RESUMEN
A slight deviation of the stoichiometry has been introduced in Na3-3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples to determine the effect on the structural and electrochemical behavior as a positive electrode in sodium-ion batteries. X-ray diffraction and XPS results provide evidence for the flexibility of the NASICON framework to allow a limited vanadium superstoichiometry. In particular, the Na2.94V2.02(PO4)3 formula reveals the best electrochemical performance at the highest rate (40C) and capacity retention upon long cycling. It is attributed to the excellent kinetic response and interphase chemical stability upon cycling. The electrochemical performance of this vanadium superstoichiometric sample in a full sodium-ion cell is also described.
RESUMEN
Off-stoichiometric Na3+3x V2-x (PO4 )3 samples have been prepared by a sol-gel route. X-ray diffraction and XPS revealed the flexibility of the NASICON framework to accommodate these deviations of the stoichiometry; at least for low x values. X-ray photoelectron spectra evidenced the presence of Na4 P2 O7 impurities. The synergic combination of the structural deviations and the presence of Na4 P2 O7 impurities induce a significant improvement of the electrochemical performance and cycling stability at high rates, as compared to the stoichiometric Na3 V2 (PO4 )3 sample. The fast kinetic response provided by the induced off-stoichiometry involves a decrease of the cell resistance.
RESUMEN
Coated C+MxOy@Na3V2(PO4)3 samples containing 1.5% or 3.5% wt. of MxOy (Al2O3, MgO or ZnO) have been synthesized by a two-step method including first a citric based sol-gel method for preparing the active material and second an ultrasonic stirring technique to deposit MxOy. The presence of the metal oxides properly coating the surface of the active material is evidenced by XPS and electron microscopy. Galvanostatic cycling of sodium half-cells reveals a significant capacity enhancement for samples coated with 1.5% of metal oxides and an exceptional cycling stability as evidenced by Coulombic efficiencies as high as 95.9% for ZnO@ Na3V2(PO4)3. It is correlated to their low surface layer and charge transfer resistance values. The formation of metal fluorides that remove traces of corrosive HF from the electrolyte is checked by XPS spectroscopy. The feasibility of sodium-ion batteries assembled with C+MxOy@Na3V2(PO4)3 is further verified by evaluating the electrochemical performance of full cells. Particularly, a Graphite//Al2O3@ Na3V2(PO4)3 battery delivers an energy density as high as 260 W h kg-1 and exhibits a Coulombic efficiency of 89.3% after 115 cycles.
RESUMEN
A novel design of a sodium-ion cell is proposed based on the use of nanocrystalline thin films composed of transition metal oxides. X-ray diffraction, Raman spectroscopy and electron microscopy were helpful techniques to unveil the microstructural properties of the pristine nanostructured electrodes. Thus, Raman spectroscopy revealed the presence of amorphous NiO, α-Fe2 O3 (hematite) and γ-Fe2 O3 (maghemite). Also, this technique allowed the calculation of an average particle size of 23.4 Å in the amorphous carbon phase in situ generated on the positive electrode. The full sodium-ion cell performed with a reversible capacity of 100 mA h g(-1) at C/2 with an output voltage of about 1.8 V, corresponding to a specific energy density of about 180 W h kg(-1) . These promising electrochemical performances allow these transition metal thin films obtained by electrochemical deposition to be envisaged as serious competitors for future negative electrodes in sodium-ion batteries.