for stable cycling in the Mg(TFSI)₂-tetraglyme (4G) electrolyte. The fully conditioned electrode in Mg(TFSI)₂-4G is able to cycle with an overpotential of <0.25 V without additional additives such as Cl^- or BH₄^-.

Organic-Inorganic Superlattices of Vanadium Oxide@Polyaniline for High-Performance Magnesium-Ion Batteries

Rechargeable magnesium batteries (RMBs) have attracted significant attention owing to the high energy density and economic viability. However, the lack of suitable cathode materials, owing to the high polarizability of divalent Mg-ion and slow Mg-ion diffusion, hinders the development of RMBs. V₂ O₅ is a promising RMBs cathode material, but its limited interlayer spacing is unfavorable for the rapid diffusion of Mg²⁺ , demonstrating unsatisfactory electrochemical performance. In this study, the superlattices of V₂ O₅ and polyaniline (PANI) with expanded interlayer spacing are assembled as the cathode material for RMBs. The intercalation of PANI in the interlayer region of V₂ O₅ significantly improves the reversible capacities, Mg²⁺ diffusion kinetics, and cycling performance of the PVO cathode. Furthermore, RMBs with PVO as the cathode and Mg metal as the anode deliver high specific capacities. The introduced polyaniline layer not only expands the interlayer spacing of V₂ O₅ , but also increases the electrical conductivity. Moreover, ex situ XRD characterization indicates that PVO does not undergo obvious phase transformation with the continuous insertion of Mg²⁺ , which may be ascribed to the π-conjugated chains of PANI that give flexibility to the structure to improve cycling stability. This study demonstrates that designing organic-inorganic superlattices is an efficient strategy for developing high-performance cathode materials for RMBs.

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.

Promises and Challenges of Next-Generation "Beyond Li-ion" Batteries for Electric Vehicles and Grid Decarbonization

The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

Joint Cationic and Anionic Redox Chemistry for Advanced Mg Batteries

Lack of appropriate cathodes severely restrains the development of high-energy Mg batteries. In this work, we proposed joint cationic and anionic redox chemistry of transition-metal (TM) sulfides as the most promising way out. A series of solid-solution pyrite Fe_xCo_1-xS₂ (0 ≤ x ≤ 1) was specially designed, in which S 3p electrons pour into the d bands of Fe and Co, generating redox-active dimerized (S₂)^2-. The Fe_0.5Co_0.5S₂ sample is highlighted to deliver a high specific energy of 240 Wh/kg at room temperature involving both cationic (Fe and Co) and anionic (S) redox. The highly delocalized electronic clouds in pyrite structures comfortably accommodate the charge of Mg²⁺, contributing to the fast kinetics and the superior cycling stability of the Fe_0.5Co_0.5S₂. It is anticipated that the joint cationic and anionic redox chemistry proposed in this work would be the ultimate answer for designing high-energy cathodes for advanced Mg batteries.

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.

Spinel-Type MgMn2O4 Nanoplates with Vanadate Coating for a Positive Electrode of Magnesium Rechargeable Batteries

Spinel-type MgMn₂O₄ nanoplates ∼10 nm thick were prepared as a positive electrode for magnesium rechargeable batteries by the transformation of metal hydroxide nanoplates. Homogeneous coating with a vanadate layer thinner than 3 nm was achieved on the spinel oxide nanoplates via coverage of the precursor and subsequent mild calcination. We found that the spinel oxide nanoplates with the homogeneous coating exhibit improved electrochemical properties, such as discharge potential, capacity, and cyclability, due to the enhanced insertion and extraction of magnesium ions and suppressed decomposition of electrolytes. The nanometric platy morphology of the spinel oxide and the vanadate coating act synergistically for the improvement of the electrochemical performance.

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V4Nb18O55

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg²⁺ insertion and extraction at high voltages; this is thought to be due to poor Mg²⁺ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg²⁺ into the tetragonal tungsten bronze V₄Nb₁₈O₅₅, with a maximum reversible electrochemical capacity of 75 mA h g^-1, which corresponds to a magnesiated composition of Mg₄V₄Nb₁₈O₅₅. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg²⁺); when magnesiation is limited to a composition of Mg₂V₄Nb₁₈O₅₅, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg²⁺ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.

