Lithium-Insertion Mechanism in Crystalline and Amorphous FePO4•nH2O

Several techniques including electrochemical measurements, in situ X-ray diffraction, Mossbauer spectroscopy, and X-ray absorption spectroscopy (XAS) were used to study the lithium-insertion/deinsertion properties of five hydrated iron phosphates. Crystalline phosphosiderite (P-FePO4 center dot 2H(2)O) undergoes a complete two-phase lithium-insertion mechanism, giving rise to isosbestic points on the superposition of normalized XAS spectra. Lithium insertion into crystalline strengite (S-FePO4 center dot 2H(2)O) reveals a two-phase region followed by a single-phase one. Amorphous compositions a-FePO4 center dot nH(2)O are described as disordered solid mixtures of [Fe(PO4)(4)(H2O)(2)] clusters: Such a description accounts for both the presence of isosbestic points on the superposition of XAS spectra recorded at different stages of Li insertion and for the sloping open-circuit potential. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3126386] All rights reserved.