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 FePO 4 ·H 2 O, isostructural to already reported VPO 4 ·H 2 O and MnPO 4 ·H 2 O phases, was obtained from tavorite LiFePO 4 (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 LiFePO 4 OH and FePO 4 ·H 2 O are very similar and both are characterized by chains of FeO 6 octahedra, interconnected through PO 4 tetrahedra, such as the resulting frameworks enclose different types of tunnels. A neutron diffraction study has allowed the localization of hydrogens in the FePO 4 ·H 2 O structure, revealing that the two hydrogen atoms are linked to the oxygen atoms shared by two adjacent FeO 6 octahedra. The presence of these “H 2 O-like” groups inserted along the FeO 6 chains leads to a considerable distortion of the FeO 6 octahedra. The nature of the −OH and −OH 2 groups in LiFePO 4 OH and FePO 4 ·H 2 O, respectively, was confirmed by vibrational spectroscopies. Lithium intercalation was shown to occur in FePO 4 ·H 2 O through the reduction of Fe 3+ to Fe 2+ at an average voltage of ∼2.8 V (vs Li + /Li) with a good cyclability.