Enhancement of electrical conductivity and thermal stability of Iron- or Tin- substituted vanadate glass and glass-ceramics nanocomposite to be applied as a high-performance cathode active material in sodium-ion batteries

Abstract

To explore novel vanadate formulations, i.e. newly effective cathode active material for sodium-ion battery (SIB), relationships among the local structure, electrical conductivity, and cathode performance of xNa2O•10 P2O5•(85-x)V2O5•5Fe2O3 and xNa2O•10 P2O5•(85-x) V2O5•5SnO2 (5 <x < 45 mol%) glasses and glass-ceramics nanocomposite abbreviated as xFeV and xSnV, respectively, were systematically investigated before and after heat treatment at 500 ℃ for 100 min. Measurements of 57Fe- and 119Sn Mössbauer spectra (FeMS, SnMS, respectively), X-ray absorption fine structure (XAFS), X-ray diffraction patterns (XRD), Fourier Transform Infrared spectroscopy (FTIR), differential thermal analysis (DTA), electrochemical impedance spectra (EIS) and charge-discharge capacity were carried out in this study. From DTA curves, decreases in glass transition temperature (Tg) and crystallization temperature (Tc) were observed for xFeV and xSnV glasses as ‘x’ increased from 5 to 45 mol% Na2O content. This indicates that the introduction of Na2O reduces thermal stability. Furthermore, the DC conductivity for xFeV and xSnV glasses and glass-ceramics nanocomposite, decreases from 2.82 • 10−5 to 6.61 • 10−7 S cm−1 and from 1.26 • 10−5 to 4.27 • 10−7 S cm−1, respectively, with increasing Na2O content from 5 to 45 mol%. The values of electrical conductivity of xFeV and xSnV are higher than that of xNa2O•10 P2O5•(90-x)V2O5, abbreviated as xV glasses. This indicates that the introduction of Fe2O3 and SnO2 produces an increase in electrical conductivity. 57Fe-Mössbauer spectra of xFeV glass showed decreased quadrupole splitting from 0.73 to 0.63 mm s−1, indicating that FeIII ion forms less distorted tetrahedra upon increasing ‘x’ from 5 to 45 mol%. Additionally, a high capacity of about 200 mAh g−1 was achieved for the SIB made of 5FeV glasses as a cathode. This capacity was larger than the previously reported value of 158 mAh g−1 in Na1.25V3O8. It can be concluded that the introduction of Fe or Sn ions into the phospho-vanadate glass as a cathode causes an increase in durability and conductivity, so these glasses can be considered promising for a high-performance cathode active material with significant improvement of cyclability in SIBs.