Electrochemically formed α’-NaV2O5: A new sodium intercalation compound

Metal-Ion Intercalation Mechanisms in Vanadium Pentoxide and Its New Perspectives

The investigation into intercalation mechanisms in vanadium pentoxide has garnered significant attention within the realm of research, primarily propelled by its remarkable theoretical capacity for energy storage. This comprehensive review delves into the latest advancements that have enriched our understanding of these intricate mechanisms. Notwithstanding its exceptional storage capacity, the compound grapples with challenges arising from inherent structural instability. Researchers are actively exploring avenues for improving electrodes, with a focus on innovative structures and the meticulous fine-tuning of particle properties. Within the scope of this review, we engage in a detailed discussion on the mechanistic intricacies involved in ion intercalation within the framework of vanadium pentoxide. Additionally, we explore recent breakthroughs in understanding its intercalation properties, aiming to refine the material's structure and morphology. These refinements are anticipated to pave the way for significantly enhanced performance in various energy storage applications.

Sodium-Vanadium Bronze Na9V14O35: An Electrode Material for Na-Ion Batteries

Na₉V₁₄O₃₅ (η-Na_xV₂O₅) has been synthesized via solid-state reaction in an evacuated sealed silica ampoule and tested as electroactive material for Na-ion batteries. According to powder X-ray diffraction, electron diffraction and atomic resolution scanning transmission electron microscopy, Na₉V₁₄O₃₅ adopts a monoclinic structure consisting of layers of corner- and edge-sharing VO₅ tetragonal pyramids and VO₄ tetrahedra with Na cations positioned between the layers, and can be considered as sodium vanadium(IV,V) oxovanadate Na₉V₁₀^4.1+O₁₉(V⁵⁺O₄)₄. Behavior of Na₉V₁₄O₃₅ as a positive and negative electrode in Na half-cells was investigated by galvanostatic cycling against metallic Na, synchrotron powder X-ray diffraction and electron energy loss spectroscopy. Being charged to 4.6 V vs. Na⁺/Na, almost 3 Na can be extracted per Na₉V₁₄O₃₅ formula, resulting in electrochemical capacity of ~60 mAh g⁻¹. Upon discharge below 1 V, Na₉V₁₄O₃₅ uptakes sodium up to Na:V = 1:1 ratio that is accompanied by drastic elongation of the separation between the layers of the VO₄ tetrahedra and VO₅ tetragonal pyramids and volume increase of about 31%. Below 0.25 V, the ordered layered Na₉V₁₄O₃₅ structure transforms into a rock-salt type disordered structure and ultimately into amorphous products of a conversion reaction at 0.1 V. The discharge capacity of 490 mAh g⁻¹ delivered at first cycle due to the conversion reaction fades with the number of charge-discharge cycles.

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.

Benchmarked capacitive performance of a 330 μm-thick NaxV2O5/CC monolithic electrode via synergism of a hierarchical pore structure and ultrahigh-mass-loading

To address the longstanding issue of conventional supercapacitors, viz. their energy and power deliveries are largely attenuated by the poor packing density of particularly the active electrodes, an ultracompact yet porous monolithic electrode is put forward in the present study. Particularly, it is built on electroactive α'-Na_xV₂O₅ with the areal mass loading amounting to 33.24 mg cm^-2 densely packed into a 330-μm-thick carbon cloth and more importantly, with a hierarchical meso-/nano-pore structure in favor of the ion transport throughout this 330 μm-thick α'-Na_xV₂O₅/CC heavy electrode. In such context, a series of superior performances including the areal, gravimetric and volumetric capacitances reaching 12.47 F cm^-2, 375.2 F g^-1 and 377.93 F cm^-3, and the energy and power densities amounting to 1.38 mW h cm^-2 and 34.1 mW cm^-2 are successfully delivered by this compact monolith at the electrode- and device-level, respectively, altogether outperforming significantly those of additional modern and promising electrodes and energy storage devices reported in the literature.

