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In this paper, a relationship between the structure and the electrical properties of a nanocrystalline composite ceramics xNa₂O·(100 − x)V₂O₅ with ‘x’ of 5, 15, 25, 35, and 45 mol%, abbreviated as xNV, was investigated by X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), Cyclic Voltammetry (CV), Electrochemical impedance spectroscopy (EIS), and cathode active performance in Na-ion battery (SIB). For the expected sodium vanadium bronzes (Na_xV₂O₅) precipitation, the preparation of xNV was performed by keeping the system in the molten state at 1200 °C for one hour, followed by a temperature decrease in the electric furnace to room temperature at a cooling rate of 10 °C min⁻¹. XRD patterns of the 15NV ceramic exhibited the formation of Na_0.33V₂O₅ and NaV₃O₈ crystalline phases. Moreover, the V K-edge XANES showed that the absorption edge energy of ceramics 15NV recorded at 5479 eV is smaller than that of V₂O₅ at 5481 eV, evidently indicating a partial reduction from V⁵⁺ to V⁴⁺ due to the precipitation of Na_0.33V₂O₅. In the cyclic voltammetry, reduction peaks of 15NV were observed at 1.12, 1.78 V, and 2.69 V, while the oxidation peak showed up only at 2.36 V. The values of the reduction peaks were related to the NaV₃O₈ crystalline phase. Moreover, the diffusion coefficient of Na⁺ (D_Na⁺) gradually decreased from 8.28 × 10⁻¹¹ cm² s⁻¹ to 1.23 × 10⁻¹² cm² s⁻¹ with increasing Na₂O content (x) from 5 to 45 mol%. In the evaluation of the active cathode performance of xNV in SIB, ceramics 15NV showed the highest discharge capacity 203 mAh g⁻¹ at a current rate of 50 mA g⁻¹. In the wider voltage range from 0.8 to 3.6 V, the capacity retention was maintained at 50% after 30 cycles, while it was significantly improved to 90% in the narrower voltage range from 1.8 to 4.0 V, although the initial capacity decreased to 56 mAh g⁻¹. It is concluded that the precipitation of the Na_0.33V₂O₅ phase improved the structural and electrical properties of 15NV, which provides a high capacity for the Na-ion battery when incorporated as a cathode active material. Full article

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. Full article