Structural and Electrochemical Study of a New Crystalline Hydrated Iron(III) Phosphate FePO₄·H₂O Obtained from LiFePO₄(OH) by Ion Exchange

A new iron(III) phosphate FePO₄·H₂O, isostructural to already reported VPO₄·H₂O and MnPO₄·H₂O phases, was obtained from tavorite LiFePO₄(OH) through a Li⁺/H⁺ exchange. The composition of this new phase was confirmed by different chemical analyses. The ion-exchange reaction was shown to be topotactic; indeed the structures of LiFePO₄OH and FePO₄·H₂O are very similar and both are characterized by chains of FeO₆ octahedra, interconnected through PO₄ tetrahedra, such as the resulting frameworks enclose different types of tunnels. A neutron diffraction study has allowed the localization of hydrogens in the FePO₄·H₂O structure, revealing that the two hydrogen atoms are linked to the oxygen atoms shared by two adjacent FeO₆ octahedra. The presence of these “H₂O-like” groups inserted along the FeO₆ chains leads to a considerable distortion of the FeO₆ octahedra. The nature of the −OH and −OH₂ groups in LiFePO₄OH and FePO₄·H₂O, respectively, was confirmed by vibrational spectroscopies. Lithium intercalation was shown to occur in FePO₄·H₂O through the reduction of Fe³⁺ to Fe²⁺ at an average voltage of ∼2.8 V (vs Li⁺/Li) with a good cyclability.