Facile synthesis and lithium storage performance of (NH4)2V3O8 nanoflakes

How About Vanadium-Based Compounds as Cathode Materials for Aqueous Zinc Ion Batteries?

Aqueous zinc-ion batteries (AZIBs) stand out among many monovalent/multivalent metal-ion batteries as promising new energy storage devices because of their good safety, low cost, and environmental friendliness. Nevertheless, there are still many great challenges to exploring new-type cathode materials that are suitable for Zn2+ intercalation. Vanadium-based compounds with various structures, large layer spacing, and different oxidation states are considered suitable cathode candidates for AZIBs. Herein, the research advances in vanadium-based compounds in recent years are systematically reviewed. The preparation methods, crystal structures, electrochemical performances, and energy storage mechanisms of vanadium-based compounds (e.g., vanadium phosphates, vanadium oxides, vanadates, vanadium sulfides, and vanadium nitrides) are mainly introduced. Finally, the limitations and development prospects of vanadium-based compounds are pointed out. Vanadium-based compounds as cathode materials for AZIBs are hoped to flourish in the coming years and attract more and more researchers' attention.

Two-Dimensional NH4V3O8 Nanoflakes as Efficient Energy Conversion and Storage Materials for the Hydrogen Evolution Reaction and Supercapacitors

Herein, for the first time, we present two-dimensional (2D) NH4V3O8 nanoflakes as an excellent material for both energy conversion of the hydrogen evolution reaction and storage of supercapacitors by a simple and fast two-step synthesis, which exhibit a completely sheet-like morphology, high crystallinity, good specific surface area, and also stability, as determined by thermogravimetric analysis. The 2D-NH4V3O8 flakes show an acceptable hydrogen evolution performance in 0.5 M H2SO4 on a glassy carbon electrode (GCE) coated with 2D-NH4V3O8, which results in a low overpotential of 314 mV at -10 mA cm-2 with an excellent Tafel slope as low as 90 mV dec-1. So far, with the main focus on energy storage, 2D-NH4V3O8 nanoflakes were found to be ideal for supercapacitor electrodes. The NH4V3O8 working electrode in 1 M Na2SO4 shows an excellent electrochemical capability of 274 F g-1 at 0.5 A g-1 for a maximum energy density of 38 W h kg-1 at a power density as high as 250 W kg-1. Moreover, the crystal structure of 2D-NH4V3O8 is demonstrated by density functional theory (DFT) computational simulation using three functionals, GGA, GGA + U, and HSE06. The simple preparation, low cost, and abundance of the NH4V3O8 material provide a promising candidate for not only energy conversion but also energy-storage applications.

Redetermination of di-ammonium trivanadate, (NH4)2V3O8

The crystal structure of (NH4)2V3O8 has been reported twice using single-crystal X-ray data [Theobald et al. (1984 ▸). J. Phys. Chem. Solids, 45, 581-587; Range et al. (1988 ▸). Z. Naturforsch. Teil B, 43, 309-317]. In both cases, the orientation of the ammonium cation in the asymmetric unit was poorly defined: in Theobald's study, the shape and dimensions were constrained for NH4 +, while in Range's study, H atoms were not included. In the present study, we collected a highly redundant data set for this ternary oxide, at 0.61 Å resolution, using Ag Kα radiation. These accurate data reveal that the NH4 + cation is disordered by rotation around a non-crystallographic axis. The rotation axis coincides with one N-H bond lying in the mirror m symmetry element of space-group type P4bm, and the remaining H sites were modelled over two disordered positions, with equal occupancy. It therefore follows that the NH4 + cations filling the space available in the (001) layered structure formed by (V3O8)2- ions do not form strong N-H⋯O hydrogen bonds with the mixed-valent oxidovanadate(IV,V) anions. This feature could have consequences for the Li-ion inter-calation properties of this material, which is used as a cathode for supercapacitors.

Uniaxial thermal expansion behaviors and ionic conduction in a layered (NH4)2V3O8

The zero/negative thermal expansion (ZTE/NTE), which is an intriguing physical property of solids, has been observed in a few families of materials. ZTE materials possess practical applications in specific circumstances such as space-related applications, engineering structures and precision instrument. Generally, NTE materials are used as additives to form a composite of the ZTE material with positive thermal expansion material. It is still a tremendous challenge to design new families of ZTE/NTE materials. Herein, we presented a temperature-dependent single crystal structure analysis in 110-300 K for a layered (NH4)2V3O8, which crystallizes in a tetragonal space group P4bm and comprises mixed valence [V3O82-]∞ monolayers and NH4+ residual in the interlayer spaces. Along the c-axis, (NH4)2V3O8 demonstrated uniaxial expansion behaviors, i.e., ZTE with αc = -1.10 × 10-6 K-1 in 110-170 K and NTE with αc = -16.25 × 10-6 K-1 in 170-220 K. Along the a-axis, (NH4)2V3O8 exhibited ZTE with αa = + 2.06 × 10-6 K-1 in 240-300 K. The mechanisms of ZTE and NTE were explored using structural analysis. The conduction of NH4+ ions in the interlayer space was studied, indicating that the conductivity rapidly rises with the expansion of interlayer space at temperatures of >293 K. This study discloses that layered vanadates are promising ZTE/NTE materials.

Ultrathin Nanoribbons of in Situ Carbon-Coated V3O7·H2O for High-Energy and Long-Life Li-Ion Batteries: Synthesis, Electrochemical Performance, and Charge-Discharge Behavior

The ever-growing demands of Li-ion batteries (LIBs) for high-energy and long-life applications, such as electrical vehicles, have prompted great research interest. Herein, by applying an interesting one-step high-temperature mixing method under hydrothermal conditions, ultrathin V₃O₇·H₂O@C nanoribbons with good crystallinity and robust configuration are in situ synthesized as promising cathode materials of high-energy, high-power, and long-life LIBs. Their capacity is up to 319 mA h/g at a current density of 100 mA/g. Moreover, the capacity of 262 mA h/g can be delivered at 500 mA/g, and 94% of capacity can be retained after 100 cycles. Even at a large current density of 3000 mA/g, they can still deliver a high capacity of 165 mA h/g, and 119% of the initial capacity can be kept after 600 cycles. Importantly, their energy density is up to 800 Wh/kg, which is 48-60% higher than those of conventional cathode materials (such as LiCoO₂, LiMn₂O₄, and LiFePO₄), and they can maintain an energy density of 355 Wh/kg at a high power density of 8000 W/kg. Furthermore, based on ex situ X-ray diffraction and X-ray photoelectron spectroscopy technology, their exact charge-discharge behavior is reasonably described for the first time. Excitingly, it is found for the first time that the as-synthesized V₃O₇·H₂O@C nanoribbons are also great promising cathode materials for Na-ion batteries.

Synthesis and electrochemical performance of NaV6O15 microflowers for lithium and sodium ion batteries

High first discharge capacity of 255 mA h g⁻¹ (vs. Li⁺/Li) and 130 mA h g⁻¹ (vs. Na⁺/Na) were observed in NaV₆O₁₅ microflowers and the capacity retention reaches 105% and 64% after 50 cycles at the current density of 100 mA g⁻¹ and 50 mA g⁻¹, respectively.