Mössbauer analysis of Fe ion state in lithium iron phosphate glasses and their glass-ceramics with olivine-type LiFePO4 crystals

The valence and local coordination state of Fe ions in 26Li2O–43FeO–5Nb2O5–26P2O5 (mol%) glasses and glass-ceramics consisting of LiFePO4 crystals (a potential candidate as cathode materials for rechargeable lithium ion batteries) are studied from Fe57 Mössbauer effect spectrum measurements. The main valence of Fe ions in the precursor and crystallized glasses is found to be Fe2+ (more than 80%). Fe2+ ions in the precursor glasses give the values of the isomer shift δ=1.16–1.19mm/s and the quadrupole splitting Δ=2.26–2.40mm/s, indicating the presence of FeO6 octahedra. Olivine-type LiFePO4 crystals in the crystallized glasses show the values of δ=1.22mm/s and Δ=2.94–2.96mm/s, demonstrating that Fe2+ sites in LiFePO4 crystals are highly distorted octahedra. The presence of Fe3+ ions with octahedral and/or tetrahedral local structures are suggested in the precursor glasses. It is proposed that similarities in the valence and local coordination state of Fe2+ in the precursor glasses and LiFePO4 crystals would be one of the reasons why LiFePO4 crystals are easily formed through the crystallization of lithium iron phosphate glasses.