Cathode properties of sodium iron phosphate glass for sodium ion batteries

• Glassy cathode active materials are successfully fabricated by melt quenching process.
• Large reversible discharge capacity at 115 mAh/g for Na cell was confirmed.
• Reversible discharge capacity is closely related with local structure of glass.

In order to clarify the possibility of cathode activity of Na2O-FeO-P2O5 glasses for sodium ion batteries, glass formation tendency, crystallization behavior, electrical conductivity as well as charge and discharge properties are examined. Glass formation in xFeO–(100-x)NaPO3 was confirmed for x ≤ 45 by melt quenching up to 1000 °C in N2 filled electric furnace. By means of electrical conductivity tests, conductivity was increased with increases of x content in xFeO–(100-x)NaPO3 and the activation energy for electrical conductivity was decreased. Charge and discharge profiles for Na anode exhibits high reversible discharge capacity as 115 mAh/g at 0.1 °C rate in 40FeO-60NaPO3 (30Na2O–40FeO–30P2O5) glass. The results on Raman scattering spectra suggests that glass structure is subjected to polymerized Q1 and Q2 phosphate units. And polymerized phosphates in glass cause lower density, open structure and sodium ion diffusive channels as well.