Electrochemical Properties and In-situ XAFS Observation of Li2O-V2O5-P2O5-Fe2O3 Quaternary-glass and Crystallized-glass Cathodes

Abstract

Electrochemical properties of Li2O-V2O5-P2O5-Fe2O3 quaternary-glass and crystallized-glass cathodes were compared in terms of the local structural changes of vanadium and iron ions with increasing discharge capacity by in-situ X-ray absorption fine structure (XAFS) measurements. 10.3Li2O-69.2V2O5-10.2P2O5-10.3Fe2O3 glass was prepared by the melt-quenching method. The crystallized glass was produced by heat treatment of the glass at 375°C in air. The crystal phase of the crystallized glass was found to be a single phase of β-vanadium bronze. The glass exhibited a discharge capacity of around 300 mAh g−1 and a capacity retention of 66% after 100cycles. On the other hand, the crystallized glass exhibited a capacity of around 340 mAh g−1 and a capacity retention of 68% after 100cycles. In the cases of glass and crystallized glass, in-situ X-ray absorption near-edge spectroscopy (XANES) spectra revealed that a redox reaction of vanadium and iron ions occurs as a compensation reaction for lithium-ion insertion. In addition, the absorption-energy shift of the vanadium and iron K-edges in the case of glass was larger than that in the case of crystallized glass despite the same discharge capacity. The analysis of pre-edge peak intensities and extended X-ray absorption fine structure (EXAFS) analysis revealed that the vanadium and iron coordination symmetry of the glass can transform more easily than that of the crystallized glass under lithium-ion insertion. These results imply that the origin of good cycleability differs in the cases of glass and crystal-phase cathodes.