Electrostatic Shielding Guides Lateral Deposition for Stable Interphase toward Reversible Magnesium Metal Anodes

Compared with lithium, magnesium shows a low propensity toward dendritic deposition due to its low surface self-diffusion barriers. However, due to the intrinsic surface roughness of the metal and the nonuniformity of the formed solid-electrolyte interphase, uneven deposition of Mg still happens, which brings about high local current density and continuous proliferation of the interphase, greatly exacerbating the passivation. Unfortunately, little attention has been paid to the deposition uniformity and the interfacial stability of Mg metal anodes, which result in a potential penalty. Herein, we modify the electrolyte with cathodically stable cations to guide smooth deposition via an electrostatic shielding strategy. The cations adsorbed on the initial protuberances effectively homogenize the charge flux by repulsing the incoming Mg²⁺ away from the tips. Importantly, we prove the lateral growth can benefit the interphase stability and electrochemical reversibility.

Recent Advances in the Rational Design and Synthesis of Two-Dimensional Materials for Multivalent Ion Batteries

With the increase of device requirements, rechargeable lithium-ion batteries are facing tremendous challenges in large-scale applications due to the high price and gradual shortage of lithium sources. In contrast, multivalent ion batteries, such as aluminum, magnesium, and zinc, are promising candidates for the next-generation energy-storage systems because of their high volumetric energy density, safe operation, and abundant reserves. The strong intercalation between multivalent ions and the host materials, however, will cause lower ion-diffusion kinetics and a poor discharge capacity. One of the main challenges is to search for a suitable cathode material with a high capacity and good structural stability to overcome the abovementioned problems. Two-dimensional layered materials, with characteristic unique structural features, good conductivity, and high electrochemically active surface, have attracted attention from researchers during the past decade. In this review, the design approach and synthetic procedures for the preparation of two-dimensional materials as cathodes for multivalent ion batteries, including interlayer engineering, two-dimensional heterostructures, pore/hole engineering, and heteroatom doping, are summarized. Meanwhile, the relationship between the design configuration and optimized electrochemical performance is rationally and systematically presented. Additionally, perspectives for the sustainable synthesis of cathode materials are proposed for multivalent metal-ion chemistry.

VOPO4·2H2O as a new cathode material for rechargeable Ca-ion batteries

VOPO₄·2H₂O, as a new cathode material for Ca-ion batteries, exhibits a discharge capacity of 100.6 mA h g^-1, excellent cycling stability (200 cycles) and good rate performance (42.7 mA h g^-1 at 200 mA g^-1). In situ X-ray diffraction and in situ Raman results demonstrate the calcium-ion-storage mechanism of VOPO₄·2H₂O is a single-phase reaction based on asymmetric Ca²⁺ insertion and deinsertion.

Methods and Protocols for Reliable Electrochemical Testing in Post-Li Batteries (Na, K, Mg, and Ca)

While less mature than the Li-ion battery, technologies based on Na, K, Mg, and Ca are attracting more and more attention from the battery community. New material (cathode, anode, or electrolyte) testing for these post-Li systems commonly involves the use of an electrochemical setup called a half-cell in which metal counter and reference electrodes are used. Here we first describe the different issues that become critical when moving away from Li with respect to the cell hardware (cell design, current collector, separator, insulator) and the nature of the counter and reference electrodes. Workarounds are given, and a versatile setup is proposed to run reliable electrochemical tests for post-Li battery materials in general, in a broad range of electrolyte compositions.

Rational Design of Mixed Solvent and Porous Graphene-Supported Spinel Oxide Electrodes for High-Rate and Long Cycle-Life Mg Batteries

The development of Mg batteries based on the interfacial charge storage mechanism, where the capacity originates from capacitive processes and the solvent-related interfacial reactions, could efficiently circumvent the challenge of intercalation-based Mg batteries with sluggish kinetics. In this work, the proposed Mg organohaloaluminate mixture electrolyte is reported to improve the charge storage performance of the graphene-supported cathodes, resulting in both high cycling stability (91% capacity retention after 2000 cycles) and high rate capability (51% capacity retention when the current density increases by 100 times). The experimental and computational studies have revealed that the exceptional cell performance originates from the optimized electrode/electrolyte interface, where the highly reversible interfacial reactions occur with the 1,2-dimethoxyethane additive in the typical all-phenyl complex electrolyte. The fast charge-transfer kinetics along the surface of highly porous and conductive graphene-supported electrodes have also been observed.

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.

