Nanostructured-VO2(B): A high-capacity magnesium-ion cathode and its electrochemical reaction mechanism

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.

Re-evaluating the Magnesium-ion Storage Capability of Vanadium Dioxide, VO2 (B): Uncovering the Influence of Water Content on the Previously Overestimated High Capacity

Magnesium batteries have emerged as a promising alternative to lithium-ion batteries due to their theoretical high energy density and abundant magnesium resources. Vanadium dioxide, VO2 (B), has been reported as a high-capacity cathode material for magnesium batteries. However, the electrochemical intercalation mechanism requires further elucidation due to a limited understanding of the structure-property relationship. In this study, we re-evaluated the magnesium storage capability of the material, with a particular focus on the influence of water content in nonaqueous electrolytes. The higher discharge capacity of 250 mAh g-1 is achieved exclusively in the wet electrolyte with 650 ppm water content. A significantly lower capacity of 51 mAh g-1 was observed in the dry electrolyte solution containing 40 ppm water content. Through X-ray structural and elemental analyses, as well as magnesium-ion diffusion pathway analysis using bond-valence-energy-landscape calculations, the restricted capacity was clarified by examining the reaction mechanism. According to this study, the impressive capacity of magnesium-ion battery cathodes may be exaggerated due to the involvement of non-magnesium-ion insertion unless the electrolytes' water content is appropriately regulated.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Controllable Phase Transition Properties in VO2 Films via Metal-Ion Intercalation

VO₂ has shown great promise for sensors, smart windows, and energy storage devices, because of its drastic semiconductor-to-metal transition (SMT) near 340 K coupled with a structural transition. To push its application toward room-temperature, effective transition temperature (T_c) tuning in VO₂ is desired. In this study, tailorable SMT characteristics in VO₂ films have been achieved by the electrochemical intercalation of foreign ions (e.g., Li ions). By controlling the relative potential with respect to Li/Li⁺ during the intercalation process, T_c of VO₂ can be effectively and systematically tuned in the window from 326.7 to 340.8 K. The effective T_c tuning could be attributed to the observed strain and lattice distortion and the change of the charge carrier density in VO₂ introduced by the intercalation process. This demonstration opens up a new approach in tuning the VO₂ phase transition toward room-temperature device applications and enables future real-time phase change property tuning.

Photo-Enhanced Magnesium-Ion Capacitors Using Photoactive Electrodes

Off-grid power sources are becoming increasingly important for applications ranging from autonomous sensor networks to fighting energy poverty. Interactions of light with certain classes of battery and capacitor materials have recently gained attention to enhance the rate performance or to even charge energy storage devices directly with light. Interestingly, these devices have the potential to reduce the volume and cost of autonomous power sources. Here, a light-enhanced magnesium (Mg)-ion capacitor is shown. The latter is interesting because of the large natural abundance of Mg and its ability to operate in low cost and non-flammable aqueous electrolytes. Photoelectrodes using a combination of vanadium dioxide and reduced graphene oxide can achieve capacitance enhancements of up to 56% under light exposure alongside a 21% higher energy density of 20.5 mAh kg^-1 .

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.

Phase B vanadium dioxide: characteristics, synthesis and applications

Starting from the numerous and unique characteristics of VO2(B), we will introduce to readers the research progress of VO2(B) in recent years, including the detailed mainstream methods for its preparation and popular fields of application.

Current Design Strategies for Rechargeable Magnesium-Based Batteries

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

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.

VO2 Nanostructures for Batteries and Supercapacitors: A Review

Vanadium dioxide (VO₂ ) received tremendous interest lately due to its unique structural, electronic, and optoelectronic properties. VO₂ has been extensively used in electrochromic displays and memristors and its VO₂ (B) polymorph is extensively utilized as electrode material in energy storage applications. More studies are focused on VO₂ (B) nanostructures which displayed different energy storage behavior than the bulk VO₂ . The present review provides a systematic overview of the progress in VO₂ nanostructures syntheses and its application in energy storage devices. Herein, a general introduction, discussion about crystal structure, and syntheses of a variety of nanostructures such as nanowires, nanorods, nanobelts, nanotubes, carambola shaped, etc. are summarized. The energy storage application of VO₂ nanostructure and its composites are also described in detail and categorically, e.g. Li-ion battery, Na-ion battery, and supercapacitors. The current status and challenges associated with VO₂ nanostructures are reported. Finally, light has been shed for the overall performance improvement of VO₂ nanostructure as potential electrode material for future application.

Hierarchical WSe2 nanoflower as a cathode material for rechargeable Mg-ion batteries

Transition metal dichalcogenides (TMDs) have emerged as a promising material in the energy field due to their unique structural arrangement. In this work, ordered flower-like WSe₂ nanosheet was synthesized through simple one-step hydrothermal method, and its cathode application for rechargeable Mg-ion batteries was assessed. The WSe₂ cathode exhibits a high reversible capacity above 265 mAh g^-1 at 50 mA g^-1, excellent cycling life of 90% initial capacitance that can be ceaselessly harvested for 100 cycles at 50 mA g^-1, and superior rate capability of 70% initial capacitance maintained even at the current density of 500 mA g^-1. This work paves the way for the application of WSe₂ cathode in Mg-ion and other rechargeable batteries.

Water-Pillared Sodium Vanadium Bronze Nanowires for Enhanced Rechargeable Magnesium Ion Storage

Owing to the advantages of high safety, low cost, high theoretical volumetric capacities, and environmental friendliness, magnesium-ion batteries (MIBs) have more feasibility for large-scale energy storage compared to lithium-ion batteries. However, lack of suitable cathode materials due to sluggish kinetics of magnesium ion is one of the biggest challenges. Herein, water-pillared sodium vanadium bronze nanowires (Na₂ V₆ O₁₆ ·1.63H₂ O) are reported as cathode material for MIBs, which display high performance in magnesium storage. The hydrated sodium ions provide excellent structural stability. The charge shielding effect of lattice water enables fast Mg²⁺ diffusion. It exhibits high specific capacity of 175 mAh g^-1 , long cycle life (450 cycles), and high coulombic efficiency (≈100%). At high current density of 200 mA g^-1 , the capacity retention is up to 71% even after 450 cycles (compared to the highest capacity), demonstrating excellent long-term cycling performance. The nature of charge storage kinetics is explored. Furthermore, a highly reversible structure change during the electrochemical process is proved by comprehensive electrochemical analysis. The remarkable electrochemical performance makes Na₂ V₆ O₁₆ ·1.63H₂ O a promising cathode material for low-cost and safe MIBs.