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