Eco-compatible oxides enabling energy storage via Li+/Mg2+ co-intercalation

The storage of energy by means of reversible intercalation of bivalent magnesium ions represents, nowadays, the shortest route for doubling the energy density of conventional lithium ion batteries. Contrary to the intercalation of monovalent lithium ions, the intercalation of Mg²⁺ is a kinetically limited process. Herein we demonstrate a new approach for improving Mg²⁺ intercalation, which is based on dual intercalation of Li⁺ and Mg²⁺ ions with a synergic effect. The concept is proved on the basis of eco-compatible oxides such as magnesium manganate spinels, MgMn₂O₄, and lithium titanates Li₂TiO₃ and Li₄Ti₅O₁₂ with a monoclinic and spinel structure. These two types of oxides are selected since they exhibit high and low potentials of ion intercalation due to the redox couples Mn^2,3+/Mn^3,4+ and Ti³⁺/Ti⁴⁺, respectively. Through a newly developed method of synthesis, we succeeded in the preparation of well-crystallized nanosized spinels with a specific cationic distribution. The intercalation properties of MgMn₂O₄, Li₂TiO₃ and Li₄Ti₅O₁₂ are first examined in model cells versus the metallic Li anode. The Li⁺ and Mg²⁺ intercalation is directed by the kind of the used electrolyte: a lithium electrolyte consisting of 1 M LiPF₆ solution in EC : DMC and a magnesium electrolyte consisting of 0.5 Mg(TFSI)₂ solution in diglyme. The mechanism of Li⁺ and Mg²⁺ co-intercalation is assessed by ex situ X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM). Finally, a new type of hybrid Li-Mg ion cell combining MgMn₂O₄ and Li₄Ti₅O₁₂ oxides as electrodes is constructed. The cell configuration allows reaching an operating voltage of around 1.7 V by using an electrolyte containing 0.5 M LiTFSI in diglyme.

High-Energy-Density Rechargeable Mg Battery Enabled by a Displacement Reaction

Because of its high theoretical volumetric capacity and dendrite-free stripping/plating of Mg, rechargeable magnesium batteries (RMBs) hold great promise for high energy density in consumer electronics. However, the lack of high-energy-density cathodes severely constrains their practical applications. Herein, for the first time, we report that a CuS cathode can fully reversibly work through a displacement reaction in CuS/Mg pouch cells at room temperature and provide a high capacity of ∼400 mA h/g in a MACC electrolyte, corresponding to the gravimetric and volumetric energy density of 608 W h/kg and1042 W h/L, respectively. Even after 80 cycles, CuS/Mg pouch cells can maintain a high capacity of 335 mA h/g. Detailed mechanistic studies reveal that CuS undergoes a displacement reaction route rather than a typical conversion mechanism. This work will provide a guide for more discovery of high-performance cathode candidates 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.

The Power of Stoichiometry: Conditioning and Speciation of MgCl2/AlCl3 in Tetraethylene Glycol Dimethyl Ether-Based Electrolytes

In many Mg-based battery systems, the reversibility of Mg deposition and dissolution is lowered by parasitic formation processes of the electrolyte. Therefore, high Coulombic efficiencies of Mg deposition and dissolution are only achieved after several "conditioning" cycles. As this phenomenon is especially reported for AlCl₃-containing solutions, this study focuses on the "conditioning" mechanisms of MgCl₂/AlCl₃ and MgHMDS₂/AlCl₃ (HMDS = hexamethyldisilazide) in tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. Electrochemical (cyclic voltammetry) and spectroscopic investigations (²⁷Al nuclear magnetic resonance spectroscopy, Raman spectroscopy, inductively coupled plasma optical emission spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy) reveal that cationic AlCl₂⁺ species in TEGDME-based electrolytes with an AlCl₃/MgCl₂ ratio higher than 1:1 corrode the Mg metal. According to a cementation reaction mechanism, the corrosion of Mg is accompanied with Al deposition. In effect, the consumption of Mg results in low Coulombic efficiencies of Mg deposition and dissolution during the electrolyte "conditioning". After understanding the mechanism of this process, we demonstrate that a careful adjustment of the stoichiometry in MgCl₂/AlCl₃ and MgHMDS₂/AlCl₃ in TEGDME formulations prevents Mg corrosion and results in "conditioning"-free, highly efficient Mg deposition and dissolution.

The Rechargeable Aluminum Battery: Opportunities and Challenges

Aluminum battery systems are considered as a system that could supplement current lithium batteries due to the low cost and high volumetric capacity of aluminum metal, and the high safety of the whole battery system. However, first the use of ionic liquid electrolytes leading to AlCl₄ ^- instead of Al³⁺ , the different intercalation reagents, the sluggish solid diffusion process and the fast capacity fading during cycling in aluminum batteries all need to be thoroughly explored. To provide a good understanding of the opportunities and challenges of the newly emerging aluminum batteries, this Review discusses the reaction mechanisms and the difficulties caused by the trivalent reaction medium in electrolytes, electrodes, and electrode-electrolyte interfaces. It is hoped that the Review will stimulate scientists and engineers to develop more reliable aluminum batteries.