New chemical route for the synthesis of β-Na(0.33)V₂O₅ and its fully reversible Li intercalation

Freestanding Sodium Vanadate/Carbon Nanotube Composite Cathodes with Excellent Structural Stability and High Rate Capability for Sodium-Ion Batteries

Sodium vanadate NaV₆O₁₅ (NVO) is one of the most promising cathode materials for sodium-ion batteries because of its low cost and high theoretical capacity. Nevertheless, NVO suffers from fast capacity fading and poor rate capability. Herein, a novel free-standing NVO/multiwalled carbon nanotube (MWCNT) composite film cathode was synthesized and designed by a simple hydrothermal method followed by a dispersion technique with high safety and low cost. The kinetics analysis based on cyclic voltammetry measurements reveals that the intimate integration of the MWCNT 3D porous conductive network with the 3D pillaring tunnel structure of NVO nanorods enhances the Na⁺ intercalation pseudocapacitive behavior, thus leading to exceptional rate capability and long lifespan. Furthermore, the NVO/MWCNT composite exhibits excellent structural stability during the charge/discharge process. With these benefits, the composite delivers a high discharge capacity of 217.2 mA h g^-1 at 0.1 A g^-1 in a potential region of 1.5-4.0 V. It demonstrates a superior rate capability of 123.7 mA h g^-1 at 10 A g^-1. More encouragingly, it displays long lifespan; impressively, 96% of the initial capacity is retained at 5 A g^-1 for over 500 cycles. Our work presents a promising strategy for developing electrode materials with a high rate capability and a long cycle life.

Theoretical Description, Synthesis, and Structural Characterization of β-Na0.33V2O5 and Its Fluorinated Derivative β-Na0.33V2O4.67F0.33: Influence of Oxygen Substitution by Fluorine on the Electrochemical Properties

The structure of β-Na_0.33V₂O_4.67F_0.33 has been investigated by both theoretical and experimental methods. It exhibits the same structure as that of the parent bronze β-Na_0.33V₂O₅. The partial substitution of oxygen by fluorine has little effect on the average structure and cell parameters, but the sodium environment changes significantly. Using DFT calculations, we determined the most stable positions of fluorine atoms in the unit cell. It was found that the partial replacement of oxide by fluoride takes mainly place in the coordination sphere of Na producing a shortening of the Na-anion bond lengths. We also analyzed the electronic properties based on density of states and Bader charge distribution. The crystallochemical situation of sodium ions in β-Na_0.33V₂O_4.67F_0.33 oxyfluoride, detected by both experimental and computational methods, affects its mobility with respect to the parent oxide. The higher ionicity in the Na coordination sphere of β-Na_0.33V₂O_4.67F_0.33 is related to a sodium ion diffusion coefficient, D_Na+, that is 1 order of magnitude lower (1.24 × 10^-13 cm² s^-1) than in the case of β-Na_0.33V₂O₅ (1.13 × 10^-12 cm² s^-1). Electrochemical sodium insertion/deinsertion properties of the oxyfluoride have been also investigated and are compared to the oxide. Insertion/deinsertion equilibrium potential for the same formal oxidation state of vanadium increases due to fluorination (for instance reduction of V^+4.3 occurs at 1.5 V in the oxide and at 1.75 V in the oxyfluoride). However, the capacity of Na_0.33V₂O_4.67F_0.33 at constant current is lower than in the case of β-Na_0.33V₂O₅ due to a less adequate morphology, a lower D_Na+, and a lower oxidation state of vanadium owing to the aliovalent O/F substitution.

Uniform β-Na0.33V2O5 nanorod cathode providing superior rate capability for lithium ion batteries

A vanadium bronze nanomaterial, β-Na_0.33V₂O₅, was synthesized using a facile sol-gel method followed by annealing at high temperature. The morphology of the sample was observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM), and the crystal phase was determined by x-ray diffraction (XRD) spectroscopy. The as-prepared sample displays a morphology of nanorods, and has a pure phase with a high crystallinity. When used as the cathode material for rechargeable lithium batteries, the β-Na_0.33V₂O₅ nanorods fired at 400 °C exhibit better electrochemical properties at a 2.0 V cutoff voltage than those at a 1.5 V cutoff voltage. Over the voltage range of 2.0-4.0 V, they can deliver an initial capacity of 221 mAh g^-1 at a 0.5 C rate, and retain 212 mAh g^-1 after 200 cycles, accounting for a capacity fading of only 0.02% per cycle. At a 5 C rate, the discharge capacity still reaches 146 mAh g^-1, displaying an outstanding rate capability. Control of the electrochemical window is proved to be an effective strategy in boosting the cycling stability of the β-Na_0.33V₂O₅ cathode in this work in spite of a discounted capacity. Results suggest the as-prepared β-Na_0.33V₂O₅ nanorods are promising for use as high-performance cathode materials for rechargeable lithium batteries.

