Amorphous xNaF-FeSO4 Systems (1 ≤ x ≤ 2) with Excellent Cathode Properties for Sodium-Ion Batteries

NaFeSO4F is a promising cathode for Na-ion batteries because of its high operating voltage and the abundance of iron resources. We synthesized amorphous xNaF-FeSO4 (1 ≤ x ≤ 2) systems with a dry-ball milling method, and heating of the amorphous NaF-FeSO4 successfully gave crystalline NaFeSO4F. The crystalline NaFeSO4F had a reversible capacity of 66 mAh g–1, which is larger than the reported value. Amorphous 1.3NaF-FeSO4 exhibited the largest reversible capacity in the xNaF-FeSO4 (1 ≤ x ≤ 2) series. Although amorphous xNaF-FeSO4 showed a lower rate capability than that of obtained crystalline NaFeSO4F, its reversible capacities in all compositions were larger than that of crystalline NaFeSO4F.