Effect of Iron Substitution in the Electrochemical Performance of Na3V2(PO4)3 as Cathode for Na-Ion Batteries

By M. J. Aragón, P. Lavela, G. Ortiz, J. Tirado. 2015

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Na 3 V 2-x Fe x (PO 4 ) 3 /C (0 ≤ x ≤ 0.5) samples were prepared by a simple sol-gel method. X-ray diffraction patterns revealed high purity and crystalline phosphate indexed in the R-3c space group. On increasing the iron content, the cell volume increases due to the slightly higher radius of Fe 3 + . Cyclic voltammetry showed two signals at ca. 3.5 and 4.0 V, which evidence a linear dependence of potential on the level of substitution. The appearance of the short plateau at ca. 4.0 V is closely related to the higher iron substitution levels. The evaluation of XPS and 57 Fe M¨ossbauer spectra of charged and discharged electrodes allow ascribing the 4.0 V plateau to V 4 + /V 5 + redox reaction and the extraction/insertion of extra sodium ions from octahedral M1 sites. Galvanostatic cycling showed capacity values as high as 115 mA h g − 1 for samples with x = 0.1, 0.2, 0.3 and exceptional cyclability. The The binding energy values were referred to the C1s line of the adven- titious carbon located at 284.6 eV. Transmission electron microscopy (TEM) images were recorded in a JEOL 200CX microscope. 57 Fe M¨ossbauer spectra were recorded in transmission mode at room tem- perature on an EG&G constant accelerator spectrometer. The isomer shifts values are relative to the center of pure α -Fe. The experimental spectra were fitted to Lorentzian lines by using a least-squares based method. The goodness of the fitting is controlled by the usual χ 2 test. The spectra of charged and discharged electrodes were kept in sealed (RCA-90) plastic bags under argon atmosphere. The electrochemical tests of sodium cells were monitored in two- electrode Swagelok type cells assembled in an argon-filled glove box under a control survey of O 2 and H 2 O traces. The working electrodes were prepared by blending the active material (80%) with carbon black (10%) and PVDF (polyvinylidene fluoride) (10%) in N-methyl- 2-pyrrolidone. To ensure a proper homogenization, the slurry was ultrasonically stirred, cast onto a 9 mm aluminum collectors and vac- uum dried at 120 ◦ C for at least two hours. A 9 mm sodium disk was used as a counter electrode. The electrolyte solution was 1 M NaPF 6 (EC:DEC) (10% FEC) soaking glass fiber separators (GF/A- Whatman). The test cells were galvanostatically cycled between 2.0 and 4.3 V. The charge and discharge rates will be expressed as C/ n , being n the number of hours needed for the insertion of one sodium per formula unit at the applied current intensity. Cyclic voltammo- grams scanned at 0.5 mV s − 1 and Potentiostatic Intermittent Titration

Subfields: Automotive Engineering, Electrical and Electronic Engineering

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