Ultrafast Carrier Transport through an Advanced Thick Electrode with a High Areal Capacity for Aqueous Lithium-Ion Batteries

Thick electrode design holds great promise to render the aqueous lithium ion battery more cost effective by boosting the packing density of the electroactive materials to enhance the energy delivery at the device level. However, a thick electrode faces the concomitant challenge of the sluggish transport of electrons and, importantly, the Li ions. To address this issue, numerous 3 D shortcuts that include a conductive percolation network and well-interconnected mesoporous channels were established in the 330 μm thick V₂ O₅ ⋅H₂ O/CC monolithic electrode developed here. In this way, electron transfer and ion transport were favored, which accounts for the outstanding charge-storage capacity that exceeded 2 mA h cm^-2 and the exceptional energy and power densities of 1.38 mW h cm^-2 and 34.1 mW cm^-2 , respectively, measured at the electrode and the device scale within a short subhour timeframe. Such a remarkable high rate performance is better than that of electrodes reported previously for commercial lithium-ion microbatteries, advanced aqueous batteries, and state-of-the-art supercapacitors designed for high-power applications.

Redox Activity of Sodium Vanadium Oxides towards Oxidation in Na Ion Batteries

The search for new materials that could be used as electrode material for Na-ion batteries is one of the most challenging issues of today. Many transition metal oxide families as well as transition metal polyanionic frameworks have been proposed over the last five years. In this work, we report the sodium extraction from Na₂V₃O₇, which is a tunnel type structure built of [V₃O₇]²⁻_∞ nanotubes held by sodium ions. We report a reversible charge capacity of 80 mAh/g at 2.8 V vs. Na⁺/Na due to the V⁵⁺/V⁴⁺ redox activity. No oxygen redox activity has been observed for this material nor for the vanadium (5+) oxide Na₄V₂O₇.

Extraordinary pseudocapacitive energy storage triggered by phase transformation in hierarchical vanadium oxides

Pseudocapacitance holds great promise for improving energy densities of electrochemical supercapacitors, but state-of-the-art pseudocapacitive materials show capacitances far below their theoretical values and deliver much lower levels of electrical power than carbon-based materials due to poor cation accessibility and/or long-range electron transferability. Here we show that in situ corundum-to-rutile phase transformation in electron-correlated vanadium sesquioxide can yield nonstoichiometric rutile vanadium dioxide layers that are composed of highly sodium ion accessible oxygen-deficiency quasi-hexagonal tunnels sandwiched between conductive rutile slabs. This unique structure serves to boost redox and intercalation kinetics for extraordinary pseudocapacitive energy storage in hierarchical isomeric vanadium oxides, leading to a high specific capacitance of ~1856 F g^–1 (almost sixfold that of the pristine vanadium sesquioxide and dioxide) and a bipolar charge/discharge capability at ultrafast rates in aqueous electrolyte. Symmetric wide voltage window pseudocapacitors of vanadium oxides deliver a power density of ~280 W cm^–3 together with an exceptionally high volumetric energy density of ~110 mWh cm^–3 as well as long-term cycling stability.

Pseudocapacitive materials could enable high-performance electrochemical supercapacitors, but their practical capacitance and power density remain low. Here the authors show that in situ phase transformation triggers extraordinary pseudocapacitive energy storage in metallic isomeric vanadium oxides.

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-Ion Batteries

P2-Na_2/3Ni_1/3Mn_2/3O₂ (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na⁺/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, whereas the PVdF electrode exhibits only 80 and 71%, respectively. The alternating current impedance measurements reveal that the PGluNa electrode shows a much lower resistance during the cycles compared with the PVdF one. From the surface analysis and peeling test of the electrodes, the PGluNa binder was found to cover the surface of the P2-NiMn particles and suppresses the electrolyte decomposition and surface degradation. The PGluNa binder further enhance the mechanical strength of the electrodes and suppresses the electrical isolation of the P2-NiMn particles during sodium extraction/insertion. The efficient binder with noticeable adhesion strength and surface coverage of active materials and carbon has paved a new way to enhance the electrochemical performances of high-voltage positive electrode materials for Na-ion batteries.