Development of electrically conductive ZrO₂-CaO-Fe₂O₃-V₂O₅ glass and glass-ceramics as a new cathode active materials for Na-ion batteries with high performance

Glass-ceramics xZrO₂•10Fe₂O₃•(90-x)V₂O₅ with ‘x’ between 0 and 30 mol% and yZrO₂･(20-y)CaO･10Fe₂O₃･70 V₂O₅ glass with ‘y’ between 0 and 20 mol%, respectively abbreviated as xZFV and yZCFV, before and after heat treatment at 500 °C for 100 min, were evaluated as potential cathode-active materials for sodium-ion batteries (SIBs). Relationships between physical properties and local structure of xZFV and yZCFV glass-ceramics were investigated by ⁵⁷Fe-Mössbauer spectroscopy, V K-edge X-ray absorption near edge structure (XANES), X-ray diffractometry (XRD), DC four-probe method and differential thermal analysis (DTA). SIBs containing heat-treated xZFV glass-ceramics showed the highest discharge capacity of 153 mA h g⁻¹ under a current density of 50 mA•g⁻¹, which exhibited a high electrical conductivity of 1.8 × 10⁻² Scm⁻¹. Precipitation of V_0.05Zr_0.95O₂ and Fe₂V₄O₁₃ nanoparticles were confirmed from the XRD pattern of the heat-treated 20ZFV glass, consistent with the lower energy of the pre-edge peak at 5467 eV in the V K-edge XANES spectrum. This result is associated with the reduction of vanadium ions from V^V to V^IV. It is concluded that the precipitation of stable vanadium bronze phases with high electrical conductivity and structural stability effectively enable the high SIB capacity of these materials.