Electrochemical Properties of Na0.66V4O10 Nanostructures as Cathode Material in Rechargeable Batteries for Energy Storage Applications

We report the electrochemical performance of nanostructures of Na0.66V4O10 as cathode material for rechargeable batteries. The Rietveld refinement of room-temperature X-ray diffraction pattern shows the monoclinic phase with C2/m space group. The cyclic voltammetry curves of prepared half-cells exhibit redox peaks at 3.1 and 2.6 V, which are due to two-phase transition reaction between V5+/4+ and can be assigned to the single-step deintercalation/intercalation of Na ion. We observe a good cycling stability with specific discharge capacity (measured vs Na+/Na) between 80 (±2) and 30 (±2) mAh g-1 at current densities of 3 and 50 mA g-1, respectively. The electrochemical performance of Na0.66V4O10 electrode was also tested with Li anode, which showed higher capacity but decayed faster than Na. Using density functional theory, we calculate the Na vacancy formation energies: 3.37 eV in the bulk of the material and 2.52 eV on the (100) surface, which underlines the importance of nanostructures.

Synthesis and electrochemical performance of NaV3O8 nanobelts for Li/Na-ion batteries and aqueous zinc-ion batteries

NaV₃O₈ nanobelts were successfully synthesized for Li/Na-ion batteries and rechargeable aqueous zinc-ion batteries (ZIBs) by a facile hydrothermal reaction and subsequent thermal transformation. Compared to the electrochemical performance of LIBs and NIBs, NaV₃O₈ nanobelt cathode materials in ZIBs have shown excellent electrochemical performance, including high specific capacity of 421 mA h g⁻¹ at 100 mA g⁻¹ and good cycle stability with a capacity retention of 94% over 500 cycles at 5 A g⁻¹. The good diffusion coefficients and high surface capacity of NaV₃O₈ nanobelts in ZIBs were in favor of fast Zn²⁺ intercalation and long-term cycle stability.

Compared to the electrochemical performance for LIBs and NIBs, NaV₃O₈ nanobelts electrode for ZIBs shows excellent electrochemical performance, including high specific capacity of 421 mA h g⁻¹ at 100 mA g⁻¹, good rate performance and cycle performance.

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.

Unidirectional growth of single crystalline β-Na0.33V2O5and α-V2O5nanowires driven by controlling the pH of aqueous solution and their electrochemical performances for Na-ion batteries

Single crystalline β-Na_0.33V₂O₅and α-V₂O₅nanowires were prepared with pH controlled precursors.

In situ soft-chemistry synthesis of β-Na0.33V2O5 nanorods as high-performance cathode for lithium-ion batteries

β-Na_0.33V₂O₅ nanorods were prepared via a facile soft-chemistry strategy using Na⁺ intercalated (NH₄)_0.5V₂O₅ nanosheets as precursor.

Facile Synthesis of Na0.33V2O5 Nanosheet-Graphene Hybrids as Ultrahigh Performance Cathode Materials for Lithium Ion Batteries

Na0.33V2O5 nanosheet-graphene hybrids were successfully fabricated for the first time via a two-step route involving a novel hydrothermal method and a freeze-drying technique. Uniform Na0.33V2O5 nanosheets with a thickness of about 30 nm are well-dispersed between graphene layers. The special sandwich-like nanostructures endow the hybrids with high discharge capacity, good cycling stability, and superior rate performance as cathodes for lithium storage. Desirable discharge capacities of 313, 232, 159, and 108 mA·h·g(-1) can be delivered at 0.3, 3, 6, and 9 A·g(-1), respectively. Moreover, the Na0.33V2O5-graphene hybrids can maintain a high discharge capacity of 199 mA·h·g(-1) after 400 cycles even at an extremely high current density of 4.5 A·g(-1), with an average fading rate of 0.03% per cycle.